diff --git a/lib/crewai/src/crewai/__init__.py b/lib/crewai/src/crewai/__init__.py
index 64d459358..a5344c8eb 100644
--- a/lib/crewai/src/crewai/__init__.py
+++ b/lib/crewai/src/crewai/__init__.py
@@ -8,14 +8,15 @@ from pydantic import PydanticUserError
 
 from crewai.agent.core import Agent
 from crewai.agent.planning_config import PlanningConfig
+from crewai.context import ExecutionContext
 from crewai.crew import Crew
 from crewai.crews.crew_output import CrewOutput
-from crewai.execution_context import ExecutionContext
 from crewai.flow.flow import Flow
 from crewai.knowledge.knowledge import Knowledge
 from crewai.llm import LLM
 from crewai.llms.base_llm import BaseLLM
 from crewai.process import Process
+from crewai.runtime_state import _entity_discriminator
 from crewai.task import Task
 from crewai.tasks.llm_guardrail import LLMGuardrail
 from crewai.tasks.task_output import TaskOutput
@@ -112,10 +113,13 @@ try:
 
     _base_namespace: dict[str, type] = {
         "Agent": Agent,
+        "BaseAgent": _BaseAgent,
         "Crew": Crew,
+        "Flow": Flow,
         "BaseLLM": BaseLLM,
         "Task": Task,
         "CrewAgentExecutorMixin": _CrewAgentExecutorMixin,
+        "ExecutionContext": ExecutionContext,
     }
 
     try:
@@ -154,13 +158,34 @@ try:
     for _mod_name in (
         _BaseAgent.__module__,
         Agent.__module__,
+        Crew.__module__,
+        Flow.__module__,
+        Task.__module__,
         _AgentExecutor.__module__,
     ):
         sys.modules[_mod_name].__dict__.update(_resolve_namespace)
 
+    from crewai.tasks.conditional_task import ConditionalTask as _ConditionalTask
+
     _BaseAgent.model_rebuild(force=True, _types_namespace=_full_namespace)
+    Task.model_rebuild(force=True, _types_namespace=_full_namespace)
+    _ConditionalTask.model_rebuild(force=True, _types_namespace=_full_namespace)
+    Crew.model_rebuild(force=True, _types_namespace=_full_namespace)
+    Flow.model_rebuild(force=True, _types_namespace=_full_namespace)
     _AgentExecutor.model_rebuild(force=True, _types_namespace=_full_namespace)
 
+    from typing import Annotated
+
+    from pydantic import Discriminator, RootModel, Tag
+
+    Entity = Annotated[
+        Annotated[Flow, Tag("flow")]  # type: ignore[type-arg]
+        | Annotated[Crew, Tag("crew")]
+        | Annotated[Agent, Tag("agent")],
+        Discriminator(_entity_discriminator),
+    ]
+    RuntimeState = RootModel[list[Entity]]
+
     try:
         Agent.model_rebuild(force=True, _types_namespace=_full_namespace)
     except PydanticUserError:
@@ -172,6 +197,7 @@ except (ImportError, PydanticUserError):
         "model_rebuild() failed; forward refs may be unresolved.",
         exc_info=True,
     )
+    RuntimeState = None  # type: ignore[assignment,misc]
 
 __all__ = [
     "LLM",
@@ -186,6 +212,7 @@ __all__ = [
     "Memory",
     "PlanningConfig",
     "Process",
+    "RuntimeState",
     "Task",
     "TaskOutput",
     "__version__",
diff --git a/lib/crewai/src/crewai/agent/core.py b/lib/crewai/src/crewai/agent/core.py
index 760268a1d..34250436f 100644
--- a/lib/crewai/src/crewai/agent/core.py
+++ b/lib/crewai/src/crewai/agent/core.py
@@ -14,6 +14,7 @@ import subprocess
 import time
 from typing import (
     TYPE_CHECKING,
+    Annotated,
     Any,
     Literal,
     NoReturn,
@@ -23,12 +24,14 @@ import warnings
 
 from pydantic import (
     BaseModel,
+    BeforeValidator,
     ConfigDict,
     Field,
     InstanceOf,
     PrivateAttr,
     model_validator,
 )
+from pydantic.functional_serializers import PlainSerializer
 from typing_extensions import Self
 
 from crewai.agent.planning_config import PlanningConfig
@@ -46,7 +49,11 @@ from crewai.agent.utils import (
     save_last_messages,
     validate_max_execution_time,
 )
-from crewai.agents.agent_builder.base_agent import BaseAgent
+from crewai.agents.agent_builder.base_agent import (
+    BaseAgent,
+    _serialize_llm_ref,
+    _validate_llm_ref,
+)
 from crewai.agents.cache.cache_handler import CacheHandler
 from crewai.agents.crew_agent_executor import CrewAgentExecutor
 from crewai.events.event_bus import crewai_event_bus
@@ -122,6 +129,24 @@ if TYPE_CHECKING:
 
 _passthrough_exceptions: tuple[type[Exception], ...] = ()
 
+_EXECUTOR_CLASS_MAP: dict[str, type] = {
+    "CrewAgentExecutor": CrewAgentExecutor,
+    "AgentExecutor": AgentExecutor,
+}
+
+
+def _validate_executor_class(value: Any) -> Any:
+    if isinstance(value, str):
+        cls = _EXECUTOR_CLASS_MAP.get(value)
+        if cls is None:
+            raise ValueError(f"Unknown executor class: {value}")
+        return cls
+    return value
+
+
+def _serialize_executor_class(value: Any) -> str:
+    return value.__name__ if isinstance(value, type) else str(value)
+
 
 class Agent(BaseAgent):
     """Represents an agent in a system.
@@ -167,12 +192,16 @@ class Agent(BaseAgent):
         default=True,
         description="Use system prompt for the agent.",
     )
-    llm: str | BaseLLM | None = Field(
-        description="Language model that will run the agent.", default=None
-    )
-    function_calling_llm: str | BaseLLM | None = Field(
-        description="Language model that will run the agent.", default=None
-    )
+    llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(description="Language model that will run the agent.", default=None)
+    function_calling_llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(description="Language model that will run the agent.", default=None)
     system_template: str | None = Field(
         default=None, description="System format for the agent."
     )
@@ -271,7 +300,11 @@ class Agent(BaseAgent):
     agent_executor: InstanceOf[CrewAgentExecutor] | InstanceOf[AgentExecutor] | None = (
         Field(default=None, description="An instance of the CrewAgentExecutor class.")
     )
-    executor_class: type[CrewAgentExecutor] | type[AgentExecutor] = Field(
+    executor_class: Annotated[
+        type[CrewAgentExecutor] | type[AgentExecutor],
+        BeforeValidator(_validate_executor_class),
+        PlainSerializer(_serialize_executor_class, return_type=str, when_used="json"),
+    ] = Field(
         default=CrewAgentExecutor,
         description="Class to use for the agent executor. Defaults to CrewAgentExecutor, can optionally use AgentExecutor.",
     )
@@ -1053,7 +1086,7 @@ class Agent(BaseAgent):
                 )
             )
 
-    def get_delegation_tools(self, agents: list[BaseAgent]) -> list[BaseTool]:
+    def get_delegation_tools(self, agents: Sequence[BaseAgent]) -> list[BaseTool]:
         agent_tools = AgentTools(agents=agents)
         return agent_tools.tools()
 
diff --git a/lib/crewai/src/crewai/agents/agent_adapters/langgraph/langgraph_adapter.py b/lib/crewai/src/crewai/agents/agent_adapters/langgraph/langgraph_adapter.py
index f90f7200d..1710b56cb 100644
--- a/lib/crewai/src/crewai/agents/agent_adapters/langgraph/langgraph_adapter.py
+++ b/lib/crewai/src/crewai/agents/agent_adapters/langgraph/langgraph_adapter.py
@@ -5,7 +5,7 @@ with CrewAI's agent system. Provides memory persistence, tool integration, and s
 output functionality.
 """
 
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from typing import Any, cast
 
 from pydantic import ConfigDict, Field, PrivateAttr
@@ -30,6 +30,7 @@ from crewai.events.types.agent_events import (
 )
 from crewai.tools.agent_tools.agent_tools import AgentTools
 from crewai.tools.base_tool import BaseTool
+from crewai.types.callback import SerializableCallable
 from crewai.utilities import Logger
 from crewai.utilities.converter import Converter
 from crewai.utilities.import_utils import require
@@ -50,7 +51,7 @@ class LangGraphAgentAdapter(BaseAgentAdapter):
     _memory: Any = PrivateAttr(default=None)
     _max_iterations: int = PrivateAttr(default=10)
     function_calling_llm: Any = Field(default=None)
-    step_callback: Callable[..., Any] | None = Field(default=None)
+    step_callback: SerializableCallable | None = Field(default=None)
 
     model: str = Field(default="gpt-4o")
     verbose: bool = Field(default=False)
@@ -272,7 +273,7 @@ class LangGraphAgentAdapter(BaseAgentAdapter):
             available_tools: list[Any] = self._tool_adapter.tools()
             self._graph.tools = available_tools
 
-    def get_delegation_tools(self, agents: list[BaseAgent]) -> list[BaseTool]:
+    def get_delegation_tools(self, agents: Sequence[BaseAgent]) -> list[BaseTool]:
         """Implement delegation tools support for LangGraph.
 
         Creates delegation tools that allow this agent to delegate tasks to other agents.
diff --git a/lib/crewai/src/crewai/agents/agent_adapters/openai_agents/openai_adapter.py b/lib/crewai/src/crewai/agents/agent_adapters/openai_agents/openai_adapter.py
index 568f5e83e..82eb8640b 100644
--- a/lib/crewai/src/crewai/agents/agent_adapters/openai_agents/openai_adapter.py
+++ b/lib/crewai/src/crewai/agents/agent_adapters/openai_agents/openai_adapter.py
@@ -4,6 +4,7 @@ This module contains the OpenAIAgentAdapter class that integrates OpenAI Assista
 with CrewAI's agent system, providing tool integration and structured output support.
 """
 
+from collections.abc import Sequence
 from typing import Any, cast
 
 from pydantic import ConfigDict, Field, PrivateAttr
@@ -221,7 +222,7 @@ class OpenAIAgentAdapter(BaseAgentAdapter):
         """
         return self._converter_adapter.post_process_result(result.final_output)
 
-    def get_delegation_tools(self, agents: list[BaseAgent]) -> list[BaseTool]:
+    def get_delegation_tools(self, agents: Sequence[BaseAgent]) -> list[BaseTool]:
         """Implement delegation tools support.
 
         Creates delegation tools that allow this agent to delegate tasks to other agents.
diff --git a/lib/crewai/src/crewai/agents/agent_builder/base_agent.py b/lib/crewai/src/crewai/agents/agent_builder/base_agent.py
index f487a0d8c..d71f27a2d 100644
--- a/lib/crewai/src/crewai/agents/agent_builder/base_agent.py
+++ b/lib/crewai/src/crewai/agents/agent_builder/base_agent.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
+from collections.abc import Sequence
 from copy import copy as shallow_copy
 from hashlib import md5
 from pathlib import Path
@@ -48,6 +49,7 @@ from crewai.utilities.string_utils import interpolate_only
 
 
 if TYPE_CHECKING:
+    from crewai.context import ExecutionContext
     from crewai.crew import Crew
 
 
@@ -61,6 +63,26 @@ def _serialize_crew_ref(value: Any) -> str | None:
     return str(value.id) if hasattr(value, "id") else str(value)
 
 
+def _validate_llm_ref(value: Any) -> Any:
+    return value
+
+
+def _resolve_agent(value: Any, info: Any) -> Any:
+    if isinstance(value, BaseAgent) or value is None or not isinstance(value, dict):
+        return value
+    from crewai.agent.core import Agent
+
+    return Agent.model_validate(value, context=getattr(info, "context", None))
+
+
+def _serialize_llm_ref(value: Any) -> str | None:
+    if value is None:
+        return None
+    if isinstance(value, str):
+        return value
+    return getattr(value, "model", str(value))
+
+
 _SLUG_RE: Final[re.Pattern[str]] = re.compile(
     r"^(?:crewai-amp:)?[a-zA-Z0-9][a-zA-Z0-9_-]*(?:#[\w-]+)?$"
 )
@@ -138,6 +160,8 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
             Set private attributes.
     """
 
