fix(flows): align MongoDB model serialization

lib/crewai/src/crewai/flow/persistence/mongodb.py

@@ -148,7 +148,10 @@ class MongoDbFlowPersistence(FlowPersistence):
     def _to_state_dict(state_data: dict[str, Any] | BaseModel) -> dict[str, Any]:
         """Convert state_data to a plain dict."""
         if isinstance(state_data, BaseModel):
-            return state_data.model_dump(mode="json")
+            try:
+                return state_data.model_dump(mode="json")
+            except Exception:
+                return state_data.model_dump(mode="python")
         if isinstance(state_data, dict):
             return state_data
         raise ValueError(

lib/crewai/tests/test_flow_persistence_mongodb.py

@@ -11,7 +11,7 @@ from datetime import datetime, timezone
 import sys
 from typing import Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, ConfigDict
 import pytest
 
 from crewai.flow.async_feedback.types import PendingFeedbackContext
@@ -213,6 +213,27 @@ def test_basemodel_state_serialized_as_json(
     assert loaded["when"].startswith("2026-01-02T03:04:05")
 
 
+def test_basemodel_state_falls_back_to_python_serialization(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    _patch_client(monkeypatch)
+
+    class NonJsonValue:
+        def __str__(self) -> str:
+            return "non-json value"
+
+    class State(BaseModel):
+        model_config = ConfigDict(arbitrary_types_allowed=True)
+
+        value: NonJsonValue
+
+    persistence = MongoDbFlowPersistence(CONN)
+
+    persistence.save_state("flow-1", "s", State(value=NonJsonValue()))
+
+    assert persistence.load_state("flow-1") == {"value": "non-json value"}
+
+
 def test_dict_state_serializes_non_json_values(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None: