Merge branch 'main' into fix/trace-serialization-pydantic-context

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

lib/crewai/src/crewai/events/base_events.py

@@ -2,14 +2,56 @@ from collections.abc import Iterator
 import contextvars
 from datetime import datetime, timezone
 import itertools
-from typing import Any
+from typing import Any, TypedDict
 import uuid
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, SerializationInfo
 
 from crewai.utilities.serialization import Serializable, to_serializable
 
 
+def _is_trace_context(info: SerializationInfo) -> bool:
+    """Check if serialization is happening in trace context."""
+    return bool(info.context and info.context.get("trace"))
+
+
+class AgentRef(TypedDict):
+    id: str
+    role: str
+
+
+class TaskRef(TypedDict):
+    id: str
+    name: str
+
+
+def _trace_agent_ref(agent: Any) -> AgentRef | None:
+    """Return a lightweight agent reference for trace serialization."""
+    if agent is None:
+        return None
+    return AgentRef(
+        id=str(getattr(agent, "id", "")),
+        role=getattr(agent, "role", ""),
+    )
+
+
+def _trace_task_ref(task: Any) -> TaskRef | None:
+    """Return a lightweight task reference for trace serialization."""
+    if task is None:
+        return None
+    return TaskRef(
+        id=str(getattr(task, "id", "")),
+        name=str(getattr(task, "name", None) or getattr(task, "description", "")),
+    )
+
+
+def _trace_tool_names(tools: Any) -> list[str] | None:
+    """Return a list of tool names for trace serialization."""
+    if not tools:
+        return None
+    return [getattr(t, "name", str(t)) for t in tools]
+
+
 _emission_counter: contextvars.ContextVar[Iterator[int]] = contextvars.ContextVar(
     "_emission_counter"
 )

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

@@ -1,7 +1,7 @@
 """Trace collection listener for orchestrating trace collection."""
 
 import os
-from typing import Any, ClassVar
+from typing import Any
 import uuid
 
 from typing_extensions import Self
@@ -126,18 +126,13 @@ from crewai.events.types.tool_usage_events import (
 from crewai.events.utils.console_formatter import ConsoleFormatter
 
 
+_TRACE_CONTEXT: dict[str, bool] = {"trace": True}
+"""Serialization context that triggers lightweight field serializers on event models."""
+
+
 class TraceCollectionListener(BaseEventListener):
     """Trace collection listener that orchestrates trace collection."""
 
-    complex_events: ClassVar[list[str]] = [
-        "task_started",
-        "task_completed",
-        "llm_call_started",
-        "llm_call_completed",
-        "agent_execution_started",
-        "agent_execution_completed",
-    ]
-
     _instance: Self | None = None
     _initialized: bool = False
     _listeners_setup: bool = False
@@ -810,9 +805,19 @@ class TraceCollectionListener(BaseEventListener):
     def _build_event_data(
         self, event_type: str, event: Any, source: Any
     ) -> dict[str, Any]:
-        """Build event data"""
-        if event_type not in self.complex_events:
-            return safe_serialize_to_dict(event)
+        """Build event data with context-based serialization to reduce trace bloat.
+
+        Field serializers on event models check for context={"trace": True} and
+        return lightweight references instead of full nested objects. This replaces
+        the old denylist approach with Pydantic v2's native context mechanism.
+
+        Only crew_kickoff_started gets a full crew structure (built separately).
+        Complex events (task_started, etc.) use custom projections for specific shapes.
+        All other events get context-aware serialization automatically.
+        """
+        if event_type == "crew_kickoff_started":
+            return self._build_crew_started_data(event)
+
         if event_type == "task_started":
             task_name = event.task.name or event.task.description
             task_display_name = (
@@ -853,19 +858,77 @@ class TraceCollectionListener(BaseEventListener):
                 "agent_backstory": event.agent.backstory,
             }
         if event_type == "llm_call_started":
-            event_data = safe_serialize_to_dict(event)
+            event_data = safe_serialize_to_dict(event, context=_TRACE_CONTEXT)
             event_data["task_name"] = event.task_name or getattr(
                 event, "task_description", None
             )
             return event_data
         if event_type == "llm_call_completed":
-            return safe_serialize_to_dict(event)
+            return safe_serialize_to_dict(event, context=_TRACE_CONTEXT)
 
