fix: crew field serializer for circular ref round-trip

lib/crewai/src/crewai/__init__.py

@@ -145,12 +145,17 @@ try:
         "ToolResult": _ToolResult,
     }
 
+    _resolve_namespace = {
+        **_full_namespace,
+        **sys.modules[_BaseAgent.__module__].__dict__,
+    }
+
     for _mod_name in (
         _BaseAgent.__module__,
         Agent.__module__,
         _AgentExecutor.__module__,
     ):
-        sys.modules[_mod_name].__dict__.update(_full_namespace)
+        sys.modules[_mod_name].__dict__.update(_resolve_namespace)
 
     _BaseAgent.model_rebuild(force=True, _types_namespace=_full_namespace)
     _AgentExecutor.model_rebuild(force=True, _types_namespace=_full_namespace)

lib/crewai/src/crewai/agents/agent_builder/base_agent.py

@@ -5,19 +5,20 @@ from copy import copy as shallow_copy
 from hashlib import md5
 from pathlib import Path
 import re
-from typing import TYPE_CHECKING, Any, Final, Literal
+from typing import TYPE_CHECKING, Annotated, Any, Final, Literal
 import uuid
 
 from pydantic import (
     UUID4,
     BaseModel,
+    BeforeValidator,
     Field,
     InstanceOf,
     PrivateAttr,
-    field_serializer,
     field_validator,
     model_validator,
 )
+from pydantic.functional_serializers import PlainSerializer
 from pydantic_core import PydanticCustomError
 from typing_extensions import Self
 
@@ -50,6 +51,16 @@ if TYPE_CHECKING:
     from crewai.crew import Crew
 
 
+def _validate_crew_ref(value: Any) -> Any:
+    return value
+
+
+def _serialize_crew_ref(value: Any) -> str | None:
+    if value is None:
+        return None
+    return str(value.id) if hasattr(value, "id") else str(value)
+
+
 _SLUG_RE: Final[re.Pattern[str]] = re.compile(
     r"^(?:crewai-amp:)?[a-zA-Z0-9][a-zA-Z0-9_-]*(?:#[\w-]+)?$"
 )
@@ -168,9 +179,13 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
     llm: str | BaseLLM | None = Field(
         default=None, description="Language model that will run the agent."
     )
-    crew: Crew | None = Field(
-        default=None, description="Crew to which the agent belongs."
-    )
+    crew: Annotated[
+        Crew | str | None,
+        BeforeValidator(_validate_crew_ref),
+        PlainSerializer(
+            _serialize_crew_ref, return_type=str | None, when_used="always"
+        ),
+    ] = Field(default=None, description="Crew to which the agent belongs.")
     i18n: I18N = Field(
         default_factory=get_i18n, description="Internationalization settings."
     )
@@ -234,11 +249,6 @@ class BaseAgent(BaseModel, ABC, metaclass=AgentMeta):
         min_length=1,
     )
 
-    @field_serializer("crew")
-    @classmethod
-    def _serialize_crew(cls, v: Crew | None) -> str | None:
-        return str(v.id) if v else None
-
     @model_validator(mode="before")
     @classmethod
     def process_model_config(cls, values: Any) -> dict[str, Any]: