Overwrite

from typing import Annotated import operator from langgraph.graph import StateGraph from langgraph.types import Overwrite  class State(TypedDict):  messages: Annotated[list, operator.add]  def node_a(state: TypedDict):  # Normal update: uses the reducer (operator.add)  return {"messages": ["a"]}  def node_b(state: State):  # Overwrite: bypasses the reducer and replaces the entire value  return {"messages": Overwrite(value=["b"])}  builder = StateGraph(State) builder.add_node("node_a", node_a) builder.add_node("node_b", node_b) builder.set_entry_point("node_a") builder.add_edge("node_a", "node_b") graph = builder.compile()  # Without Overwrite in node_b, messages would be ["START", "a", "b"] # With Overwrite, messages is just ["b"] result = graph.invoke({"messages": ["START"]}) assert result == {"messages": ["b"]}