-        return {
-            "event_type": event_type,
-            "event": safe_serialize_to_dict(event),
-            "source": source,
-        }
+        return safe_serialize_to_dict(event, context=_TRACE_CONTEXT)
+
+    def _build_crew_started_data(self, event: Any) -> dict[str, Any]:
+        """Build comprehensive crew structure for crew_kickoff_started event.
+
+        This is the ONE place where we serialize the full crew structure.
+        Subsequent events use lightweight references via field serializers.
+        """
+        event_data = safe_serialize_to_dict(event, context=_TRACE_CONTEXT)
+
+        crew = getattr(event, "crew", None)
+        if crew is not None:
+            agents_data = []
+            for agent in getattr(crew, "agents", []) or []:
+                agent_data = {
+                    "id": str(getattr(agent, "id", "")),
+                    "role": getattr(agent, "role", ""),
+                    "goal": getattr(agent, "goal", ""),
+                    "backstory": getattr(agent, "backstory", ""),
+                    "verbose": getattr(agent, "verbose", False),
+                    "allow_delegation": getattr(agent, "allow_delegation", False),
+                    "max_iter": getattr(agent, "max_iter", None),
+                    "max_rpm": getattr(agent, "max_rpm", None),
+                }
+                tools = getattr(agent, "tools", None)
+                if tools:
+                    agent_data["tool_names"] = [
+                        getattr(t, "name", str(t)) for t in tools
+                    ]
+                agents_data.append(agent_data)
+
+            tasks_data = []
+            for task in getattr(crew, "tasks", []) or []:
+                task_data = {
+                    "id": str(getattr(task, "id", "")),
+                    "name": getattr(task, "name", None),
+                    "description": getattr(task, "description", ""),
+                    "expected_output": getattr(task, "expected_output", ""),
+                    "async_execution": getattr(task, "async_execution", False),
+                    "human_input": getattr(task, "human_input", False),
+                }
+                task_agent = getattr(task, "agent", None)
+                if task_agent:
+                    task_data["agent_ref"] = {
+                        "id": str(getattr(task_agent, "id", "")),
+                        "role": getattr(task_agent, "role", ""),
+                    }
+                context_tasks = getattr(task, "context", None)
+                if context_tasks:
+                    task_data["context_task_ids"] = [
+                        str(getattr(ct, "id", "")) for ct in context_tasks
+                    ]
+                tasks_data.append(task_data)
+
+            event_data["crew_structure"] = {
+                "agents": agents_data,
+                "tasks": tasks_data,
+                "process": str(getattr(crew, "process", "")),
+                "verbose": getattr(crew, "verbose", False),
+                "memory": getattr(crew, "memory", False),
+            }
+
+        return event_data
 
     def _show_tracing_disabled_message(self) -> None:
         """Show a message when tracing is disabled."""

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

@@ -429,10 +429,22 @@ def mark_first_execution_done(user_consented: bool = False) -> None:
         p.write_text(json.dumps(data, indent=2))
 
 
-def safe_serialize_to_dict(obj: Any, exclude: set[str] | None = None) -> dict[str, Any]:
-    """Safely serialize an object to a dictionary for event data."""
+def safe_serialize_to_dict(
+    obj: Any,
+    exclude: set[str] | None = None,
+    context: dict[str, Any] | None = None,
+) -> dict[str, Any]:
+    """Safely serialize an object to a dictionary for event data.
+
+    Args:
+        obj: Object to serialize.
+        exclude: Set of keys to exclude from the result.
+        context: Optional context dict passed through to Pydantic's model_dump().
+            Field serializers can inspect this to customize output
+            (e.g. context={"trace": True} for lightweight trace serialization).
+    """
     try:
-        serialized = to_serializable(obj, exclude)
+        serialized = to_serializable(obj, exclude, context=context)
         if isinstance(serialized, dict):
             return serialized
         return {"serialized_data": serialized}

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

@@ -5,11 +5,17 @@ from __future__ import annotations
 from collections.abc import Sequence
 from typing import Any
 
-from pydantic import ConfigDict, model_validator
+from pydantic import ConfigDict, SerializationInfo, field_serializer, model_validator
 from typing_extensions import Self
 
 from crewai.agents.agent_builder.base_agent import BaseAgent
-from crewai.events.base_events import BaseEvent
+from crewai.events.base_events import (
+    BaseEvent,
+    _is_trace_context,
+    _trace_agent_ref,
+    _trace_task_ref,
+    _trace_tool_names,
+)
 from crewai.tools.base_tool import BaseTool
 from crewai.tools.structured_tool import CrewStructuredTool
 
@@ -31,6 +37,21 @@ class AgentExecutionStartedEvent(BaseEvent):
         _set_agent_fingerprint(self, self.agent)
         return self
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
+    @field_serializer("tools")
+    @classmethod
+    def _serialize_tools(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_tool_names(v) if _is_trace_context(info) else v
+
 
 class AgentExecutionCompletedEvent(BaseEvent):
     """Event emitted when an agent completes executing a task"""
@@ -48,6 +69,16 @@ class AgentExecutionCompletedEvent(BaseEvent):
         _set_agent_fingerprint(self, self.agent)
         return self
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
 
 class AgentExecutionErrorEvent(BaseEvent):
     """Event emitted when an agent encounters an error during execution"""
@@ -65,6 +96,16 @@ class AgentExecutionErrorEvent(BaseEvent):
         _set_agent_fingerprint(self, self.agent)
         return self
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
 
 # New event classes for LiteAgent
 class LiteAgentExecutionStartedEvent(BaseEvent):
@@ -77,6 +118,11 @@ class LiteAgentExecutionStartedEvent(BaseEvent):
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
+    @field_serializer("tools")
+    @classmethod
+    def _serialize_tools(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_tool_names(v) if _is_trace_context(info) else v
+
 
 class LiteAgentExecutionCompletedEvent(BaseEvent):
     """Event emitted when a LiteAgent completes execution"""

