test_middleware_can_return_system_message() — langchain Function Reference

Source Code

libs/langchain_v1/tests/unit_tests/agents/test_system_message.py lines 763–795

    def test_middleware_can_return_system_message(self) -> None:
        """Test that middleware can return a SystemMessage with dynamic content."""

        def dynamic_system_prompt_middleware(request: ModelRequest) -> SystemMessage:
            """Return a SystemMessage with dynamic content."""
            region = getattr(request.runtime.context, "region", "n/a")
            return SystemMessage(content=f"You are a helpful assistant. Region: {region}")

        @dataclass
        class RegionContext:
            region: str

        runtime = Runtime(context=RegionContext(region="EU"))
        request = ModelRequest(
            model=GenericFakeChatModel(messages=iter([AIMessage(content="response")])),
            system_message=None,
            messages=[HumanMessage(content="Hello")],
            tool_choice=None,
            tools=[],
            response_format=None,
            state=AgentState(messages=[]),
            runtime=runtime,
            model_settings={},
        )

        new_system_message = dynamic_system_prompt_middleware(request)

        assert isinstance(new_system_message, SystemMessage)
        assert len(new_system_message.content_blocks) == 1
        assert (
            new_system_message.content_blocks[0].get("text")
            == "You are a helpful assistant. Region: EU"
        )