`InvalidUpdateError` with `INVALID_CONCURRENT_GRAPH_UPDATE` when parallel branches/nodes write to shared state key (e.g., `messages`) without reducer. Message history duplicates or corrupts; tool calls mismatch in parallel tool execution.

Parallel nodes or branches (including in subgraphs invoked concurrently) attempt concurrent updates to the same non-reducer-annotated state key (default 'last write wins'), triggering `InvalidUpdateError: INVALID_CONCURRENT_GRAPH_UPDATE`. Exacerbated by `operator.add` on messages (corrupts history) or nodes returning full state (duplicates).