+    entity_type: Literal["agent"] = "agent"
+
     __hash__ = object.__hash__
     _logger: Logger = PrivateAttr(default_factory=lambda: Logger(verbose=False))
     _rpm_controller: RPMController | None = PrivateAttr(default=None)
@@ -176,9 +200,11 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
     agent_executor: InstanceOf[CrewAgentExecutorMixin] | None = Field(
         default=None, description="An instance of the CrewAgentExecutor class."
     )
-    llm: str | BaseLLM | None = Field(
-        default=None, description="Language model that will run the agent."
-    )
+    llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(default=None, description="Language model that will run the agent.")
     crew: Annotated[
         Crew | str | None,
         BeforeValidator(_validate_crew_ref),
@@ -197,7 +223,7 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
         description="An instance of the ToolsHandler class.",
     )
     tools_results: list[dict[str, Any]] = Field(
-        default=[], description="Results of the tools used by the agent."
+        default_factory=list, description="Results of the tools used by the agent."
     )
     max_tokens: int | None = Field(
         default=None, description="Maximum number of tokens for the agent's execution."
@@ -248,6 +274,7 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
         description="Agent Skills. Accepts paths for discovery or pre-loaded Skill objects.",
         min_length=1,
     )
+    execution_context: ExecutionContext | None = Field(default=None)
 
     @model_validator(mode="before")
     @classmethod
@@ -362,11 +389,12 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
 
     @field_validator("id", mode="before")
     @classmethod
-    def _deny_user_set_id(cls, v: UUID4 | None) -> None:
-        if v:
+    def _deny_user_set_id(cls, v: UUID4 | None, info: Any) -> UUID4 | None:
+        if v and not (info.context or {}).get("from_checkpoint"):
             raise PydanticCustomError(
                 "may_not_set_field", "This field is not to be set by the user.", {}
             )
+        return v
 
     @model_validator(mode="after")
     def set_private_attrs(self) -> Self:
@@ -423,7 +451,7 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
         pass
 
     @abstractmethod
-    def get_delegation_tools(self, agents: list[BaseAgent]) -> list[BaseTool]:
+    def get_delegation_tools(self, agents: Sequence[BaseAgent]) -> list[BaseTool]:
         """Set the task tools that init BaseAgenTools class."""
 
     @abstractmethod
diff --git a/lib/crewai/src/crewai/agents/tools_handler.py b/lib/crewai/src/crewai/agents/tools_handler.py
index 8b39196e5..8ab759b85 100644
--- a/lib/crewai/src/crewai/agents/tools_handler.py
+++ b/lib/crewai/src/crewai/agents/tools_handler.py
@@ -3,20 +3,15 @@
 from __future__ import annotations
 
 import json
-from typing import TYPE_CHECKING, Any
 
-from pydantic import GetCoreSchemaHandler
-from pydantic_core import CoreSchema, core_schema
+from pydantic import BaseModel, Field
 
+from crewai.agents.cache.cache_handler import CacheHandler
 from crewai.tools.cache_tools.cache_tools import CacheTools
+from crewai.tools.tool_calling import InstructorToolCalling, ToolCalling
 
 
-if TYPE_CHECKING:
-    from crewai.agents.cache.cache_handler import CacheHandler
-    from crewai.tools.tool_calling import InstructorToolCalling, ToolCalling
-
-
-class ToolsHandler:
+class ToolsHandler(BaseModel):
     """Callback handler for tool usage.
 
     Attributes:
@@ -24,14 +19,8 @@ class ToolsHandler:
         cache: Optional cache handler for storing tool outputs.
     """
 
-    def __init__(self, cache: CacheHandler | None = None) -> None:
-        """Initialize the callback handler.
-
-        Args:
-            cache: Optional cache handler for storing tool outputs.
-        """
-        self.cache: CacheHandler | None = cache
-        self.last_used_tool: ToolCalling | InstructorToolCalling | None = None
+    cache: CacheHandler | None = Field(default=None)
+    last_used_tool: ToolCalling | InstructorToolCalling | None = Field(default=None)
 
     def on_tool_use(
         self,
@@ -48,7 +37,6 @@ class ToolsHandler:
         """
         self.last_used_tool = calling
         if self.cache and should_cache and calling.tool_name != CacheTools().name:
-            # Convert arguments to string for cache
             input_str = ""
             if calling.arguments:
                 if isinstance(calling.arguments, dict):
@@ -61,14 +49,3 @@ class ToolsHandler:
                 input=input_str,
                 output=output,
             )
-
-    @classmethod
-    def __get_pydantic_core_schema__(
-        cls, _source_type: Any, _handler: GetCoreSchemaHandler
-    ) -> CoreSchema:
-        """Generate Pydantic core schema for BaseClient Protocol.
-
-        This allows the Protocol to be used in Pydantic models without
-        requiring arbitrary_types_allowed=True.
-        """
-        return core_schema.any_schema()
diff --git a/lib/crewai/src/crewai/context.py b/lib/crewai/src/crewai/context.py
index bf73a221c..e6efe4349 100644
--- a/lib/crewai/src/crewai/context.py
+++ b/lib/crewai/src/crewai/context.py
@@ -4,6 +4,23 @@ import contextvars
 import os
 from typing import Any
 
+from pydantic import BaseModel, Field
+
+from crewai.events.base_events import (
+    get_emission_sequence,
+    set_emission_counter,
+)
+from crewai.events.event_context import (
+    _event_id_stack,
+    _last_event_id,
+    _triggering_event_id,
+)
+from crewai.flow.flow_context import (
+    current_flow_id,
+    current_flow_method_name,
+    current_flow_request_id,
+)
+
 
