feat: improve event bus thread safety and async support

Add thread-safe, async-compatible event bus with read–write locking and
handler dependency ordering. Remove blinker dependency and implement
direct dispatch. Improve type safety, error handling, and deterministic
event synchronization.

Refactor tests to auto-wait for async handlers, ensure clean teardown,
and add comprehensive concurrency coverage. Replace thread-local state
in AgentEvaluator with instance-based locking for correct cross-thread
access. Enhance tracing reliability and event finalization.

lib/crewai/pyproject.toml

@@ -35,7 +35,6 @@ dependencies = [
     "uv>=0.4.25",
     "tomli-w>=1.1.0",
     "tomli>=2.0.2",
-    "blinker>=1.9.0",
     "json5>=0.10.0",
     "portalocker==2.7.0",
     "pydantic-settings>=2.10.1",

lib/crewai/src/crewai/events/__init__.py

@@ -5,10 +5,13 @@ This module provides the event infrastructure that allows users to:
 - Track memory operations and performance
 - Build custom logging and analytics
 - Extend CrewAI with custom event handlers
+- Declare handler dependencies for ordered execution
 """
 
 from crewai.events.base_event_listener import BaseEventListener
+from crewai.events.depends import Depends
 from crewai.events.event_bus import crewai_event_bus
+from crewai.events.handler_graph import CircularDependencyError
 from crewai.events.types.agent_events import (
     AgentEvaluationCompletedEvent,
     AgentEvaluationFailedEvent,
@@ -109,6 +112,7 @@ __all__ = [
     "AgentReasoningFailedEvent",
     "AgentReasoningStartedEvent",
     "BaseEventListener",
+    "CircularDependencyError",
     "CrewKickoffCompletedEvent",
     "CrewKickoffFailedEvent",
     "CrewKickoffStartedEvent",
@@ -119,6 +123,7 @@ __all__ = [
     "CrewTrainCompletedEvent",
     "CrewTrainFailedEvent",
     "CrewTrainStartedEvent",
+    "Depends",
     "FlowCreatedEvent",
     "FlowEvent",
     "FlowFinishedEvent",

lib/crewai/src/crewai/events/base_event_listener.py

@@ -9,6 +9,7 @@ class BaseEventListener(ABC):
     def __init__(self):
         super().__init__()
         self.setup_listeners(crewai_event_bus)
+        crewai_event_bus.validate_dependencies()
 
     @abstractmethod
     def setup_listeners(self, crewai_event_bus: CrewAIEventsBus):

lib/crewai/src/crewai/events/depends.py

@@ -0,0 +1,105 @@
+"""Dependency injection system for event handlers.
+
+This module provides a FastAPI-style dependency system that allows event handlers
+to declare dependencies on other handlers, ensuring proper execution order while
+maintaining parallelism where possible.
+"""
+
+from collections.abc import Coroutine
+from typing import Any, Generic, Protocol, TypeVar
+
+from crewai.events.base_events import BaseEvent
+
+
+EventT_co = TypeVar("EventT_co", bound=BaseEvent, contravariant=True)
+
+
+class EventHandler(Protocol[EventT_co]):
+    """Protocol for event handler functions.
+
+    Generic protocol that accepts any subclass of BaseEvent.
+    Handlers can be either synchronous (returning None) or asynchronous
+    (returning a coroutine).
+    """
+
+    def __call__(
+        self, source: Any, event: EventT_co, /
+    ) -> None | Coroutine[Any, Any, None]:
+        """Event handler signature.
+
+        Args:
+            source: The object that emitted the event
+            event: The event instance (any BaseEvent subclass)
+
+        Returns:
+            None for sync handlers, Coroutine for async handlers
+        """
+        ...
+
+
+T = TypeVar("T", bound=EventHandler[Any])
+
+
+class Depends(Generic[T]):
+    """Declares a dependency on another event handler.
+
+    Similar to FastAPI's Depends, this allows handlers to specify that they
+    depend on other handlers completing first. Handlers with dependencies will
+    execute after their dependencies, while independent handlers can run in parallel.
+
+    Args:
+        handler: The handler function that this handler depends on
+
+    Example:
+        >>> from crewai.events import Depends, crewai_event_bus
+        >>> from crewai.events import LLMCallStartedEvent
+        >>> @crewai_event_bus.on(LLMCallStartedEvent)
+        >>> def setup_context(source, event):
+        ...     return {"initialized": True}
+        >>>
+        >>> @crewai_event_bus.on(LLMCallStartedEvent, depends_on=Depends(setup_context))
+        >>> def process(source, event):
+        ...     # Runs after setup_context completes
+        ...     pass
+    """
+
+    def __init__(self, handler: T) -> None:
+        """Initialize a dependency on a handler.
+
+        Args:
+            handler: The handler function this depends on
+        """
+        self.handler = handler
+
+    def __repr__(self) -> str:
+        """Return a string representation of the dependency.
+
+        Returns:
+            A string showing the dependent handler name
+        """
+        handler_name = getattr(self.handler, "__name__", repr(self.handler))
+        return f"Depends({handler_name})"
+
+    def __eq__(self, other: object) -> bool:
+        """Check equality based on the handler reference.
+
+        Args:
+            other: Another Depends instance to compare
+
+        Returns:
+            True if both depend on the same handler, False otherwise
+        """
+        if not isinstance(other, Depends):
+            return False
+        return self.handler is other.handler
+
+    def __hash__(self) -> int:
+        """Return hash based on handler identity.
+
+        Since equality is based on identity (is), we hash the handler
+        object directly rather than its id for consistency.
+
+        Returns:
+            Hash of the handler object
+        """
+        return id(self.handler)

lib/crewai/src/crewai/events/event_bus.py

@@ -1,125 +1,507 @@
-from __future__ import annotations
+"""Event bus for managing and dispatching events in CrewAI.
 
-import threading
-from collections.abc import Callable
+This module provides a singleton event bus that allows registration and handling
+of events throughout the CrewAI system, supporting both synchronous and asynchronous
+event handlers with optional dependency management.
+"""
+
+import asyncio
+import atexit
+from collections.abc import Callable, Generator
+from concurrent.futures import Future, ThreadPoolExecutor
 from contextlib import contextmanager
-from typing import Any, TypeVar, cast
+import threading
+from typing import Any, Final, ParamSpec, TypeVar
 
-from blinker import Signal
+from typing_extensions import Self
 
 from crewai.events.base_events import BaseEvent
-from crewai.events.event_types import EventTypes
+from crewai.events.depends import Depends
+from crewai.events.handler_graph import build_execution_plan
+from crewai.events.types.event_bus_types import (
+    AsyncHandler,
+    AsyncHandlerSet,
+    ExecutionPlan,
+    Handler,
+    SyncHandler,
+    SyncHandlerSet,
+)
+from crewai.events.types.llm_events import LLMStreamChunkEvent
+from crewai.events.utils.console_formatter import ConsoleFormatter
+from crewai.events.utils.handlers import is_async_handler, is_call_handler_safe
+from crewai.events.utils.rw_lock import RWLock
 
-EventT = TypeVar("EventT", bound=BaseEvent)
+
+P = ParamSpec("P")
+R = TypeVar("R")
 
 
 class CrewAIEventsBus:
-    """
-    A singleton event bus that uses blinker signals for event handling.
-    Allows both internal (Flow/Crew) and external event handling.
+    """Singleton event bus for handling events in CrewAI.
+
+    This class manages event registration and emission for both synchronous
+    and asynchronous event handlers, automatically scheduling async handlers
+    in a dedicated background event loop.
+
+    Synchronous handlers execute in a thread pool executor to ensure completion
+    before program exit. Asynchronous handlers execute in a dedicated event loop
+    running in a daemon thread, with graceful shutdown waiting for completion.
+
+    Attributes:
+        _instance: Singleton instance of the event bus
+        _instance_lock: Reentrant lock for singleton initialization (class-level)
+        _rwlock: Read-write lock for handler registration and access (instance-level)
+        _sync_handlers: Mapping of event types to registered synchronous handlers
+        _async_handlers: Mapping of event types to registered asynchronous handlers
+        _sync_executor: Thread pool executor for running synchronous handlers
+        _loop: Dedicated asyncio event loop for async handler execution
+        _loop_thread: Background daemon thread running the event loop
+        _console: Console formatter for error output
     """
 
-    _instance = None
-    _lock = threading.Lock()
+    _instance: Self | None = None
+    _instance_lock: threading.RLock = threading.RLock()
+    _rwlock: RWLock
+    _sync_handlers: dict[type[BaseEvent], SyncHandlerSet]
+    _async_handlers: dict[type[BaseEvent], AsyncHandlerSet]
+    _handler_dependencies: dict[type[BaseEvent], dict[Handler, list[Depends]]]
+    _execution_plan_cache: dict[type[BaseEvent], ExecutionPlan]
+    _console: ConsoleFormatter
+    _shutting_down: bool
 
-    def __new__(cls):
+    def __new__(cls) -> Self:
+        """Create or return the singleton instance.
+
+        Returns:
+            The singleton CrewAIEventsBus instance
+        """
         if cls._instance is None:
-            with cls._lock:
-                if cls._instance is None:  # prevent race condition
+            with cls._instance_lock:
+                if cls._instance is None:
                     cls._instance = super().__new__(cls)
                     cls._instance._initialize()
         return cls._instance
 
     def _initialize(self) -> None:
-        """Initialize the event bus internal state"""
-        self._signal = Signal("crewai_event_bus")
-        self._handlers: dict[type[BaseEvent], list[Callable]] = {}
+        """Initialize the event bus internal state.
+
+        Creates handler dictionaries and starts a dedicated background
+        event loop for async handler execution.
+        """
+        self._shutting_down = False
+        self._rwlock = RWLock()
+        self._sync_handlers: dict[type[BaseEvent], SyncHandlerSet] = {}
+        self._async_handlers: dict[type[BaseEvent], AsyncHandlerSet] = {}
+        self._handler_dependencies: dict[type[BaseEvent], dict[Handler, list[Depends]]] = {}
+        self._execution_plan_cache: dict[type[BaseEvent], ExecutionPlan] = {}
+        self._sync_executor = ThreadPoolExecutor(
+            max_workers=10,
+            thread_name_prefix="CrewAISyncHandler",
+        )
+        self._console = ConsoleFormatter()
+
+        self._loop = asyncio.new_event_loop()
+        self._loop_thread = threading.Thread(
+            target=self._run_loop,
+            name="CrewAIEventsLoop",
+            daemon=True,
+        )
+        self._loop_thread.start()
+
+    def _run_loop(self) -> None:
+        """Run the background async event loop."""
+        asyncio.set_event_loop(self._loop)
+        self._loop.run_forever()
+
+    def _register_handler(
+        self,
+        event_type: type[BaseEvent],
+        handler: Callable[..., Any],
+        dependencies: list[Depends] | None = None,
+    ) -> None:
+        """Register a handler for the given event type.
+
+        Args:
+            event_type: The event class to listen for
+            handler: The handler function to register
+            dependencies: Optional list of dependencies
+        """
+        with self._rwlock.w_locked():
+            if is_async_handler(handler):
+                existing_async = self._async_handlers.get(event_type, frozenset())
+                self._async_handlers[event_type] = existing_async | {handler}
+            else:
+                existing_sync = self._sync_handlers.get(event_type, frozenset())
+                self._sync_handlers[event_type] = existing_sync | {handler}
+
+            if dependencies:
+                if event_type not in self._handler_dependencies:
+                    self._handler_dependencies[event_type] = {}
+                self._handler_dependencies[event_type][handler] = dependencies
+
+            self._execution_plan_cache.pop(event_type, None)
 
     def on(
-        self, event_type: type[EventT]
-    ) -> Callable[[Callable[[Any, EventT], None]], Callable[[Any, EventT], None]]:
-        """
-        Decorator to register an event handler for a specific event type.
+        self,
+        event_type: type[BaseEvent],
+        depends_on: Depends | list[Depends] | None = None,
+    ) -> Callable[[Callable[P, R]], Callable[P, R]]:
+        """Decorator to register an event handler for a specific event type.
 
-        Usage:
-            @crewai_event_bus.on(AgentExecutionCompletedEvent)
-            def on_agent_execution_completed(
-                source: Any, event: AgentExecutionCompletedEvent
-            ):
-                print(f"👍 Agent '{event.agent}' completed task")
-                print(f"   Output: {event.output}")
+        Args:
+            event_type: The event class to listen for
+            depends_on: Optional dependency or list of dependencies. Handlers with
+                       dependencies will execute after their dependencies complete.
+
+        Returns:
+            Decorator function that registers the handler
+
+        Example:
+            >>> from crewai.events import crewai_event_bus, Depends
+            >>> from crewai.events.types.llm_events import LLMCallStartedEvent
+            >>>
+            >>> @crewai_event_bus.on(LLMCallStartedEvent)
+            >>> def setup_context(source, event):
+            ...     print("Setting up context")
+            >>>
+            >>> @crewai_event_bus.on(LLMCallStartedEvent, depends_on=Depends(setup_context))
+            >>> def process(source, event):
+            ...     print("Processing (runs after setup_context)")
         """
 
-        def decorator(
-            handler: Callable[[Any, EventT], None],
-        ) -> Callable[[Any, EventT], None]:
-            if event_type not in self._handlers:
-                self._handlers[event_type] = []
-            self._handlers[event_type].append(
-                cast(Callable[[Any, EventT], None], handler)
-            )
+        def decorator(handler: Callable[P, R]) -> Callable[P, R]:
+            """Register the handler and return it unchanged.
+
+            Args:
+                handler: Event handler function to register
+
+            Returns:
+                The same handler function unchanged
+            """
+            deps = None
+            if depends_on is not None:
+                deps = [depends_on] if isinstance(depends_on, Depends) else depends_on
+
+            self._register_handler(event_type, handler, dependencies=deps)
             return handler
 
         return decorator
 
-    @staticmethod
-    def _call_handler(
-        handler: Callable, source: Any, event: BaseEvent, event_type: type
+    def _call_handlers(
+        self,
+        source: Any,
+        event: BaseEvent,
+        handlers: SyncHandlerSet,
     ) -> None:
-        """Call a single handler with error handling."""
-        try:
-            handler(source, event)
-        except Exception as e:
-            print(
-                f"[EventBus Error] Handler '{handler.__name__}' failed for event '{event_type.__name__}': {e}"
+        """Call provided synchronous handlers.
+
+        Args:
+            source: The emitting object
+            event: The event instance
+            handlers: Frozenset of sync handlers to call
+        """
+        errors: list[tuple[SyncHandler, Exception]] = [
+            (handler, error)
+            for handler in handlers
+            if (error := is_call_handler_safe(handler, source, event)) is not None
+        ]
+
+        if errors:
+            for handler, error in errors:
+                self._console.print(
+                    f"[CrewAIEventsBus] Sync handler error in {handler.__name__}: {error}"
+                )
+
+    async def _acall_handlers(
+        self,
+        source: Any,
+        event: BaseEvent,
+        handlers: AsyncHandlerSet,
+    ) -> None:
+        """Asynchronously call provided async handlers.
+
+        Args:
+            source: The object that emitted the event
+            event: The event instance
+            handlers: Frozenset of async handlers to call
+        """
+        coros = [handler(source, event) for handler in handlers]
+        results = await asyncio.gather(*coros, return_exceptions=True)
+        for handler, result in zip(handlers, results, strict=False):
+            if isinstance(result, Exception):
+                self._console.print(
+                    f"[CrewAIEventsBus] Async handler error in {getattr(handler, '__name__', handler)}: {result}"
+                )
+
+    async def _emit_with_dependencies(self, source: Any, event: BaseEvent) -> None:
+        """Emit an event with dependency-aware handler execution.
+
+        Handlers are grouped into execution levels based on their dependencies.
+        Within each level, async handlers run concurrently while sync handlers
+        run sequentially (or in thread pool). Each level completes before the
+        next level starts.
+
+        Uses a cached execution plan for performance. The plan is built once
+        per event type and cached until handlers are modified.
+
+        Args:
+            source: The emitting object
+            event: The event instance to emit
+        """
+        event_type = type(event)
+
+        with self._rwlock.r_locked():
+            if self._shutting_down:
+                return
+            cached_plan = self._execution_plan_cache.get(event_type)
+            if cached_plan is not None:
+                sync_handlers = self._sync_handlers.get(event_type, frozenset())
+                async_handlers = self._async_handlers.get(event_type, frozenset())
+
+        if cached_plan is None:
+            with self._rwlock.w_locked():
+                if self._shutting_down:
+                    return
+                cached_plan = self._execution_plan_cache.get(event_type)
+                if cached_plan is None:
+                    sync_handlers = self._sync_handlers.get(event_type, frozenset())
+                    async_handlers = self._async_handlers.get(event_type, frozenset())
+                    dependencies = dict(self._handler_dependencies.get(event_type, {}))
+                    all_handlers = list(sync_handlers | async_handlers)
+
+                    if not all_handlers:
+                        return
+
+                    cached_plan = build_execution_plan(all_handlers, dependencies)
+                    self._execution_plan_cache[event_type] = cached_plan
+                else:
+                    sync_handlers = self._sync_handlers.get(event_type, frozenset())
+                    async_handlers = self._async_handlers.get(event_type, frozenset())
+
+        for level in cached_plan:
+            level_sync = frozenset(h for h in level if h in sync_handlers)
+            level_async = frozenset(h for h in level if h in async_handlers)
+
+            if level_sync:
+                if event_type is LLMStreamChunkEvent:
+                    self._call_handlers(source, event, level_sync)
+                else:
+                    future = self._sync_executor.submit(
+                        self._call_handlers, source, event, level_sync
+                    )
+                    await asyncio.get_running_loop().run_in_executor(
+                        None, future.result
+                    )
+
+            if level_async:
+                await self._acall_handlers(source, event, level_async)
+
+    def emit(self, source: Any, event: BaseEvent) -> Future[None] | None:
+        """Emit an event to all registered handlers.
+
+        If handlers have dependencies (registered with depends_on), they execute
+        in dependency order. Otherwise, handlers execute as before (sync in thread
+        pool, async fire-and-forget).
+
+        Stream chunk events always execute synchronously to preserve ordering.
+
+        Args:
+            source: The emitting object
+            event: The event instance to emit
+
+        Returns:
+            Future that completes when handlers finish. Returns:
+            - Future for sync-only handlers (ThreadPoolExecutor future)
+            - Future for async handlers or mixed handlers (asyncio future)
+            - Future for dependency-managed handlers (asyncio future)
+            - None if no handlers or sync stream chunk events
+
+        Example:
+            >>> future = crewai_event_bus.emit(source, event)
+            >>> if future:
+            ...     await asyncio.wrap_future(future)  # In async test
+            ...     # or future.result(timeout=5.0) in sync code
+        """
+        event_type = type(event)
+
+        with self._rwlock.r_locked():
+            if self._shutting_down:
+                self._console.print(
+                    "[CrewAIEventsBus] Warning: Attempted to emit event during shutdown. Ignoring."
+                )
+                return None
+            has_dependencies = event_type in self._handler_dependencies
+            sync_handlers = self._sync_handlers.get(event_type, frozenset())
+            async_handlers = self._async_handlers.get(event_type, frozenset())
+
+        if has_dependencies:
+            return asyncio.run_coroutine_threadsafe(
+                self._emit_with_dependencies(source, event),
+                self._loop,
             )
 
-    def emit(self, source: Any, event: BaseEvent) -> None:
-        """
-        Emit an event to all registered handlers
+        if sync_handlers:
+            if event_type is LLMStreamChunkEvent:
+                self._call_handlers(source, event, sync_handlers)
+            else:
+                sync_future = self._sync_executor.submit(
+                    self._call_handlers, source, event, sync_handlers
+                )
+                if not async_handlers:
+                    return sync_future
+
+        if async_handlers:
+            return asyncio.run_coroutine_threadsafe(
+                self._acall_handlers(source, event, async_handlers),
+                self._loop,
+            )
+
+        return None
+
+    async def aemit(self, source: Any, event: BaseEvent) -> None:
+        """Asynchronously emit an event to registered async handlers.
+
+        Only processes async handlers. Use in async contexts.
 
         Args:
             source: The object emitting the event
             event: The event instance to emit
         """
-        for event_type, handlers in self._handlers.items():
-            if isinstance(event, event_type):
-                for handler in handlers:
-                    self._call_handler(handler, source, event, event_type)
+        event_type = type(event)
 
-        self._signal.send(source, event=event)
+        with self._rwlock.r_locked():
+            if self._shutting_down:
+                self._console.print(
+                    "[CrewAIEventsBus] Warning: Attempted to emit event during shutdown. Ignoring."
+                )
+                return
+            async_handlers = self._async_handlers.get(event_type, frozenset())
+
+        if async_handlers:
+            await self._acall_handlers(source, event, async_handlers)
 
     def register_handler(
-        self, event_type: type[EventTypes], handler: Callable[[Any, EventTypes], None]
+        self,
+        event_type: type[BaseEvent],
+        handler: SyncHandler | AsyncHandler,
     ) -> None:
-        """Register an event handler for a specific event type"""
-        if event_type not in self._handlers:
-            self._handlers[event_type] = []
-        self._handlers[event_type].append(
-            cast(Callable[[Any, EventTypes], None], handler)
-        )
+        """Register an event handler for a specific event type.
+
+        Args:
+            event_type: The event class to listen for
+            handler: The handler function to register
+        """
+        self._register_handler(event_type, handler)
+
+    def validate_dependencies(self) -> None:
+        """Validate all registered handler dependencies.
+
+        Attempts to build execution plans for all event types with dependencies.
+        This detects circular dependencies and cross-event-type dependencies
+        before events are emitted.
+
+        Raises:
+            CircularDependencyError: If circular dependencies or unresolved
+                dependencies (e.g., cross-event-type) are detected
+        """
+        with self._rwlock.r_locked():
+            for event_type in self._handler_dependencies:
+                sync_handlers = self._sync_handlers.get(event_type, frozenset())
+                async_handlers = self._async_handlers.get(event_type, frozenset())
+                dependencies = dict(self._handler_dependencies.get(event_type, {}))
+                all_handlers = list(sync_handlers | async_handlers)
+
+                if all_handlers and dependencies:
+                    build_execution_plan(all_handlers, dependencies)
 
     @contextmanager
-    def scoped_handlers(self):
-        """
-        Context manager for temporary event handling scope.
-        Useful for testing or temporary event handling.
+    def scoped_handlers(self) -> Generator[None, Any, None]:
+        """Context manager for temporary event handling scope.
 