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

@@ -1,6 +1,8 @@
 from typing import TYPE_CHECKING, Any
 
-from crewai.events.base_events import BaseEvent
+from pydantic import SerializationInfo, field_serializer
+
+from crewai.events.base_events import BaseEvent, _is_trace_context
 
 
 if TYPE_CHECKING:
@@ -26,6 +28,14 @@ class CrewBaseEvent(BaseEvent):
             if self.crew.fingerprint.metadata:
                 self.fingerprint_metadata = self.crew.fingerprint.metadata
 
+    @field_serializer("crew")
+    @classmethod
+    def _serialize_crew(cls, v: Any, info: SerializationInfo) -> Any:
+        """Exclude crew in trace context — crew_kickoff_started builds structure separately."""
+        if _is_trace_context(info):
+            return None
+        return v
+
     def to_json(self, exclude: set[str] | None = None) -> Any:
         if exclude is None:
             exclude = set()

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

@@ -1,9 +1,9 @@
 from enum import Enum
 from typing import Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, SerializationInfo, field_serializer
 
-from crewai.events.base_events import BaseEvent
+from crewai.events.base_events import BaseEvent, _is_trace_context
 
 
 class LLMEventBase(BaseEvent):
@@ -49,6 +49,16 @@ class LLMCallStartedEvent(LLMEventBase):
     callbacks: list[Any] | None = None
     available_functions: dict[str, Any] | None = None
 
+    @field_serializer("callbacks")
+    @classmethod
+    def _serialize_callbacks(cls, v: Any, info: SerializationInfo) -> Any:
+        return None if _is_trace_context(info) else v
+
+    @field_serializer("available_functions")
+    @classmethod
+    def _serialize_available_functions(cls, v: Any, info: SerializationInfo) -> Any:
+        return None if _is_trace_context(info) else v
+
 
 class LLMCallCompletedEvent(LLMEventBase):
     """Event emitted when a LLM call completes"""

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

@@ -1,6 +1,8 @@
 from typing import Any
 
-from crewai.events.base_events import BaseEvent
+from pydantic import SerializationInfo, field_serializer
+
+from crewai.events.base_events import BaseEvent, _is_trace_context, _trace_task_ref
 from crewai.tasks.task_output import TaskOutput
 
 
@@ -24,6 +26,11 @@ class TaskStartedEvent(BaseEvent):
         super().__init__(**data)
         _set_task_fingerprint(self, self.task)
 
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
 
 class TaskCompletedEvent(BaseEvent):
     """Event emitted when a task completes"""
@@ -36,6 +43,11 @@ class TaskCompletedEvent(BaseEvent):
         super().__init__(**data)
         _set_task_fingerprint(self, self.task)
 
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
 
 class TaskFailedEvent(BaseEvent):
     """Event emitted when a task fails"""
@@ -48,6 +60,11 @@ class TaskFailedEvent(BaseEvent):
         super().__init__(**data)
         _set_task_fingerprint(self, self.task)
 
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v
+
 
 class TaskEvaluationEvent(BaseEvent):
     """Event emitted when a task evaluation is completed"""
@@ -59,3 +76,8 @@ class TaskEvaluationEvent(BaseEvent):
     def __init__(self, **data: Any) -> None:
         super().__init__(**data)
         _set_task_fingerprint(self, self.task)
+
+    @field_serializer("task")
+    @classmethod
+    def _serialize_task(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_task_ref(v) if _is_trace_context(info) else v

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

@@ -2,9 +2,9 @@ from collections.abc import Callable
 from datetime import datetime
 from typing import Any
 
-from pydantic import ConfigDict
+from pydantic import ConfigDict, SerializationInfo, field_serializer
 
-from crewai.events.base_events import BaseEvent
+from crewai.events.base_events import BaseEvent, _is_trace_context, _trace_agent_ref
 
 
 class ToolUsageEvent(BaseEvent):
@@ -26,6 +26,11 @@ class ToolUsageEvent(BaseEvent):
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
     def __init__(self, **data: Any) -> None:
         if data.get("from_task"):
             task = data["from_task"]
@@ -99,6 +104,11 @@ class ToolExecutionErrorEvent(BaseEvent):
     tool_class: Callable[..., Any]
     agent: Any | None = None
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
     def __init__(self, **data: Any) -> None:
         super().__init__(**data)
         # Set fingerprint data from the agent

lib/crewai/src/crewai/utilities/serialization.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from datetime import date, datetime
+from enum import Enum
 import json
 from typing import Any, TypeAlias
 import uuid
@@ -20,6 +21,7 @@ def to_serializable(
     max_depth: int = 5,
     _current_depth: int = 0,
     _ancestors: set[int] | None = None,
+    context: dict[str, Any] | None = None,
 ) -> Serializable:
     """Converts a Python object into a JSON-compatible representation.
 
