LangChain errors / Schema
langgraph.errors.InvalidUpdateError: Must write to at least one of [state keys]
Cause
A graph node returns a dict that omits all keys defined in the typed state schema, so LangGraph's write-validation rejects the update.
Fix
Ensure every node returns a dict whose keys match at least one field in your TypedDict state class. Check for typos between the return value and the schema. If a field uses a reducer, verify the reducer accepts the value type the node returns.
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