fix: require messages input parameter in Agent component

haystack/components/agents/agent.py

@@ -215,6 +215,7 @@ def translate(
 
     # The template variables 'language' and 'document' become inputs to the run method
     result = agent.run(
+        messages=[],
         language="French",
         document="The weather is lovely today and the sun is shining.",
     )
@@ -502,7 +503,7 @@ def _create_agent_span(self) -> Any:
 
     def _initialize_fresh_execution(
         self,
-        messages: list[ChatMessage] | None,
+        messages: list[ChatMessage],
         streaming_callback: StreamingCallbackT | None,
         requires_async: bool,
         *,
@@ -532,11 +533,6 @@ def _initialize_fresh_execution(
         """
         user_prompt = user_prompt or self.user_prompt
         system_prompt = system_prompt or self.system_prompt
-        if messages is None and user_prompt is None and system_prompt is None:
-            raise ValueError(
-                "No messages provided to the Agent and neither user_prompt nor system_prompt is set. "
-                "Please provide at least one of these inputs."
-            )
         messages = messages or []
 
         if user_prompt is not None:
@@ -744,7 +740,7 @@ def _runtime_checks(self, break_point: AgentBreakpoint | None, tools: ToolsType)
 
     def run(  # noqa: PLR0915
         self,
-        messages: list[ChatMessage] | None = None,
+        messages: list[ChatMessage],
         streaming_callback: StreamingCallbackT | None = None,
         *,
         generation_kwargs: dict[str, Any] | None = None,
@@ -975,7 +971,7 @@ def run(  # noqa: PLR0915
 
     async def run_async(  # noqa: PLR0915
         self,
-        messages: list[ChatMessage] | None = None,
+        messages: list[ChatMessage],
         streaming_callback: StreamingCallbackT | None = None,
         *,
         generation_kwargs: dict[str, Any] | None = None,

haystack/components/generators/chat/llm.py

@@ -132,7 +132,7 @@ def run(
             - "last_message": The last message exchanged during the LLM's run.
         """
         return super(LLM, self).run(  # noqa: UP008
-            messages=messages,
+            messages=messages or [],
             streaming_callback=streaming_callback,
             generation_kwargs=generation_kwargs,
             system_prompt=system_prompt,
@@ -169,7 +169,7 @@ async def run_async(
             - "last_message": The last message exchanged during the LLM's run.
         """
         return await super(LLM, self).run_async(  # noqa: UP008
-            messages=messages,
+            messages=messages or [],
             streaming_callback=streaming_callback,
             generation_kwargs=generation_kwargs,
             system_prompt=system_prompt,

releasenotes/notes/revert-agent-messages-e44f5005e48d371e.yaml

@@ -0,0 +1,16 @@
+---
+upgrade:
+  - |
+    ``Agent.run()`` and ``Agent.run_async()`` now require ``messages`` as an explicit argument (no longer
+    optional). If you were relying on the default ``None`` value in Haystack version 2.26 or 2.27, pass an empty list instead:
+
+    .. code:: python
+
+      agent.run(messages=[], ...)
+
+    ``LLM.run()`` and ``LLM.run_async()`` are unaffected — they still accept ``None`` and default to
+    an empty list internally.
+fixes:
+  - |
+    Reverts the change that made ``Agent`` messages optional as it caused issues with pipeline execution.
+    As a consequence, the ``LLM`` component now defaults to an empty messages list unless provided at runtime.

test/components/agents/test_agent.py

@@ -1517,17 +1517,10 @@ def test_system_prompt_runtime_override(self, make_agent):
         assert result["messages"][0].text == "You are an Haystack expert."
         assert result["messages"][1].text == "Hi"
 
-    def test_user_prompt_raises_when_no_messages_and_no_prompt(self, weather_tool):
-        agent = Agent(chat_generator=MockChatGenerator(), tools=[weather_tool])
-        with pytest.raises(
-            ValueError, match="No messages provided to the Agent and neither user_prompt nor system_prompt is set"
-        ):
-            agent.run()
-
     def test_user_prompt_only_variables_forwarded_to_builder(self, make_agent):
         agent = make_agent(user_prompt=_user_msg("Question: {{question}}"))
         # 'irrelevant_kwarg' is not a template variable — must not raise
-        result = agent.run(question="Will it snow?", irrelevant_kwarg="unused")
+        result = agent.run(messages=[], question="Will it snow?", irrelevant_kwarg="unused")
         assert "messages" in result
 
     def test_user_prompt_with_template_variables(self, make_agent):
@@ -1538,7 +1531,7 @@ def test_user_prompt_with_template_variables(self, make_agent):
                 + " on {{date}}?"
             )
         )
-        result = agent.run(name="Alice", cities=["Berlin", "Paris", "Rome"], date="2024-01-15")
+        result = agent.run(messages=[], name="Alice", cities=["Berlin", "Paris", "Rome"], date="2024-01-15")
         user_messages = [m for m in result["messages"] if m.is_from(ChatRole.USER)]
         assert user_messages[0].text == "Hello ALICE, check weather for: Berlin, Paris, Rome on 2024-01-15?"
 
@@ -1549,7 +1542,7 @@ def test_user_prompt_with_template_variables(self, make_agent):
 
     def test_runtime_user_prompt_overrides_init_prompt(self, make_agent):
         agent = make_agent(user_prompt=_user_msg("Default prompt for {{city}}."))
-        result = agent.run(user_prompt=_user_msg("Runtime prompt for {{city}}."), city="Berlin")
+        result = agent.run(messages=[], user_prompt=_user_msg("Runtime prompt for {{city}}."), city="Berlin")
         user_messages = [m for m in result["messages"] if m.is_from(ChatRole.USER)]
         assert user_messages[0].text == "Runtime prompt for Berlin."
 
@@ -1580,7 +1573,7 @@ def test_system_prompt_and_user_prompt(self, make_agent):
         assert agent._system_chat_prompt_builder is not None
         assert agent._user_chat_prompt_builder is not None
 
-        result = agent.run(project="Haystack", topic="pipelines")
+        result = agent.run(messages=[], project="Haystack", topic="pipelines")
         messages = result["messages"]
         assert messages[0].is_from(ChatRole.SYSTEM)
         assert messages[0].text == "You help users of Haystack."
@@ -1629,7 +1622,7 @@ def _factory(user_prompt: str | None = None):
     def test_rag_pipeline_user_prompt_init_only(self, make_rag_pipeline):
         pipeline = make_rag_pipeline()
         query = "Where is the Colosseum?"
-        result = pipeline.run(data={"retriever": {"query": query}, "agent": {"query": query}})
+        result = pipeline.run(data={"retriever": {"query": query}, "agent": {"query": query, "messages": []}})
         assert "agent" in result
         agent_output = result["agent"]
         assert "messages" in agent_output
@@ -1662,6 +1655,7 @@ def test_rag_pipeline_user_prompt_runtime_override(self, make_rag_pipeline):
                         "Answer: {{query}}"
                     ),
                     "query": query,
+                    "messages": [],
                 },
             }
         )