@@ -33,6 +35,9 @@ def to_serializable(
         max_depth: Maximum recursion depth. Defaults to 5.
         _current_depth: Current recursion depth (for internal use).
         _ancestors: Set of ancestor object ids for cycle detection (for internal use).
+        context: Optional context dict passed to Pydantic's model_dump(context=...).
+            Field serializers on the model can inspect this to customize output
+            (e.g. context={"trace": True} for lightweight trace serialization).
 
     Returns:
         Serializable: A JSON-compatible structure.
@@ -48,6 +53,15 @@ def to_serializable(
 
     if isinstance(obj, (str, int, float, bool, type(None))):
         return obj
+    if isinstance(obj, Enum):
+        return to_serializable(
+            obj.value,
+            exclude=exclude,
+            max_depth=max_depth,
+            _current_depth=_current_depth,
+            _ancestors=_ancestors,
+            context=context,
+        )
     if isinstance(obj, uuid.UUID):
         return str(obj)
     if isinstance(obj, (date, datetime)):
@@ -66,6 +80,7 @@ def to_serializable(
                 max_depth=max_depth,
                 _current_depth=_current_depth + 1,
                 _ancestors=new_ancestors,
+                context=context,
             )
             for item in obj
         ]
@@ -77,17 +92,24 @@ def to_serializable(
                 max_depth=max_depth,
                 _current_depth=_current_depth + 1,
                 _ancestors=new_ancestors,
+                context=context,
             )
             for key, value in obj.items()
             if key not in exclude
         }
     if isinstance(obj, BaseModel):
         try:
+            dump_kwargs: dict[str, Any] = {}
+            if exclude:
+                dump_kwargs["exclude"] = exclude
+            if context is not None:
+                dump_kwargs["context"] = context
             return to_serializable(
-                obj=obj.model_dump(exclude=exclude),
+                obj=obj.model_dump(**dump_kwargs),
                 max_depth=max_depth,
                 _current_depth=_current_depth + 1,
                 _ancestors=new_ancestors,
+                context=context,
             )
         except Exception:
             try:
@@ -97,12 +119,30 @@ def to_serializable(
                         max_depth=max_depth,
                         _current_depth=_current_depth + 1,
                         _ancestors=new_ancestors,
+                        context=context,
                     )
                     for k, v in obj.__dict__.items()
                     if k not in (exclude or set())
                 }
             except Exception:
                 return repr(obj)
+    if callable(obj):
+        return repr(obj)
+    if hasattr(obj, "__dict__"):
+        try:
+            return {
+                _to_serializable_key(k): to_serializable(
+                    v,
+                    max_depth=max_depth,
+                    _current_depth=_current_depth + 1,
+                    _ancestors=new_ancestors,
+                    context=context,
+                )
+                for k, v in obj.__dict__.items()
+                if not k.startswith("_")
+            }
+        except Exception:
+            return repr(obj)
     return repr(obj)
 
 

lib/crewai/tests/tracing/test_trace_serialization.py

@@ -0,0 +1,612 @@
+"""Tests for trace serialization optimization using Pydantic v2 context-based serialization.
+
+These tests verify that trace events use @field_serializer with SerializationInfo.context
+to produce lightweight representations, reducing event sizes from 50-100KB to a few KB.
+"""
+
+import json
+import uuid
+from typing import Any
+from unittest.mock import MagicMock
+
+import pytest
+from pydantic import ConfigDict
+
+from crewai.agents.agent_builder.base_agent import BaseAgent
+from crewai.events.base_events import _trace_agent_ref, _trace_task_ref, _trace_tool_names
+from crewai.events.listeners.tracing.utils import safe_serialize_to_dict
+from crewai.utilities.serialization import to_serializable
+
+
+# ---------------------------------------------------------------------------
+# Lightweight BaseAgent subclass for tests (avoids heavy dependencies)
+# ---------------------------------------------------------------------------
+
+class _StubAgent(BaseAgent):
+    """Minimal BaseAgent subclass that satisfies validation without heavy deps."""
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    def execute_task(self, *a: Any, **kw: Any) -> str:
+        return ""
+
+    def create_agent_executor(self, *a: Any, **kw: Any) -> None:
+        pass
+
+    def _parse_tools(self, *a: Any, **kw: Any) -> list:
+        return []
+
+    def get_delegation_tools(self, *a: Any, **kw: Any) -> list:
+        return []
+
+    def get_output_converter(self, *a: Any, **kw: Any) -> Any:
+        return None
+
+    def get_multimodal_tools(self, *a: Any, **kw: Any) -> list:
+        return []
+
+    async def aexecute_task(self, *a: Any, **kw: Any) -> str:
+        return ""
+
+    def get_mcp_tools(self, *a: Any, **kw: Any) -> list:
+        return []
+
+    def get_platform_tools(self, *a: Any, **kw: Any) -> list:
+        return []
+
+
+def _make_stub_agent(**overrides) -> _StubAgent:
+    """Create a minimal BaseAgent instance for testing."""
+    defaults = {
+        "role": "Researcher",
+        "goal": "Research things",
+        "backstory": "Expert researcher",
+        "tools": [],
+    }
+    defaults.update(overrides)
+    return _StubAgent(**defaults)
+
+
+# ---------------------------------------------------------------------------
+# Helpers to build realistic mock objects for event fields
+# ---------------------------------------------------------------------------
+
+def _make_mock_task(**overrides):
+    task = MagicMock()
+    task.id = overrides.get("id", uuid.uuid4())
+    task.name = overrides.get("name", "Research Task")
+    task.description = overrides.get("description", "Do research")
+    task.expected_output = overrides.get("expected_output", "Research results")
+    task.async_execution = overrides.get("async_execution", False)
+    task.human_input = overrides.get("human_input", False)
+    task.agent = overrides.get("agent", _make_stub_agent())
+    task.context = overrides.get("context", None)
+    task.crew = MagicMock()
+    task.tools = overrides.get("tools", [MagicMock(), MagicMock()])
+
+    fp = MagicMock()
+    fp.uuid_str = str(uuid.uuid4())
+    fp.metadata = {"name": task.name}
+    task.fingerprint = fp
+
+    return task
+
+
+def _make_stub_tool(tool_name="web_search") -> Any:
+    """Create a minimal BaseTool instance for testing."""
+    from crewai.tools.base_tool import BaseTool
+
+    class _StubTool(BaseTool):
+        name: str = "stub"
+        description: str = "stub tool"
+
+        def _run(self, *a: Any, **kw: Any) -> str:
+            return ""
+
+    return _StubTool(name=tool_name, description=f"{tool_name} tool")
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: trace ref helpers
+# ---------------------------------------------------------------------------
+
+class TestTraceRefHelpers:
+    def test_trace_agent_ref(self):
+        agent = _make_stub_agent(role="Analyst")
+        ref = _trace_agent_ref(agent)
+        assert ref["role"] == "Analyst"
+        assert "id" in ref
+        assert len(ref) == 2  # only id and role
+
+    def test_trace_agent_ref_none(self):
+        assert _trace_agent_ref(None) is None
+
+    def test_trace_task_ref(self):
+        task = _make_mock_task(name="Write Report")
+        ref = _trace_task_ref(task)
+        assert ref["name"] == "Write Report"
+        assert "id" in ref
+        assert len(ref) == 2
+
+    def test_trace_task_ref_falls_back_to_description(self):
+        task = _make_mock_task(name=None, description="Describe the report")
+        ref = _trace_task_ref(task)
+        assert ref["name"] == "Describe the report"
+
+    def test_trace_task_ref_none(self):
+        assert _trace_task_ref(None) is None
+
+    def test_trace_tool_names(self):
+        tools = [_make_stub_tool("search"), _make_stub_tool("read")]
+        names = _trace_tool_names(tools)
+        assert names == ["search", "read"]
+
+    def test_trace_tool_names_empty(self):
+        assert _trace_tool_names([]) is None
+        assert _trace_tool_names(None) is None
+
+
+# ---------------------------------------------------------------------------
+# Integration tests: field serializers on real event classes
+# ---------------------------------------------------------------------------
+
+class TestAgentEventFieldSerializers:
+    """Test that agent event field serializers respond to trace context."""
+
+    def test_agent_execution_started_trace_context(self):
+        from crewai.events.types.agent_events import AgentExecutionStartedEvent
+
+        agent = _make_stub_agent(role="Researcher")
+        task = _make_mock_task(name="Research Task")
+        tools = [_make_stub_tool("search"), _make_stub_tool("read")]
+
+        event = AgentExecutionStartedEvent(
+            agent=agent, task=task, tools=tools, task_prompt="Do research"
+        )
+
+        # With trace context: lightweight refs