-        Usage:
-            with crewai_event_bus.scoped_handlers():
-                @crewai_event_bus.on(CrewKickoffStarted)
-                def temp_handler(source, event):
-                    print("Temporary handler")
-                # Do stuff...
-            # Handlers are cleared after the context
+        Useful for testing or temporary event handling. All handlers registered
+        within this context are cleared when the context exits.
+
+        Example:
+            >>> from crewai.events.event_bus import crewai_event_bus
+            >>> from crewai.events.event_types import CrewKickoffStartedEvent
+            >>> with crewai_event_bus.scoped_handlers():
+            ...
+            ...     @crewai_event_bus.on(CrewKickoffStartedEvent)
+            ...     def temp_handler(source, event):
+            ...         print("Temporary handler")
+            ...
+            ...     # Do stuff...
+            ... # Handlers are cleared after the context
         """
-        previous_handlers = self._handlers.copy()
-        self._handlers.clear()
+        with self._rwlock.w_locked():
+            prev_sync = self._sync_handlers
+            prev_async = self._async_handlers
+            prev_deps = self._handler_dependencies
+            prev_cache = self._execution_plan_cache
+            self._sync_handlers = {}
+            self._async_handlers = {}
+            self._handler_dependencies = {}
+            self._execution_plan_cache = {}
+
         try:
             yield
         finally:
-            self._handlers = previous_handlers
+            with self._rwlock.w_locked():
+                self._sync_handlers = prev_sync
+                self._async_handlers = prev_async
+                self._handler_dependencies = prev_deps
+                self._execution_plan_cache = prev_cache
+
+    def shutdown(self, wait: bool = True) -> None:
+        """Gracefully shutdown the event loop and wait for all tasks to finish.
+
+        Args:
+            wait: If True, wait for all pending tasks to complete before stopping.
+                  If False, cancel all pending tasks immediately.
+        """
+        with self._rwlock.w_locked():
+            self._shutting_down = True
+            loop = getattr(self, "_loop", None)
+
+        if loop is None or loop.is_closed():
+            return
+
+        if wait:
+
+            async def _wait_for_all_tasks() -> None:
+                tasks = {
+                    t
+                    for t in asyncio.all_tasks(loop)
+                    if t is not asyncio.current_task()
+                }
+                if tasks:
+                    await asyncio.gather(*tasks, return_exceptions=True)
+
+            future = asyncio.run_coroutine_threadsafe(_wait_for_all_tasks(), loop)
+            try:
+                future.result()
+            except Exception as e:
+                self._console.print(f"[CrewAIEventsBus] Error waiting for tasks: {e}")
+        else:
+
+            def _cancel_tasks() -> None:
+                for task in asyncio.all_tasks(loop):
+                    if task is not asyncio.current_task():
+                        task.cancel()
+
+            loop.call_soon_threadsafe(_cancel_tasks)
+
+        loop.call_soon_threadsafe(loop.stop)
+        self._loop_thread.join()
+        loop.close()
+        self._sync_executor.shutdown(wait=wait)
+
+        with self._rwlock.w_locked():
+            self._sync_handlers.clear()
+            self._async_handlers.clear()
+            self._execution_plan_cache.clear()
 
 
-# Global instance
-crewai_event_bus = CrewAIEventsBus()
+crewai_event_bus: Final[CrewAIEventsBus] = CrewAIEventsBus()
+
+atexit.register(crewai_event_bus.shutdown)

lib/crewai/src/crewai/events/handler_graph.py

@@ -0,0 +1,130 @@
+"""Dependency graph resolution for event handlers.
+
+This module resolves handler dependencies into execution levels, ensuring
+handlers execute in correct order while maximizing parallelism.
+"""
+
+from collections import defaultdict, deque
+from collections.abc import Sequence
+
+from crewai.events.depends import Depends
+from crewai.events.types.event_bus_types import ExecutionPlan, Handler
+
+
+class CircularDependencyError(Exception):
+    """Exception raised when circular dependencies are detected in event handlers.
+
+    Attributes:
+        handlers: The handlers involved in the circular dependency
+    """
+
+    def __init__(self, handlers: list[Handler]) -> None:
+        """Initialize the circular dependency error.
+
+        Args:
+            handlers: The handlers involved in the circular dependency
+        """
+        handler_names = ", ".join(
+            getattr(h, "__name__", repr(h)) for h in handlers[:5]
+        )
+        message = f"Circular dependency detected in event handlers: {handler_names}"
+        super().__init__(message)
+        self.handlers = handlers
+
+
+class HandlerGraph:
+    """Resolves handler dependencies into parallel execution levels.
+
+    Handlers are organized into levels where:
+    - Level 0: Handlers with no dependencies (can run first)
+    - Level N: Handlers that depend on handlers in levels 0...N-1
+
+    Handlers within the same level can execute in parallel.
+
+    Attributes:
+        levels: List of handler sets, where each level can execute in parallel
+    """
+
+    def __init__(
+        self,
+        handlers: dict[Handler, list[Depends]],
+    ) -> None:
+        """Initialize the dependency graph.
+
+        Args:
+            handlers: Mapping of handler -> list of `crewai.events.depends.Depends` objects
+        """
+        self.handlers = handlers
+        self.levels: ExecutionPlan = []
+        self._resolve()
+
+    def _resolve(self) -> None:
+        """Resolve dependencies into execution levels using topological sort."""
+        dependents: dict[Handler, set[Handler]] = defaultdict(set)
+        in_degree: dict[Handler, int] = {}
+
+        for handler in self.handlers:
+            in_degree[handler] = 0
+
+        for handler, deps in self.handlers.items():
+            in_degree[handler] = len(deps)
+            for dep in deps:
+                dependents[dep.handler].add(handler)
+
+        queue: deque[Handler] = deque(
+            [h for h, deg in in_degree.items() if deg == 0]
+        )
+
+        while queue:
+            current_level: set[Handler] = set()
+
+            for _ in range(len(queue)):
+                handler = queue.popleft()
+                current_level.add(handler)
+
+                for dependent in dependents[handler]:
+                    in_degree[dependent] -= 1
+                    if in_degree[dependent] == 0:
+                        queue.append(dependent)
+
+            if current_level:
+                self.levels.append(current_level)
+
+        remaining = [h for h, deg in in_degree.items() if deg > 0]
+        if remaining:
+            raise CircularDependencyError(remaining)
+
+    def get_execution_plan(self) -> ExecutionPlan:
+        """Get the ordered execution plan.
+
+        Returns:
+            List of handler sets, where each set represents handlers that can
+            execute in parallel. Sets are ordered such that dependencies are
+            satisfied.
+        """
+        return self.levels
+
+
+def build_execution_plan(
+    handlers: Sequence[Handler],
+    dependencies: dict[Handler, list[Depends]],
+) -> ExecutionPlan:
+    """Build an execution plan from handlers and their dependencies.
+
+    Args:
+        handlers: All handlers for an event type
+        dependencies: Mapping of handler -> list of dependencies
+
+    Returns:
+        Execution plan as list of levels, where each level is a set of
+        handlers that can execute in parallel
+
+    Raises:
+        CircularDependencyError: If circular dependencies are detected
+    """
+    handler_dict: dict[Handler, list[Depends]] = {
+        h: dependencies.get(h, []) for h in handlers
+    }
+
+    graph = HandlerGraph(handler_dict)
+    return graph.get_execution_plan()

lib/crewai/src/crewai/events/listeners/tracing/trace_batch_manager.py

@@ -1,8 +1,9 @@
-import uuid
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from logging import getLogger
+from threading import Condition, Lock
 from typing import Any
+import uuid
 
 from rich.console import Console
 from rich.panel import Panel
@@ -14,6 +15,7 @@ from crewai.events.listeners.tracing.types import TraceEvent
 from crewai.events.listeners.tracing.utils import should_auto_collect_first_time_traces
 from crewai.utilities.constants import CREWAI_BASE_URL
 
+
 logger = getLogger(__name__)
 
 
@@ -41,6 +43,11 @@ class TraceBatchManager:
     """Single responsibility: Manage batches and event buffering"""
 
     def __init__(self):
+        self._init_lock = Lock()
+        self._pending_events_lock = Lock()
+        self._pending_events_cv = Condition(self._pending_events_lock)
+        self._pending_events_count = 0
+
         self.is_current_batch_ephemeral: bool = False
         self.trace_batch_id: str | None = None
         self.current_batch: TraceBatch | None = None
@@ -64,24 +71,28 @@ class TraceBatchManager:
         execution_metadata: dict[str, Any],
         use_ephemeral: bool = False,
     ) -> TraceBatch:
-        """Initialize a new trace batch"""
-        self.current_batch = TraceBatch(
-            user_context=user_context, execution_metadata=execution_metadata
-        )
-        self.event_buffer.clear()
-        self.is_current_batch_ephemeral = use_ephemeral
+        """Initialize a new trace batch (thread-safe)"""
+        with self._init_lock:
+            if self.current_batch is not None:
+                logger.debug("Batch already initialized, skipping duplicate initialization")
+                return self.current_batch
 
-        self.record_start_time("execution")
-
-        if should_auto_collect_first_time_traces():
-            self.trace_batch_id = self.current_batch.batch_id
-        else:
-            self._initialize_backend_batch(
-                user_context, execution_metadata, use_ephemeral
+            self.current_batch = TraceBatch(
+                user_context=user_context, execution_metadata=execution_metadata
             )
-            self.backend_initialized = True
+            self.is_current_batch_ephemeral = use_ephemeral
 
-        return self.current_batch
+            self.record_start_time("execution")
+
+            if should_auto_collect_first_time_traces():
+                self.trace_batch_id = self.current_batch.batch_id
+            else:
+                self._initialize_backend_batch(
+                    user_context, execution_metadata, use_ephemeral
+                )
+                self.backend_initialized = True
+
+            return self.current_batch
 
     def _initialize_backend_batch(
         self,
@@ -148,6 +159,38 @@ class TraceBatchManager:
                 f"Error initializing trace batch: {e}. Continuing without tracing."
             )
 
+    def begin_event_processing(self):
+        """Mark that an event handler started processing (for synchronization)"""
+        with self._pending_events_lock:
+            self._pending_events_count += 1
+
+    def end_event_processing(self):
+        """Mark that an event handler finished processing (for synchronization)"""
+        with self._pending_events_cv:
+            self._pending_events_count -= 1
+            if self._pending_events_count == 0:
+                self._pending_events_cv.notify_all()
+
+    def wait_for_pending_events(self, timeout: float = 2.0) -> bool:
+        """Wait for all pending event handlers to finish processing
+
+        Args:
+            timeout: Maximum time to wait in seconds (default: 2.0)
+
+        Returns:
+            True if all handlers completed, False if timeout occurred
+        """
+        with self._pending_events_cv:
+            if self._pending_events_count > 0:
+                logger.debug(f"Waiting for {self._pending_events_count} pending event handlers...")
+                self._pending_events_cv.wait(timeout)
+                if self._pending_events_count > 0:
+                    logger.error(
+                        f"Timeout waiting for event handlers. {self._pending_events_count} still pending. Events may be incomplete!"
+                    )
+                    return False
+        return True
+
     def add_event(self, trace_event: TraceEvent):
         """Add event to buffer"""
         self.event_buffer.append(trace_event)
@@ -180,8 +223,8 @@ class TraceBatchManager:
                 self.event_buffer.clear()
                 return 200
 
-            logger.warning(
-                f"Failed to send events: {response.status_code}. Events will be lost."
+            logger.error(
+                f"Failed to send events: {response.status_code}. Response: {response.text}. Events will be lost."
             )
             return 500
 
@@ -196,15 +239,33 @@ class TraceBatchManager:
         if not self.current_batch:
             return None
 
-        self.current_batch.events = self.event_buffer.copy()
-        if self.event_buffer:
+        all_handlers_completed = self.wait_for_pending_events(timeout=2.0)
+
+        if not all_handlers_completed:
+            logger.error("Event handler timeout - marking batch as failed due to incomplete events")
+            self.plus_api.mark_trace_batch_as_failed(
+                self.trace_batch_id, "Timeout waiting for event handlers - events incomplete"
+            )
+            return None
+
+        sorted_events = sorted(
+            self.event_buffer,
+            key=lambda e: e.timestamp if hasattr(e, 'timestamp') and e.timestamp else ''
+        )
+
+        self.current_batch.events = sorted_events
+        events_sent_count = len(sorted_events)
+        if sorted_events:
+            original_buffer = self.event_buffer
+            self.event_buffer = sorted_events
             events_sent_to_backend_status = self._send_events_to_backend()
+            self.event_buffer = original_buffer
             if events_sent_to_backend_status == 500:
                 self.plus_api.mark_trace_batch_as_failed(
                     self.trace_batch_id, "Error sending events to backend"
                 )
                 return None
-        self._finalize_backend_batch()
+        self._finalize_backend_batch(events_sent_count)
 
         finalized_batch = self.current_batch
 
@@ -220,18 +281,20 @@ class TraceBatchManager:
 
         return finalized_batch
 
-    def _finalize_backend_batch(self):
-        """Send batch finalization to backend"""
+    def _finalize_backend_batch(self, events_count: int = 0):
+        """Send batch finalization to backend
+
+        Args:
+            events_count: Number of events that were successfully sent
+        """
         if not self.plus_api or not self.trace_batch_id:
             return
 
         try:
-            total_events = len(self.current_batch.events) if self.current_batch else 0
-
             payload = {
                 "status": "completed",
                 "duration_ms": self.calculate_duration("execution"),
-                "final_event_count": total_events,
+                "final_event_count": events_count,
             }
 
             response = (

lib/crewai/src/crewai/events/listeners/tracing/trace_listener.py

@@ -170,14 +170,6 @@ class TraceCollectionListener(BaseEventListener):
         def on_flow_finished(source, event):
             self._handle_trace_event("flow_finished", source, event)
 
-            if self.batch_manager.batch_owner_type == "flow":
-                if self.first_time_handler.is_first_time:
-                    self.first_time_handler.mark_events_collected()
-                    self.first_time_handler.handle_execution_completion()
-                else:
-                    # Normal flow finalization
-                    self.batch_manager.finalize_batch()
-
         @event_bus.on(FlowPlotEvent)
         def on_flow_plot(source, event):
             self._handle_action_event("flow_plot", source, event)
@@ -383,10 +375,12 @@ class TraceCollectionListener(BaseEventListener):
 
     def _handle_trace_event(self, event_type: str, source: Any, event: Any):
         """Generic handler for context end events"""
-
-        trace_event = self._create_trace_event(event_type, source, event)
-
-        self.batch_manager.add_event(trace_event)
+        self.batch_manager.begin_event_processing()
+        try:
+            trace_event = self._create_trace_event(event_type, source, event)
+            self.batch_manager.add_event(trace_event)
+        finally:
+            self.batch_manager.end_event_processing()
 
     def _handle_action_event(self, event_type: str, source: Any, event: Any):
         """Generic handler for action events (LLM calls, tool usage)"""
@@ -399,18 +393,29 @@ class TraceCollectionListener(BaseEventListener):
             }
             self.batch_manager.initialize_batch(user_context, execution_metadata)
 
-        trace_event = self._create_trace_event(event_type, source, event)
-        self.batch_manager.add_event(trace_event)
+        self.batch_manager.begin_event_processing()
+        try:
+            trace_event = self._create_trace_event(event_type, source, event)
+            self.batch_manager.add_event(trace_event)
+        finally:
+            self.batch_manager.end_event_processing()
 
     def _create_trace_event(
         self, event_type: str, source: Any, event: Any
     ) -> TraceEvent:
         """Create a trace event"""
-        trace_event = TraceEvent(
-            type=event_type,
-        )
+        if hasattr(event, 'timestamp') and event.timestamp:
+            trace_event = TraceEvent(
+                type=event_type,
+                timestamp=event.timestamp.isoformat(),
+            )
+        else:
+            trace_event = TraceEvent(
+                type=event_type,
+            )
 
         trace_event.event_data = self._build_event_data(event_type, event, source)
+
         return trace_event
 
     def _build_event_data(

lib/crewai/src/crewai/events/types/event_bus_types.py

@@ -0,0 +1,14 @@
+"""Type definitions for event handlers."""
+
+from collections.abc import Callable, Coroutine
+from typing import Any, TypeAlias
+
+from crewai.events.base_events import BaseEvent
+
+SyncHandler: TypeAlias = Callable[[Any, BaseEvent], None]
+AsyncHandler: TypeAlias = Callable[[Any, BaseEvent], Coroutine[Any, Any, None]]
+SyncHandlerSet: TypeAlias = frozenset[SyncHandler]
+AsyncHandlerSet: TypeAlias = frozenset[AsyncHandler]
+
+Handler: TypeAlias = Callable[[Any, BaseEvent], Any]
+ExecutionPlan: TypeAlias = list[set[Handler]]

lib/crewai/src/crewai/events/utils/handlers.py

@@ -0,0 +1,59 @@
+"""Handler utility functions for event processing."""
+
+import functools
+import inspect
+from typing import Any
+
+from typing_extensions import TypeIs
+
+from crewai.events.base_events import BaseEvent
+from crewai.events.types.event_bus_types import AsyncHandler, SyncHandler
+
+
+def is_async_handler(
+    handler: Any,
+) -> TypeIs[AsyncHandler]:
+    """Type guard to check if handler is an async handler.
+
+    Args:
+        handler: The handler to check
+
+    Returns:
+        True if handler is an async coroutine function
+    """
+    try:
+        if inspect.iscoroutinefunction(handler) or (
+            callable(handler) and inspect.iscoroutinefunction(handler.__call__)
+        ):
+            return True
+    except AttributeError:
+        return False
+
+    if isinstance(handler, functools.partial) and inspect.iscoroutinefunction(
+        handler.func
+    ):
+        return True
+
+    return False
+
+
+def is_call_handler_safe(
+    handler: SyncHandler,
+    source: Any,
+    event: BaseEvent,
+) -> Exception | None:
+    """Safely call a single handler and return any exception.
+
+    Args:
+        handler: The handler function to call
+        source: The object that emitted the event
+        event: The event instance
+
+    Returns:
+        Exception if handler raised one, None otherwise
+    """
+    try:
+        handler(source, event)
+        return None
+    except Exception as e:
+        return e

lib/crewai/src/crewai/events/utils/rw_lock.py

@@ -0,0 +1,81 @@
+"""Read-write lock for thread-safe concurrent access.
+
+This module provides a reader-writer lock implementation that allows multiple
+concurrent readers or a single exclusive writer.
+"""
+
+from collections.abc import Generator
+from contextlib import contextmanager
+from threading import Condition
+
+
+class RWLock:
+    """Read-write lock for managing concurrent read and exclusive write access.
+
+    Allows multiple threads to acquire read locks simultaneously, but ensures
+    exclusive access for write operations. Writers are prioritized when waiting.
+
+    Attributes:
+        _cond: Condition variable for coordinating lock access
+        _readers: Count of active readers
+        _writer: Whether a writer currently holds the lock
+    """
+
+    def __init__(self) -> None:
+        """Initialize the read-write lock."""
+        self._cond = Condition()
+        self._readers = 0
+        self._writer = False
+
+    def r_acquire(self) -> None:
+        """Acquire a read lock, blocking if a writer holds the lock."""
+        with self._cond:
+            while self._writer:
+                self._cond.wait()
+            self._readers += 1
+
+    def r_release(self) -> None:
+        """Release a read lock and notify waiting writers if last reader."""
+        with self._cond:
+            self._readers -= 1
+            if self._readers == 0:
+                self._cond.notify_all()
+
+    @contextmanager
+    def r_locked(self) -> Generator[None, None, None]:
+        """Context manager for acquiring a read lock.
+
+        Yields:
+            None
+        """
+        try:
+            self.r_acquire()
+            yield
+        finally:
+            self.r_release()
+
+    def w_acquire(self) -> None:
+        """Acquire a write lock, blocking if any readers or writers are active."""
+        with self._cond:
+            while self._writer or self._readers > 0:
+                self._cond.wait()
+            self._writer = True
+
+    def w_release(self) -> None:
+        """Release a write lock and notify all waiting threads."""
+        with self._cond:
+            self._writer = False
+            self._cond.notify_all()
+
+    @contextmanager
+    def w_locked(self) -> Generator[None, None, None]:
+        """Context manager for acquiring a write lock.
+
+        Yields:
+            None
+        """
+        try:
+            self.w_acquire()
+            yield
+        finally:
+            self.w_release()

lib/crewai/src/crewai/experimental/evaluation/agent_evaluator.py

@@ -52,19 +52,14 @@ class AgentEvaluator:
         self.console_formatter = ConsoleFormatter()
         self.display_formatter = EvaluationDisplayFormatter()
 
-        self._thread_local: threading.local = threading.local()
+        self._execution_state = ExecutionState()
+        self._state_lock = threading.Lock()
 
         for agent in self.agents:
             self._execution_state.agent_evaluators[str(agent.id)] = self.evaluators
 
         self._subscribe_to_events()
 
-    @property
-    def _execution_state(self) -> ExecutionState:
-        if not hasattr(self._thread_local, "execution_state"):
-            self._thread_local.execution_state = ExecutionState()
-        return self._thread_local.execution_state
-
     def _subscribe_to_events(self) -> None:
         from typing import cast
 
@@ -112,21 +107,22 @@ class AgentEvaluator:
                 state=state,
             )
 
-            current_iteration = self._execution_state.iteration
-            if current_iteration not in self._execution_state.iterations_results:
-                self._execution_state.iterations_results[current_iteration] = {}
+            with self._state_lock:
+                current_iteration = self._execution_state.iteration
+                if current_iteration not in self._execution_state.iterations_results:
+                    self._execution_state.iterations_results[current_iteration] = {}
+
+                if (
+                    agent.role
+                    not in self._execution_state.iterations_results[current_iteration]
+                ):
+                    self._execution_state.iterations_results[current_iteration][
+                        agent.role
+                    ] = []
 
-            if (
-                agent.role
-                not in self._execution_state.iterations_results[current_iteration]
-            ):
                 self._execution_state.iterations_results[current_iteration][
                     agent.role
-                ] = []
-
-            self._execution_state.iterations_results[current_iteration][
-                agent.role
-            ].append(result)
+                ].append(result)
 
     def _handle_lite_agent_completed(
         self, source: object, event: LiteAgentExecutionCompletedEvent
@@ -164,22 +160,23 @@ class AgentEvaluator:
                 state=state,
             )
 
