fix: Handle thread locks in Flow state serialization

- Add state serialization in Flow events to avoid pickling RLock objects
- Update event emission to use serialized state
- Add test case for Flow with thread locks

Fixes #2120

Co-Authored-By: Joe Moura <joao@crewai.com>

src/crewai/flow/flow.py

@@ -819,9 +819,14 @@ class Flow(Generic[T], metaclass=FlowMeta):
         self, method_name: str, method: Callable, *args: Any, **kwargs: Any
     ) -> Any:
         try:
-            dumped_params = {f"_{i}": arg for i, arg in enumerate(args)} | (
-                kwargs or {}
+            # Serialize state before event emission to avoid pickling issues
+            state_copy = (
+                type(self._state)(**self._state.model_dump())
+                if isinstance(self._state, BaseModel)
+                else dict(self._state)
             )
+
+            dumped_params = {f"_{i}": arg for i, arg in enumerate(args)} | (kwargs or {})
             crewai_event_bus.emit(
                 self,
                 MethodExecutionStartedEvent(
@@ -829,7 +834,7 @@ class Flow(Generic[T], metaclass=FlowMeta):
                     method_name=method_name,
                     flow_name=self.__class__.__name__,
                     params=dumped_params,
-                    state=self._copy_state(),
+                    state=state_copy,
                 ),
             )
 
@@ -844,6 +849,24 @@ class Flow(Generic[T], metaclass=FlowMeta):
                 self._method_execution_counts.get(method_name, 0) + 1
             )
 
+            # Serialize state after execution to avoid pickling issues
+            state_copy = (
+                type(self._state)(**self._state.model_dump())
+                if isinstance(self._state, BaseModel)
+                else dict(self._state)
+            )
+
+            crewai_event_bus.emit(
+                self,
+                MethodExecutionFinishedEvent(
+                    type="method_execution_finished",
+                    method_name=method_name,
+                    flow_name=self.__class__.__name__,
+                    state=state_copy,
+                    result=result,
+                ),
+            )
+
             crewai_event_bus.emit(
                 self,
                 MethodExecutionFinishedEvent(

tests/flow_test.py

@@ -350,6 +350,35 @@ def test_flow_uuid_structured():
     assert flow.state.message == "final"
 
 
+def test_flow_with_thread_lock():
+    """Test that Flow properly handles thread locks in state."""
+    import threading
+    
+    class LockFlow(Flow):
+        def __init__(self):
+            super().__init__()
+            self.lock = threading.RLock()
+            self.counter = 0
+            
+        @start()
+        async def step_1(self):
+            with self.lock:
+                self.counter += 1
+                return "step 1"
+                
+        @listen(step_1)
+        async def step_2(self, result):
+            with self.lock:
+                self.counter += 1
+                return result + " -> step 2"
+
+    flow = LockFlow()
+    result = flow.kickoff()
+    
+    assert result == "step 1 -> step 2"
+    assert flow.counter == 2
+
+
 def test_router_with_multiple_conditions():
     """Test a router that triggers when any of multiple steps complete (OR condition),
     and another router that triggers only after all specified steps complete (AND condition).