+        trace_dump = event.model_dump(context={"trace": True})
+        assert trace_dump["agent"] == {"id": str(agent.id), "role": "Researcher"}
+        assert trace_dump["task"] == {"id": str(task.id), "name": "Research Task"}
+        assert trace_dump["tools"] == ["search", "read"]
+
+    def test_agent_execution_started_no_context(self):
+        from crewai.events.types.agent_events import AgentExecutionStartedEvent
+
+        agent = _make_stub_agent(role="SpecificRole")
+        task = _make_mock_task()
+
+        event = AgentExecutionStartedEvent(
+            agent=agent, task=task, tools=None, task_prompt="Do research"
+        )
+
+        # Without context: full agent dict (Pydantic model_dump expands it)
+        normal_dump = event.model_dump()
+        assert isinstance(normal_dump["agent"], dict)
+        assert normal_dump["agent"]["role"] == "SpecificRole"
+        # Should have ALL agent fields, not just the lightweight ref
+        assert "goal" in normal_dump["agent"]
+        assert "backstory" in normal_dump["agent"]
+        assert "max_iter" in normal_dump["agent"]
+
+    def test_agent_execution_error_preserves_identification(self):
+        from crewai.events.types.agent_events import AgentExecutionErrorEvent
+
+        agent = _make_stub_agent(role="Analyst")
+        task = _make_mock_task(name="Analysis Task")
+
+        event = AgentExecutionErrorEvent(
+            agent=agent, task=task, error="Something went wrong"
+        )
+
+        trace_dump = event.model_dump(context={"trace": True})
+        # Error events should still have agent/task identification as refs
+        assert trace_dump["agent"]["role"] == "Analyst"
+        assert trace_dump["task"]["name"] == "Analysis Task"
+        assert trace_dump["error"] == "Something went wrong"
+
+    def test_agent_execution_completed_trace_context(self):
+        from crewai.events.types.agent_events import AgentExecutionCompletedEvent
+
+        agent = _make_stub_agent(role="Writer")
+        task = _make_mock_task(name="Writing Task")
+
+        event = AgentExecutionCompletedEvent(
+            agent=agent, task=task, output="Final output"
+        )
+
+        trace_dump = event.model_dump(context={"trace": True})
+        assert trace_dump["agent"]["role"] == "Writer"
+        assert trace_dump["task"]["name"] == "Writing Task"
+        assert trace_dump["output"] == "Final output"
+
+
+class TestTaskEventFieldSerializers:
+    """Test that task event field serializers respond to trace context."""
+
+    def test_task_started_trace_context(self):
+        from crewai.events.types.task_events import TaskStartedEvent
+
+        task = _make_mock_task(name="Test Task")
+        event = TaskStartedEvent(task=task, context="some context")
+
+        trace_dump = event.model_dump(context={"trace": True})
+        assert trace_dump["task"] == {"id": str(task.id), "name": "Test Task"}
+        assert trace_dump["context"] == "some context"
+
+    def test_task_failed_trace_context(self):
+        from crewai.events.types.task_events import TaskFailedEvent
+
+        task = _make_mock_task(name="Failing Task")
+        event = TaskFailedEvent(task=task, error="Task failed")
+
+        trace_dump = event.model_dump(context={"trace": True})
+        assert trace_dump["task"]["name"] == "Failing Task"
+        assert trace_dump["error"] == "Task failed"
+
+
+class TestCrewEventFieldSerializers:
+    """Test that crew event field serializers respond to trace context."""
+
+    def test_crew_kickoff_started_excludes_crew_in_trace(self):
+        from crewai.events.types.crew_events import CrewKickoffStartedEvent
+
+        crew = MagicMock()
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        event = CrewKickoffStartedEvent(
+            crew=crew, crew_name="TestCrew", inputs={"key": "value"}
+        )
+
+        trace_dump = event.model_dump(context={"trace": True})
+        # crew field should be None in trace context
+        assert trace_dump["crew"] is None
+        # scalar fields preserved
+        assert trace_dump["crew_name"] == "TestCrew"
+        assert trace_dump["inputs"] == {"key": "value"}
+
+    def test_crew_event_no_context_preserves_crew(self):
+        from crewai.events.types.crew_events import CrewKickoffStartedEvent
+
+        crew = MagicMock()
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        event = CrewKickoffStartedEvent(
+            crew=crew, crew_name="TestCrew", inputs=None
+        )
+
+        normal_dump = event.model_dump()
+        # Without trace context, crew should NOT be None (field serializer didn't fire)
+        assert normal_dump["crew"] is not None
+
+
+class TestLLMEventFieldSerializers:
+    """Test that LLM event field serializers respond to trace context."""
+
+    def test_llm_call_started_excludes_callbacks_in_trace(self):
+        from crewai.events.types.llm_events import LLMCallStartedEvent
+
+        event = LLMCallStartedEvent(
+            call_id="test-call",
+            messages=[{"role": "user", "content": "Hello"}],
+            tools=[{"name": "search", "description": "Search tool"}],
+            callbacks=[MagicMock(), MagicMock()],
+            available_functions={"search": MagicMock()},
+        )
+
+        trace_dump = event.model_dump(context={"trace": True})
+        # callbacks and available_functions excluded
+        assert trace_dump["callbacks"] is None
+        assert trace_dump["available_functions"] is None
+        # tools preserved (lightweight list of dicts)
+        assert trace_dump["tools"] == [{"name": "search", "description": "Search tool"}]
+        # messages preserved
+        assert trace_dump["messages"] == [{"role": "user", "content": "Hello"}]
+
+
+# ---------------------------------------------------------------------------
+# Integration tests: safe_serialize_to_dict with context
+# ---------------------------------------------------------------------------
+
+class TestSafeSerializeWithContext:
+    """Test that safe_serialize_to_dict properly passes context through."""
+
+    def test_context_flows_through_to_field_serializers(self):
+        from crewai.events.types.agent_events import AgentExecutionErrorEvent
+
+        agent = _make_stub_agent(role="Worker")
+        task = _make_mock_task(name="Work Task")
+
+        event = AgentExecutionErrorEvent(
+            agent=agent, task=task, error="error msg"
+        )
+
+        result = safe_serialize_to_dict(event, context={"trace": True})
+        # Field serializers should have fired
+        assert result["agent"] == {"id": str(agent.id), "role": "Worker"}
+        assert result["task"] == {"id": str(task.id), "name": "Work Task"}
+        assert result["error"] == "error msg"
+
+    def test_no_context_preserves_full_serialization(self):
+        from crewai.events.types.task_events import TaskFailedEvent
+
+        task = _make_mock_task(name="Test")
+        event = TaskFailedEvent(task=task, error="fail")
+
+        result = safe_serialize_to_dict(event)
+        # Without context, task should not be a lightweight ref
+        assert result.get("task") is not None
+        # It should be the raw object (model_dump returns it as-is for Any fields)
+        # to_serializable will then repr() or process it further
+
+
+# ---------------------------------------------------------------------------
+# Integration tests: TraceCollectionListener._build_event_data
+# ---------------------------------------------------------------------------
+
+class TestBuildEventData:
+    @pytest.fixture
+    def listener(self):
+        from crewai.events.listeners.tracing.trace_listener import (
+            TraceCollectionListener,
+        )
+        TraceCollectionListener._instance = None
+        TraceCollectionListener._initialized = False
+        TraceCollectionListener._listeners_setup = False
+        return TraceCollectionListener()
+
+    def test_crew_kickoff_started_has_crew_structure(self, listener):
+        agent = _make_stub_agent(role="Researcher")
+        agent.tools = [_make_stub_tool("search"), _make_stub_tool("read")]
+
+        task = _make_mock_task(name="Research Task", agent=agent)
+        task.context = None
+
+        crew = MagicMock()
+        crew.agents = [agent]
+        crew.tasks = [task]
+        crew.process = "sequential"
+        crew.verbose = True
+        crew.memory = False
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        from crewai.events.types.crew_events import CrewKickoffStartedEvent
+        event = CrewKickoffStartedEvent(
+            crew=crew, crew_name="TestCrew", inputs={"key": "value"}
+        )
+
+        result = listener._build_event_data("crew_kickoff_started", event, None)
+
+        assert "crew_structure" in result
+        cs = result["crew_structure"]
+        assert len(cs["agents"]) == 1
+        assert cs["agents"][0]["role"] == "Researcher"
+        assert cs["agents"][0]["tool_names"] == ["search", "read"]
+        assert len(cs["tasks"]) == 1
+        assert cs["tasks"][0]["name"] == "Research Task"
+        assert "agent_ref" in cs["tasks"][0]
+        assert cs["tasks"][0]["agent_ref"]["role"] == "Researcher"
+
+    def test_crew_kickoff_started_context_task_ids(self, listener):
+        agent = _make_stub_agent()
+        task1 = _make_mock_task(name="Task 1", agent=agent)
+        task1.context = None
+        task2 = _make_mock_task(name="Task 2", agent=agent)
+        task2.context = [task1]
+
+        crew = MagicMock()
+        crew.agents = [agent]
+        crew.tasks = [task1, task2]
+        crew.process = "sequential"
+        crew.verbose = False
+        crew.memory = False
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        from crewai.events.types.crew_events import CrewKickoffStartedEvent
+        event = CrewKickoffStartedEvent(
+            crew=crew, crew_name="TestCrew", inputs=None
+        )
+
+        result = listener._build_event_data("crew_kickoff_started", event, None)
+        task2_data = result["crew_structure"]["tasks"][1]
+        assert "context_task_ids" in task2_data
+        assert str(task1.id) in task2_data["context_task_ids"]
+
+    def test_generic_event_uses_trace_context(self, listener):
+        """Non-complex events should use context-based serialization."""
+        from crewai.events.types.crew_events import CrewKickoffCompletedEvent
+
+        crew = MagicMock()
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        event = CrewKickoffCompletedEvent(
+            crew=crew, crew_name="TestCrew", output="Final result", total_tokens=5000
+        )
+
+        result = listener._build_event_data("crew_kickoff_completed", event, None)
+
+        # Scalar fields preserved
+        assert result.get("crew_name") == "TestCrew"
+        assert result.get("total_tokens") == 5000
+        # crew excluded by field serializer
+        assert result.get("crew") is None
+        # No crew_structure (that's only for kickoff_started)
+        assert "crew_structure" not in result
+
+    def test_task_started_custom_projection(self, listener):
+        task = _make_mock_task(name="Test Task")
+        from crewai.events.types.task_events import TaskStartedEvent
+        event = TaskStartedEvent(task=task, context="test context")
+        source = MagicMock()
+        source.agent = _make_stub_agent(role="Worker")
+
+        result = listener._build_event_data("task_started", event, source)
+
+        assert result["task_name"] == "Test Task"
+        assert result["agent_role"] == "Worker"
+        assert result["task_id"] == str(task.id)
+        assert result["context"] == "test context"
+
+    def test_llm_call_started_uses_trace_context(self, listener):
+        from crewai.events.types.llm_events import LLMCallStartedEvent
+
+        event = LLMCallStartedEvent(
+            call_id="test",
+            messages=[{"role": "user", "content": "Hello"}],
