This error is raised in the prebuilt createReactAgent when the callModel 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: [new 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 null or a ToolMessage, e.g. graph.invoke({messages: [new HumanMessage(...)]}, config). This interrupt could have been triggered in one of the following ways:
  • You manually set interruptBefore: ['tools'] in createReactAgent
  • 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: [new 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.getState(config)
      2. modify the list of messages to either remove unanswered tool calls from AIMessages
or add ToolMessages with toolCallIds that match unanswered tool calls 3. call graph.updateState(config, {messages: ...}) with the modified list of messages 4. resume the graph, e.g. call graph.invoke(null, config)