Support instructions-only agent run with OpenAIResponsesModel

pydantic_ai_slim/pydantic_ai/models/openai.py

@@ -1223,11 +1223,22 @@ async def _responses_create(
         if model_settings.get('openai_include_web_search_sources'):
             include.append('web_search_call.action.sources')
 
+        # When there are no input messages and we're not reusing a previous response,
+        # the OpenAI API will reject a request without any input. To avoid this provide
+        # an explicit empty user message.
+        if not openai_messages and not previous_response_id:
+            openai_messages = [
+                responses.EasyInputMessageParam(
+                    role='user',
+                    content='',
+                )
+            ]
+
         try:
             extra_headers = model_settings.get('extra_headers', {})
             extra_headers.setdefault('User-Agent', get_user_agent())
             return await self.client.responses.create(
-                input=openai_messages,
+                input=cast(responses.ResponseInputParam, openai_messages),
                 model=self._model_name,
                 instructions=instructions,
                 parallel_tool_calls=model_settings.get('parallel_tool_calls', OMIT),

tests/models/test_openai_responses.py

@@ -7442,3 +7442,52 @@ def get_meaning_of_life() -> int:
             },
         ]
     )
+
+
+async def test_openai_responses_runs_with_deps_only_and_sends_input(
+    allow_model_requests: None,
+):
+    from pydantic import BaseModel
+
+    c = response_message(
+        [
+            ResponseOutputMessage(
+                id='output-1',
+                content=[ResponseOutputText(text='ok', type='output_text', annotations=[])],
+                role='assistant',
+                status='completed',
+                type='message',
+            )
+        ]
+    )
+
+    mock_client = MockOpenAIResponses.create_mock(c)
+    model = OpenAIResponsesModel('gpt-4o', provider=OpenAIProvider(openai_client=mock_client))
+
+    class Payload(BaseModel):
+        topic: str
+        sentences: int
+
+    payload = Payload(topic='artificial intelligence', sentences=3)
+
+    agent = Agent(model=model, instructions='Generate an article.', deps_type=Payload)
+
+    @agent.instructions
+    def instr(ctx: Any) -> str:
+        # no explicit input passed to run(); this uses ctx.deps
+        return f'Topic: {ctx.deps.topic}\nSentences: {ctx.deps.sentences}'
+
+    # Run with only deps
+    result = await agent.run(deps=payload)
+    assert result.output == 'ok'
+
+    response_kwargs = get_mock_responses_kwargs(mock_client)
+    assert response_kwargs, 'Responses API was not called'
+
+    kw = response_kwargs[0]
+    assert 'input' in kw or 'text' in kw, "Expected 'input' or 'text' in responses.create kwargs"
+
+    if 'input' in kw:
+        assert kw['input'], "Responses 'input' should not be empty when deps are provided"
+    else:
+        assert kw['text'], "Responses 'text' config should be set when no input messages were built"