-            current_iteration = self._execution_state.iteration
-            if current_iteration not in self._execution_state.iterations_results:
-                self._execution_state.iterations_results[current_iteration] = {}
+            with self._state_lock:
+                current_iteration = self._execution_state.iteration
+                if current_iteration not in self._execution_state.iterations_results:
+                    self._execution_state.iterations_results[current_iteration] = {}
+
+                agent_role = target_agent.role
+                if (
+                    agent_role
+                    not in self._execution_state.iterations_results[current_iteration]
+                ):
+                    self._execution_state.iterations_results[current_iteration][
+                        agent_role
+                    ] = []
 
-            agent_role = target_agent.role
-            if (
-                agent_role
-                not in self._execution_state.iterations_results[current_iteration]
-            ):
                 self._execution_state.iterations_results[current_iteration][
                     agent_role
-                ] = []
-
-            self._execution_state.iterations_results[current_iteration][
-                agent_role
-            ].append(result)
+                ].append(result)
 
     def set_iteration(self, iteration: int) -> None:
         self._execution_state.iteration = iteration

lib/crewai/src/crewai/flow/flow.py

@@ -3,6 +3,7 @@ import copy
 import inspect
 import logging
 from collections.abc import Callable
+from concurrent.futures import Future
 from typing import Any, ClassVar, Generic, TypeVar, cast
 from uuid import uuid4
 
@@ -463,6 +464,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
         self._completed_methods: set[str] = set()  # Track completed methods for reload
         self._persistence: FlowPersistence | None = persistence
         self._is_execution_resuming: bool = False
+        self._event_futures: list[Future[None]] = []
 
         # Initialize state with initial values
         self._state = self._create_initial_state()
@@ -855,7 +857,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     self._initialize_state(filtered_inputs)
 
             # Emit FlowStartedEvent and log the start of the flow.
-            crewai_event_bus.emit(
+            future = crewai_event_bus.emit(
                 self,
                 FlowStartedEvent(
                     type="flow_started",
@@ -863,6 +865,8 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     inputs=inputs,
                 ),
             )
+            if future:
+                self._event_futures.append(future)
             self._log_flow_event(
                 f"Flow started with ID: {self.flow_id}", color="bold_magenta"
             )
@@ -881,7 +885,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
 
             final_output = self._method_outputs[-1] if self._method_outputs else None
 
-            crewai_event_bus.emit(
+            future = crewai_event_bus.emit(
                 self,
                 FlowFinishedEvent(
                     type="flow_finished",
@@ -889,6 +893,25 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     result=final_output,
                 ),
             )
+            if future:
+                self._event_futures.append(future)
+
+            if self._event_futures:
+                await asyncio.gather(*[asyncio.wrap_future(f) for f in self._event_futures])
+                self._event_futures.clear()
+
+            if (
+                is_tracing_enabled()
+                or self.tracing
+                or should_auto_collect_first_time_traces()
+            ):
+                trace_listener = TraceCollectionListener()
+                if trace_listener.batch_manager.batch_owner_type == "flow":
+                    if trace_listener.first_time_handler.is_first_time:
+                        trace_listener.first_time_handler.mark_events_collected()
+                        trace_listener.first_time_handler.handle_execution_completion()
+                    else:
+                        trace_listener.batch_manager.finalize_batch()
 
             return final_output
         finally:
@@ -971,7 +994,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
             dumped_params = {f"_{i}": arg for i, arg in enumerate(args)} | (
                 kwargs or {}
             )
-            crewai_event_bus.emit(
+            future = crewai_event_bus.emit(
                 self,
                 MethodExecutionStartedEvent(
                     type="method_execution_started",
@@ -981,6 +1004,8 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     state=self._copy_state(),
                 ),
             )
+            if future:
+                self._event_futures.append(future)
 
             result = (
                 await method(*args, **kwargs)
@@ -994,7 +1019,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
             )
 
             self._completed_methods.add(method_name)
-            crewai_event_bus.emit(
+            future = crewai_event_bus.emit(
                 self,
                 MethodExecutionFinishedEvent(
                     type="method_execution_finished",
@@ -1004,10 +1029,12 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     result=result,
                 ),
             )
+            if future:
+                self._event_futures.append(future)
 
             return result
         except Exception as e:
-            crewai_event_bus.emit(
+            future = crewai_event_bus.emit(
                 self,
                 MethodExecutionFailedEvent(
                     type="method_execution_failed",
@@ -1016,6 +1043,8 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     error=e,
                 ),
             )
+            if future:
+                self._event_futures.append(future)
             raise e
 
     async def _execute_listeners(self, trigger_method: str, result: Any) -> None:

lib/crewai/tests/agents/test_agent.py

@@ -1,6 +1,7 @@
 """Test Agent creation and execution basic functionality."""
 
 import os
+import threading
 from unittest import mock
 from unittest.mock import MagicMock, patch
 
@@ -185,14 +186,17 @@ def test_agent_execution_with_tools():
         expected_output="The result of the multiplication.",
     )
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageFinishedEvent)
     def handle_tool_end(source, event):
         received_events.append(event)
+        event_received.set()
 
     output = agent.execute_task(task)
     assert output == "The result of the multiplication is 12."
 
+    assert event_received.wait(timeout=5), "Timeout waiting for tool usage event"
     assert len(received_events) == 1
     assert isinstance(received_events[0], ToolUsageFinishedEvent)
     assert received_events[0].tool_name == "multiplier"
@@ -284,10 +288,12 @@ def test_cache_hitting():
         'multiplier-{"first_number": 12, "second_number": 3}': 36,
     }
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageFinishedEvent)
     def handle_tool_end(source, event):
         received_events.append(event)
+        event_received.set()
 
     with (
         patch.object(CacheHandler, "read") as read,
@@ -303,6 +309,7 @@ def test_cache_hitting():
         read.assert_called_with(
             tool="multiplier", input='{"first_number": 2, "second_number": 6}'
         )
+        assert event_received.wait(timeout=5), "Timeout waiting for tool usage event"
         assert len(received_events) == 1
         assert isinstance(received_events[0], ToolUsageFinishedEvent)
         assert received_events[0].from_cache

lib/crewai/tests/agents/test_lite_agent.py

@@ -1,4 +1,5 @@
 # mypy: ignore-errors
+import threading
 from collections import defaultdict
 from typing import cast
 from unittest.mock import Mock, patch
@@ -156,14 +157,17 @@ def test_lite_agent_with_tools():
     )
 
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageStartedEvent)
     def event_handler(source, event):
         received_events.append(event)
+        event_received.set()
 
     agent.kickoff("What are the effects of climate change on coral reefs?")
 
     # Verify tool usage events were emitted
+    assert event_received.wait(timeout=5), "Timeout waiting for tool usage events"
     assert len(received_events) > 0, "Tool usage events should be emitted"
     event = received_events[0]
     assert isinstance(event, ToolUsageStartedEvent)
@@ -316,15 +320,18 @@ def test_sets_parent_flow_when_inside_flow():
             return agent.kickoff("Test query")
 
     flow = MyFlow()
-    with crewai_event_bus.scoped_handlers():
+    event_received = threading.Event()
 
-        @crewai_event_bus.on(LiteAgentExecutionStartedEvent)
-        def capture_agent(source, event):
-            nonlocal captured_agent
-            captured_agent = source
+    @crewai_event_bus.on(LiteAgentExecutionStartedEvent)
+    def capture_agent(source, event):
+        nonlocal captured_agent
+        captured_agent = source
+        event_received.set()
 
-        flow.kickoff()
-        assert captured_agent.parent_flow is flow
+    flow.kickoff()
+
+    assert event_received.wait(timeout=5), "Timeout waiting for agent execution event"
+    assert captured_agent.parent_flow is flow
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -342,30 +349,43 @@ def test_guardrail_is_called_using_string():
         guardrail="""Only include Brazilian players, both women and men""",
     )
 
-    with crewai_event_bus.scoped_handlers():
+    all_events_received = threading.Event()
 
-        @crewai_event_bus.on(LLMGuardrailStartedEvent)
-        def capture_guardrail_started(source, event):
-            assert isinstance(source, LiteAgent)
-            assert source.original_agent == agent
-            guardrail_events["started"].append(event)
+    @crewai_event_bus.on(LLMGuardrailStartedEvent)
+    def capture_guardrail_started(source, event):
+        assert isinstance(source, LiteAgent)
+        assert source.original_agent == agent
+        guardrail_events["started"].append(event)
+        if (
+            len(guardrail_events["started"]) == 2
+            and len(guardrail_events["completed"]) == 2
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(LLMGuardrailCompletedEvent)
-        def capture_guardrail_completed(source, event):
-            assert isinstance(source, LiteAgent)
-            assert source.original_agent == agent
-            guardrail_events["completed"].append(event)
+    @crewai_event_bus.on(LLMGuardrailCompletedEvent)
+    def capture_guardrail_completed(source, event):
+        assert isinstance(source, LiteAgent)
+        assert source.original_agent == agent
+        guardrail_events["completed"].append(event)
+        if (
+            len(guardrail_events["started"]) == 2
+            and len(guardrail_events["completed"]) == 2
+        ):
+            all_events_received.set()
 
-        result = agent.kickoff(messages="Top 10 best players in the world?")
+    result = agent.kickoff(messages="Top 10 best players in the world?")
 
-        assert len(guardrail_events["started"]) == 2
-        assert len(guardrail_events["completed"]) == 2
-        assert not guardrail_events["completed"][0].success
-        assert guardrail_events["completed"][1].success
-        assert (
-            "Here are the top 10 best soccer players in the world, focusing exclusively on Brazilian players"
-            in result.raw
-        )
+    assert all_events_received.wait(timeout=10), (
+        "Timeout waiting for all guardrail events"
+    )
+    assert len(guardrail_events["started"]) == 2
+    assert len(guardrail_events["completed"]) == 2
+    assert not guardrail_events["completed"][0].success
+    assert guardrail_events["completed"][1].success
+    assert (
+        "Here are the top 10 best soccer players in the world, focusing exclusively on Brazilian players"
+        in result.raw
+    )
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -376,29 +396,42 @@ def test_guardrail_is_called_using_callable():
         LLMGuardrailStartedEvent,
     )
 
-    with crewai_event_bus.scoped_handlers():
+    all_events_received = threading.Event()
 
-        @crewai_event_bus.on(LLMGuardrailStartedEvent)
-        def capture_guardrail_started(source, event):
-            guardrail_events["started"].append(event)
+    @crewai_event_bus.on(LLMGuardrailStartedEvent)
+    def capture_guardrail_started(source, event):
+        guardrail_events["started"].append(event)
+        if (
+            len(guardrail_events["started"]) == 1
+            and len(guardrail_events["completed"]) == 1
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(LLMGuardrailCompletedEvent)
-        def capture_guardrail_completed(source, event):
-            guardrail_events["completed"].append(event)
+    @crewai_event_bus.on(LLMGuardrailCompletedEvent)
+    def capture_guardrail_completed(source, event):
+        guardrail_events["completed"].append(event)
+        if (
+            len(guardrail_events["started"]) == 1
+            and len(guardrail_events["completed"]) == 1
+        ):
+            all_events_received.set()
 
-        agent = Agent(
-            role="Sports Analyst",
-            goal="Gather information about the best soccer players",
-            backstory="""You are an expert at gathering and organizing information. You carefully collect details and present them in a structured way.""",
-            guardrail=lambda output: (True, "Pelé - Santos, 1958"),
-        )
+    agent = Agent(
+        role="Sports Analyst",
+        goal="Gather information about the best soccer players",
+        backstory="""You are an expert at gathering and organizing information. You carefully collect details and present them in a structured way.""",
+        guardrail=lambda output: (True, "Pelé - Santos, 1958"),
+    )
 
-        result = agent.kickoff(messages="Top 1 best players in the world?")
+    result = agent.kickoff(messages="Top 1 best players in the world?")
 
-        assert len(guardrail_events["started"]) == 1
-        assert len(guardrail_events["completed"]) == 1
-        assert guardrail_events["completed"][0].success
-        assert "Pelé - Santos, 1958" in result.raw
+    assert all_events_received.wait(timeout=10), (
+        "Timeout waiting for all guardrail events"
+    )
+    assert len(guardrail_events["started"]) == 1
+    assert len(guardrail_events["completed"]) == 1
+    assert guardrail_events["completed"][0].success
+    assert "Pelé - Santos, 1958" in result.raw
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -409,37 +442,50 @@ def test_guardrail_reached_attempt_limit():
         LLMGuardrailStartedEvent,
     )
 
-    with crewai_event_bus.scoped_handlers():
+    all_events_received = threading.Event()
 
-        @crewai_event_bus.on(LLMGuardrailStartedEvent)
-        def capture_guardrail_started(source, event):
-            guardrail_events["started"].append(event)
-
-        @crewai_event_bus.on(LLMGuardrailCompletedEvent)
-        def capture_guardrail_completed(source, event):
-            guardrail_events["completed"].append(event)
-
-        agent = Agent(
-            role="Sports Analyst",
-            goal="Gather information about the best soccer players",
-            backstory="""You are an expert at gathering and organizing information. You carefully collect details and present them in a structured way.""",
-            guardrail=lambda output: (
-                False,
-                "You are not allowed to include Brazilian players",
-            ),
-            guardrail_max_retries=2,
-        )
-
-        with pytest.raises(
-            Exception, match="Agent's guardrail failed validation after 2 retries"
+    @crewai_event_bus.on(LLMGuardrailStartedEvent)
+    def capture_guardrail_started(source, event):
+        guardrail_events["started"].append(event)
+        if (
+            len(guardrail_events["started"]) == 3
+            and len(guardrail_events["completed"]) == 3
         ):
-            agent.kickoff(messages="Top 10 best players in the world?")
+            all_events_received.set()
 
-        assert len(guardrail_events["started"]) == 3  # 2 retries + 1 initial call
-        assert len(guardrail_events["completed"]) == 3  # 2 retries + 1 initial call
-        assert not guardrail_events["completed"][0].success
-        assert not guardrail_events["completed"][1].success
-        assert not guardrail_events["completed"][2].success
+    @crewai_event_bus.on(LLMGuardrailCompletedEvent)
+    def capture_guardrail_completed(source, event):
+        guardrail_events["completed"].append(event)
+        if (
+            len(guardrail_events["started"]) == 3
+            and len(guardrail_events["completed"]) == 3
+        ):
+            all_events_received.set()
+
+    agent = Agent(
+        role="Sports Analyst",
+        goal="Gather information about the best soccer players",
+        backstory="""You are an expert at gathering and organizing information. You carefully collect details and present them in a structured way.""",
+        guardrail=lambda output: (
+            False,
+            "You are not allowed to include Brazilian players",
+        ),
+        guardrail_max_retries=2,
+    )
+
+    with pytest.raises(
+        Exception, match="Agent's guardrail failed validation after 2 retries"
+    ):
+        agent.kickoff(messages="Top 10 best players in the world?")
+
+    assert all_events_received.wait(timeout=10), (
+        "Timeout waiting for all guardrail events"
+    )
+    assert len(guardrail_events["started"]) == 3  # 2 retries + 1 initial call
+    assert len(guardrail_events["completed"]) == 3  # 2 retries + 1 initial call
+    assert not guardrail_events["completed"][0].success
+    assert not guardrail_events["completed"][1].success
+    assert not guardrail_events["completed"][2].success
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])

lib/crewai/tests/conftest.py

@@ -33,7 +33,7 @@ def setup_test_environment():
         except (OSError, IOError) as e:
             raise RuntimeError(
                 f"Test storage directory {storage_dir} is not writable: {e}"
-            )
+            ) from e
 
         os.environ["CREWAI_STORAGE_DIR"] = str(storage_dir)
         os.environ["CREWAI_TESTING"] = "true"
@@ -159,6 +159,29 @@ def mock_opentelemetry_components():
         }
 
 
+@pytest.fixture(autouse=True)
+def clear_event_bus_handlers():
+    """Clear event bus handlers after each test for isolation.
+
+    Handlers registered during the test are allowed to run, then cleaned up
+    after the test completes.
+    """
+    from crewai.events.event_bus import crewai_event_bus
+    from crewai.experimental.evaluation.evaluation_listener import (
+        EvaluationTraceCallback,
+    )
+
+    yield
+
+    crewai_event_bus.shutdown(wait=True)
+    crewai_event_bus._initialize()
+
+    callback = EvaluationTraceCallback()
+    callback.traces.clear()
+    callback.current_agent_id = None
+    callback.current_task_id = None
+
+
 @pytest.fixture(scope="module")
 def vcr_config(request) -> dict:
     import os

lib/crewai/tests/events/test_depends.py

@@ -0,0 +1,286 @@
+"""Tests for FastAPI-style dependency injection in event handlers."""
+
+import asyncio
+
+import pytest
+
+from crewai.events import Depends, crewai_event_bus
+from crewai.events.base_events import BaseEvent
+
+
+class DependsTestEvent(BaseEvent):
+    """Test event for dependency tests."""
+
+    value: int = 0
+    type: str = "test_event"
+
+
+@pytest.mark.asyncio
+async def test_basic_dependency():
+    """Test that handler with dependency runs after its dependency."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def setup(source, event: DependsTestEvent):
+            execution_order.append("setup")
+
+        @crewai_event_bus.on(DependsTestEvent, Depends(setup))
+        def process(source, event: DependsTestEvent):
+            execution_order.append("process")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        assert execution_order == ["setup", "process"]
+
+
+@pytest.mark.asyncio
+async def test_multiple_dependencies():
+    """Test handler with multiple dependencies."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def setup_a(source, event: DependsTestEvent):
+            execution_order.append("setup_a")
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def setup_b(source, event: DependsTestEvent):
+            execution_order.append("setup_b")
+
+        @crewai_event_bus.on(
+            DependsTestEvent, depends_on=[Depends(setup_a), Depends(setup_b)]
+        )
+        def process(source, event: DependsTestEvent):
+            execution_order.append("process")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        # setup_a and setup_b can run in any order (same level)
+        assert "process" in execution_order
+        assert execution_order.index("process") > execution_order.index("setup_a")
+        assert execution_order.index("process") > execution_order.index("setup_b")
+
+
+@pytest.mark.asyncio
+async def test_chain_of_dependencies():
+    """Test chain of dependencies (A -> B -> C)."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def handler_a(source, event: DependsTestEvent):
+            execution_order.append("handler_a")
+
+        @crewai_event_bus.on(DependsTestEvent, depends_on=Depends(handler_a))
+        def handler_b(source, event: DependsTestEvent):
+            execution_order.append("handler_b")
+
+        @crewai_event_bus.on(DependsTestEvent, depends_on=Depends(handler_b))
+        def handler_c(source, event: DependsTestEvent):
+            execution_order.append("handler_c")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        assert execution_order == ["handler_a", "handler_b", "handler_c"]
+
+
+@pytest.mark.asyncio
+async def test_async_handler_with_dependency():
+    """Test async handler with dependency on sync handler."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def sync_setup(source, event: DependsTestEvent):
+            execution_order.append("sync_setup")
+
+        @crewai_event_bus.on(DependsTestEvent, depends_on=Depends(sync_setup))
+        async def async_process(source, event: DependsTestEvent):
+            await asyncio.sleep(0.01)
+            execution_order.append("async_process")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        assert execution_order == ["sync_setup", "async_process"]
+
+
+@pytest.mark.asyncio
+async def test_mixed_handlers_with_dependencies():
+    """Test mix of sync and async handlers with dependencies."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def setup(source, event: DependsTestEvent):
+            execution_order.append("setup")
+
+        @crewai_event_bus.on(DependsTestEvent, depends_on=Depends(setup))
+        def sync_process(source, event: DependsTestEvent):
+            execution_order.append("sync_process")
+
+        @crewai_event_bus.on(DependsTestEvent, depends_on=Depends(setup))
+        async def async_process(source, event: DependsTestEvent):
+            await asyncio.sleep(0.01)
+            execution_order.append("async_process")
+
+        @crewai_event_bus.on(
+            DependsTestEvent, depends_on=[Depends(sync_process), Depends(async_process)]
+        )
+        def finalize(source, event: DependsTestEvent):
+            execution_order.append("finalize")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        # Verify execution order
+        assert execution_order[0] == "setup"
+        assert "finalize" in execution_order
+        assert execution_order.index("finalize") > execution_order.index("sync_process")
+        assert execution_order.index("finalize") > execution_order.index("async_process")
+
+
+@pytest.mark.asyncio
+async def test_independent_handlers_run_concurrently():
+    """Test that handlers without dependencies can run concurrently."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        async def handler_a(source, event: DependsTestEvent):
+            await asyncio.sleep(0.01)
+            execution_order.append("handler_a")
+
+        @crewai_event_bus.on(DependsTestEvent)
+        async def handler_b(source, event: DependsTestEvent):
+            await asyncio.sleep(0.01)
+            execution_order.append("handler_b")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        # Both handlers should have executed
+        assert len(execution_order) == 2
+        assert "handler_a" in execution_order
+        assert "handler_b" in execution_order
+
+
+@pytest.mark.asyncio
+async def test_circular_dependency_detection():
+    """Test that circular dependencies are detected and raise an error."""
+    from crewai.events.handler_graph import CircularDependencyError, build_execution_plan
+
+    # Create circular dependency: handler_a -> handler_b -> handler_c -> handler_a
+    def handler_a(source, event: DependsTestEvent):
+        pass
+
+    def handler_b(source, event: DependsTestEvent):
+        pass
+
+    def handler_c(source, event: DependsTestEvent):
+        pass
+
+    # Build a dependency graph with a cycle
+    handlers = [handler_a, handler_b, handler_c]
+    dependencies = {
+        handler_a: [Depends(handler_b)],
+        handler_b: [Depends(handler_c)],
+        handler_c: [Depends(handler_a)],  # Creates the cycle
+    }
+
+    # Should raise CircularDependencyError about circular dependency
+    with pytest.raises(CircularDependencyError, match="Circular dependency"):
+        build_execution_plan(handlers, dependencies)
+
+
+@pytest.mark.asyncio
+async def test_handler_without_dependency_runs_normally():
+    """Test that handlers without dependencies still work as before."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def simple_handler(source, event: DependsTestEvent):
+            execution_order.append("simple_handler")
+
+        event = DependsTestEvent(value=1)
+        future = crewai_event_bus.emit("test_source", event)
+
+        if future:
+            await asyncio.wrap_future(future)
+
+        assert execution_order == ["simple_handler"]
+
+
+@pytest.mark.asyncio
+async def test_depends_equality():
+    """Test Depends equality and hashing."""
+
+    def handler_a(source, event):
+        pass
+
+    def handler_b(source, event):
+        pass
+
+    dep_a1 = Depends(handler_a)
+    dep_a2 = Depends(handler_a)
+    dep_b = Depends(handler_b)
+
+    # Same handler should be equal
+    assert dep_a1 == dep_a2
+    assert hash(dep_a1) == hash(dep_a2)
+
+    # Different handlers should not be equal
+    assert dep_a1 != dep_b
+    assert hash(dep_a1) != hash(dep_b)
+
+
+@pytest.mark.asyncio
+async def test_aemit_ignores_dependencies():
+    """Test that aemit only processes async handlers (no dependency support yet)."""
+    execution_order = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(DependsTestEvent)
+        def sync_handler(source, event: DependsTestEvent):
+            execution_order.append("sync_handler")
+
+        @crewai_event_bus.on(DependsTestEvent)
+        async def async_handler(source, event: DependsTestEvent):
+            execution_order.append("async_handler")
+
+        event = DependsTestEvent(value=1)
+        await crewai_event_bus.aemit("test_source", event)
+
+        # Only async handler should execute
+        assert execution_order == ["async_handler"]

lib/crewai/tests/experimental/evaluation/test_agent_evaluator.py

@@ -1,3 +1,5 @@
+import threading
+
 import pytest
 from crewai.agent import Agent
 from crewai.crew import Crew
@@ -19,7 +21,10 @@ from crewai.experimental.evaluation import (
     create_default_evaluator,
 )
 from crewai.experimental.evaluation.agent_evaluator import AgentEvaluator
-from crewai.experimental.evaluation.base_evaluator import AgentEvaluationResult
+from crewai.experimental.evaluation.base_evaluator import (
+    AgentEvaluationResult,
+    BaseEvaluator,
+)
 from crewai.task import Task
 
