test(integration): openai chat drops tool_choice when the request has no tools (Pylon #8022)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/integration/contracts.json

@@ -235,6 +235,9 @@
     "tests/integration/providers/test_fal_ai_video_wire.py::test_fal_provider_hanging_up_on_the_result_probe_keeps_the_completed_status": [
       "other.provider_wire.fal_ai.video_result_probe_hangup_stays_completed"
     ],
+    "tests/integration/providers/test_openai_chat_wire.py::test_openai_chat_tool_choice_without_tools_is_not_forwarded": [
+      "providers.openai_chat_wire.tool_choice_without_tools_is_dropped_before_the_wire"
+    ],
     "tests/integration/mcp/test_mcp_lifecycle.py::test_saved_headers_reach_real_mcp_tool_and_survive_unrelated_edit": [
       "mcp.call_tool.saved_headers.reach_actual_transport"
     ],

tests/integration/providers/test_openai_chat_wire.py

@@ -0,0 +1,66 @@
+import json
+import uuid
+from typing import Final
+
+import pytest
+from integration._support.client import Gateway
+from integration._support.wire import Reply, Request, wire_server
+from pydantic import JsonValue, TypeAdapter
+
+_BACKEND: Final = "gpt-5.4-mini"
+_API_KEY: Final = "synthetic-openai-key"
+_PROMPT: Final = "Summarize this conversation in one sentence."
+_JSON_OBJECT: Final = TypeAdapter(dict[str, JsonValue])
+
+
+def _completion(identity: str, content: str) -> bytes:
+    return json.dumps(
+        {
+            "id": identity,
+            "object": "chat.completion",
+            "created": 1,
+            "model": _BACKEND,
+            "choices": [{"index": 0, "message": {"role": "assistant", "content": content}, "finish_reason": "stop"}],
+            "usage": {"prompt_tokens": 19, "completion_tokens": 7, "total_tokens": 26},
+        }
+    ).encode()
+
+
+@pytest.mark.covers("providers.openai_chat_wire.tool_choice_without_tools_is_dropped_before_the_wire")
+def test_openai_chat_tool_choice_without_tools_is_not_forwarded(gateway: Gateway) -> None:
+    identity: Final = f"openai-toolless-{uuid.uuid4().hex}"
+
+    def respond(request: Request) -> Reply:
+        assert request.method == "POST"
+        assert request.target == "/chat/completions"
+        assert request.headers["authorization"] == f"Bearer {_API_KEY}"
+        body: Final = _JSON_OBJECT.validate_json(request.body)
+        assert body["model"] == _BACKEND
+        assert body["messages"] == [{"role": "user", "content": _PROMPT}]
+        assert "tool_choice" not in body, body
+        assert "tools" not in body, body
+        return Reply(body=_completion(identity, "One sentence."))
+
+    with wire_server(respond) as wire, gateway.scenario() as scenario:
+        model: Final = scenario.model(model=f"openai/{_BACKEND}", api_base=wire.url, api_key=_API_KEY)
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": _PROMPT}], "tool_choice": "none"},
+        )
+        assert response.status_code == 200, response.text
+        payload: Final = _JSON_OBJECT.validate_json(response.content)
+        assert payload["id"] == identity
+        assert payload["choices"] == [
+            {
+                "finish_reason": "stop",
+                "index": 0,
+                "message": {
+                    "role": "assistant",
+                    "content": "One sentence.",
+                    "provider_specific_fields": {"refusal": None},
+                },
+                "provider_specific_fields": {},
+            }
+        ]
+        assert [(request.method, request.target) for request in wire.drain()] == [("POST", "/chat/completions")]