Raise exception for reducers w wrong signature (#1261)

libs/langgraph/langgraph/graph/state.py

@@ -725,14 +725,14 @@ def _is_field_binop(typ: Type[Any]) -> Optional[BinaryOperatorAggregate]:
         if len(meta) >= 1 and callable(meta[-1]):
             sig = signature(meta[0])
             params = list(sig.parameters.values())
-            if len(params) == 2 and len(
-                [
-                    p
-                    for p in params
-                    if p.kind in (p.POSITIONAL_ONLY, p.POSITIONAL_OR_KEYWORD)
-                ]
+            if len(params) == 2 and all(
+                p.kind in (p.POSITIONAL_ONLY, p.POSITIONAL_OR_KEYWORD) for p in params
             ):
                 return BinaryOperatorAggregate(typ, meta[0])
+            else:
+                raise ValueError(
+                    f"Invalid reducer signature. Expected (a, b) -> c. Got {sig}"
+                )
     return None
 
 

libs/langgraph/tests/test_pregel.py

@@ -185,6 +185,15 @@ def test_graph_validation() -> None:
     with pytest.raises(ValueError, match="Found edge starting at unknown node "):
         graph.compile()
 
+    def bad_reducer(a):
+        ...
+
+    class BadReducerState(TypedDict):
+        hello: Annotated[str, bad_reducer]
+
+    with pytest.raises(ValueError, match="Invalid reducer"):
+        StateGraph(BadReducerState)
+
 
 def test_checkpoint_errors() -> None:
     class FaultyGetCheckpointer(MemorySaver):