 
@@ -51,12 +56,25 @@ class TestAgentEvaluator:
 
     @pytest.mark.vcr(filter_headers=["authorization"])
     def test_evaluate_current_iteration(self, mock_crew):
+        from crewai.events.types.task_events import TaskCompletedEvent
+
         agent_evaluator = AgentEvaluator(
             agents=mock_crew.agents, evaluators=[GoalAlignmentEvaluator()]
         )
 
+        task_completed_event = threading.Event()
+
+        @crewai_event_bus.on(TaskCompletedEvent)
+        async def on_task_completed(source, event):
+            # TaskCompletedEvent fires AFTER evaluation results are stored
+            task_completed_event.set()
+
         mock_crew.kickoff()
 
+        assert task_completed_event.wait(timeout=5), (
+            "Timeout waiting for task completion"
+        )
+
         results = agent_evaluator.get_evaluation_results()
 
         assert isinstance(results, dict)
@@ -98,73 +116,15 @@ class TestAgentEvaluator:
         ]
 
         assert len(agent_evaluator.evaluators) == len(expected_types)
-        for evaluator, expected_type in zip(agent_evaluator.evaluators, expected_types):
+        for evaluator, expected_type in zip(
+            agent_evaluator.evaluators, expected_types, strict=False
+        ):
             assert isinstance(evaluator, expected_type)
 
-    @pytest.mark.vcr(filter_headers=["authorization"])
-    def test_eval_lite_agent(self):
-        agent = Agent(
-            role="Test Agent",
-            goal="Complete test tasks successfully",
-            backstory="An agent created for testing purposes",
-        )
-
-        with crewai_event_bus.scoped_handlers():
-            events = {}
-
-            @crewai_event_bus.on(AgentEvaluationStartedEvent)
-            def capture_started(source, event):
-                events["started"] = event
-
-            @crewai_event_bus.on(AgentEvaluationCompletedEvent)
-            def capture_completed(source, event):
-                events["completed"] = event
-
-            @crewai_event_bus.on(AgentEvaluationFailedEvent)
-            def capture_failed(source, event):
-                events["failed"] = event
-
-            agent_evaluator = AgentEvaluator(
-                agents=[agent], evaluators=[GoalAlignmentEvaluator()]
-            )
-
-            agent.kickoff(messages="Complete this task successfully")
-
-            assert events.keys() == {"started", "completed"}
-            assert events["started"].agent_id == str(agent.id)
-            assert events["started"].agent_role == agent.role
-            assert events["started"].task_id is None
-            assert events["started"].iteration == 1
-
-            assert events["completed"].agent_id == str(agent.id)
-            assert events["completed"].agent_role == agent.role
-            assert events["completed"].task_id is None
-            assert events["completed"].iteration == 1
-            assert events["completed"].metric_category == MetricCategory.GOAL_ALIGNMENT
-            assert isinstance(events["completed"].score, EvaluationScore)
-            assert events["completed"].score.score == 2.0
-
-            results = agent_evaluator.get_evaluation_results()
-
-            assert isinstance(results, dict)
-
-            (result,) = results[agent.role]
-            assert isinstance(result, AgentEvaluationResult)
-
-            assert result.agent_id == str(agent.id)
-            assert result.task_id == "lite_task"
-
-            (goal_alignment,) = result.metrics.values()
-            assert goal_alignment.score == 2.0
-
-            expected_feedback = "The agent did not demonstrate a clear understanding of the task goal, which is to complete test tasks successfully"
-            assert expected_feedback in goal_alignment.feedback
-
-            assert goal_alignment.raw_response is not None
-            assert '"score": 2' in goal_alignment.raw_response
-
     @pytest.mark.vcr(filter_headers=["authorization"])
     def test_eval_specific_agents_from_crew(self, mock_crew):
+        from crewai.events.types.task_events import TaskCompletedEvent
+
         agent = Agent(
             role="Test Agent Eval",
             goal="Complete test tasks successfully",
@@ -178,111 +138,132 @@ class TestAgentEvaluator:
         mock_crew.agents.append(agent)
         mock_crew.tasks.append(task)
 
-        with crewai_event_bus.scoped_handlers():
-            events = {}
+        events = {}
+        started_event = threading.Event()
+        completed_event = threading.Event()
+        task_completed_event = threading.Event()
 
-            @crewai_event_bus.on(AgentEvaluationStartedEvent)
-            def capture_started(source, event):
+        agent_evaluator = AgentEvaluator(
+            agents=[agent], evaluators=[GoalAlignmentEvaluator()]
+        )
+
+        @crewai_event_bus.on(AgentEvaluationStartedEvent)
+        async def capture_started(source, event):
+            if event.agent_id == str(agent.id):
                 events["started"] = event
+                started_event.set()
 
-            @crewai_event_bus.on(AgentEvaluationCompletedEvent)
-            def capture_completed(source, event):
+        @crewai_event_bus.on(AgentEvaluationCompletedEvent)
+        async def capture_completed(source, event):
+            if event.agent_id == str(agent.id):
                 events["completed"] = event
+                completed_event.set()
 
-            @crewai_event_bus.on(AgentEvaluationFailedEvent)
-            def capture_failed(source, event):
-                events["failed"] = event
+        @crewai_event_bus.on(AgentEvaluationFailedEvent)
+        def capture_failed(source, event):
+            events["failed"] = event
 
-            agent_evaluator = AgentEvaluator(
-                agents=[agent], evaluators=[GoalAlignmentEvaluator()]
-            )
-            mock_crew.kickoff()
+        @crewai_event_bus.on(TaskCompletedEvent)
+        async def on_task_completed(source, event):
+            # TaskCompletedEvent fires AFTER evaluation results are stored
+            if event.task and event.task.id == task.id:
+                task_completed_event.set()
 
-            assert events.keys() == {"started", "completed"}
-            assert events["started"].agent_id == str(agent.id)
-            assert events["started"].agent_role == agent.role
-            assert events["started"].task_id == str(task.id)
-            assert events["started"].iteration == 1
+        mock_crew.kickoff()
 
-            assert events["completed"].agent_id == str(agent.id)
-            assert events["completed"].agent_role == agent.role
-            assert events["completed"].task_id == str(task.id)
-            assert events["completed"].iteration == 1
-            assert events["completed"].metric_category == MetricCategory.GOAL_ALIGNMENT
-            assert isinstance(events["completed"].score, EvaluationScore)
-            assert events["completed"].score.score == 5.0
+        assert started_event.wait(timeout=5), "Timeout waiting for started event"
+        assert completed_event.wait(timeout=5), "Timeout waiting for completed event"
+        assert task_completed_event.wait(timeout=5), (
+            "Timeout waiting for task completion"
+        )
 
-            results = agent_evaluator.get_evaluation_results()
+        assert events.keys() == {"started", "completed"}
+        assert events["started"].agent_id == str(agent.id)
+        assert events["started"].agent_role == agent.role
+        assert events["started"].task_id == str(task.id)
+        assert events["started"].iteration == 1
 
-            assert isinstance(results, dict)
-            assert len(results.keys()) == 1
-            (result,) = results[agent.role]
-            assert isinstance(result, AgentEvaluationResult)
+        assert events["completed"].agent_id == str(agent.id)
+        assert events["completed"].agent_role == agent.role
+        assert events["completed"].task_id == str(task.id)
+        assert events["completed"].iteration == 1
+        assert events["completed"].metric_category == MetricCategory.GOAL_ALIGNMENT
+        assert isinstance(events["completed"].score, EvaluationScore)
+        assert events["completed"].score.score == 5.0
 
-            assert result.agent_id == str(agent.id)
-            assert result.task_id == str(task.id)
+        results = agent_evaluator.get_evaluation_results()
 
-            (goal_alignment,) = result.metrics.values()
-            assert goal_alignment.score == 5.0
+        assert isinstance(results, dict)
+        assert len(results.keys()) == 1
+        (result,) = results[agent.role]
+        assert isinstance(result, AgentEvaluationResult)
 
-            expected_feedback = "The agent provided a thorough guide on how to conduct a test task but failed to produce specific expected output"
-            assert expected_feedback in goal_alignment.feedback
+        assert result.agent_id == str(agent.id)
+        assert result.task_id == str(task.id)
 
-            assert goal_alignment.raw_response is not None
-            assert '"score": 5' in goal_alignment.raw_response
+        (goal_alignment,) = result.metrics.values()
+        assert goal_alignment.score == 5.0
+
+        expected_feedback = "The agent provided a thorough guide on how to conduct a test task but failed to produce specific expected output"
+        assert expected_feedback in goal_alignment.feedback
+
+        assert goal_alignment.raw_response is not None
+        assert '"score": 5' in goal_alignment.raw_response
 
     @pytest.mark.vcr(filter_headers=["authorization"])
     def test_failed_evaluation(self, mock_crew):
         (agent,) = mock_crew.agents
         (task,) = mock_crew.tasks
 
-        with crewai_event_bus.scoped_handlers():
-            events = {}
+        events = {}
+        started_event = threading.Event()
+        failed_event = threading.Event()
 
-            @crewai_event_bus.on(AgentEvaluationStartedEvent)
-            def capture_started(source, event):
-                events["started"] = event
+        @crewai_event_bus.on(AgentEvaluationStartedEvent)
+        def capture_started(source, event):
+            events["started"] = event
+            started_event.set()
 
-            @crewai_event_bus.on(AgentEvaluationCompletedEvent)
-            def capture_completed(source, event):
-                events["completed"] = event
+        @crewai_event_bus.on(AgentEvaluationCompletedEvent)
+        def capture_completed(source, event):
+            events["completed"] = event
 
-            @crewai_event_bus.on(AgentEvaluationFailedEvent)
-            def capture_failed(source, event):
-                events["failed"] = event
+        @crewai_event_bus.on(AgentEvaluationFailedEvent)
+        def capture_failed(source, event):
+            events["failed"] = event
+            failed_event.set()
 
-            # Create a mock evaluator that will raise an exception
-            from crewai.experimental.evaluation import MetricCategory
-            from crewai.experimental.evaluation.base_evaluator import BaseEvaluator
+        class FailingEvaluator(BaseEvaluator):
+            metric_category = MetricCategory.GOAL_ALIGNMENT
 
-            class FailingEvaluator(BaseEvaluator):
-                metric_category = MetricCategory.GOAL_ALIGNMENT
+            def evaluate(self, agent, task, execution_trace, final_output):
+                raise ValueError("Forced evaluation failure")
 
-                def evaluate(self, agent, task, execution_trace, final_output):
-                    raise ValueError("Forced evaluation failure")
+        agent_evaluator = AgentEvaluator(
+            agents=[agent], evaluators=[FailingEvaluator()]
+        )
+        mock_crew.kickoff()
 
-            agent_evaluator = AgentEvaluator(
-                agents=[agent], evaluators=[FailingEvaluator()]
-            )
-            mock_crew.kickoff()
+        assert started_event.wait(timeout=5), "Timeout waiting for started event"
+        assert failed_event.wait(timeout=5), "Timeout waiting for failed event"
 
-            assert events.keys() == {"started", "failed"}
-            assert events["started"].agent_id == str(agent.id)
-            assert events["started"].agent_role == agent.role
-            assert events["started"].task_id == str(task.id)
-            assert events["started"].iteration == 1
+        assert events.keys() == {"started", "failed"}
+        assert events["started"].agent_id == str(agent.id)
+        assert events["started"].agent_role == agent.role
+        assert events["started"].task_id == str(task.id)
+        assert events["started"].iteration == 1
 
-            assert events["failed"].agent_id == str(agent.id)
-            assert events["failed"].agent_role == agent.role
-            assert events["failed"].task_id == str(task.id)
-            assert events["failed"].iteration == 1
-            assert events["failed"].error == "Forced evaluation failure"
+        assert events["failed"].agent_id == str(agent.id)
+        assert events["failed"].agent_role == agent.role
+        assert events["failed"].task_id == str(task.id)
+        assert events["failed"].iteration == 1
+        assert events["failed"].error == "Forced evaluation failure"
 
-            results = agent_evaluator.get_evaluation_results()
-            (result,) = results[agent.role]
-            assert isinstance(result, AgentEvaluationResult)
+        results = agent_evaluator.get_evaluation_results()
+        (result,) = results[agent.role]
+        assert isinstance(result, AgentEvaluationResult)
 
-            assert result.agent_id == str(agent.id)
-            assert result.task_id == str(task.id)
+        assert result.agent_id == str(agent.id)
+        assert result.task_id == str(task.id)
 
-            assert result.metrics == {}
+        assert result.metrics == {}

lib/crewai/tests/memory/test_external_memory.py

@@ -1,23 +1,36 @@
-from unittest.mock import MagicMock, patch, ANY
+import threading
 from collections import defaultdict
-from crewai.events.event_bus import crewai_event_bus
-from crewai.events.types.memory_events import (
-    MemorySaveStartedEvent,
-    MemorySaveCompletedEvent,
-    MemoryQueryStartedEvent,
-    MemoryQueryCompletedEvent,
-)
+from unittest.mock import ANY, MagicMock, patch
+
 import pytest
 from mem0.memory.main import Memory
 
 from crewai.agent import Agent
 from crewai.crew import Crew, Process
+from crewai.events.event_bus import crewai_event_bus
+from crewai.events.types.memory_events import (
+    MemoryQueryCompletedEvent,
+    MemoryQueryStartedEvent,
+    MemorySaveCompletedEvent,
+    MemorySaveStartedEvent,
+)
 from crewai.memory.external.external_memory import ExternalMemory
 from crewai.memory.external.external_memory_item import ExternalMemoryItem
 from crewai.memory.storage.interface import Storage
 from crewai.task import Task
 
 
+@pytest.fixture(autouse=True)
+def cleanup_event_handlers():
+    """Cleanup event handlers after each test"""
+    yield
+    with crewai_event_bus._rwlock.w_locked():
+        crewai_event_bus._sync_handlers = {}
+        crewai_event_bus._async_handlers = {}
+        crewai_event_bus._handler_dependencies = {}
+        crewai_event_bus._execution_plan_cache = {}
+
+
 @pytest.fixture
 def mock_mem0_memory():
     mock_memory = MagicMock(spec=Memory)
@@ -238,24 +251,26 @@ def test_external_memory_search_events(
     custom_storage, external_memory_with_mocked_config
 ):
     events = defaultdict(list)
+    event_received = threading.Event()
 
     external_memory_with_mocked_config.storage = custom_storage
-    with crewai_event_bus.scoped_handlers():
 
-        @crewai_event_bus.on(MemoryQueryStartedEvent)
-        def on_search_started(source, event):
-            events["MemoryQueryStartedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryStartedEvent)
+    def on_search_started(source, event):
+        events["MemoryQueryStartedEvent"].append(event)
 
-        @crewai_event_bus.on(MemoryQueryCompletedEvent)
-        def on_search_completed(source, event):
-            events["MemoryQueryCompletedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryCompletedEvent)
+    def on_search_completed(source, event):
+        events["MemoryQueryCompletedEvent"].append(event)
+        event_received.set()
 
-        external_memory_with_mocked_config.search(
-            query="test value",
-            limit=3,
-            score_threshold=0.35,
-        )
+    external_memory_with_mocked_config.search(
+        query="test value",
+        limit=3,
+        score_threshold=0.35,
+    )
 
+    assert event_received.wait(timeout=5), "Timeout waiting for search events"
     assert len(events["MemoryQueryStartedEvent"]) == 1
     assert len(events["MemoryQueryCompletedEvent"]) == 1
 
@@ -300,24 +315,25 @@ def test_external_memory_save_events(
     custom_storage, external_memory_with_mocked_config
 ):
     events = defaultdict(list)
+    event_received = threading.Event()
 
     external_memory_with_mocked_config.storage = custom_storage
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(MemorySaveStartedEvent)
+    def on_save_started(source, event):
+        events["MemorySaveStartedEvent"].append(event)
 
-        @crewai_event_bus.on(MemorySaveStartedEvent)
-        def on_save_started(source, event):
-            events["MemorySaveStartedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveCompletedEvent)
+    def on_save_completed(source, event):
+        events["MemorySaveCompletedEvent"].append(event)
+        event_received.set()
 
-        @crewai_event_bus.on(MemorySaveCompletedEvent)
-        def on_save_completed(source, event):
-            events["MemorySaveCompletedEvent"].append(event)
-
-        external_memory_with_mocked_config.save(
-            value="saving value",
-            metadata={"task": "test_task"},
-        )
+    external_memory_with_mocked_config.save(
+        value="saving value",
+        metadata={"task": "test_task"},
+    )
 
+    assert event_received.wait(timeout=5), "Timeout waiting for save events"
     assert len(events["MemorySaveStartedEvent"]) == 1
     assert len(events["MemorySaveCompletedEvent"]) == 1
 

lib/crewai/tests/memory/test_long_term_memory.py

@@ -1,7 +1,9 @@
+import threading
 from collections import defaultdict
 from unittest.mock import ANY
 
 import pytest
+
 from crewai.events.event_bus import crewai_event_bus
 from crewai.events.types.memory_events import (
     MemoryQueryCompletedEvent,
@@ -21,27 +23,37 @@ def long_term_memory():
 
 def test_long_term_memory_save_events(long_term_memory):
     events = defaultdict(list)
+    all_events_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(MemorySaveStartedEvent)
+    def on_save_started(source, event):
+        events["MemorySaveStartedEvent"].append(event)
+        if (
+            len(events["MemorySaveStartedEvent"]) == 1
+            and len(events["MemorySaveCompletedEvent"]) == 1
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(MemorySaveStartedEvent)
-        def on_save_started(source, event):
-            events["MemorySaveStartedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveCompletedEvent)
+    def on_save_completed(source, event):
+        events["MemorySaveCompletedEvent"].append(event)
+        if (
+            len(events["MemorySaveStartedEvent"]) == 1
+            and len(events["MemorySaveCompletedEvent"]) == 1
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(MemorySaveCompletedEvent)
-        def on_save_completed(source, event):
-            events["MemorySaveCompletedEvent"].append(event)
-
-        memory = LongTermMemoryItem(
-            agent="test_agent",
-            task="test_task",
-            expected_output="test_output",
-            datetime="test_datetime",
-            quality=0.5,
-            metadata={"task": "test_task", "quality": 0.5},
-        )
-        long_term_memory.save(memory)
+    memory = LongTermMemoryItem(
+        agent="test_agent",
+        task="test_task",
+        expected_output="test_output",
+        datetime="test_datetime",
+        quality=0.5,
+        metadata={"task": "test_task", "quality": 0.5},
+    )
+    long_term_memory.save(memory)
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for save events"
     assert len(events["MemorySaveStartedEvent"]) == 1
     assert len(events["MemorySaveCompletedEvent"]) == 1
     assert len(events["MemorySaveFailedEvent"]) == 0
@@ -86,21 +98,31 @@ def test_long_term_memory_save_events(long_term_memory):
 
 def test_long_term_memory_search_events(long_term_memory):
     events = defaultdict(list)
+    all_events_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(MemoryQueryStartedEvent)
+    def on_search_started(source, event):
+        events["MemoryQueryStartedEvent"].append(event)
+        if (
+            len(events["MemoryQueryStartedEvent"]) == 1
+            and len(events["MemoryQueryCompletedEvent"]) == 1
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(MemoryQueryStartedEvent)
-        def on_search_started(source, event):
-            events["MemoryQueryStartedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryCompletedEvent)
+    def on_search_completed(source, event):
+        events["MemoryQueryCompletedEvent"].append(event)
+        if (
+            len(events["MemoryQueryStartedEvent"]) == 1
+            and len(events["MemoryQueryCompletedEvent"]) == 1
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(MemoryQueryCompletedEvent)
-        def on_search_completed(source, event):
-            events["MemoryQueryCompletedEvent"].append(event)
+    test_query = "test query"
 
-        test_query = "test query"
-
-        long_term_memory.search(test_query, latest_n=5)
+    long_term_memory.search(test_query, latest_n=5)
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for search events"
     assert len(events["MemoryQueryStartedEvent"]) == 1
     assert len(events["MemoryQueryCompletedEvent"]) == 1
     assert len(events["MemoryQueryFailedEvent"]) == 0

lib/crewai/tests/memory/test_short_term_memory.py

@@ -1,3 +1,4 @@
+import threading
 from collections import defaultdict
 from unittest.mock import ANY, patch
 
@@ -37,24 +38,33 @@ def short_term_memory():
 
 def test_short_term_memory_search_events(short_term_memory):
     events = defaultdict(list)
+    search_started = threading.Event()
+    search_completed = threading.Event()
 
     with patch.object(short_term_memory.storage, "search", return_value=[]):
-        with crewai_event_bus.scoped_handlers():
 
-            @crewai_event_bus.on(MemoryQueryStartedEvent)
-            def on_search_started(source, event):
-                events["MemoryQueryStartedEvent"].append(event)
+        @crewai_event_bus.on(MemoryQueryStartedEvent)
+        def on_search_started(source, event):
+            events["MemoryQueryStartedEvent"].append(event)
+            search_started.set()
 
-            @crewai_event_bus.on(MemoryQueryCompletedEvent)
-            def on_search_completed(source, event):
-                events["MemoryQueryCompletedEvent"].append(event)
+        @crewai_event_bus.on(MemoryQueryCompletedEvent)
+        def on_search_completed(source, event):
+            events["MemoryQueryCompletedEvent"].append(event)
+            search_completed.set()
 
-            # Call the save method
-            short_term_memory.search(
-                query="test value",
-                limit=3,
-                score_threshold=0.35,
-            )
+        short_term_memory.search(
+            query="test value",
+            limit=3,
+            score_threshold=0.35,
+        )
+
+        assert search_started.wait(timeout=2), (
+            "Timeout waiting for search started event"
+        )
+        assert search_completed.wait(timeout=2), (
+            "Timeout waiting for search completed event"
+        )
 
     assert len(events["MemoryQueryStartedEvent"]) == 1
     assert len(events["MemoryQueryCompletedEvent"]) == 1
@@ -98,20 +108,26 @@ def test_short_term_memory_search_events(short_term_memory):
 
 def test_short_term_memory_save_events(short_term_memory):
     events = defaultdict(list)
-    with crewai_event_bus.scoped_handlers():
+    save_started = threading.Event()
+    save_completed = threading.Event()
 
-        @crewai_event_bus.on(MemorySaveStartedEvent)
-        def on_save_started(source, event):
-            events["MemorySaveStartedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveStartedEvent)
+    def on_save_started(source, event):
+        events["MemorySaveStartedEvent"].append(event)
+        save_started.set()
 
-        @crewai_event_bus.on(MemorySaveCompletedEvent)
-        def on_save_completed(source, event):
-            events["MemorySaveCompletedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveCompletedEvent)
+    def on_save_completed(source, event):
+        events["MemorySaveCompletedEvent"].append(event)
+        save_completed.set()
 
-        short_term_memory.save(
-            value="test value",
-            metadata={"task": "test_task"},
-        )
+    short_term_memory.save(
+        value="test value",
+        metadata={"task": "test_task"},
+    )
+
+    assert save_started.wait(timeout=2), "Timeout waiting for save started event"
+    assert save_completed.wait(timeout=2), "Timeout waiting for save completed event"
 
     assert len(events["MemorySaveStartedEvent"]) == 1
     assert len(events["MemorySaveCompletedEvent"]) == 1

lib/crewai/tests/test_crew.py

@@ -1,9 +1,10 @@
 """Test Agent creation and execution basic functionality."""
 
