Revert "feat: enforce message history starts with user message (#3440)"

This reverts commit 468e60a.

Author: DouweM

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -216,12 +216,6 @@ async def run(  # noqa: C901
         ctx.state.message_history = messages
         ctx.deps.new_message_index = len(messages)
 
-        # Validate that message history starts with a user message
-        if messages and isinstance(messages[0], _messages.ModelResponse):
-            raise exceptions.UserError(
-                'Message history cannot start with a `ModelResponse`. Conversations must begin with a user message.'
-            )
-
         if self.deferred_tool_results is not None:
             return await self._handle_deferred_tool_results(self.deferred_tool_results, messages, ctx)
 

tests/models/test_outlines.py

@@ -577,7 +577,6 @@ def test_input_format(transformers_multimodal_model: OutlinesModel, binary_image
 
     # unsupported: tool calls
     tool_call_message_history: list[ModelMessage] = [
-        ModelRequest(parts=[UserPromptPart(content='some user prompt')]),
         ModelResponse(parts=[ToolCallPart(tool_call_id='1', tool_name='get_location')]),
         ModelRequest(parts=[ToolReturnPart(tool_name='get_location', content='London', tool_call_id='1')]),
     ]
@@ -593,8 +592,7 @@ def test_input_format(transformers_multimodal_model: OutlinesModel, binary_image
 
     # unsupported: non-image file parts
     file_part_message_history: list[ModelMessage] = [
-        ModelRequest(parts=[UserPromptPart(content='some user prompt')]),
-        ModelResponse(parts=[FilePart(content=BinaryContent(data=b'test', media_type='text/plain'))]),
+        ModelResponse(parts=[FilePart(content=BinaryContent(data=b'test', media_type='text/plain'))])
     ]
     with pytest.raises(
         UserError, match='File parts other than `BinaryImage` are not supported for Outlines models yet.'

tests/test_agent.py

@@ -6132,19 +6132,3 @@ def llm(messages: list[ModelMessage], _info: AgentInfo) -> ModelResponse:
             ]
         )
         assert run.all_messages_json().startswith(b'[{"parts":[{"content":"Hello",')
-
-
-def test_message_history_cannot_start_with_model_response():
-    """Test that message history starting with ModelResponse raises UserError."""
-
-    agent = Agent('test')
-
-    invalid_history = [
-        ModelResponse(parts=[TextPart(content='ai response')]),
-    ]
-
-    with pytest.raises(
-        UserError,
-        match='Message history cannot start with a `ModelResponse`.',
-    ):
-        agent.run_sync('hello', message_history=invalid_history)