 _platform_integration_token: contextvars.ContextVar[str | None] = (
     contextvars.ContextVar("platform_integration_token", default=None)
@@ -63,3 +80,53 @@ def reset_current_task_id(token: contextvars.Token[str | None]) -> None:
 def get_current_task_id() -> str | None:
     """Get the current task ID from the context."""
     return _current_task_id.get()
+
+
+class ExecutionContext(BaseModel):
+    """Snapshot of ContextVar execution state."""
+
+    current_task_id: str | None = Field(default=None)
+    flow_request_id: str | None = Field(default=None)
+    flow_id: str | None = Field(default=None)
+    flow_method_name: str = Field(default="unknown")
+
+    event_id_stack: tuple[tuple[str, str], ...] = Field(default=())
+    last_event_id: str | None = Field(default=None)
+    triggering_event_id: str | None = Field(default=None)
+    emission_sequence: int = Field(default=0)
+
+    feedback_callback_info: dict[str, Any] | None = Field(default=None)
+    platform_token: str | None = Field(default=None)
+
+
+def capture_execution_context(
+    feedback_callback_info: dict[str, Any] | None = None,
+) -> ExecutionContext:
+    """Read current ContextVars into an ExecutionContext."""
+    return ExecutionContext(
+        current_task_id=_current_task_id.get(),
+        flow_request_id=current_flow_request_id.get(),
+        flow_id=current_flow_id.get(),
+        flow_method_name=current_flow_method_name.get(),
+        event_id_stack=_event_id_stack.get(),
+        last_event_id=_last_event_id.get(),
+        triggering_event_id=_triggering_event_id.get(),
+        emission_sequence=get_emission_sequence(),
+        feedback_callback_info=feedback_callback_info,
+        platform_token=_platform_integration_token.get(),
+    )
+
+
+def apply_execution_context(ctx: ExecutionContext) -> None:
+    """Write an ExecutionContext back into the ContextVars."""
+    _current_task_id.set(ctx.current_task_id)
+    current_flow_request_id.set(ctx.flow_request_id)
+    current_flow_id.set(ctx.flow_id)
+    current_flow_method_name.set(ctx.flow_method_name)
+
+    _event_id_stack.set(ctx.event_id_stack)
+    _last_event_id.set(ctx.last_event_id)
+    _triggering_event_id.set(ctx.triggering_event_id)
+    set_emission_counter(ctx.emission_sequence)
+
+    _platform_integration_token.set(ctx.platform_token)
diff --git a/lib/crewai/src/crewai/crew.py b/lib/crewai/src/crewai/crew.py
index 3b18a2753..bd84f3067 100644
--- a/lib/crewai/src/crewai/crew.py
+++ b/lib/crewai/src/crewai/crew.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import asyncio
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from concurrent.futures import Future
 from copy import copy as shallow_copy
 from hashlib import md5
@@ -10,7 +10,9 @@ from pathlib import Path
 import re
 from typing import (
     TYPE_CHECKING,
+    Annotated,
     Any,
+    Literal,
     cast,
 )
 import uuid
@@ -21,12 +23,14 @@ from opentelemetry.context import attach, detach
 from pydantic import (
     UUID4,
     BaseModel,
+    BeforeValidator,
     Field,
     Json,
     PrivateAttr,
     field_validator,
     model_validator,
 )
+from pydantic.functional_serializers import PlainSerializer
 from pydantic_core import PydanticCustomError
 from rich.console import Console
 from rich.panel import Panel
@@ -37,6 +41,8 @@ if TYPE_CHECKING:
     from crewai_files import FileInput
     from opentelemetry.trace import Span
 
+    from crewai.context import ExecutionContext
+
 try:
     from crewai_files import get_supported_content_types
 
@@ -49,7 +55,12 @@ except ImportError:
 
 
 from crewai.agent import Agent
-from crewai.agents.agent_builder.base_agent import BaseAgent
+from crewai.agents.agent_builder.base_agent import (
+    BaseAgent,
+    _resolve_agent,
+    _serialize_llm_ref,
+    _validate_llm_ref,
+)
 from crewai.agents.cache.cache_handler import CacheHandler
 from crewai.crews.crew_output import CrewOutput
 from crewai.crews.utils import (
@@ -132,6 +143,12 @@ from crewai.utilities.training_handler import CrewTrainingHandler
 warnings.filterwarnings("ignore", category=SyntaxWarning, module="pysbd")
 
 
+def _resolve_agents(value: Any, info: Any) -> Any:
+    if not isinstance(value, list):
+        return value
+    return [_resolve_agent(a, info) for a in value]
+
+
 class Crew(FlowTrackable, BaseModel):
     """
     Represents a group of agents, defining how they should collaborate and the
@@ -170,6 +187,8 @@ class Crew(FlowTrackable, BaseModel):
             fingerprinting.
     """
 