+import json
+import threading
 from collections import defaultdict
 from concurrent.futures import Future
 from hashlib import md5
-import json
 import re
 from unittest import mock
 from unittest.mock import ANY, MagicMock, patch
@@ -2476,62 +2477,63 @@ def test_using_contextual_memory():
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_memory_events_are_emitted():
     events = defaultdict(list)
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(MemorySaveStartedEvent)
+    def handle_memory_save_started(source, event):
+        events["MemorySaveStartedEvent"].append(event)
 
-        @crewai_event_bus.on(MemorySaveStartedEvent)
-        def handle_memory_save_started(source, event):
-            events["MemorySaveStartedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveCompletedEvent)
+    def handle_memory_save_completed(source, event):
+        events["MemorySaveCompletedEvent"].append(event)
 
-        @crewai_event_bus.on(MemorySaveCompletedEvent)
-        def handle_memory_save_completed(source, event):
-            events["MemorySaveCompletedEvent"].append(event)
+    @crewai_event_bus.on(MemorySaveFailedEvent)
+    def handle_memory_save_failed(source, event):
+        events["MemorySaveFailedEvent"].append(event)
 
-        @crewai_event_bus.on(MemorySaveFailedEvent)
-        def handle_memory_save_failed(source, event):
-            events["MemorySaveFailedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryStartedEvent)
+    def handle_memory_query_started(source, event):
+        events["MemoryQueryStartedEvent"].append(event)
 
-        @crewai_event_bus.on(MemoryQueryStartedEvent)
-        def handle_memory_query_started(source, event):
-            events["MemoryQueryStartedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryCompletedEvent)
+    def handle_memory_query_completed(source, event):
+        events["MemoryQueryCompletedEvent"].append(event)
 
-        @crewai_event_bus.on(MemoryQueryCompletedEvent)
-        def handle_memory_query_completed(source, event):
-            events["MemoryQueryCompletedEvent"].append(event)
+    @crewai_event_bus.on(MemoryQueryFailedEvent)
+    def handle_memory_query_failed(source, event):
+        events["MemoryQueryFailedEvent"].append(event)
 
-        @crewai_event_bus.on(MemoryQueryFailedEvent)
-        def handle_memory_query_failed(source, event):
-            events["MemoryQueryFailedEvent"].append(event)
+    @crewai_event_bus.on(MemoryRetrievalStartedEvent)
+    def handle_memory_retrieval_started(source, event):
+        events["MemoryRetrievalStartedEvent"].append(event)
 
-        @crewai_event_bus.on(MemoryRetrievalStartedEvent)
-        def handle_memory_retrieval_started(source, event):
-            events["MemoryRetrievalStartedEvent"].append(event)
+    @crewai_event_bus.on(MemoryRetrievalCompletedEvent)
+    def handle_memory_retrieval_completed(source, event):
+        events["MemoryRetrievalCompletedEvent"].append(event)
+        event_received.set()
 
-        @crewai_event_bus.on(MemoryRetrievalCompletedEvent)
-        def handle_memory_retrieval_completed(source, event):
-            events["MemoryRetrievalCompletedEvent"].append(event)
+    math_researcher = Agent(
+        role="Researcher",
+        goal="You research about math.",
+        backstory="You're an expert in research and you love to learn new things.",
+        allow_delegation=False,
+    )
 
-        math_researcher = Agent(
-            role="Researcher",
-            goal="You research about math.",
-            backstory="You're an expert in research and you love to learn new things.",
-            allow_delegation=False,
-        )
+    task1 = Task(
+        description="Research a topic to teach a kid aged 6 about math.",
+        expected_output="A topic, explanation, angle, and examples.",
+        agent=math_researcher,
+    )
 
-        task1 = Task(
-            description="Research a topic to teach a kid aged 6 about math.",
-            expected_output="A topic, explanation, angle, and examples.",
-            agent=math_researcher,
-        )
+    crew = Crew(
+        agents=[math_researcher],
+        tasks=[task1],
+        memory=True,
+    )
 
-        crew = Crew(
-            agents=[math_researcher],
-            tasks=[task1],
-            memory=True,
-        )
-
-        crew.kickoff()
+    crew.kickoff()
 
+    assert event_received.wait(timeout=5), "Timeout waiting for memory events"
     assert len(events["MemorySaveStartedEvent"]) == 3
     assert len(events["MemorySaveCompletedEvent"]) == 3
     assert len(events["MemorySaveFailedEvent"]) == 0
@@ -2907,19 +2909,29 @@ def test_crew_train_success(
     copy_mock.return_value = crew
 
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
 
     @crewai_event_bus.on(CrewTrainStartedEvent)
     def on_crew_train_started(source, event: CrewTrainStartedEvent):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == 2:
+                all_events_received.set()
 
     @crewai_event_bus.on(CrewTrainCompletedEvent)
     def on_crew_train_completed(source, event: CrewTrainCompletedEvent):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == 2:
+                all_events_received.set()
 
     crew.train(
         n_iterations=2, inputs={"topic": "AI"}, filename="trained_agents_data.pkl"
     )
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for all train events"
+
     # Ensure kickoff is called on the copied crew
     kickoff_mock.assert_has_calls(
         [mock.call(inputs={"topic": "AI"}), mock.call(inputs={"topic": "AI"})]
@@ -3726,17 +3738,27 @@ def test_crew_testing_function(kickoff_mock, copy_mock, crew_evaluator, research
     llm_instance = LLM("gpt-4o-mini")
 
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
 
     @crewai_event_bus.on(CrewTestStartedEvent)
     def on_crew_test_started(source, event: CrewTestStartedEvent):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == 2:
+                all_events_received.set()
 
     @crewai_event_bus.on(CrewTestCompletedEvent)
     def on_crew_test_completed(source, event: CrewTestCompletedEvent):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == 2:
+                all_events_received.set()
 
     crew.test(n_iterations, llm_instance, inputs={"topic": "AI"})
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for all test events"
+
     # Ensure kickoff is called on the copied crew
     kickoff_mock.assert_has_calls(
         [mock.call(inputs={"topic": "AI"}), mock.call(inputs={"topic": "AI"})]

lib/crewai/tests/test_flow.py

@@ -1,9 +1,12 @@
 """Test Flow creation and execution basic functionality."""
 
 import asyncio
+import threading
 from datetime import datetime
 
 import pytest
+from pydantic import BaseModel
+
 from crewai.events.event_bus import crewai_event_bus
 from crewai.events.types.flow_events import (
     FlowFinishedEvent,
@@ -13,7 +16,6 @@ from crewai.events.types.flow_events import (
     MethodExecutionStartedEvent,
 )
 from crewai.flow.flow import Flow, and_, listen, or_, router, start
-from pydantic import BaseModel
 
 
 def test_simple_sequential_flow():
@@ -439,20 +441,42 @@ def test_unstructured_flow_event_emission():
 
     flow = PoemFlow()
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
+    expected_event_count = (
+        7  # 1 FlowStarted + 5 MethodExecutionStarted + 1 FlowFinished
+    )
 
     @crewai_event_bus.on(FlowStartedEvent)
     def handle_flow_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(MethodExecutionStartedEvent)
     def handle_method_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(FlowFinishedEvent)
     def handle_flow_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     flow.kickoff(inputs={"separator": ", "})
+
+    assert all_events_received.wait(timeout=5), "Timeout waiting for all flow events"
+
+    # Sort events by timestamp to ensure deterministic order
+    # (async handlers may append out of order)
+    with lock:
+        received_events.sort(key=lambda e: e.timestamp)
+
     assert isinstance(received_events[0], FlowStartedEvent)
     assert received_events[0].flow_name == "PoemFlow"
     assert received_events[0].inputs == {"separator": ", "}
@@ -642,28 +666,48 @@ def test_structured_flow_event_emission():
             return f"Welcome, {self.state.name}!"
 
     flow = OnboardingFlow()
-    flow.kickoff(inputs={"name": "Anakin"})
 
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
+    expected_event_count = 6  # 1 FlowStarted + 2 MethodExecutionStarted + 2 MethodExecutionFinished + 1 FlowFinished
 
     @crewai_event_bus.on(FlowStartedEvent)
     def handle_flow_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(MethodExecutionStartedEvent)
     def handle_method_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(MethodExecutionFinishedEvent)
     def handle_method_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(FlowFinishedEvent)
     def handle_flow_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     flow.kickoff(inputs={"name": "Anakin"})
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for all flow events"
+
+    # Sort events by timestamp to ensure deterministic order
+    with lock:
+        received_events.sort(key=lambda e: e.timestamp)
+
     assert isinstance(received_events[0], FlowStartedEvent)
     assert received_events[0].flow_name == "OnboardingFlow"
     assert received_events[0].inputs == {"name": "Anakin"}
@@ -711,25 +755,46 @@ def test_stateless_flow_event_emission():
 
     flow = StatelessFlow()
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
+    expected_event_count = 6  # 1 FlowStarted + 2 MethodExecutionStarted + 2 MethodExecutionFinished + 1 FlowFinished
 
     @crewai_event_bus.on(FlowStartedEvent)
     def handle_flow_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(MethodExecutionStartedEvent)
     def handle_method_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(MethodExecutionFinishedEvent)
     def handle_method_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     @crewai_event_bus.on(FlowFinishedEvent)
     def handle_flow_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) == expected_event_count:
+                all_events_received.set()
 
     flow.kickoff()
 
+    assert all_events_received.wait(timeout=5), "Timeout waiting for all flow events"
+
+    # Sort events by timestamp to ensure deterministic order
+    with lock:
+        received_events.sort(key=lambda e: e.timestamp)
+
     assert isinstance(received_events[0], FlowStartedEvent)
     assert received_events[0].flow_name == "StatelessFlow"
     assert received_events[0].inputs is None
@@ -769,13 +834,16 @@ def test_flow_plotting():
     flow = StatelessFlow()
     flow.kickoff()
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(FlowPlotEvent)
     def handle_flow_plot(source, event):
         received_events.append(event)
+        event_received.set()
 
     flow.plot("test_flow")
 
+    assert event_received.wait(timeout=5), "Timeout waiting for plot event"
     assert len(received_events) == 1
     assert isinstance(received_events[0], FlowPlotEvent)
     assert received_events[0].flow_name == "StatelessFlow"

lib/crewai/tests/test_task_guardrails.py

@@ -1,3 +1,4 @@
+import threading
 from unittest.mock import Mock, patch
 
 import pytest
@@ -175,78 +176,92 @@ def test_task_guardrail_process_output(task_output):
 def test_guardrail_emits_events(sample_agent):
     started_guardrail = []
     completed_guardrail = []
+    all_events_received = threading.Event()
+    expected_started = 3  # 2 from first task, 1 from second
+    expected_completed = 3  # 2 from first task, 1 from second
 
-    task = Task(
+    task1 = Task(
         description="Gather information about available books on the First World War",
         agent=sample_agent,
         expected_output="A list of available books on the First World War",
         guardrail="Ensure the authors are from Italy",
     )
 
-    with crewai_event_bus.scoped_handlers():
-
-        @crewai_event_bus.on(LLMGuardrailStartedEvent)
-        def handle_guardrail_started(source, event):
-            assert source == task
-            started_guardrail.append(
-                {"guardrail": event.guardrail, "retry_count": event.retry_count}
-            )
-
-        @crewai_event_bus.on(LLMGuardrailCompletedEvent)
-        def handle_guardrail_completed(source, event):
-            assert source == task
-            completed_guardrail.append(
-                {
-                    "success": event.success,
-                    "result": event.result,
-                    "error": event.error,
-                    "retry_count": event.retry_count,
-                }
-            )
-
-        result = task.execute_sync(agent=sample_agent)
-
-        def custom_guardrail(result: TaskOutput):
-            return (True, "good result from callable function")
-
-        task = Task(
-            description="Test task",
-            expected_output="Output",
-            guardrail=custom_guardrail,
+    @crewai_event_bus.on(LLMGuardrailStartedEvent)
+    def handle_guardrail_started(source, event):
+        started_guardrail.append(
+            {"guardrail": event.guardrail, "retry_count": event.retry_count}
         )
+        if (
+            len(started_guardrail) >= expected_started
+            and len(completed_guardrail) >= expected_completed
+        ):
+            all_events_received.set()
 
-        task.execute_sync(agent=sample_agent)
+    @crewai_event_bus.on(LLMGuardrailCompletedEvent)
+    def handle_guardrail_completed(source, event):
+        completed_guardrail.append(
+            {
+                "success": event.success,
+                "result": event.result,
+                "error": event.error,
+                "retry_count": event.retry_count,
+            }
+        )
+        if (
+            len(started_guardrail) >= expected_started
+            and len(completed_guardrail) >= expected_completed
+        ):
+            all_events_received.set()
 
-        expected_started_events = [
-            {"guardrail": "Ensure the authors are from Italy", "retry_count": 0},
-            {"guardrail": "Ensure the authors are from Italy", "retry_count": 1},
-            {
-                "guardrail": """def custom_guardrail(result: TaskOutput):
-            return (True, "good result from callable function")""",
-                "retry_count": 0,
-            },
-        ]
+    result = task1.execute_sync(agent=sample_agent)
 
-        expected_completed_events = [
-            {
-                "success": False,
-                "result": None,
-                "error": "The task result does not comply with the guardrail because none of "
-                "the listed authors are from Italy. All authors mentioned are from "
-                "different countries, including Germany, the UK, the USA, and others, "
-                "which violates the requirement that authors must be Italian.",
-                "retry_count": 0,
-            },
-            {"success": True, "result": result.raw, "error": None, "retry_count": 1},
-            {
-                "success": True,
-                "result": "good result from callable function",
-                "error": None,
-                "retry_count": 0,
-            },
-        ]
-        assert started_guardrail == expected_started_events
-        assert completed_guardrail == expected_completed_events
+    def custom_guardrail(result: TaskOutput):
+        return (True, "good result from callable function")
+
+    task2 = Task(
+        description="Test task",
+        expected_output="Output",
+        guardrail=custom_guardrail,
+    )
+
+    task2.execute_sync(agent=sample_agent)
+
+    # Wait for all events to be received
+    assert all_events_received.wait(timeout=10), (
+        "Timeout waiting for all guardrail events"
+    )
+
+    expected_started_events = [
+        {"guardrail": "Ensure the authors are from Italy", "retry_count": 0},
+        {"guardrail": "Ensure the authors are from Italy", "retry_count": 1},
+        {
+            "guardrail": """def custom_guardrail(result: TaskOutput):
+        return (True, "good result from callable function")""",
+            "retry_count": 0,
+        },
+    ]
+
+    expected_completed_events = [
+        {
+            "success": False,
+            "result": None,
+            "error": "The task result does not comply with the guardrail because none of "
+            "the listed authors are from Italy. All authors mentioned are from "
+            "different countries, including Germany, the UK, the USA, and others, "
+            "which violates the requirement that authors must be Italian.",
+            "retry_count": 0,
+        },
+        {"success": True, "result": result.raw, "error": None, "retry_count": 1},
+        {
+            "success": True,
+            "result": "good result from callable function",
+            "error": None,
+            "retry_count": 0,
+        },
+    ]
+    assert started_guardrail == expected_started_events
+    assert completed_guardrail == expected_completed_events
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])

lib/crewai/tests/tools/test_tool_usage.py

@@ -1,6 +1,7 @@
 import datetime
 import json
 import random
+import threading
 import time
 from unittest.mock import MagicMock, patch
 
@@ -32,7 +33,7 @@ class RandomNumberTool(BaseTool):
     args_schema: type[BaseModel] = RandomNumberToolInput
 
     def _run(self, min_value: int, max_value: int) -> int:
-        return random.randint(min_value, max_value)
+        return random.randint(min_value, max_value)  # noqa: S311
 
 
 # Example agent and task
@@ -470,13 +471,21 @@ def test_tool_selection_error_event_direct():
     )
 
     received_events = []
+    first_event_received = threading.Event()
+    second_event_received = threading.Event()
 
     @crewai_event_bus.on(ToolSelectionErrorEvent)
     def event_handler(source, event):
         received_events.append(event)
+        if event.tool_name == "Non Existent Tool":
+            first_event_received.set()
+        elif event.tool_name == "":
+            second_event_received.set()
 
-    with pytest.raises(Exception):
+    with pytest.raises(Exception):  # noqa: B017
         tool_usage._select_tool("Non Existent Tool")
+
+    assert first_event_received.wait(timeout=5), "Timeout waiting for first event"
     assert len(received_events) == 1
     event = received_events[0]
     assert isinstance(event, ToolSelectionErrorEvent)
@@ -488,12 +497,12 @@ def test_tool_selection_error_event_direct():
     assert "A test tool" in event.tool_class
     assert "don't exist" in event.error
 
-    received_events.clear()
-    with pytest.raises(Exception):
+    with pytest.raises(Exception):  # noqa: B017
         tool_usage._select_tool("")
 
-    assert len(received_events) == 1
-    event = received_events[0]
+    assert second_event_received.wait(timeout=5), "Timeout waiting for second event"
+    assert len(received_events) == 2
+    event = received_events[1]
     assert isinstance(event, ToolSelectionErrorEvent)
     assert event.agent_key == "test_key"
     assert event.agent_role == "test_role"
@@ -562,7 +571,7 @@ def test_tool_validate_input_error_event():
 
         # Test invalid input
         invalid_input = "invalid json {[}"
-        with pytest.raises(Exception):
+        with pytest.raises(Exception):  # noqa: B017
             tool_usage._validate_tool_input(invalid_input)
 
         # Verify event was emitted
@@ -616,12 +625,13 @@ def test_tool_usage_finished_event_with_result():
         action=MagicMock(),
     )
 
-    # Track received events
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageFinishedEvent)
     def event_handler(source, event):
         received_events.append(event)
+        event_received.set()
 
     # Call on_tool_use_finished with test data
     started_at = time.time()
@@ -634,7 +644,7 @@ def test_tool_usage_finished_event_with_result():
         result=result,
     )
 
-    # Verify event was emitted
+    assert event_received.wait(timeout=5), "Timeout waiting for event"
     assert len(received_events) == 1, "Expected one event to be emitted"
     event = received_events[0]
     assert isinstance(event, ToolUsageFinishedEvent)
@@ -695,12 +705,13 @@ def test_tool_usage_finished_event_with_cached_result():
         action=MagicMock(),
     )
 
-    # Track received events
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageFinishedEvent)
     def event_handler(source, event):
         received_events.append(event)
+        event_received.set()
 
     # Call on_tool_use_finished with test data and from_cache=True
     started_at = time.time()
@@ -713,7 +724,7 @@ def test_tool_usage_finished_event_with_cached_result():
         result=result,
     )
 
-    # Verify event was emitted
+    assert event_received.wait(timeout=5), "Timeout waiting for event"
     assert len(received_events) == 1, "Expected one event to be emitted"
     event = received_events[0]
     assert isinstance(event, ToolUsageFinishedEvent)

lib/crewai/tests/tracing/test_tracing.py

@@ -14,6 +14,7 @@ from crewai.events.listeners.tracing.trace_listener import (
 )
 from crewai.events.listeners.tracing.types import TraceEvent
 from crewai.flow.flow import Flow, start
+from tests.utils import wait_for_event_handlers
 
 
 class TestTraceListenerSetup:
@@ -39,38 +40,44 @@ class TestTraceListenerSetup:
         ):
             yield
 
-    @pytest.fixture(autouse=True)
-    def clear_event_bus(self):
-        """Clear event bus listeners before and after each test"""
-        from crewai.events.event_bus import crewai_event_bus
-
-        # Store original handlers
-        original_handlers = crewai_event_bus._handlers.copy()
-
-        # Clear for test
-        crewai_event_bus._handlers.clear()
-
-        yield
-
-        # Restore original state
-        crewai_event_bus._handlers.clear()
-        crewai_event_bus._handlers.update(original_handlers)
-
     @pytest.fixture(autouse=True)
     def reset_tracing_singletons(self):
         """Reset tracing singleton instances between tests"""
+        from crewai.events.event_bus import crewai_event_bus
+        from crewai.events.event_listener import EventListener
+
+        # Clear event bus handlers BEFORE creating any new singletons
+        with crewai_event_bus._rwlock.w_locked():
+            crewai_event_bus._sync_handlers = {}
+            crewai_event_bus._async_handlers = {}
+            crewai_event_bus._handler_dependencies = {}
+            crewai_event_bus._execution_plan_cache = {}
+
         # Reset TraceCollectionListener singleton
         if hasattr(TraceCollectionListener, "_instance"):
             TraceCollectionListener._instance = None
             TraceCollectionListener._initialized = False
 
