feat: Warn users if Agent is called with only system messages (#9514)

* Add warning message and raise error in agent run method

* Add tests

* Add reno

* Updates

* Updates

Author: sjrl

haystack/components/agents/agent.py

@@ -12,7 +12,7 @@
 from haystack.core.pipeline.pipeline import Pipeline
 from haystack.core.pipeline.utils import _deepcopy_with_exceptions
 from haystack.core.serialization import component_to_dict
-from haystack.dataclasses import ChatMessage
+from haystack.dataclasses import ChatMessage, ChatRole
 from haystack.dataclasses.streaming_chunk import StreamingCallbackT, select_streaming_callback
 from haystack.tools import Tool, Toolset, deserialize_tools_or_toolset_inplace, serialize_tools_or_toolset
 from haystack.utils.callable_serialization import deserialize_callable, serialize_callable
@@ -69,7 +69,7 @@ def __init__(
         max_agent_steps: int = 100,
         raise_on_tool_invocation_failure: bool = False,
         streaming_callback: Optional[StreamingCallbackT] = None,
-    ):
+    ) -> None:
         """
         Initialize the agent component.
 
@@ -237,13 +237,20 @@ def run(
             - "messages": List of all messages exchanged during the agent's run.
             - "last_message": The last message exchanged during the agent's run.
             - Any additional keys defined in the `state_schema`.
+        :raises RuntimeError: If the Agent component wasn't warmed up before calling `run()`.
         """
         if not self._is_warmed_up and hasattr(self.chat_generator, "warm_up"):
             raise RuntimeError("The component Agent wasn't warmed up. Run 'warm_up()' before calling 'run()'.")
 
         if self.system_prompt is not None:
             messages = [ChatMessage.from_system(self.system_prompt)] + messages
 
+        if all(m.is_from(ChatRole.SYSTEM) for m in messages):
+            logger.warning(
+                "All messages provided to the Agent component are system messages. This is not recommended as the "
+                "Agent will not perform any actions specific to user input. Consider adding user messages to the input."
+            )
+
         state = State(schema=self.state_schema, data=kwargs)
         state.set("messages", messages)
         component_visits = dict.fromkeys(["chat_generator", "tool_invoker"], 0)
@@ -332,13 +339,20 @@ async def run_async(
             - "messages": List of all messages exchanged during the agent's run.
             - "last_message": The last message exchanged during the agent's run.
             - Any additional keys defined in the `state_schema`.
+        :raises RuntimeError: If the Agent component wasn't warmed up before calling `run_async()`.
         """
         if not self._is_warmed_up and hasattr(self.chat_generator, "warm_up"):
             raise RuntimeError("The component Agent wasn't warmed up. Run 'warm_up()' before calling 'run_async()'.")
 
         if self.system_prompt is not None:
             messages = [ChatMessage.from_system(self.system_prompt)] + messages
 
+        if all(m.is_from(ChatRole.SYSTEM) for m in messages):
+            logger.warning(
+                "All messages provided to the Agent component are system messages. This is not recommended as the "
+                "Agent will not perform any actions specific to user input. Consider adding user messages to the input."
+            )
+
         state = State(schema=self.state_schema, data=kwargs)
         state.set("messages", messages)
         component_visits = dict.fromkeys(["chat_generator", "tool_invoker"], 0)

releasenotes/notes/warning-or-error-missing-messages-agent-c9bb7f1fd3708df1.yaml

@@ -0,0 +1,4 @@
+---
+enhancements:
+  - |
+    - If only system messages are provided as input a warning will be logged to the user indicating that this likely not intended and that they should probably also provide user messages.

test/components/agents/test_agent.py

@@ -719,6 +719,21 @@ def test_run_not_warmed_up(self, weather_tool):
         with pytest.raises(RuntimeError, match="The component Agent wasn't warmed up."):
             agent.run([ChatMessage.from_user("What is the weather in Berlin?")])
 
+    def test_run_no_messages(self, monkeypatch):
+        monkeypatch.setenv("OPENAI_API_KEY", "fake-key")
+        chat_generator = OpenAIChatGenerator()
+        agent = Agent(chat_generator=chat_generator, tools=[])
+        agent.warm_up()
+        result = agent.run([])
+        assert result["messages"] == []
+
+    def test_run_only_system_prompt(self, caplog):
+        chat_generator = MockChatGeneratorWithoutRunAsync()
+        agent = Agent(chat_generator=chat_generator, tools=[], system_prompt="This is a system prompt.")
+        agent.warm_up()
+        _ = agent.run([])
+        assert "All messages provided to the Agent component are system messages." in caplog.text
+
     @pytest.mark.skipif(not os.environ.get("OPENAI_API_KEY"), reason="OPENAI_API_KEY not set")
     @pytest.mark.integration
     def test_run(self, weather_tool):