+    entity_type: Literal["crew"] = "crew"
+
     __hash__ = object.__hash__
     _execution_span: Span | None = PrivateAttr()
     _rpm_controller: RPMController = PrivateAttr()
@@ -191,7 +210,10 @@ class Crew(FlowTrackable, BaseModel):
     name: str | None = Field(default="crew")
     cache: bool = Field(default=True)
     tasks: list[Task] = Field(default_factory=list)
-    agents: list[BaseAgent] = Field(default_factory=list)
+    agents: Annotated[
+        list[BaseAgent],
+        BeforeValidator(_resolve_agents),
+    ] = Field(default_factory=list)
     process: Process = Field(default=Process.sequential)
     verbose: bool = Field(default=False)
     memory: bool | Memory | MemoryScope | MemorySlice | None = Field(
@@ -209,15 +231,20 @@ class Crew(FlowTrackable, BaseModel):
         default=None,
         description="Metrics for the LLM usage during all tasks execution.",
     )
-    manager_llm: str | BaseLLM | None = Field(
-        description="Language model that will run the agent.", default=None
-    )
-    manager_agent: BaseAgent | None = Field(
-        description="Custom agent that will be used as manager.", default=None
-    )
-    function_calling_llm: str | LLM | None = Field(
-        description="Language model that will run the agent.", default=None
-    )
+    manager_llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(description="Language model that will run the agent.", default=None)
+    manager_agent: Annotated[
+        BaseAgent | None,
+        BeforeValidator(_resolve_agent),
+    ] = Field(description="Custom agent that will be used as manager.", default=None)
+    function_calling_llm: Annotated[
+        str | LLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(description="Language model that will run the agent.", default=None)
     config: Json[dict[str, Any]] | dict[str, Any] | None = Field(default=None)
     id: UUID4 = Field(default_factory=uuid.uuid4, frozen=True)
     share_crew: bool | None = Field(default=False)
@@ -266,7 +293,11 @@ class Crew(FlowTrackable, BaseModel):
         default=False,
         description="Plan the crew execution and add the plan to the crew.",
     )
-    planning_llm: str | BaseLLM | None = Field(
+    planning_llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(
         default=None,
         description=(
             "Language model that will run the AgentPlanner if planning is True."
@@ -287,7 +318,11 @@ class Crew(FlowTrackable, BaseModel):
             "knowledge object."
         ),
     )
-    chat_llm: str | BaseLLM | None = Field(
+    chat_llm: Annotated[
+        str | BaseLLM | None,
+        BeforeValidator(_validate_llm_ref),
+        PlainSerializer(_serialize_llm_ref, return_type=str | None, when_used="json"),
+    ] = Field(
         default=None,
         description="LLM used to handle chatting with the crew.",
     )
@@ -313,14 +348,20 @@ class Crew(FlowTrackable, BaseModel):
         description="Whether to enable tracing for the crew. True=always enable, False=always disable, None=check environment/user settings.",
     )
 
+    execution_context: ExecutionContext | None = Field(default=None)
+    checkpoint_inputs: dict[str, Any] | None = Field(default=None)
+    checkpoint_train: bool | None = Field(default=None)
+    checkpoint_kickoff_event_id: str | None = Field(default=None)
+
     @field_validator("id", mode="before")
     @classmethod
-    def _deny_user_set_id(cls, v: UUID4 | None) -> None:
+    def _deny_user_set_id(cls, v: UUID4 | None, info: Any) -> UUID4 | None:
         """Prevent manual setting of the 'id' field by users."""
-        if v:
+        if v and not (info.context or {}).get("from_checkpoint"):
             raise PydanticCustomError(
                 "may_not_set_field", "The 'id' field cannot be set by the user.", {}
             )
+        return v
 
     @field_validator("config", mode="before")
     @classmethod
@@ -1388,7 +1429,7 @@ class Crew(FlowTrackable, BaseModel):
         self,
         tools: list[BaseTool],
         task_agent: BaseAgent,
-        agents: list[BaseAgent],
+        agents: Sequence[BaseAgent],
     ) -> list[BaseTool]:
         if hasattr(task_agent, "get_delegation_tools"):
             delegation_tools = task_agent.get_delegation_tools(agents)
diff --git a/lib/crewai/src/crewai/crews/crew_output.py b/lib/crewai/src/crewai/crews/crew_output.py
index 38e9bb2f8..4541ae02a 100644
--- a/lib/crewai/src/crewai/crews/crew_output.py
+++ b/lib/crewai/src/crewai/crews/crew_output.py
@@ -21,7 +21,7 @@ class CrewOutput(BaseModel):
         description="JSON dict output of Crew", default=None
     )
     tasks_output: list[TaskOutput] = Field(
-        description="Output of each task", default=[]
+        description="Output of each task", default_factory=list
     )
     token_usage: UsageMetrics = Field(
         description="Processed token summary", default_factory=UsageMetrics
diff --git a/lib/crewai/src/crewai/execution_context.py b/lib/crewai/src/crewai/execution_context.py
deleted file mode 100644
index 7bad1fd2c..000000000
--- a/lib/crewai/src/crewai/execution_context.py
+++ /dev/null
@@ -1,80 +0,0 @@
-"""Checkpointable execution context for the crewAI runtime.
-
-Captures the ContextVar state needed to resume execution from a checkpoint.
-Used by the RootModel (step 5) to include execution context in snapshots.
-"""
-
-from __future__ import annotations
-
-from typing import Any
-
-from pydantic import BaseModel, Field
-
-from crewai.context import (
-    _current_task_id,
-    _platform_integration_token,
-)
-from crewai.events.base_events import (
-    get_emission_sequence,
-    set_emission_counter,
-)
-from crewai.events.event_context import (
-    _event_id_stack,
-    _last_event_id,
-    _triggering_event_id,
-)
-from crewai.flow.flow_context import (
-    current_flow_id,
-    current_flow_method_name,
-    current_flow_request_id,
-)
-
-
-class ExecutionContext(BaseModel):
-    """Snapshot of ContextVar state required for checkpoint/resume."""
-
-    current_task_id: str | None = Field(default=None)
-    flow_request_id: str | None = Field(default=None)
-    flow_id: str | None = Field(default=None)
-    flow_method_name: str = Field(default="unknown")
-
-    event_id_stack: tuple[tuple[str, str], ...] = Field(default=())
-    last_event_id: str | None = Field(default=None)
-    triggering_event_id: str | None = Field(default=None)
-    emission_sequence: int = Field(default=0)
-
-    feedback_callback_info: dict[str, Any] | None = Field(default=None)
-    platform_token: str | None = Field(default=None)
-
-
-def capture_execution_context(
-    feedback_callback_info: dict[str, Any] | None = None,
-) -> ExecutionContext:
-    """Read all checkpoint-required ContextVars into an ExecutionContext."""
-    return ExecutionContext(
-        current_task_id=_current_task_id.get(),
-        flow_request_id=current_flow_request_id.get(),
-        flow_id=current_flow_id.get(),
-        flow_method_name=current_flow_method_name.get(),
-        event_id_stack=_event_id_stack.get(),
-        last_event_id=_last_event_id.get(),
-        triggering_event_id=_triggering_event_id.get(),
-        emission_sequence=get_emission_sequence(),
-        feedback_callback_info=feedback_callback_info,
-        platform_token=_platform_integration_token.get(),
-    )
-
-
-def apply_execution_context(ctx: ExecutionContext) -> None:
-    """Write an ExecutionContext back into the ContextVars."""
-    _current_task_id.set(ctx.current_task_id)
-    current_flow_request_id.set(ctx.flow_request_id)
-    current_flow_id.set(ctx.flow_id)
-    current_flow_method_name.set(ctx.flow_method_name)
-
-    _event_id_stack.set(ctx.event_id_stack)
-    _last_event_id.set(ctx.last_event_id)
-    _triggering_event_id.set(ctx.triggering_event_id)
-    set_emission_counter(ctx.emission_sequence)
-
-    _platform_integration_token.set(ctx.platform_token)
diff --git a/lib/crewai/src/crewai/flow/flow.py b/lib/crewai/src/crewai/flow/flow.py
index def7d1ba9..a1be6317a 100644
--- a/lib/crewai/src/crewai/flow/flow.py
+++ b/lib/crewai/src/crewai/flow/flow.py
@@ -25,6 +25,7 @@ import logging
 import threading
 from typing import (
     TYPE_CHECKING,
+    Annotated,
     Any,
     ClassVar,
     Generic,
@@ -41,9 +42,11 @@ from opentelemetry import baggage
 from opentelemetry.context import attach, detach
 from pydantic import (
     BaseModel,
+    BeforeValidator,
     ConfigDict,
     Field,
     PrivateAttr,
+    SerializeAsAny,
     ValidationError,
 )
 from pydantic._internal._model_construction import ModelMetaclass
@@ -115,6 +118,7 @@ from crewai.memory.unified_memory import Memory
 if TYPE_CHECKING:
     from crewai_files import FileInput
 