+        # Reset EventListener singleton
+        if hasattr(EventListener, "_instance"):
+            EventListener._instance = None
+
         yield
 
         # Clean up after test
+        with crewai_event_bus._rwlock.w_locked():
+            crewai_event_bus._sync_handlers = {}
+            crewai_event_bus._async_handlers = {}
+            crewai_event_bus._handler_dependencies = {}
+            crewai_event_bus._execution_plan_cache = {}
+
         if hasattr(TraceCollectionListener, "_instance"):
             TraceCollectionListener._instance = None
             TraceCollectionListener._initialized = False
 
+        if hasattr(EventListener, "_instance"):
+            EventListener._instance = None
+
     @pytest.fixture(autouse=True)
     def mock_plus_api_calls(self):
         """Mock all PlusAPI HTTP calls to avoid network requests"""
@@ -167,15 +174,26 @@ class TestTraceListenerSetup:
             from crewai.events.event_bus import crewai_event_bus
 
             trace_listener = None
-            for handler_list in crewai_event_bus._handlers.values():
-                for handler in handler_list:
-                    if hasattr(handler, "__self__") and isinstance(
-                        handler.__self__, TraceCollectionListener
-                    ):
-                        trace_listener = handler.__self__
+            with crewai_event_bus._rwlock.r_locked():
+                for handler_set in crewai_event_bus._sync_handlers.values():
+                    for handler in handler_set:
+                        if hasattr(handler, "__self__") and isinstance(
+                            handler.__self__, TraceCollectionListener
+                        ):
+                            trace_listener = handler.__self__
+                            break
+                    if trace_listener:
                         break
-                if trace_listener:
-                    break
+                if not trace_listener:
+                    for handler_set in crewai_event_bus._async_handlers.values():
+                        for handler in handler_set:
+                            if hasattr(handler, "__self__") and isinstance(
+                                handler.__self__, TraceCollectionListener
+                            ):
+                                trace_listener = handler.__self__
+                                break
+                        if trace_listener:
+                            break
 
             if not trace_listener:
                 pytest.skip(
@@ -221,6 +239,7 @@ class TestTraceListenerSetup:
                 wraps=trace_listener.batch_manager.add_event,
             ) as add_event_mock:
                 crew.kickoff()
+                wait_for_event_handlers()
 
                 assert add_event_mock.call_count >= 2
 
@@ -267,24 +286,22 @@ class TestTraceListenerSetup:
             from crewai.events.event_bus import crewai_event_bus
 
             trace_handlers = []
-            for handlers in crewai_event_bus._handlers.values():
-                for handler in handlers:
-                    if hasattr(handler, "__self__") and isinstance(
-                        handler.__self__, TraceCollectionListener
-                    ):
-                        trace_handlers.append(handler)
-                    elif hasattr(handler, "__name__") and any(
-                        trace_name in handler.__name__
-                        for trace_name in [
-                            "on_crew_started",
-                            "on_crew_completed",
-                            "on_flow_started",
-                        ]
-                    ):
-                        trace_handlers.append(handler)
+            with crewai_event_bus._rwlock.r_locked():
+                for handlers in crewai_event_bus._sync_handlers.values():
+                    for handler in handlers:
+                        if hasattr(handler, "__self__") and isinstance(
+                            handler.__self__, TraceCollectionListener
+                        ):
+                            trace_handlers.append(handler)
+                for handlers in crewai_event_bus._async_handlers.values():
+                    for handler in handlers:
+                        if hasattr(handler, "__self__") and isinstance(
+                            handler.__self__, TraceCollectionListener
+                        ):
+                            trace_handlers.append(handler)
 
             assert len(trace_handlers) == 0, (
-                f"Found {len(trace_handlers)} trace handlers when tracing should be disabled"
+                f"Found {len(trace_handlers)} TraceCollectionListener handlers when tracing should be disabled"
             )
 
     def test_trace_listener_setup_correctly_for_crew(self):
@@ -385,6 +402,7 @@ class TestTraceListenerSetup:
             ):
                 crew = Crew(agents=[agent], tasks=[task], tracing=True)
                 crew.kickoff()
+                wait_for_event_handlers()
 
                 mock_plus_api_class.assert_called_with(api_key="mock_token_12345")
 
@@ -396,15 +414,33 @@ class TestTraceListenerSetup:
     def teardown_method(self):
         """Cleanup after each test method"""
         from crewai.events.event_bus import crewai_event_bus
+        from crewai.events.event_listener import EventListener
 
-        crewai_event_bus._handlers.clear()
+        with crewai_event_bus._rwlock.w_locked():
+            crewai_event_bus._sync_handlers = {}
+            crewai_event_bus._async_handlers = {}
+            crewai_event_bus._handler_dependencies = {}
+            crewai_event_bus._execution_plan_cache = {}
+
+        # Reset EventListener singleton
+        if hasattr(EventListener, "_instance"):
+            EventListener._instance = None
 
     @classmethod
     def teardown_class(cls):
         """Final cleanup after all tests in this class"""
         from crewai.events.event_bus import crewai_event_bus
+        from crewai.events.event_listener import EventListener
 
-        crewai_event_bus._handlers.clear()
+        with crewai_event_bus._rwlock.w_locked():
+            crewai_event_bus._sync_handlers = {}
+            crewai_event_bus._async_handlers = {}
+            crewai_event_bus._handler_dependencies = {}
+            crewai_event_bus._execution_plan_cache = {}
+
+        # Reset EventListener singleton
+        if hasattr(EventListener, "_instance"):
+            EventListener._instance = None
 
     @pytest.mark.vcr(filter_headers=["authorization"])
     def test_first_time_user_trace_collection_with_timeout(self, mock_plus_api_calls):
@@ -466,6 +502,7 @@ class TestTraceListenerSetup:
                 ) as mock_add_event,
             ):
                 result = crew.kickoff()
+                wait_for_event_handlers()
                 assert result is not None
 
                 assert mock_handle_completion.call_count >= 1
@@ -543,6 +580,7 @@ class TestTraceListenerSetup:
                 )
 
                 crew.kickoff()
+                wait_for_event_handlers()
 
                 assert mock_handle_completion.call_count >= 1, (
                     "handle_execution_completion should be called"
@@ -561,7 +599,6 @@ class TestTraceListenerSetup:
     @pytest.mark.vcr(filter_headers=["authorization"])
     def test_first_time_user_trace_consolidation_logic(self, mock_plus_api_calls):
         """Test the consolidation logic for first-time users vs regular tracing"""
-
         with (
             patch.dict(os.environ, {"CREWAI_TRACING_ENABLED": "false"}),
             patch(
@@ -579,7 +616,9 @@ class TestTraceListenerSetup:
         ):
             from crewai.events.event_bus import crewai_event_bus
 
-            crewai_event_bus._handlers.clear()
+            with crewai_event_bus._rwlock.w_locked():
+                crewai_event_bus._sync_handlers = {}
+                crewai_event_bus._async_handlers = {}
 
             trace_listener = TraceCollectionListener()
             trace_listener.setup_listeners(crewai_event_bus)
@@ -600,6 +639,9 @@ class TestTraceListenerSetup:
             with patch.object(TraceBatchManager, "initialize_batch") as mock_initialize:
                 result = crew.kickoff()
 
+                assert trace_listener.batch_manager.wait_for_pending_events(timeout=5.0), (
+                    "Timeout waiting for trace event handlers to complete"
+                )
                 assert mock_initialize.call_count >= 1
                 assert mock_initialize.call_args_list[0][1]["use_ephemeral"] is True
                 assert result is not None
@@ -700,6 +742,7 @@ class TestTraceListenerSetup:
                 ) as mock_mark_failed,
             ):
                 crew.kickoff()
+                wait_for_event_handlers()
 
                 mock_mark_failed.assert_called_once()
                 call_args = mock_mark_failed.call_args_list[0]

lib/crewai/tests/utilities/events/test_async_event_bus.py

@@ -0,0 +1,206 @@
+"""Tests for async event handling in CrewAI event bus.
+
+This module tests async handler registration, execution, and the aemit method.
+"""
+
+import asyncio
+
+import pytest
+
+from crewai.events.base_events import BaseEvent
+from crewai.events.event_bus import crewai_event_bus
+
+
+class AsyncTestEvent(BaseEvent):
+    pass
+
+
+@pytest.mark.asyncio
+async def test_async_handler_execution():
+    received_events = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            received_events.append(event)
+
+        event = AsyncTestEvent(type="async_test")
+        crewai_event_bus.emit("test_source", event)
+
+        await asyncio.sleep(0.1)
+
+        assert len(received_events) == 1
+        assert received_events[0] == event
+
+
+@pytest.mark.asyncio
+async def test_aemit_with_async_handlers():
+    received_events = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            received_events.append(event)
+
+        event = AsyncTestEvent(type="async_test")
+        await crewai_event_bus.aemit("test_source", event)
+
+        assert len(received_events) == 1
+        assert received_events[0] == event
+
+
+@pytest.mark.asyncio
+async def test_multiple_async_handlers():
+    received_events_1 = []
+    received_events_2 = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def handler_1(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            received_events_1.append(event)
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def handler_2(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.02)
+            received_events_2.append(event)
+
+        event = AsyncTestEvent(type="async_test")
+        await crewai_event_bus.aemit("test_source", event)
+
+        assert len(received_events_1) == 1
+        assert len(received_events_2) == 1
+
+
+@pytest.mark.asyncio
+async def test_mixed_sync_and_async_handlers():
+    sync_events = []
+    async_events = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        def sync_handler(source: object, event: BaseEvent) -> None:
+            sync_events.append(event)
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            async_events.append(event)
+
+        event = AsyncTestEvent(type="mixed_test")
+        crewai_event_bus.emit("test_source", event)
+
+        await asyncio.sleep(0.1)
+
+        assert len(sync_events) == 1
+        assert len(async_events) == 1
+
+
+@pytest.mark.asyncio
+async def test_async_handler_error_handling():
+    successful_handler_called = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def failing_handler(source: object, event: BaseEvent) -> None:
+            raise ValueError("Async handler error")
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def successful_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            successful_handler_called.append(True)
+
+        event = AsyncTestEvent(type="error_test")
+        await crewai_event_bus.aemit("test_source", event)
+
+        assert len(successful_handler_called) == 1
+
+
+@pytest.mark.asyncio
+async def test_aemit_with_no_handlers():
+    with crewai_event_bus.scoped_handlers():
+        event = AsyncTestEvent(type="no_handlers")
+        await crewai_event_bus.aemit("test_source", event)
+
+
+@pytest.mark.asyncio
+async def test_async_handler_registration_via_register_handler():
+    received_events = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        async def custom_async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            received_events.append(event)
+
+        crewai_event_bus.register_handler(AsyncTestEvent, custom_async_handler)
+
+        event = AsyncTestEvent(type="register_test")
+        await crewai_event_bus.aemit("test_source", event)
+
+        assert len(received_events) == 1
+        assert received_events[0] == event
+
+
+@pytest.mark.asyncio
+async def test_emit_async_handlers_fire_and_forget():
+    received_events = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def slow_async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.05)
+            received_events.append(event)
+
+        event = AsyncTestEvent(type="fire_forget_test")
+        crewai_event_bus.emit("test_source", event)
+
+        assert len(received_events) == 0
+
+        await asyncio.sleep(0.1)
+
+        assert len(received_events) == 1
+
+
+@pytest.mark.asyncio
+async def test_scoped_handlers_with_async():
+    received_before = []
+    received_during = []
+    received_after = []
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def before_handler(source: object, event: BaseEvent) -> None:
+            received_before.append(event)
+
+        with crewai_event_bus.scoped_handlers():
+
+            @crewai_event_bus.on(AsyncTestEvent)
+            async def scoped_handler(source: object, event: BaseEvent) -> None:
+                received_during.append(event)
+
+            event1 = AsyncTestEvent(type="during_scope")
+            await crewai_event_bus.aemit("test_source", event1)
+
+            assert len(received_before) == 0
+            assert len(received_during) == 1
+
+        @crewai_event_bus.on(AsyncTestEvent)
+        async def after_handler(source: object, event: BaseEvent) -> None:
+            received_after.append(event)
+
+        event2 = AsyncTestEvent(type="after_scope")
+        await crewai_event_bus.aemit("test_source", event2)
+
+        assert len(received_before) == 1
+        assert len(received_during) == 1
+        assert len(received_after) == 1

lib/crewai/tests/utilities/events/test_crewai_event_bus.py

@@ -1,3 +1,4 @@
+import threading
 from unittest.mock import Mock
 
 from crewai.events.base_events import BaseEvent
@@ -21,27 +22,42 @@ def test_specific_event_handler():
     mock_handler.assert_called_once_with("source_object", event)
 
 
-def test_wildcard_event_handler():
-    mock_handler = Mock()
+def test_multiple_handlers_same_event():
+    """Test that multiple handlers can be registered for the same event type."""
+    mock_handler1 = Mock()
+    mock_handler2 = Mock()
 
-    @crewai_event_bus.on(BaseEvent)
-    def handler(source, event):
-        mock_handler(source, event)
+    @crewai_event_bus.on(TestEvent)
+    def handler1(source, event):
+        mock_handler1(source, event)
+
+    @crewai_event_bus.on(TestEvent)
+    def handler2(source, event):
+        mock_handler2(source, event)
 
     event = TestEvent(type="test_event")
     crewai_event_bus.emit("source_object", event)
 
-    mock_handler.assert_called_once_with("source_object", event)
+    mock_handler1.assert_called_once_with("source_object", event)
+    mock_handler2.assert_called_once_with("source_object", event)
 
 
-def test_event_bus_error_handling(capfd):
-    @crewai_event_bus.on(BaseEvent)
+def test_event_bus_error_handling():
+    """Test that handler exceptions are caught and don't break the event bus."""
+    called = threading.Event()
+    error_caught = threading.Event()
+
+    @crewai_event_bus.on(TestEvent)
     def broken_handler(source, event):
+        called.set()
         raise ValueError("Simulated handler failure")
 
+    @crewai_event_bus.on(TestEvent)
+    def working_handler(source, event):
+        error_caught.set()
+
     event = TestEvent(type="test_event")
     crewai_event_bus.emit("source_object", event)
 
-    out, err = capfd.readouterr()
-    assert "Simulated handler failure" in out
-    assert "Handler 'broken_handler' failed" in out
+    assert called.wait(timeout=2), "Broken handler was never called"
+    assert error_caught.wait(timeout=2), "Working handler was never called after error"

lib/crewai/tests/utilities/events/test_rw_lock.py

@@ -0,0 +1,264 @@
+"""Tests for read-write lock implementation.
+
+This module tests the RWLock class for correct concurrent read and write behavior.
+"""
+
+import threading
+import time
+
+from crewai.events.utils.rw_lock import RWLock
+
+
+def test_multiple_readers_concurrent():
+    lock = RWLock()
+    active_readers = [0]
+    max_concurrent_readers = [0]
+    lock_for_counters = threading.Lock()
+
+    def reader(reader_id: int) -> None:
+        with lock.r_locked():
+            with lock_for_counters:
+                active_readers[0] += 1
+                max_concurrent_readers[0] = max(
+                    max_concurrent_readers[0], active_readers[0]
+                )
+
+            time.sleep(0.1)
+
+            with lock_for_counters:
+                active_readers[0] -= 1
+
+    threads = [threading.Thread(target=reader, args=(i,)) for i in range(5)]
+
+    for thread in threads:
+        thread.start()
+
+    for thread in threads:
+        thread.join()
+
+    assert max_concurrent_readers[0] == 5
+
+
+def test_writer_blocks_readers():
+    lock = RWLock()
+    writer_holding_lock = [False]
+    reader_accessed_during_write = [False]
+
+    def writer() -> None:
+        with lock.w_locked():
+            writer_holding_lock[0] = True
+            time.sleep(0.2)
+            writer_holding_lock[0] = False
+
+    def reader() -> None:
+        time.sleep(0.05)
+        with lock.r_locked():
+            if writer_holding_lock[0]:
+                reader_accessed_during_write[0] = True
+
+    writer_thread = threading.Thread(target=writer)
+    reader_thread = threading.Thread(target=reader)
+
+    writer_thread.start()
+    reader_thread.start()
+
+    writer_thread.join()
+    reader_thread.join()
+
+    assert not reader_accessed_during_write[0]
+
+
+def test_writer_blocks_other_writers():
+    lock = RWLock()
+    execution_order: list[int] = []
+    lock_for_order = threading.Lock()
+
+    def writer(writer_id: int) -> None:
+        with lock.w_locked():
+            with lock_for_order:
+                execution_order.append(writer_id)
+            time.sleep(0.1)
+
+    threads = [threading.Thread(target=writer, args=(i,)) for i in range(3)]
+
+    for thread in threads:
+        thread.start()
+
+    for thread in threads:
+        thread.join()
+
+    assert len(execution_order) == 3
+    assert len(set(execution_order)) == 3
+
+
+def test_readers_block_writers():
+    lock = RWLock()
+    reader_count = [0]
+    writer_accessed_during_read = [False]
+    lock_for_counters = threading.Lock()
+
+    def reader() -> None:
+        with lock.r_locked():
+            with lock_for_counters:
+                reader_count[0] += 1
+            time.sleep(0.2)
+            with lock_for_counters:
+                reader_count[0] -= 1
+
+    def writer() -> None:
+        time.sleep(0.05)
+        with lock.w_locked():
+            with lock_for_counters:
+                if reader_count[0] > 0:
+                    writer_accessed_during_read[0] = True
+
+    reader_thread = threading.Thread(target=reader)
+    writer_thread = threading.Thread(target=writer)
+
+    reader_thread.start()
+    writer_thread.start()
+
+    reader_thread.join()
+    writer_thread.join()
+
+    assert not writer_accessed_during_read[0]
+
+
+def test_alternating_readers_and_writers():
+    lock = RWLock()
+    operations: list[str] = []
+    lock_for_operations = threading.Lock()
+
+    def reader(reader_id: int) -> None:
+        with lock.r_locked():
+            with lock_for_operations:
+                operations.append(f"r{reader_id}_start")
+            time.sleep(0.05)
+            with lock_for_operations:
+                operations.append(f"r{reader_id}_end")
+
+    def writer(writer_id: int) -> None:
+        with lock.w_locked():
+            with lock_for_operations:
+                operations.append(f"w{writer_id}_start")
+            time.sleep(0.05)
+            with lock_for_operations:
+                operations.append(f"w{writer_id}_end")
+
+    threads = [
+        threading.Thread(target=reader, args=(0,)),
+        threading.Thread(target=writer, args=(0,)),
+        threading.Thread(target=reader, args=(1,)),
+        threading.Thread(target=writer, args=(1,)),
+        threading.Thread(target=reader, args=(2,)),
+    ]
+
+    for thread in threads:
+        thread.start()
+
+    for thread in threads:
+        thread.join()
+
+    assert len(operations) == 10
+
+    start_ops = [op for op in operations if "_start" in op]
+    end_ops = [op for op in operations if "_end" in op]
+    assert len(start_ops) == 5
+    assert len(end_ops) == 5
+
+
+def test_context_manager_releases_on_exception():
+    lock = RWLock()
+    exception_raised = False
+
+    try:
+        with lock.r_locked():
+            raise ValueError("Test exception")
+    except ValueError:
+        exception_raised = True
+
+    assert exception_raised
+
+    acquired = False
+    with lock.w_locked():
+        acquired = True
+
+    assert acquired
+
+
+def test_write_lock_releases_on_exception():
+    lock = RWLock()
+    exception_raised = False
+
+    try:
+        with lock.w_locked():
+            raise ValueError("Test exception")
+    except ValueError:
+        exception_raised = True
+
+    assert exception_raised
+
+    acquired = False
+    with lock.r_locked():
+        acquired = True
+
+    assert acquired
+
+
+def test_stress_many_readers_few_writers():
+    lock = RWLock()
+    read_count = [0]
+    write_count = [0]
+    lock_for_counters = threading.Lock()
+
+    def reader() -> None:
+        for _ in range(10):
+            with lock.r_locked():
+                with lock_for_counters:
+                    read_count[0] += 1
+                time.sleep(0.001)
+
+    def writer() -> None:
+        for _ in range(5):
+            with lock.w_locked():
+                with lock_for_counters:
+                    write_count[0] += 1
+                time.sleep(0.01)
+
+    reader_threads = [threading.Thread(target=reader) for _ in range(10)]
+    writer_threads = [threading.Thread(target=writer) for _ in range(2)]
+
+    all_threads = reader_threads + writer_threads
+
+    for thread in all_threads:
+        thread.start()
+
+    for thread in all_threads:
+        thread.join()
+
+    assert read_count[0] == 100
+    assert write_count[0] == 10
+
+
+def test_nested_read_locks_same_thread():
+    lock = RWLock()
+    nested_acquired = False
+
+    with lock.r_locked():
+        with lock.r_locked():
+            nested_acquired = True
+
+    assert nested_acquired
+
+
+def test_manual_acquire_release():
+    lock = RWLock()
+
+    lock.r_acquire()
+    lock.r_release()
+
+    lock.w_acquire()
+    lock.w_release()
+
+    with lock.r_locked():
+        pass

