This error is raised in the prebuilt create_react_agent when the call_model graph node receives a malformed list of messages. Specifically, it is malformed when there are AIMessages with tool_calls (LLM requesting to call a tool) that do not have a corresponding ToolMessage (result of a tool invocation to return to the LLM). There could be a few reasons you’re seeing this error:
  1. You manually passed a malformed list of messages when invoking the graph, e.g. graph.invoke({'messages': [AIMessage(..., tool_calls=[...])]})
  2. The graph was interrupted before receiving updates from the tools node (i.e. a list of ToolMessages) and you invoked it with an input that is not None or a ToolMessage, e.g. graph.invoke({'messages': [HumanMessage(...)]}, config). This interrupt could have been triggered in one of the following ways:
  • You manually set interrupt_before = ['tools'] in create_react_agent
  • One of the tools raised an error that wasn’t handled by the ToolNode ("tools")

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Troubleshooting

To resolve this, you can do one of the following:
  1. Don’t invoke the graph with a malformed list of messages
  2. In case of an interrupt (manual or due to an error) you can:
  • provide ToolMessages that match existing tool calls and call graph.invoke({'messages': [ToolMessage(...)]}). NOTE: this will append the messages to the history and run the graph from the START node.
    • manually update the state and resume the graph from the interrupt:
      1. get the list of most recent messages from the graph state with graph.get_state(config)
      2. modify the list of messages to either remove unanswered tool calls from AIMessages
or add ToolMessages with tool_call_ids that match unanswered tool calls 3. call graph.update_state(config, {'messages': ...}) with the modified list of messages 4. resume the graph, e.g. call graph.invoke(None, config)