+    from crewai.context import ExecutionContext
     from crewai.flow.async_feedback.types import PendingFeedbackContext
     from crewai.llms.base_llm import BaseLLM
 
@@ -134,6 +138,19 @@ from crewai.utilities.streaming import (
 logger = logging.getLogger(__name__)
 
 
+def _resolve_persistence(value: Any) -> Any:
+    if value is None or isinstance(value, FlowPersistence):
+        return value
+    if isinstance(value, dict):
+        from crewai.flow.persistence.base import _persistence_registry
+
+        type_name = value.get("persistence_type", "SQLiteFlowPersistence")
+        cls = _persistence_registry.get(type_name)
+        if cls is not None:
+            return cls.model_validate(value)
+    return value
+
+
 class FlowState(BaseModel):
     """Base model for all flow states, ensuring each state has a unique ID."""
 
@@ -883,6 +900,8 @@ class Flow(BaseModel, Generic[T], metaclass=FlowMeta):
     _routers: ClassVar[set[FlowMethodName]] = set()
     _router_paths: ClassVar[dict[FlowMethodName, list[FlowMethodName]]] = {}
 
+    entity_type: Literal["flow"] = "flow"
+
     initial_state: Any = Field(default=None)
     name: str | None = Field(default=None)
     tracing: bool | None = Field(default=None)
@@ -893,8 +912,17 @@ class Flow(BaseModel, Generic[T], metaclass=FlowMeta):
     human_feedback_history: list[HumanFeedbackResult] = Field(default_factory=list)
     last_human_feedback: HumanFeedbackResult | None = Field(default=None)
 
-    persistence: Any = Field(default=None, exclude=True)
-    max_method_calls: int = Field(default=100, exclude=True)
+    persistence: Annotated[
+        SerializeAsAny[FlowPersistence] | Any,
+        BeforeValidator(lambda v, _: _resolve_persistence(v)),
+    ] = Field(default=None)
+    max_method_calls: int = Field(default=100)
+
+    execution_context: ExecutionContext | None = Field(default=None)
+    checkpoint_completed_methods: set[str] | None = Field(default=None)
+    checkpoint_method_outputs: list[Any] | None = Field(default=None)
+    checkpoint_method_counts: dict[str, int] | None = Field(default=None)
+    checkpoint_state: dict[str, Any] | None = Field(default=None)
 
     _methods: dict[FlowMethodName, FlowMethod[Any, Any]] = PrivateAttr(
         default_factory=dict
diff --git a/lib/crewai/src/crewai/flow/persistence/base.py b/lib/crewai/src/crewai/flow/persistence/base.py
index 376c9352b..1114359a1 100644
--- a/lib/crewai/src/crewai/flow/persistence/base.py
+++ b/lib/crewai/src/crewai/flow/persistence/base.py
@@ -5,14 +5,17 @@ from __future__ import annotations
 from abc import ABC, abstractmethod
 from typing import TYPE_CHECKING, Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 
 if TYPE_CHECKING:
     from crewai.flow.async_feedback.types import PendingFeedbackContext
 
 
-class FlowPersistence(ABC):
+_persistence_registry: dict[str, type[FlowPersistence]] = {}
+
+
+class FlowPersistence(BaseModel, ABC):
     """Abstract base class for flow state persistence.
 
     This class defines the interface that all persistence implementations must follow.
@@ -24,6 +27,13 @@ class FlowPersistence(ABC):
     - clear_pending_feedback(): Clears pending feedback after resume
     """
 
+    persistence_type: str = Field(default="base")
+
+    def __init_subclass__(cls, **kwargs: Any) -> None:
+        super().__init_subclass__(**kwargs)
+        if not getattr(cls, "__abstractmethods__", set()):
+            _persistence_registry[cls.__name__] = cls
+
     @abstractmethod
     def init_db(self) -> None:
         """Initialize the persistence backend.
@@ -95,7 +105,7 @@ class FlowPersistence(ABC):
         """
         return None
 
-    def clear_pending_feedback(self, flow_uuid: str) -> None:  # noqa: B027
+    def clear_pending_feedback(self, flow_uuid: str) -> None:
         """Clear the pending feedback marker after successful resume.
 