lib/crewai/tests/utilities/events/test_shutdown.py

@@ -0,0 +1,247 @@
+"""Tests for event bus shutdown and cleanup behavior.
+
+This module tests graceful shutdown, task completion, and cleanup operations.
+"""
+
+import asyncio
+import threading
+import time
+
+import pytest
+
+from crewai.events.base_events import BaseEvent
+from crewai.events.event_bus import CrewAIEventsBus
+
+
+class ShutdownTestEvent(BaseEvent):
+    pass
+
+
+def test_shutdown_prevents_new_events():
+    bus = CrewAIEventsBus()
+    received_events = []
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        def handler(source: object, event: BaseEvent) -> None:
+            received_events.append(event)
+
+        bus._shutting_down = True
+
+        event = ShutdownTestEvent(type="after_shutdown")
+        bus.emit("test_source", event)
+
+        time.sleep(0.1)
+
+        assert len(received_events) == 0
+
+        bus._shutting_down = False
+
+
+@pytest.mark.asyncio
+async def test_aemit_during_shutdown():
+    bus = CrewAIEventsBus()
+    received_events = []
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        async def handler(source: object, event: BaseEvent) -> None:
+            received_events.append(event)
+
+        bus._shutting_down = True
+
+        event = ShutdownTestEvent(type="aemit_during_shutdown")
+        await bus.aemit("test_source", event)
+
+        assert len(received_events) == 0
+
+        bus._shutting_down = False
+
+
+def test_shutdown_flag_prevents_emit():
+    bus = CrewAIEventsBus()
+    emitted_count = [0]
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        def handler(source: object, event: BaseEvent) -> None:
+            emitted_count[0] += 1
+
+        event1 = ShutdownTestEvent(type="before_shutdown")
+        bus.emit("test_source", event1)
+
+        time.sleep(0.1)
+        assert emitted_count[0] == 1
+
+        bus._shutting_down = True
+
+        event2 = ShutdownTestEvent(type="during_shutdown")
+        bus.emit("test_source", event2)
+
+        time.sleep(0.1)
+        assert emitted_count[0] == 1
+
+        bus._shutting_down = False
+
+
+def test_concurrent_access_during_shutdown_flag():
+    bus = CrewAIEventsBus()
+    received_events = []
+    lock = threading.Lock()
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        def handler(source: object, event: BaseEvent) -> None:
+            with lock:
+                received_events.append(event)
+
+        def emit_events() -> None:
+            for i in range(10):
+                event = ShutdownTestEvent(type=f"event_{i}")
+                bus.emit("source", event)
+                time.sleep(0.01)
+
+        def set_shutdown_flag() -> None:
+            time.sleep(0.05)
+            bus._shutting_down = True
+
+        emit_thread = threading.Thread(target=emit_events)
+        shutdown_thread = threading.Thread(target=set_shutdown_flag)
+
+        emit_thread.start()
+        shutdown_thread.start()
+
+        emit_thread.join()
+        shutdown_thread.join()
+
+        time.sleep(0.2)
+
+        assert len(received_events) < 10
+        assert len(received_events) > 0
+
+        bus._shutting_down = False
+
+
+@pytest.mark.asyncio
+async def test_async_handlers_complete_before_shutdown_flag():
+    bus = CrewAIEventsBus()
+    completed_handlers = []
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        async def async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.05)
+            if not bus._shutting_down:
+                completed_handlers.append(event)
+
+        for i in range(5):
+            event = ShutdownTestEvent(type=f"event_{i}")
+            bus.emit("source", event)
+
+        await asyncio.sleep(0.3)
+
+        assert len(completed_handlers) == 5
+
+
+def test_scoped_handlers_cleanup():
+    bus = CrewAIEventsBus()
+    received_before = []
+    received_during = []
+    received_after = []
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        def before_handler(source: object, event: BaseEvent) -> None:
+            received_before.append(event)
+
+        with bus.scoped_handlers():
+
+            @bus.on(ShutdownTestEvent)
+            def during_handler(source: object, event: BaseEvent) -> None:
+                received_during.append(event)
+
+            event1 = ShutdownTestEvent(type="during")
+            bus.emit("source", event1)
+            time.sleep(0.1)
+
+            assert len(received_before) == 0
+            assert len(received_during) == 1
+
+        event2 = ShutdownTestEvent(type="after_inner_scope")
+        bus.emit("source", event2)
+        time.sleep(0.1)
+
+        assert len(received_before) == 1
+        assert len(received_during) == 1
+
+    event3 = ShutdownTestEvent(type="after_outer_scope")
+    bus.emit("source", event3)
+    time.sleep(0.1)
+
+    assert len(received_before) == 1
+    assert len(received_during) == 1
+    assert len(received_after) == 0
+
+
+def test_handler_registration_thread_safety():
+    bus = CrewAIEventsBus()
+    handlers_registered = [0]
+    lock = threading.Lock()
+
+    with bus.scoped_handlers():
+
+        def register_handlers() -> None:
+            for _ in range(20):
+
+                @bus.on(ShutdownTestEvent)
+                def handler(source: object, event: BaseEvent) -> None:
+                    pass
+
+                with lock:
+                    handlers_registered[0] += 1
+
+                time.sleep(0.001)
+
+        threads = [threading.Thread(target=register_handlers) for _ in range(3)]
+
+        for thread in threads:
+            thread.start()
+
+        for thread in threads:
+            thread.join()
+
+        assert handlers_registered[0] == 60
+
+
+@pytest.mark.asyncio
+async def test_mixed_sync_async_handler_execution():
+    bus = CrewAIEventsBus()
+    sync_executed = []
+    async_executed = []
+
+    with bus.scoped_handlers():
+
+        @bus.on(ShutdownTestEvent)
+        def sync_handler(source: object, event: BaseEvent) -> None:
+            time.sleep(0.01)
+            sync_executed.append(event)
+
+        @bus.on(ShutdownTestEvent)
+        async def async_handler(source: object, event: BaseEvent) -> None:
+            await asyncio.sleep(0.01)
+            async_executed.append(event)
+
+        for i in range(5):
+            event = ShutdownTestEvent(type=f"event_{i}")
+            bus.emit("source", event)
+
+        await asyncio.sleep(0.2)
+
+        assert len(sync_executed) == 5
+        assert len(async_executed) == 5

lib/crewai/tests/utilities/events/test_thread_safety.py

@@ -0,0 +1,189 @@
+"""Tests for thread safety in CrewAI event bus.
+
+This module tests concurrent event emission and handler registration.
+"""
+
+import threading
+import time
+from collections.abc import Callable
+
+from crewai.events.base_events import BaseEvent
+from crewai.events.event_bus import crewai_event_bus
+
+
+class ThreadSafetyTestEvent(BaseEvent):
+    pass
+
+
+def test_concurrent_emit_from_multiple_threads():
+    received_events: list[BaseEvent] = []
+    lock = threading.Lock()
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(ThreadSafetyTestEvent)
+        def handler(source: object, event: BaseEvent) -> None:
+            with lock:
+                received_events.append(event)
+
+        threads: list[threading.Thread] = []
+        num_threads = 10
+        events_per_thread = 10
+
+        def emit_events(thread_id: int) -> None:
+            for i in range(events_per_thread):
+                event = ThreadSafetyTestEvent(type=f"thread_{thread_id}_event_{i}")
+                crewai_event_bus.emit(f"source_{thread_id}", event)
+
+        for i in range(num_threads):
+            thread = threading.Thread(target=emit_events, args=(i,))
+            threads.append(thread)
+            thread.start()
+
+        for thread in threads:
+            thread.join()
+
+        time.sleep(0.5)
+
+        assert len(received_events) == num_threads * events_per_thread
+
+
+def test_concurrent_handler_registration():
+    handlers_executed: list[int] = []
+    lock = threading.Lock()
+
+    def create_handler(handler_id: int) -> Callable[[object, BaseEvent], None]:
+        def handler(source: object, event: BaseEvent) -> None:
+            with lock:
+                handlers_executed.append(handler_id)
+
+        return handler
+
+    with crewai_event_bus.scoped_handlers():
+        threads: list[threading.Thread] = []
+        num_handlers = 20
+
+        def register_handler(handler_id: int) -> None:
+            crewai_event_bus.register_handler(
+                ThreadSafetyTestEvent, create_handler(handler_id)
+            )
+
+        for i in range(num_handlers):
+            thread = threading.Thread(target=register_handler, args=(i,))
+            threads.append(thread)
+            thread.start()
+
+        for thread in threads:
+            thread.join()
+
+        event = ThreadSafetyTestEvent(type="registration_test")
+        crewai_event_bus.emit("test_source", event)
+
+        time.sleep(0.5)
+
+        assert len(handlers_executed) == num_handlers
+        assert set(handlers_executed) == set(range(num_handlers))
+
+
+def test_concurrent_emit_and_registration():
+    received_events: list[BaseEvent] = []
+    lock = threading.Lock()
+
+    with crewai_event_bus.scoped_handlers():
+
+        def emit_continuously() -> None:
+            for i in range(50):
+                event = ThreadSafetyTestEvent(type=f"emit_event_{i}")
+                crewai_event_bus.emit("emitter", event)
+                time.sleep(0.001)
+
+        def register_continuously() -> None:
+            for _ in range(10):
+
+                @crewai_event_bus.on(ThreadSafetyTestEvent)
+                def handler(source: object, event: BaseEvent) -> None:
+                    with lock:
+                        received_events.append(event)
+
+                time.sleep(0.005)
+
+        emit_thread = threading.Thread(target=emit_continuously)
+        register_thread = threading.Thread(target=register_continuously)
+
+        emit_thread.start()
+        register_thread.start()
+
+        emit_thread.join()
+        register_thread.join()
+
+        time.sleep(0.5)
+
+        assert len(received_events) > 0
+
+
+def test_stress_test_rapid_emit():
+    received_count = [0]
+    lock = threading.Lock()
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(ThreadSafetyTestEvent)
+        def counter_handler(source: object, event: BaseEvent) -> None:
+            with lock:
+                received_count[0] += 1
+
+        num_events = 1000
+
+        for i in range(num_events):
+            event = ThreadSafetyTestEvent(type=f"rapid_event_{i}")
+            crewai_event_bus.emit("rapid_source", event)
+
+        time.sleep(1.0)
+
+        assert received_count[0] == num_events
+
+
+def test_multiple_event_types_concurrent():
+    class EventTypeA(BaseEvent):
+        pass
+
+    class EventTypeB(BaseEvent):
+        pass
+
+    received_a: list[BaseEvent] = []
+    received_b: list[BaseEvent] = []
+    lock = threading.Lock()
+
+    with crewai_event_bus.scoped_handlers():
+
+        @crewai_event_bus.on(EventTypeA)
+        def handler_a(source: object, event: BaseEvent) -> None:
+            with lock:
+                received_a.append(event)
+
+        @crewai_event_bus.on(EventTypeB)
+        def handler_b(source: object, event: BaseEvent) -> None:
+            with lock:
+                received_b.append(event)
+
+        def emit_type_a() -> None:
+            for i in range(50):
+                crewai_event_bus.emit("source_a", EventTypeA(type=f"type_a_{i}"))
+
+        def emit_type_b() -> None:
+            for i in range(50):
+                crewai_event_bus.emit("source_b", EventTypeB(type=f"type_b_{i}"))
+
+        thread_a = threading.Thread(target=emit_type_a)
+        thread_b = threading.Thread(target=emit_type_b)
+
+        thread_a.start()
+        thread_b.start()
+
+        thread_a.join()
+        thread_b.join()
+
+        time.sleep(0.5)
+
+        assert len(received_a) == 50
+        assert len(received_b) == 50

lib/crewai/tests/utilities/test_events.py

@@ -1,3 +1,4 @@
+import threading
 from datetime import datetime
 import os
 from unittest.mock import Mock, patch
@@ -49,6 +50,8 @@ from crewai.tools.base_tool import BaseTool
 from pydantic import Field
 import pytest
 
+from ..utils import wait_for_event_handlers
+
 
 @pytest.fixture(scope="module")
 def vcr_config(request) -> dict:
@@ -118,6 +121,7 @@ def test_crew_emits_start_kickoff_event(
         # Now when Crew creates EventListener, it will use our mocked telemetry
         crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
         crew.kickoff()
+    wait_for_event_handlers()
 
     mock_telemetry.crew_execution_span.assert_called_once_with(crew, None)
     mock_telemetry.end_crew.assert_called_once_with(crew, "hi")
@@ -131,15 +135,20 @@ def test_crew_emits_start_kickoff_event(
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_crew_emits_end_kickoff_event(base_agent, base_task):
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(CrewKickoffCompletedEvent)
     def handle_crew_end(source, event):
         received_events.append(event)
+        event_received.set()
 
     crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
 
     crew.kickoff()
 
+    assert event_received.wait(timeout=5), (
+        "Timeout waiting for crew kickoff completed event"
+    )
     assert len(received_events) == 1
     assert received_events[0].crew_name == "TestCrew"
     assert isinstance(received_events[0].timestamp, datetime)
@@ -165,6 +174,7 @@ def test_crew_emits_test_kickoff_type_event(base_agent, base_task):
     eval_llm = LLM(model="gpt-4o-mini")
     crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
     crew.test(n_iterations=1, eval_llm=eval_llm)
+    wait_for_event_handlers()
 
     assert len(received_events) == 3
     assert received_events[0].crew_name == "TestCrew"
@@ -181,40 +191,44 @@ def test_crew_emits_test_kickoff_type_event(base_agent, base_task):
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_crew_emits_kickoff_failed_event(base_agent, base_task):
     received_events = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(CrewKickoffFailedEvent)
+    def handle_crew_failed(source, event):
+        received_events.append(event)
+        event_received.set()
 
-        @crewai_event_bus.on(CrewKickoffFailedEvent)
-        def handle_crew_failed(source, event):
-            received_events.append(event)
+    crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
 
-        crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
+    with patch.object(Crew, "_execute_tasks") as mock_execute:
+        error_message = "Simulated crew kickoff failure"
+        mock_execute.side_effect = Exception(error_message)
 
-        with patch.object(Crew, "_execute_tasks") as mock_execute:
-            error_message = "Simulated crew kickoff failure"
-            mock_execute.side_effect = Exception(error_message)
+        with pytest.raises(Exception):  # noqa: B017
+            crew.kickoff()
 
-            with pytest.raises(Exception):  # noqa: B017
-                crew.kickoff()
-
-        assert len(received_events) == 1
-        assert received_events[0].error == error_message
-        assert isinstance(received_events[0].timestamp, datetime)
-        assert received_events[0].type == "crew_kickoff_failed"
+    assert event_received.wait(timeout=5), "Timeout waiting for failed event"
+    assert len(received_events) == 1
+    assert received_events[0].error == error_message
+    assert isinstance(received_events[0].timestamp, datetime)
+    assert received_events[0].type == "crew_kickoff_failed"
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_crew_emits_start_task_event(base_agent, base_task):
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(TaskStartedEvent)
     def handle_task_start(source, event):
         received_events.append(event)
+        event_received.set()
 
     crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
 
     crew.kickoff()
 
+    assert event_received.wait(timeout=5), "Timeout waiting for task started event"
     assert len(received_events) == 1
     assert isinstance(received_events[0].timestamp, datetime)
     assert received_events[0].type == "task_started"
@@ -225,10 +239,12 @@ def test_crew_emits_end_task_event(
     base_agent, base_task, reset_event_listener_singleton
 ):
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(TaskCompletedEvent)
     def handle_task_end(source, event):
         received_events.append(event)
+        event_received.set()
 
     mock_span = Mock()
 
@@ -246,6 +262,7 @@ def test_crew_emits_end_task_event(
         mock_telemetry.task_started.assert_called_once_with(crew=crew, task=base_task)
         mock_telemetry.task_ended.assert_called_once_with(mock_span, base_task, crew)
 
+    assert event_received.wait(timeout=5), "Timeout waiting for task completed event"
     assert len(received_events) == 1
     assert isinstance(received_events[0].timestamp, datetime)
     assert received_events[0].type == "task_completed"
@@ -255,11 +272,13 @@ def test_crew_emits_end_task_event(
 def test_task_emits_failed_event_on_execution_error(base_agent, base_task):
     received_events = []
     received_sources = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(TaskFailedEvent)
     def handle_task_failed(source, event):
         received_events.append(event)
         received_sources.append(source)
+        event_received.set()
 
     with patch.object(
         Task,
@@ -281,6 +300,9 @@ def test_task_emits_failed_event_on_execution_error(base_agent, base_task):
         with pytest.raises(Exception):  # noqa: B017
             agent.execute_task(task=task)
 
+            assert event_received.wait(timeout=5), (
+                "Timeout waiting for task failed event"
+            )
             assert len(received_events) == 1
             assert received_sources[0] == task
             assert received_events[0].error == error_message
@@ -291,17 +313,27 @@ def test_task_emits_failed_event_on_execution_error(base_agent, base_task):
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_agent_emits_execution_started_and_completed_events(base_agent, base_task):
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
 
     @crewai_event_bus.on(AgentExecutionStartedEvent)
     def handle_agent_start(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
 
     @crewai_event_bus.on(AgentExecutionCompletedEvent)
     def handle_agent_completed(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) >= 2:
+                all_events_received.set()
 
     crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
     crew.kickoff()
+
+    assert all_events_received.wait(timeout=5), (
+        "Timeout waiting for agent execution events"
+    )
     assert len(received_events) == 2
     assert received_events[0].agent == base_agent
     assert received_events[0].task == base_task
@@ -320,10 +352,12 @@ def test_agent_emits_execution_started_and_completed_events(base_agent, base_tas
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_agent_emits_execution_error_event(base_agent, base_task):
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(AgentExecutionErrorEvent)
     def handle_agent_start(source, event):
         received_events.append(event)
+        event_received.set()
 
     error_message = "Error happening while sending prompt to model."
     base_agent.max_retry_limit = 0
@@ -337,6 +371,9 @@ def test_agent_emits_execution_error_event(base_agent, base_task):
                 task=base_task,
             )
 
+        assert event_received.wait(timeout=5), (
+            "Timeout waiting for agent execution error event"
+        )
         assert len(received_events) == 1
         assert received_events[0].agent == base_agent
         assert received_events[0].task == base_task
@@ -358,10 +395,12 @@ class SayHiTool(BaseTool):
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_tools_emits_finished_events():
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageFinishedEvent)
     def handle_tool_end(source, event):
         received_events.append(event)
+        event_received.set()
 
     agent = Agent(
         role="base_agent",
@@ -377,6 +416,10 @@ def test_tools_emits_finished_events():
     )
     crew = Crew(agents=[agent], tasks=[task], name="TestCrew")
     crew.kickoff()
+
+    assert event_received.wait(timeout=5), (
+        "Timeout waiting for tool usage finished event"
+    )
     assert len(received_events) == 1
     assert received_events[0].agent_key == agent.key
     assert received_events[0].agent_role == agent.role
@@ -389,10 +432,15 @@ def test_tools_emits_finished_events():
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_tools_emits_error_events():
     received_events = []
+    lock = threading.Lock()
+    all_events_received = threading.Event()
 
     @crewai_event_bus.on(ToolUsageErrorEvent)
     def handle_tool_end(source, event):
-        received_events.append(event)
+        with lock:
+            received_events.append(event)
+            if len(received_events) >= 48:
+                all_events_received.set()
 
     class ErrorTool(BaseTool):
         name: str = Field(
@@ -423,6 +471,9 @@ def test_tools_emits_error_events():
     crew = Crew(agents=[agent], tasks=[task], name="TestCrew")
     crew.kickoff()
 
+    assert all_events_received.wait(timeout=5), (
+        "Timeout waiting for tool usage error events"
+    )
     assert len(received_events) == 48
     assert received_events[0].agent_key == agent.key
     assert received_events[0].agent_role == agent.role
@@ -435,11 +486,13 @@ def test_tools_emits_error_events():
 
 def test_flow_emits_start_event(reset_event_listener_singleton):
     received_events = []
+    event_received = threading.Event()
     mock_span = Mock()
 
     @crewai_event_bus.on(FlowStartedEvent)
     def handle_flow_start(source, event):
         received_events.append(event)
+        event_received.set()
 
     class TestFlow(Flow[dict]):
         @start()
@@ -458,6 +511,7 @@ def test_flow_emits_start_event(reset_event_listener_singleton):
         flow = TestFlow()
         flow.kickoff()
 
+    assert event_received.wait(timeout=5), "Timeout waiting for flow started event"
     mock_telemetry.flow_execution_span.assert_called_once_with("TestFlow", ["begin"])
     assert len(received_events) == 1
     assert received_events[0].flow_name == "TestFlow"
@@ -466,6 +520,7 @@ def test_flow_emits_start_event(reset_event_listener_singleton):
 
 def test_flow_name_emitted_to_event_bus():
     received_events = []
+    event_received = threading.Event()
 
     class MyFlowClass(Flow):
         name = "PRODUCTION_FLOW"
@@ -477,118 +532,133 @@ def test_flow_name_emitted_to_event_bus():
     @crewai_event_bus.on(FlowStartedEvent)
     def handle_flow_start(source, event):
         received_events.append(event)
+        event_received.set()
 
     flow = MyFlowClass()
     flow.kickoff()
 
+    assert event_received.wait(timeout=5), "Timeout waiting for flow started event"
     assert len(received_events) == 1
     assert received_events[0].flow_name == "PRODUCTION_FLOW"
 
 
 def test_flow_emits_finish_event():
     received_events = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(FlowFinishedEvent)
+    def handle_flow_finish(source, event):
+        received_events.append(event)
+        event_received.set()
 
-        @crewai_event_bus.on(FlowFinishedEvent)
-        def handle_flow_finish(source, event):
-            received_events.append(event)
+    class TestFlow(Flow[dict]):
+        @start()
+        def begin(self):
+            return "completed"
 
-        class TestFlow(Flow[dict]):
-            @start()
-            def begin(self):
-                return "completed"
+    flow = TestFlow()
+    result = flow.kickoff()
 
-        flow = TestFlow()
-        result = flow.kickoff()
-
-        assert len(received_events) == 1
-        assert received_events[0].flow_name == "TestFlow"
-        assert received_events[0].type == "flow_finished"
-        assert received_events[0].result == "completed"
-        assert result == "completed"
+    assert event_received.wait(timeout=5), "Timeout waiting for finish event"
+    assert len(received_events) == 1
+    assert received_events[0].flow_name == "TestFlow"
+    assert received_events[0].type == "flow_finished"
+    assert received_events[0].result == "completed"
+    assert result == "completed"
 
 
 def test_flow_emits_method_execution_started_event():
     received_events = []
+    lock = threading.Lock()
+    second_event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
-
-        @crewai_event_bus.on(MethodExecutionStartedEvent)
-        def handle_method_start(source, event):
+    @crewai_event_bus.on(MethodExecutionStartedEvent)
+    async def handle_method_start(source, event):
+        with lock:
             received_events.append(event)
+            if event.method_name == "second_method":
+                second_event_received.set()
 
-        class TestFlow(Flow[dict]):
-            @start()
-            def begin(self):
-                return "started"
+    class TestFlow(Flow[dict]):
+        @start()
+        def begin(self):
+            return "started"
 
-            @listen("begin")
-            def second_method(self):
-                return "executed"
+        @listen("begin")
+        def second_method(self):
+            return "executed"
 
-        flow = TestFlow()
-        flow.kickoff()
+    flow = TestFlow()
+    flow.kickoff()
 
-        assert len(received_events) == 2
+    assert second_event_received.wait(timeout=5), (
+        "Timeout waiting for second_method event"
+    )
+    assert len(received_events) == 2
 
