langgraph: fix None handling for pydantic state updates

libs/langgraph/langgraph/graph/state.py

@@ -33,7 +33,7 @@ from langgraph.channels.dynamic_barrier_value import DynamicBarrierValue, WaitFo
 from langgraph.channels.ephemeral_value import EphemeralValue
 from langgraph.channels.last_value import LastValue
 from langgraph.channels.named_barrier_value import NamedBarrierValue
-from langgraph.constants import EMPTY_SEQ, NS_END, NS_SEP, SELF, TAG_HIDDEN
+from langgraph.constants import EMPTY_SEQ, MISSING, NS_END, NS_SEP, SELF, TAG_HIDDEN
 from langgraph.errors import (
     ErrorCode,
     InvalidUpdateError,
@@ -694,7 +694,7 @@ class CompiledStateGraph(CompiledGraph):
                 return [
                     (k, getattr(input, k))
                     for k in output_keys
-                    if getattr(input, k, None) is not None
+                    if getattr(input, k, MISSING) is not MISSING
                 ]
             else:
                 msg = create_error_message(

libs/langgraph/tests/test_pregel.py

@@ -6482,3 +6482,16 @@ def test_node_destinations() -> None:
             Edge(source="child", target="node_b", data="foo", conditional=True),
             Edge(source="child", target="node_c", data="bar", conditional=True),
         ] == graph.edges
+
+
+def test_pydantic_none_state_update() -> None:
+    from pydantic import BaseModel
+
+    class State(BaseModel):
+        foo: str | None
+
+    def node_a(state: State) -> State:
+        return State(foo=None)
+
+    graph = StateGraph(State).add_node(node_a).add_edge(START, "node_a").compile()
+    assert graph.invoke({"foo": ""}) == {"foo": None}