+            tools=[{"name": "search"}],
+            callbacks=[MagicMock()],
+            available_functions={"fn": MagicMock()},
+        )
+
+        result = listener._build_event_data("llm_call_started", event, None)
+
+        # callbacks and available_functions excluded via field serializer
+        assert result.get("callbacks") is None
+        assert result.get("available_functions") is None
+        # tools preserved (lightweight schemas)
+        assert result.get("tools") == [{"name": "search"}]
+
+    def test_agent_execution_error_preserves_identification(self, listener):
+        """Error events should preserve agent/task identification via field serializers."""
+        from crewai.events.types.agent_events import AgentExecutionErrorEvent
+
+        agent = _make_stub_agent(role="Analyst")
+        task = _make_mock_task(name="Analysis")
+
+        event = AgentExecutionErrorEvent(
+            agent=agent, task=task, error="Something broke"
+        )
+
+        result = listener._build_event_data("agent_execution_error", event, None)
+
+        # Field serializers return lightweight refs, not None
+        assert result["agent"] == {"id": str(agent.id), "role": "Analyst"}
+        assert result["task"] == {"id": str(task.id), "name": "Analysis"}
+        assert result["error"] == "Something broke"
+
+    def test_task_failed_preserves_identification(self, listener):
+        from crewai.events.types.task_events import TaskFailedEvent
+
+        task = _make_mock_task(name="Failed Task")
+        event = TaskFailedEvent(task=task, error="Task failed")
+
+        result = listener._build_event_data("task_failed", event, None)
+
+        assert result["task"] == {"id": str(task.id), "name": "Failed Task"}
+        assert result["error"] == "Task failed"
+
+
+# ---------------------------------------------------------------------------
+# Size reduction verification
+# ---------------------------------------------------------------------------
+
+class TestSizeReduction:
+    @pytest.fixture
+    def listener(self):
+        from crewai.events.listeners.tracing.trace_listener import (
+            TraceCollectionListener,
+        )
+        TraceCollectionListener._instance = None
+        TraceCollectionListener._initialized = False
+        TraceCollectionListener._listeners_setup = False
+        return TraceCollectionListener()
+
+    def test_task_started_event_size(self, listener):
+        """task_started event data should be well under 2KB."""
+        agent = _make_stub_agent(
+            role="Researcher",
+            goal="Research" * 50,
+            backstory="Expert" * 100,
+        )
+        agent.tools = [_make_stub_tool(f"tool_{i}") for i in range(5)]
+
+        task = _make_mock_task(
+            name="Research Task",
+            description="Detailed description" * 20,
+            expected_output="Expected" * 10,
+            agent=agent,
+        )
+        task.context = [_make_mock_task() for _ in range(3)]
+        task.tools = [_make_stub_tool(f"t_{i}") for i in range(3)]
+
+        from crewai.events.types.task_events import TaskStartedEvent
+        event = TaskStartedEvent(task=task, context="test context")
+        source = MagicMock()
+        source.agent = agent
+
+        result = listener._build_event_data("task_started", event, source)
+        serialized = json.dumps(result, default=str)
+
+        assert len(serialized) < 2000, f"task_started too large: {len(serialized)} bytes"
+        assert "task_name" in result
+        assert "agent_role" in result
+
+    def test_error_event_size(self, listener):
+        """Error events should be small despite having agent/task refs."""
+        from crewai.events.types.agent_events import AgentExecutionErrorEvent
+
+        agent = _make_stub_agent(
+            goal="Very long goal " * 100,
+            backstory="Very long backstory " * 100,
+        )
+        task = _make_mock_task(description="Very long description " * 100)
+
+        event = AgentExecutionErrorEvent(
+            agent=agent, task=task, error="error"
+        )
+
+        result = listener._build_event_data("agent_execution_error", event, None)
+        serialized = json.dumps(result, default=str)
+
+        # Should be small - agent/task are just {id, role/name} refs
+        assert len(serialized) < 5000, f"error event too large: {len(serialized)} bytes"
+
+
+# ---------------------------------------------------------------------------
+# to_serializable context threading
+# ---------------------------------------------------------------------------
+
+class TestToSerializableContext:
+    """Test that context parameter flows through to_serializable correctly."""
+
+    def test_context_passed_to_model_dump(self):
+        from crewai.events.types.agent_events import AgentExecutionErrorEvent
+
+        agent = _make_stub_agent(role="Tester")
+        task = _make_mock_task(name="Test Task")
+
+        event = AgentExecutionErrorEvent(
+            agent=agent, task=task, error="test error"
+        )
+
+        # Directly use to_serializable with context
+        result = to_serializable(event, context={"trace": True})
+        assert isinstance(result, dict)
+        assert result["agent"] == {"id": str(agent.id), "role": "Tester"}
+        assert result["task"] == {"id": str(task.id), "name": "Test Task"}
+
+    def test_no_context_does_not_trigger_serializers(self):
+        from crewai.events.types.crew_events import CrewKickoffStartedEvent
+
+        crew = MagicMock()
+        crew.fingerprint = MagicMock()
+        crew.fingerprint.uuid_str = str(uuid.uuid4())
+        crew.fingerprint.metadata = {}
+
+        event = CrewKickoffStartedEvent(
+            crew=crew, crew_name="Test", inputs=None
+        )
+
+        # Without context, crew should NOT be None
+        result = event.model_dump()
+        assert result["crew"] is not None