agentsop-state-reducer

LangGraph raises an InvalidUpdateError when multiple parallel branches or multi-agent workers attempt to update the same state key concurrently in a single step without a defined merge strategy.

To fix concurrent state update conflicts in LangGraph, define per-key reducer semantics in your StateGraph schema so parallel writes merge deterministically instead of crashing the graph.

For merging message lists in LangGraph, the add_messages reducer is the standard pattern, while operator.add is typically selected for appending counters or standard list types.

To prevent state crashes with Send-based map reduce patterns in LangGraph, enumerate which writer nodes update each shared key and assign a pure reducer to that key in your TypedDict schema.

Only shared state keys targeted by concurrent parallel writes require a custom reducer. Keys updated sequentially or by a single writer node do not need explicit reducer semantics to avoid InvalidUpdateError.