         This is called after feedback is received and the flow resumes.
diff --git a/lib/crewai/src/crewai/flow/persistence/sqlite.py b/lib/crewai/src/crewai/flow/persistence/sqlite.py
index edf379660..fa2e4e127 100644
--- a/lib/crewai/src/crewai/flow/persistence/sqlite.py
+++ b/lib/crewai/src/crewai/flow/persistence/sqlite.py
@@ -9,7 +9,8 @@ from pathlib import Path
 import sqlite3
 from typing import TYPE_CHECKING, Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field, PrivateAttr, model_validator
+from typing_extensions import Self
 
 from crewai.flow.persistence.base import FlowPersistence
 from crewai.utilities.lock_store import lock as store_lock
@@ -50,26 +51,22 @@ class SQLiteFlowPersistence(FlowPersistence):
         ```
     """
 
-    def __init__(self, db_path: str | None = None) -> None:
-        """Initialize SQLite persistence.
+    persistence_type: str = Field(default="SQLiteFlowPersistence")
+    db_path: str = Field(
+        default_factory=lambda: str(Path(db_storage_path()) / "flow_states.db")
+    )
+    _lock_name: str = PrivateAttr()
 
-        Args:
-            db_path: Path to the SQLite database file. If not provided, uses
-                    db_storage_path() from utilities.paths.
+    def __init__(self, db_path: str | None = None, /, **kwargs: Any) -> None:
+        if db_path is not None:
+            kwargs["db_path"] = db_path
+        super().__init__(**kwargs)
 
-        Raises:
-            ValueError: If db_path is invalid
-        """
-
-        # Get path from argument or default location
-        path = db_path or str(Path(db_storage_path()) / "flow_states.db")
-
-        if not path:
-            raise ValueError("Database path must be provided")
-
-        self.db_path = path  # Now mypy knows this is str
+    @model_validator(mode="after")
+    def _setup(self) -> Self:
         self._lock_name = f"sqlite:{os.path.realpath(self.db_path)}"
         self.init_db()
+        return self
 
     def init_db(self) -> None:
         """Create the necessary tables if they don't exist."""
diff --git a/lib/crewai/src/crewai/lite_agent_output.py b/lib/crewai/src/crewai/lite_agent_output.py
index af0d51808..1ac79d422 100644
--- a/lib/crewai/src/crewai/lite_agent_output.py
+++ b/lib/crewai/src/crewai/lite_agent_output.py
@@ -40,7 +40,9 @@ class LiteAgentOutput(BaseModel):
     usage_metrics: dict[str, Any] | None = Field(
         description="Token usage metrics for this execution", default=None
     )
-    messages: list[LLMMessage] = Field(description="Messages of the agent", default=[])
+    messages: list[LLMMessage] = Field(
+        description="Messages of the agent", default_factory=list
+    )
 
     plan: str | None = Field(
         default=None, description="The execution plan that was generated, if any"
diff --git a/lib/crewai/src/crewai/memory/memory_scope.py b/lib/crewai/src/crewai/memory/memory_scope.py
index de074ce25..b5418e03f 100644
--- a/lib/crewai/src/crewai/memory/memory_scope.py
+++ b/lib/crewai/src/crewai/memory/memory_scope.py
@@ -32,6 +32,10 @@ class MemoryScope(BaseModel):
         """Extract memory dependency and normalize root path before validation."""
         if isinstance(data, MemoryScope):
             return data
+        if not isinstance(data, dict):
+            raise ValueError(f"Expected dict or MemoryScope, got {type(data).__name__}")
+        if "memory" not in data:
+            raise ValueError("MemoryScope requires a 'memory' key")
         memory = data.pop("memory")
         instance: MemoryScope = handler(data)
         instance._memory = memory
@@ -199,6 +203,10 @@ class MemorySlice(BaseModel):
         """Extract memory dependency and normalize scopes before validation."""
         if isinstance(data, MemorySlice):
             return data
+        if not isinstance(data, dict):
+            raise ValueError(f"Expected dict or MemorySlice, got {type(data).__name__}")
+        if "memory" not in data:
+            raise ValueError("MemorySlice requires a 'memory' key")
         memory = data.pop("memory")
         data["scopes"] = [s.rstrip("/") or "/" for s in data.get("scopes", [])]
         instance: MemorySlice = handler(data)
diff --git a/lib/crewai/src/crewai/runtime_state.py b/lib/crewai/src/crewai/runtime_state.py
new file mode 100644
index 000000000..5e0079ae2
--- /dev/null
+++ b/lib/crewai/src/crewai/runtime_state.py
@@ -0,0 +1,18 @@
+"""Unified runtime state for crewAI.
+
+``RuntimeState`` is a ``RootModel`` whose ``model_dump_json()`` produces a
+complete, self-contained snapshot of every active entity in the program.
+
+The ``Entity`` type alias and ``RuntimeState`` model are built at import time
+in ``crewai/__init__.py`` after all forward references are resolved.
+"""
+
+from typing import Any
+
+
+def _entity_discriminator(v: dict[str, Any] | object) -> str:
+    if isinstance(v, dict):
+        raw = v.get("entity_type", "agent")
+    else:
+        raw = getattr(v, "entity_type", "agent")
+    return str(raw)
diff --git a/lib/crewai/src/crewai/task.py b/lib/crewai/src/crewai/task.py
index 44d61729d..7cd0bdca5 100644
--- a/lib/crewai/src/crewai/task.py
+++ b/lib/crewai/src/crewai/task.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+from collections.abc import Sequence
 from concurrent.futures import Future
 import contextvars
 from copy import copy as shallow_copy
@@ -12,6 +13,7 @@ import logging
 from pathlib import Path
 import threading
 from typing import (
+    Annotated,
     Any,
     ClassVar,
     cast,
@@ -24,6 +26,7 @@ import warnings
 from pydantic import (
     UUID4,
     BaseModel,
+    BeforeValidator,
     Field,
     PrivateAttr,
     field_validator,
@@ -32,7 +35,7 @@ from pydantic import (
 from pydantic_core import PydanticCustomError
 from typing_extensions import Self
 
