Add test to ensure openai allows conversations starting with model responses

tests/models/cassettes/test_openai/test_message_history_can_start_with_model_response.yaml

@@ -0,0 +1,81 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '180'
+      content-type:
+      - application/json
+      host:
+      - api.openai.com
+    method: POST
+    parsed_body:
+      messages:
+      - content: Where do you want to go today?
+        role: assistant
+      - content: Answer in 5 words only. Who is Tux?
+        role: user
+      model: gpt-4.1-mini
+      stream: false
+    uri: https://api.openai.com/v1/chat/completions
+  response:
+    headers:
+      access-control-expose-headers:
+      - X-Request-ID
+      alt-svc:
+      - h3=":443"; ma=86400
+      connection:
+      - keep-alive
+      content-length:
+      - '841'
+      content-type:
+      - application/json
+      openai-organization:
+      - deeplytalented
+      openai-processing-ms:
+      - '423'
+      openai-project:
+      - proj_1aziXuKoVAC897wPxnvH0q7Z
+      openai-version:
+      - '2020-10-01'
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+      transfer-encoding:
+      - chunked
+    parsed_body:
+      choices:
+      - finish_reason: stop
+        index: 0
+        logprobs: null
+        message:
+          annotations: []
+          content: Linux mascot, a penguin character.
+          refusal: null
+          role: assistant
+      created: 1763805700
+      id: chatcmpl-Ceeiy4ivEE0hcL1EX5ZfLuW5xNUXB
+      model: gpt-4.1-mini-2025-04-14
+      object: chat.completion
+      service_tier: default
+      system_fingerprint: fp_9766e549b2
+      usage:
+        completion_tokens: 8
+        completion_tokens_details:
+          accepted_prediction_tokens: 0
+          audio_tokens: 0
+          reasoning_tokens: 0
+          rejected_prediction_tokens: 0
+        prompt_tokens: 31
+        prompt_tokens_details:
+          audio_tokens: 0
+          cached_tokens: 0
+        total_tokens: 39
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/test_openai.py

@@ -1215,6 +1215,57 @@ async def get_capital(country: str) -> str:
     assert result.output == snapshot('The capital of England is London.')
 
 
+async def test_message_history_can_start_with_model_response(allow_model_requests: None, openai_api_key: str):
+    """Test that an agent run with message_history starting with ModelResponse is executed correctly."""
+
+    openai_model = OpenAIChatModel('gpt-4.1-mini', provider=OpenAIProvider(api_key=openai_api_key))
+
+    message_history = [ModelResponse(parts=[TextPart('Where do you want to go today?')])]
+
+    agent = Agent(model=openai_model)
+
+    result = await agent.run('Answer in 5 words only. Who is Tux?', message_history=message_history)
+
+    assert result.output == snapshot('Linux mascot, a penguin character.')
+    assert result.all_messages() == snapshot(
+        [
+            ModelResponse(
+                parts=[TextPart(content='Where do you want to go today?')],
+                timestamp=IsDatetime(),
+            ),
+            ModelRequest(
+                parts=[
+                    UserPromptPart(
+                        content='Answer in 5 words only. Who is Tux?',
+                        timestamp=IsDatetime(),
+                    )
+                ],
+                run_id=IsStr(),
+            ),
+            ModelResponse(
+                parts=[TextPart(content='Linux mascot, a penguin character.')],
+                usage=RequestUsage(
+                    input_tokens=31,
+                    output_tokens=8,
+                    details={
+                        'accepted_prediction_tokens': 0,
+                        'audio_tokens': 0,
+                        'reasoning_tokens': 0,
+                        'rejected_prediction_tokens': 0,
+                    },
+                ),
+                model_name='gpt-4.1-mini-2025-04-14',
+                timestamp=IsDatetime(),
+                provider_name='openai',
+                provider_details={'finish_reason': 'stop'},
+                provider_response_id='chatcmpl-Ceeiy4ivEE0hcL1EX5ZfLuW5xNUXB',
+                finish_reason='stop',
+                run_id=IsStr(),
+            ),
+        ]
+    )
+
+
 async def test_extra_headers(allow_model_requests: None, openai_api_key: str):
     # This test doesn't do anything, it's just here to ensure that calls with `extra_headers` don't cause errors, including type.
     m = OpenAIChatModel('gpt-4o', provider=OpenAIProvider(api_key=openai_api_key))