langchain-ai/langgraph · error · ValueError

Last message is not an AIMessage

Error message

Last message is not an AIMessage

What it means

Error "Last message is not an AIMessage" thrown in langchain-ai/langgraph.

Source

Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:181

            else:
                raise ValueError(
                    f"Unsupported input to ValidationNode. Expected BaseModel, tool or function. Got: {type(schema)}."
                )

    def _get_message(
        self, input: list[AnyMessage] | dict[str, Any]
    ) -> tuple[str, AIMessage]:
        """Extract the last AIMessage from the input."""
        if isinstance(input, list):
            output_type = "list"
            messages: list = input
        elif messages := input.get("messages", []):
            output_type = "dict"
        else:
            raise ValueError("No message found in input")
        message: AnyMessage = messages[-1]
        if not isinstance(message, AIMessage):
            raise ValueError("Last message is not an AIMessage")
        return output_type, message

    def _func(
        self, input: list[AnyMessage] | dict[str, Any], config: RunnableConfig
    ) -> Any:
        """Validate and run tool calls synchronously."""
        output_type, message = self._get_message(input)

        def run_one(call: ToolCall) -> ToolMessage:
            schema = self.schemas_by_name[call["name"]]
            try:
                if issubclass(schema, BaseModel):
                    output = schema.model_validate(call["args"])
                    content = output.model_dump_json()
                elif issubclass(schema, BaseModelV1):
                    output = schema.validate(call["args"])
                    content = output.json()
                else:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:181 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/ede89a261a431e9a. Report an issue: GitHub.