-from crewai.agents.agent_builder.base_agent import BaseAgent
+from crewai.agents.agent_builder.base_agent import BaseAgent, _resolve_agent
 from crewai.context import reset_current_task_id, set_current_task_id
 from crewai.core.providers.content_processor import process_content
 from crewai.events.event_bus import crewai_event_bus
@@ -129,9 +132,10 @@ class Task(BaseModel):
     callback: SerializableCallable | None = Field(
         description="Callback to be executed after the task is completed.", default=None
     )
-    agent: BaseAgent | None = Field(
-        description="Agent responsible for execution the task.", default=None
-    )
+    agent: Annotated[
+        BaseAgent | None,
+        BeforeValidator(_resolve_agent),
+    ] = Field(description="Agent responsible for execution the task.", default=None)
     context: list[Task] | None | _NotSpecified = Field(
         description="Other tasks that will have their output used as context for this task.",
         default=NOT_SPECIFIED,
@@ -392,11 +396,12 @@ class Task(BaseModel):
 
     @field_validator("id", mode="before")
     @classmethod
-    def _deny_user_set_id(cls, v: UUID4 | None) -> None:
-        if v:
+    def _deny_user_set_id(cls, v: UUID4 | None, info: Any) -> UUID4 | None:
+        if v and not (info.context or {}).get("from_checkpoint"):
             raise PydanticCustomError(
                 "may_not_set_field", "This field is not to be set by the user.", {}
             )
+        return v
 
     @field_validator("input_files", mode="before")
     @classmethod
@@ -997,7 +1002,7 @@ Follow these guidelines:
         self.delegations += 1
 
     def copy(  # type: ignore
-        self, agents: list[BaseAgent], task_mapping: dict[str, Task]
+        self, agents: Sequence[BaseAgent], task_mapping: dict[str, Task]
     ) -> Task:
         """Creates a deep copy of the Task while preserving its original class type.
 
diff --git a/lib/crewai/src/crewai/tasks/conditional_task.py b/lib/crewai/src/crewai/tasks/conditional_task.py
index 909be3a1d..22f2454e1 100644
--- a/lib/crewai/src/crewai/tasks/conditional_task.py
+++ b/lib/crewai/src/crewai/tasks/conditional_task.py
@@ -8,6 +8,7 @@ from pydantic import Field
 from crewai.task import Task
 from crewai.tasks.output_format import OutputFormat
 from crewai.tasks.task_output import TaskOutput
+from crewai.types.callback import SerializableCallable
 
 
 class ConditionalTask(Task):
@@ -24,7 +25,7 @@ class ConditionalTask(Task):
         - Cannot be the first task since it needs context from the previous task
     """
 
-    condition: Callable[[TaskOutput], bool] | None = Field(
+    condition: SerializableCallable | None = Field(
         default=None,
         description="Function that determines whether the task should be executed based on previous task output.",
     )
@@ -51,7 +52,7 @@ class ConditionalTask(Task):
         """
         if self.condition is None:
             raise ValueError("No condition function set for conditional task")
-        return self.condition(context)
+        return bool(self.condition(context))
 
     def get_skipped_task_output(self) -> TaskOutput:
         """Generate a TaskOutput for when the conditional task is skipped.
diff --git a/lib/crewai/src/crewai/tasks/task_output.py b/lib/crewai/src/crewai/tasks/task_output.py
index 38712dfa7..3bfd4d33d 100644
--- a/lib/crewai/src/crewai/tasks/task_output.py
+++ b/lib/crewai/src/crewai/tasks/task_output.py
@@ -43,7 +43,9 @@ class TaskOutput(BaseModel):
     output_format: OutputFormat = Field(
         description="Output format of the task", default=OutputFormat.RAW
     )
-    messages: list[LLMMessage] = Field(description="Messages of the task", default=[])
+    messages: list[LLMMessage] = Field(
+        description="Messages of the task", default_factory=list
+    )
 
     @model_validator(mode="after")
     def set_summary(self) -> TaskOutput:
diff --git a/lib/crewai/src/crewai/tools/agent_tools/agent_tools.py b/lib/crewai/src/crewai/tools/agent_tools/agent_tools.py
index 0a1fd32e3..51552f7a8 100644
--- a/lib/crewai/src/crewai/tools/agent_tools/agent_tools.py
+++ b/lib/crewai/src/crewai/tools/agent_tools/agent_tools.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from collections.abc import Sequence
 from typing import TYPE_CHECKING
 
 from crewai.tools.agent_tools.ask_question_tool import AskQuestionTool
@@ -16,7 +17,7 @@ if TYPE_CHECKING:
 class AgentTools:
     """Manager class for agent-related tools"""
 
-    def __init__(self, agents: list[BaseAgent], i18n: I18N | None = None) -> None:
+    def __init__(self, agents: Sequence[BaseAgent], i18n: I18N | None = None) -> None:
         self.agents = agents
         self.i18n = i18n if i18n is not None else get_i18n()
 
diff --git a/lib/crewai/src/crewai/utilities/constants.py b/lib/crewai/src/crewai/utilities/constants.py
index 366c1c4f2..800de5a20 100644
--- a/lib/crewai/src/crewai/utilities/constants.py
+++ b/lib/crewai/src/crewai/utilities/constants.py
@@ -1,5 +1,7 @@
 from typing import Annotated, Final
 
+from pydantic_core import CoreSchema
+
 from crewai.utilities.printer import PrinterColor
 
 
@@ -36,6 +38,25 @@ class _NotSpecified:
     def __repr__(self) -> str:
         return "NOT_SPECIFIED"
 
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls, _source_type: object, _handler: object
+    ) -> CoreSchema:
+        from pydantic_core import core_schema
+
+        def _validate(v: object) -> _NotSpecified:
+            if isinstance(v, _NotSpecified) or v == "NOT_SPECIFIED":
+                return NOT_SPECIFIED
+            raise ValueError(f"Expected NOT_SPECIFIED sentinel, got {type(v).__name__}")
+
+        return core_schema.no_info_plain_validator_function(
+            _validate,
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                lambda v: "NOT_SPECIFIED",
+                info_arg=False,
+            ),
+        )
+
 
 NOT_SPECIFIED: Final[
     Annotated[