-        assert received_events[0].method_name == "begin"
-        assert received_events[0].flow_name == "TestFlow"
-        assert received_events[0].type == "method_execution_started"
+    # Events may arrive in any order due to async handlers, so check both are present
+    method_names = {event.method_name for event in received_events}
+    assert method_names == {"begin", "second_method"}
 
-        assert received_events[1].method_name == "second_method"
-        assert received_events[1].flow_name == "TestFlow"
-        assert received_events[1].type == "method_execution_started"
+    for event in received_events:
+        assert event.flow_name == "TestFlow"
+        assert event.type == "method_execution_started"
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_register_handler_adds_new_handler(base_agent, base_task):
     received_events = []
+    event_received = threading.Event()
 
     def custom_handler(source, event):
         received_events.append(event)
+        event_received.set()
 
-    with crewai_event_bus.scoped_handlers():
-        crewai_event_bus.register_handler(CrewKickoffStartedEvent, custom_handler)
+    crewai_event_bus.register_handler(CrewKickoffStartedEvent, custom_handler)
 
-        crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
-        crew.kickoff()
+    crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
+    crew.kickoff()
 
-        assert len(received_events) == 1
-        assert isinstance(received_events[0].timestamp, datetime)
-        assert received_events[0].type == "crew_kickoff_started"
+    assert event_received.wait(timeout=5), "Timeout waiting for handler event"
+    assert len(received_events) == 1
+    assert isinstance(received_events[0].timestamp, datetime)
+    assert received_events[0].type == "crew_kickoff_started"
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_multiple_handlers_for_same_event(base_agent, base_task):
     received_events_1 = []
     received_events_2 = []
+    event_received = threading.Event()
 
     def handler_1(source, event):
         received_events_1.append(event)
 
     def handler_2(source, event):
         received_events_2.append(event)
+        event_received.set()
 
-    with crewai_event_bus.scoped_handlers():
-        crewai_event_bus.register_handler(CrewKickoffStartedEvent, handler_1)
-        crewai_event_bus.register_handler(CrewKickoffStartedEvent, handler_2)
+    crewai_event_bus.register_handler(CrewKickoffStartedEvent, handler_1)
+    crewai_event_bus.register_handler(CrewKickoffStartedEvent, handler_2)
 
-        crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
-        crew.kickoff()
+    crew = Crew(agents=[base_agent], tasks=[base_task], name="TestCrew")
+    crew.kickoff()
 
-        assert len(received_events_1) == 1
-        assert len(received_events_2) == 1
-        assert received_events_1[0].type == "crew_kickoff_started"
-        assert received_events_2[0].type == "crew_kickoff_started"
+    assert event_received.wait(timeout=5), "Timeout waiting for handler events"
+    assert len(received_events_1) == 1
+    assert len(received_events_2) == 1
+    assert received_events_1[0].type == "crew_kickoff_started"
+    assert received_events_2[0].type == "crew_kickoff_started"
 
 
 def test_flow_emits_created_event():
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(FlowCreatedEvent)
     def handle_flow_created(source, event):
         received_events.append(event)
+        event_received.set()
 
     class TestFlow(Flow[dict]):
         @start()
@@ -598,6 +668,7 @@ def test_flow_emits_created_event():
     flow = TestFlow()
     flow.kickoff()
 
+    assert event_received.wait(timeout=5), "Timeout waiting for flow created event"
     assert len(received_events) == 1
     assert received_events[0].flow_name == "TestFlow"
     assert received_events[0].type == "flow_created"
@@ -605,11 +676,13 @@ def test_flow_emits_created_event():
 
 def test_flow_emits_method_execution_failed_event():
     received_events = []
+    event_received = threading.Event()
     error = Exception("Simulated method failure")
 
     @crewai_event_bus.on(MethodExecutionFailedEvent)
     def handle_method_failed(source, event):
         received_events.append(event)
+        event_received.set()
 
     class TestFlow(Flow[dict]):
         @start()
@@ -620,6 +693,9 @@ def test_flow_emits_method_execution_failed_event():
     with pytest.raises(Exception):  # noqa: B017
         flow.kickoff()
 
+    assert event_received.wait(timeout=5), (
+        "Timeout waiting for method execution failed event"
+    )
     assert len(received_events) == 1
     assert received_events[0].method_name == "begin"
     assert received_events[0].flow_name == "TestFlow"
@@ -641,6 +717,7 @@ def test_llm_emits_call_started_event():
 
     llm = LLM(model="gpt-4o-mini")
     llm.call("Hello, how are you?")
+    wait_for_event_handlers()
 
     assert len(received_events) == 2
     assert received_events[0].type == "llm_call_started"
@@ -656,10 +733,12 @@ def test_llm_emits_call_started_event():
 @pytest.mark.isolated
 def test_llm_emits_call_failed_event():
     received_events = []
+    event_received = threading.Event()
 
     @crewai_event_bus.on(LLMCallFailedEvent)
     def handle_llm_call_failed(source, event):
         received_events.append(event)
+        event_received.set()
 
     error_message = "OpenAI API call failed: Simulated API failure"
 
@@ -673,6 +752,7 @@ def test_llm_emits_call_failed_event():
             llm.call("Hello, how are you?")
 
         assert str(exc_info.value) == "Simulated API failure"
+        assert event_received.wait(timeout=5), "Timeout waiting for failed event"
         assert len(received_events) == 1
         assert received_events[0].type == "llm_call_failed"
         assert received_events[0].error == error_message
@@ -686,24 +766,28 @@ def test_llm_emits_call_failed_event():
 def test_llm_emits_stream_chunk_events():
     """Test that LLM emits stream chunk events when streaming is enabled."""
     received_chunks = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_stream_chunk(source, event):
+        received_chunks.append(event.chunk)
+        if len(received_chunks) >= 1:
+            event_received.set()
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_stream_chunk(source, event):
-            received_chunks.append(event.chunk)
+    # Create an LLM with streaming enabled
+    llm = LLM(model="gpt-4o", stream=True)
 
-        # Create an LLM with streaming enabled
-        llm = LLM(model="gpt-4o", stream=True)
+    # Call the LLM with a simple message
+    response = llm.call("Tell me a short joke")
 
-        # Call the LLM with a simple message
-        response = llm.call("Tell me a short joke")
+    # Wait for at least one chunk
+    assert event_received.wait(timeout=5), "Timeout waiting for stream chunks"
 
-        # Verify that we received chunks
-        assert len(received_chunks) > 0
+    # Verify that we received chunks
+    assert len(received_chunks) > 0
 
-        # Verify that concatenating all chunks equals the final response
-        assert "".join(received_chunks) == response
+    # Verify that concatenating all chunks equals the final response
+    assert "".join(received_chunks) == response
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -711,23 +795,21 @@ def test_llm_no_stream_chunks_when_streaming_disabled():
     """Test that LLM doesn't emit stream chunk events when streaming is disabled."""
     received_chunks = []
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_stream_chunk(source, event):
+        received_chunks.append(event.chunk)
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_stream_chunk(source, event):
-            received_chunks.append(event.chunk)
+    # Create an LLM with streaming disabled
+    llm = LLM(model="gpt-4o", stream=False)
 
-        # Create an LLM with streaming disabled
-        llm = LLM(model="gpt-4o", stream=False)
+    # Call the LLM with a simple message
+    response = llm.call("Tell me a short joke")
 
-        # Call the LLM with a simple message
-        response = llm.call("Tell me a short joke")
+    # Verify that we didn't receive any chunks
+    assert len(received_chunks) == 0
 
-        # Verify that we didn't receive any chunks
-        assert len(received_chunks) == 0
-
-        # Verify we got a response
-        assert response and isinstance(response, str)
+    # Verify we got a response
+    assert response and isinstance(response, str)
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -735,98 +817,105 @@ def test_streaming_fallback_to_non_streaming():
     """Test that streaming falls back to non-streaming when there's an error."""
     received_chunks = []
     fallback_called = False
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_stream_chunk(source, event):
+        received_chunks.append(event.chunk)
+        if len(received_chunks) >= 2:
+            event_received.set()
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_stream_chunk(source, event):
-            received_chunks.append(event.chunk)
+    # Create an LLM with streaming enabled
+    llm = LLM(model="gpt-4o", stream=True)
 
-        # Create an LLM with streaming enabled
-        llm = LLM(model="gpt-4o", stream=True)
+    # Store original methods
+    original_call = llm.call
 
-        # Store original methods
-        original_call = llm.call
+    # Create a mock call method that handles the streaming error
+    def mock_call(messages, tools=None, callbacks=None, available_functions=None):
+        nonlocal fallback_called
+        # Emit a couple of chunks to simulate partial streaming
+        crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk="Test chunk 1"))
+        crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk="Test chunk 2"))
 
-        # Create a mock call method that handles the streaming error
-        def mock_call(messages, tools=None, callbacks=None, available_functions=None):
-            nonlocal fallback_called
-            # Emit a couple of chunks to simulate partial streaming
-            crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk="Test chunk 1"))
-            crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk="Test chunk 2"))
+        # Mark that fallback would be called
+        fallback_called = True
 
-            # Mark that fallback would be called
-            fallback_called = True
+        # Return a response as if fallback succeeded
+        return "Fallback response after streaming error"
 
-            # Return a response as if fallback succeeded
-            return "Fallback response after streaming error"
+    # Replace the call method with our mock
+    llm.call = mock_call
 
-        # Replace the call method with our mock
-        llm.call = mock_call
+    try:
+        # Call the LLM
+        response = llm.call("Tell me a short joke")
+        wait_for_event_handlers()
 
-        try:
-            # Call the LLM
-            response = llm.call("Tell me a short joke")
+        assert event_received.wait(timeout=5), "Timeout waiting for stream chunks"
 
-            # Verify that we received some chunks
-            assert len(received_chunks) == 2
-            assert received_chunks[0] == "Test chunk 1"
-            assert received_chunks[1] == "Test chunk 2"
+        # Verify that we received some chunks
+        assert len(received_chunks) == 2
+        assert received_chunks[0] == "Test chunk 1"
+        assert received_chunks[1] == "Test chunk 2"
 
-            # Verify fallback was triggered
-            assert fallback_called
+        # Verify fallback was triggered
+        assert fallback_called
 
-            # Verify we got the fallback response
-            assert response == "Fallback response after streaming error"
+        # Verify we got the fallback response
+        assert response == "Fallback response after streaming error"
 
-        finally:
-            # Restore the original method
-            llm.call = original_call
+    finally:
+        # Restore the original method
+        llm.call = original_call
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_streaming_empty_response_handling():
     """Test that streaming handles empty responses correctly."""
     received_chunks = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_stream_chunk(source, event):
+        received_chunks.append(event.chunk)
+        if len(received_chunks) >= 3:
+            event_received.set()
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_stream_chunk(source, event):
-            received_chunks.append(event.chunk)
+    # Create an LLM with streaming enabled
+    llm = LLM(model="gpt-3.5-turbo", stream=True)
 
-        # Create an LLM with streaming enabled
-        llm = LLM(model="gpt-3.5-turbo", stream=True)
+    # Store original methods
+    original_call = llm.call
 
-        # Store original methods
-        original_call = llm.call
+    # Create a mock call method that simulates empty chunks
+    def mock_call(messages, tools=None, callbacks=None, available_functions=None):
+        # Emit a few empty chunks
+        for _ in range(3):
+            crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk=""))
 
-        # Create a mock call method that simulates empty chunks
-        def mock_call(messages, tools=None, callbacks=None, available_functions=None):
-            # Emit a few empty chunks
-            for _ in range(3):
-                crewai_event_bus.emit(llm, event=LLMStreamChunkEvent(chunk=""))
+        # Return the default message for empty responses
+        return "I apologize, but I couldn't generate a proper response. Please try again or rephrase your request."
 
-            # Return the default message for empty responses
-            return "I apologize, but I couldn't generate a proper response. Please try again or rephrase your request."
+    # Replace the call method with our mock
+    llm.call = mock_call
 
-        # Replace the call method with our mock
-        llm.call = mock_call
+    try:
+        # Call the LLM - this should handle empty response
+        response = llm.call("Tell me a short joke")
 
-        try:
-            # Call the LLM - this should handle empty response
-            response = llm.call("Tell me a short joke")
+        assert event_received.wait(timeout=5), "Timeout waiting for empty chunks"
 
-            # Verify that we received empty chunks
-            assert len(received_chunks) == 3
-            assert all(chunk == "" for chunk in received_chunks)
+        # Verify that we received empty chunks
+        assert len(received_chunks) == 3
+        assert all(chunk == "" for chunk in received_chunks)
 
-            # Verify the response is the default message for empty responses
-            assert "I apologize" in response and "couldn't generate" in response
+        # Verify the response is the default message for empty responses
+        assert "I apologize" in response and "couldn't generate" in response
 
-        finally:
-            # Restore the original method
-            llm.call = original_call
+    finally:
+        # Restore the original method
+        llm.call = original_call
 
 
 @pytest.mark.vcr(filter_headers=["authorization"])
@@ -835,41 +924,49 @@ def test_stream_llm_emits_event_with_task_and_agent_info():
     failed_event = []
     started_event = []
     stream_event = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMCallFailedEvent)
+    def handle_llm_failed(source, event):
+        failed_event.append(event)
 
-        @crewai_event_bus.on(LLMCallFailedEvent)
-        def handle_llm_failed(source, event):
-            failed_event.append(event)
+    @crewai_event_bus.on(LLMCallStartedEvent)
+    def handle_llm_started(source, event):
+        started_event.append(event)
 
-        @crewai_event_bus.on(LLMCallStartedEvent)
-        def handle_llm_started(source, event):
-            started_event.append(event)
+    @crewai_event_bus.on(LLMCallCompletedEvent)
+    def handle_llm_completed(source, event):
+        completed_event.append(event)
+        if len(started_event) >= 1 and len(stream_event) >= 12:
+            event_received.set()
 
-        @crewai_event_bus.on(LLMCallCompletedEvent)
-        def handle_llm_completed(source, event):
-            completed_event.append(event)
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_llm_stream_chunk(source, event):
+        stream_event.append(event)
+        if (
+            len(completed_event) >= 1
+            and len(started_event) >= 1
+            and len(stream_event) >= 12
+        ):
+            event_received.set()
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_llm_stream_chunk(source, event):
-            stream_event.append(event)
+    agent = Agent(
+        role="TestAgent",
+        llm=LLM(model="gpt-4o-mini", stream=True),
+        goal="Just say hi",
+        backstory="You are a helpful assistant that just says hi",
+    )
+    task = Task(
+        description="Just say hi",
+        expected_output="hi",
+        llm=LLM(model="gpt-4o-mini", stream=True),
+        agent=agent,
+    )
 
-        agent = Agent(
-            role="TestAgent",
-            llm=LLM(model="gpt-4o-mini", stream=True),
-            goal="Just say hi",
-            backstory="You are a helpful assistant that just says hi",
-        )
-        task = Task(
-            description="Just say hi",
-            expected_output="hi",
-            llm=LLM(model="gpt-4o-mini", stream=True),
-            agent=agent,
-        )
-
-        crew = Crew(agents=[agent], tasks=[task])
-        crew.kickoff()
+    crew = Crew(agents=[agent], tasks=[task])
+    crew.kickoff()
 
+    assert event_received.wait(timeout=10), "Timeout waiting for LLM events"
     assert len(completed_event) == 1
     assert len(failed_event) == 0
     assert len(started_event) == 1
@@ -899,28 +996,30 @@ def test_llm_emits_event_with_task_and_agent_info(base_agent, base_task):
     failed_event = []
     started_event = []
     stream_event = []
+    event_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMCallFailedEvent)
+    def handle_llm_failed(source, event):
+        failed_event.append(event)
 
-        @crewai_event_bus.on(LLMCallFailedEvent)
-        def handle_llm_failed(source, event):
-            failed_event.append(event)
+    @crewai_event_bus.on(LLMCallStartedEvent)
+    def handle_llm_started(source, event):
+        started_event.append(event)
 
-        @crewai_event_bus.on(LLMCallStartedEvent)
-        def handle_llm_started(source, event):
-            started_event.append(event)
+    @crewai_event_bus.on(LLMCallCompletedEvent)
+    def handle_llm_completed(source, event):
+        completed_event.append(event)
+        if len(started_event) >= 1:
+            event_received.set()
 
-        @crewai_event_bus.on(LLMCallCompletedEvent)
-        def handle_llm_completed(source, event):
-            completed_event.append(event)
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_llm_stream_chunk(source, event):
+        stream_event.append(event)
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_llm_stream_chunk(source, event):
-            stream_event.append(event)
-
-        crew = Crew(agents=[base_agent], tasks=[base_task])
-        crew.kickoff()
+    crew = Crew(agents=[base_agent], tasks=[base_task])
+    crew.kickoff()
 
+    assert event_received.wait(timeout=10), "Timeout waiting for LLM events"
     assert len(completed_event) == 1
     assert len(failed_event) == 0
     assert len(started_event) == 1
@@ -950,32 +1049,41 @@ def test_llm_emits_event_with_lite_agent():
     failed_event = []
     started_event = []
     stream_event = []
+    all_events_received = threading.Event()
 
-    with crewai_event_bus.scoped_handlers():
+    @crewai_event_bus.on(LLMCallFailedEvent)
+    def handle_llm_failed(source, event):
+        failed_event.append(event)
 
-        @crewai_event_bus.on(LLMCallFailedEvent)
-        def handle_llm_failed(source, event):
-            failed_event.append(event)
+    @crewai_event_bus.on(LLMCallStartedEvent)
+    def handle_llm_started(source, event):
+        started_event.append(event)
 
-        @crewai_event_bus.on(LLMCallStartedEvent)
-        def handle_llm_started(source, event):
-            started_event.append(event)
+    @crewai_event_bus.on(LLMCallCompletedEvent)
+    def handle_llm_completed(source, event):
+        completed_event.append(event)
+        if len(started_event) >= 1 and len(stream_event) >= 15:
+            all_events_received.set()
 
-        @crewai_event_bus.on(LLMCallCompletedEvent)
-        def handle_llm_completed(source, event):
-            completed_event.append(event)
+    @crewai_event_bus.on(LLMStreamChunkEvent)
+    def handle_llm_stream_chunk(source, event):
+        stream_event.append(event)
+        if (
+            len(completed_event) >= 1
+            and len(started_event) >= 1
+            and len(stream_event) >= 15
+        ):
+            all_events_received.set()
 
-        @crewai_event_bus.on(LLMStreamChunkEvent)
-        def handle_llm_stream_chunk(source, event):
-            stream_event.append(event)
+    agent = Agent(
+        role="Speaker",
+        llm=LLM(model="gpt-4o-mini", stream=True),
+        goal="Just say hi",
+        backstory="You are a helpful assistant that just says hi",
+    )
+    agent.kickoff(messages=[{"role": "user", "content": "say hi!"}])
 
-        agent = Agent(
-            role="Speaker",
-            llm=LLM(model="gpt-4o-mini", stream=True),
-            goal="Just say hi",
-            backstory="You are a helpful assistant that just says hi",
-        )
-        agent.kickoff(messages=[{"role": "user", "content": "say hi!"}])
+    assert all_events_received.wait(timeout=10), "Timeout waiting for all events"
 
     assert len(completed_event) == 1
     assert len(failed_event) == 0

lib/crewai/tests/utils.py

@@ -0,0 +1,39 @@
+"""Test utilities for CrewAI tests."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+
+
+def wait_for_event_handlers(timeout: float = 5.0) -> None:
+    """Wait for all pending event handlers to complete.
+
+    This helper ensures all sync and async handlers finish processing before
+    proceeding. Useful in tests to make assertions deterministic.
+
+    Args:
+        timeout: Maximum time to wait in seconds.
+    """
+    from crewai.events.event_bus import crewai_event_bus
+
+    loop = getattr(crewai_event_bus, "_loop", None)
+
+    if loop and not loop.is_closed():
+
+        async def _wait_for_async_tasks() -> None:
+            tasks = {
+                t for t in asyncio.all_tasks(loop) if t is not asyncio.current_task()
+            }
+            if tasks:
+                await asyncio.gather(*tasks, return_exceptions=True)
+
+        future = asyncio.run_coroutine_threadsafe(_wait_for_async_tasks(), loop)
+        try:
+            future.result(timeout=timeout)
+        except Exception:  # noqa: S110
+            pass
+
+    crewai_event_bus._sync_executor.shutdown(wait=True)
+    crewai_event_bus._sync_executor = ThreadPoolExecutor(
+        max_workers=10,
+        thread_name_prefix="CrewAISyncHandler",
+    )

uv.lock

@@ -465,15 +465,6 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/01/f3/a9d961cfba236dc85f27f2f2c6eab88e12698754aaa02459ba7dfafc5062/bedrock_agentcore-0.1.7-py3-none-any.whl", hash = "sha256:441dde64fea596e9571e47ae37ee3b033e58d8d255018f13bdcde8ae8bef2075", size = 77216, upload-time = "2025-10-01T16:18:38.153Z" },
 ]
 
-[[package]]
-name = "blinker"
-version = "1.9.0"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/21/28/9b3f50ce0e048515135495f198351908d99540d69bfdc8c1d15b73dc55ce/blinker-1.9.0.tar.gz", hash = "sha256:b4ce2265a7abece45e7cc896e98dbebe6cead56bcf805a3d23136d145f5445bf", size = 22460, upload-time = "2024-11-08T17:25:47.436Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/10/cb/f2ad4230dc2eb1a74edf38f1a38b9b52277f75bef262d8908e60d957e13c/blinker-1.9.0-py3-none-any.whl", hash = "sha256:ba0efaa9080b619ff2f3459d1d500c57bddea4a6b424b60a91141db6fd2f08bc", size = 8458, upload-time = "2024-11-08T17:25:46.184Z" },
-]
-
 [[package]]
 name = "boto3"
 version = "1.40.45"
@@ -987,7 +978,6 @@ name = "crewai"
 source = { editable = "lib/crewai" }
 dependencies = [
     { name = "appdirs" },
-    { name = "blinker" },
     { name = "chromadb" },
     { name = "click" },
     { name = "instructor" },
@@ -1061,7 +1051,6 @@ watson = [
 requires-dist = [
     { name = "aisuite", marker = "extra == 'aisuite'", specifier = ">=0.1.10" },
     { name = "appdirs", specifier = ">=1.4.4" },
-    { name = "blinker", specifier = ">=1.9.0" },
     { name = "boto3", marker = "extra == 'aws'", specifier = ">=1.40.38" },
     { name = "boto3", marker = "extra == 'boto3'", specifier = ">=1.40.45" },
     { name = "chromadb", specifier = "~=1.1.0" },