INVALID_CONCURRENT_GRAPH_UPDATE: parallel nodes writing the same state key

TL;DR: Two nodes in one step (fan-out, Send) returned values for the same state key, and the key has no reducer. The runtime cannot decide which write wins, so it throws.

The error

## Rules

- Any state key written by parallel branches needs a reducer. Sequential writes are fine without one; the error only fires on concurrent writes to the same key.
-

The fix

1. Any state key written by parallel branches needs a reducer. Sequential writes are fine without one; the error only fires on concurrent writes to the same key.
2. operator.add on a list is the standard append reducer. Custom reducers are just functions of (existing, update).
3. If the two writes are actually the same logical value and order does not matter, the reducer still has to exist; the runtime will not pick a winner for you.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: INVALIDCONCURRENTGRAPH_UPDATE.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Not pinned to a specific version; follows current INVALIDCONCURRENTGRAPH_UPDATE behavior.

Error catalog entry grounded in the official INVALIDCONCURRENTGRAPHUPDATE error page, verified 2026-09-26. Error page: https://docs.langchain.com/oss/python/langgraph/errors/INVALIDCONCURRENTGRAPHUPDATE