INVALID_CHAT_HISTORY: resuming an agent with unanswered tool calls

TL;DR: The prebuilt agent's model node received a message list where an AIMessage has toolcalls with no matching ToolMessages. This happens when you resume after an interrupt (manual or from a tool error) with the wrong input. Remove the unanswered tool calls from the AIMessages, or add ToolMessages with matching toolcall_ids. Rule On resume after an interrupt, the input must answer the pending tool calls.

The fix

1. You invoke the graph with a hand-built message list containing an AIMessage with tool_calls and no ToolMessages.
2. The graph was interrupted before the tools node ran (you set interrupt_before=["tools"], or a tool raised an unhandled error), and you resumed with a plain HumanMessage or new input instead of the pending tool results.
3. graph.get_state(config) and look at the recent messages.
4. Remove the unanswered tool calls from the AIMessages, or add ToolMessages with matching toolcallids.
5. graph.update_state(config, {"messages": [...]}) with the fixed list.
6. Resume with graph.invoke(None, config).

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: INVALIDCHATHISTORY.
• 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 INVALIDCHATHISTORY behavior.

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