Translate Chat Completions tool_choice to the Responses API shape

chatmock/routes_ollama.py

@@ -18,7 +18,11 @@
 )
 from .transform import convert_ollama_messages, normalize_ollama_tools
 from .upstream import normalize_model_name, start_upstream_request
-from .utils import convert_chat_messages_to_responses_input, convert_tools_chat_to_responses
+from .utils import (
+    convert_chat_messages_to_responses_input,
+    convert_tool_choice_chat_to_responses,
+    convert_tools_chat_to_responses,
+)
 
 
 ollama_bp = Blueprint("ollama", __name__)
@@ -195,7 +199,7 @@ def ollama_chat() -> Response:
     stream_req = bool(stream_req)
     tools_req = payload.get("tools") if isinstance(payload.get("tools"), list) else []
     tools_responses = convert_tools_chat_to_responses(normalize_ollama_tools(tools_req))
-    tool_choice = payload.get("tool_choice", "auto")
+    tool_choice = convert_tool_choice_chat_to_responses(payload.get("tool_choice", "auto"))
     parallel_tool_calls = bool(payload.get("parallel_tool_calls", False))
 
     # Passthrough Responses API tools (web_search) via ChatMock extension fields
@@ -297,7 +301,7 @@ def ollama_chat() -> Response:
             if verbose:
                 print("[Passthrough] Upstream rejected tools; retrying without extras (args redacted)")
             base_tools_only = convert_tools_chat_to_responses(normalize_ollama_tools(tools_req))
-            safe_choice = payload.get("tool_choice", "auto")
+            safe_choice = convert_tool_choice_chat_to_responses(payload.get("tool_choice", "auto"))
             upstream2, err2 = start_upstream_request(
                 normalize_model_name(model, current_app.config.get("DEBUG_MODEL")),
                 input_items,

chatmock/routes_openai.py

@@ -32,6 +32,7 @@
 from .upstream import normalize_model_name, start_upstream_raw_request, start_upstream_request
 from .utils import (
     convert_chat_messages_to_responses_input,
+    convert_tool_choice_chat_to_responses,
     convert_tools_chat_to_responses,
     sse_translate_chat,
     sse_translate_text,
@@ -147,7 +148,7 @@ def chat_completions() -> Response:
     include_usage = bool(stream_options.get("include_usage", False))
 
     tools_responses = convert_tools_chat_to_responses(payload.get("tools"))
-    tool_choice = payload.get("tool_choice", "auto")
+    tool_choice = convert_tool_choice_chat_to_responses(payload.get("tool_choice", "auto"))
     parallel_tool_calls = bool(payload.get("parallel_tool_calls", False))
     responses_tools_payload = payload.get("responses_tools") if isinstance(payload.get("responses_tools"), list) else []
     extra_tools: List[Dict[str, Any]] = []
@@ -246,7 +247,7 @@ def chat_completions() -> Response:
             if verbose:
                 print("[Passthrough] Upstream rejected tools; retrying without extra tools (args redacted)")
             base_tools_only = convert_tools_chat_to_responses(payload.get("tools"))
-            safe_choice = payload.get("tool_choice", "auto")
+            safe_choice = convert_tool_choice_chat_to_responses(payload.get("tool_choice", "auto"))
             upstream2, err2 = start_upstream_request(
                 model,
                 input_items,

chatmock/upstream.py

@@ -71,7 +71,7 @@ def start_upstream_request(
         "model": model,
         "input": input_items,
         "tools": tools or [],
-        "tool_choice": tool_choice if tool_choice in ("auto", "none") or isinstance(tool_choice, dict) else "auto",
+        "tool_choice": tool_choice if tool_choice in ("auto", "none", "required") or isinstance(tool_choice, dict) else "auto",
         "parallel_tool_calls": bool(parallel_tool_calls),
         "store": False,
         "stream": True,

chatmock/utils.py

@@ -218,6 +218,28 @@ def _normalize_image_data_url(url: str) -> str:
     return input_items
 
 
+def convert_tool_choice_chat_to_responses(tool_choice: Any) -> Any:
+    """Translate a Chat Completions ``tool_choice`` into the Responses API shape.
+
+    Chat Completions forces a function with ``{"type": "function", "function": {"name": ...}}``
+    while the Responses API expects ``{"type": "function", "name": ...}``. String modes
+    (``auto``/``none``/``required``) pass through unchanged and anything unrecognised
+    falls back to ``auto``.
+    """
+    if isinstance(tool_choice, str):
+        mode = tool_choice.strip().lower()
+        return mode if mode in ("auto", "none", "required") else "auto"
+    if isinstance(tool_choice, dict):
+        if tool_choice.get("type") == "function":
+            fn = tool_choice.get("function")
+            name = fn.get("name") if isinstance(fn, dict) else tool_choice.get("name")
+            if isinstance(name, str) and name.strip():
+                return {"type": "function", "name": name.strip()}
+            return "auto"
+        return tool_choice
+    return "auto"
+
+
 def convert_tools_chat_to_responses(tools: Any) -> List[Dict[str, Any]]:
     out: List[Dict[str, Any]] = []
     if not isinstance(tools, list):

tests/test_routes.py

@@ -97,6 +97,58 @@ def test_chat_completions(self, mock_start) -> None:
         self.assertEqual(body["choices"][0]["message"]["content"], "hello")
         self.assertEqual(body["model"], "gpt5.4-mini")
 
+    @patch("chatmock.routes_openai.start_upstream_request")
+    def test_chat_completions_translates_forced_tool_choice(self, mock_start) -> None:
+        mock_start.return_value = (
+            FakeUpstream([{"type": "response.completed", "response": {"id": "resp-openai"}}]),
+            None,
+        )
+        tools = [{"type": "function", "function": {"name": "lookup", "parameters": {"type": "object"}}}]
+        response = self.client.post(
+            "/v1/chat/completions",
+            json={
+                "model": "gpt-5.5",
+                "messages": [{"role": "user", "content": "hi"}],
+                "tools": tools,
+                "tool_choice": {"type": "function", "function": {"name": "lookup"}},
+            },
+        )
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(mock_start.call_args.kwargs["tool_choice"], {"type": "function", "name": "lookup"})
+
+    @patch("chatmock.routes_openai.start_upstream_request")
+    def test_chat_completions_passes_required_tool_choice(self, mock_start) -> None:
+        mock_start.return_value = (
+            FakeUpstream([{"type": "response.completed", "response": {"id": "resp-openai"}}]),
+            None,
+        )
+        response = self.client.post(
+            "/v1/chat/completions",
+            json={"model": "gpt-5.5", "messages": [{"role": "user", "content": "hi"}], "tool_choice": "required"},
+        )
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(mock_start.call_args.kwargs["tool_choice"], "required")
+
+    @patch("chatmock.routes_ollama.start_upstream_request")
+    def test_ollama_chat_translates_forced_tool_choice(self, mock_start) -> None:
+        mock_start.return_value = (
+            FakeUpstream([{"type": "response.completed", "response": {"id": "resp-ollama"}}]),
+            None,
+        )
+        tools = [{"type": "function", "function": {"name": "lookup", "parameters": {"type": "object"}}}]
+        response = self.client.post(
+            "/api/chat",
+            json={
+                "model": "gpt-5.5",
+                "messages": [{"role": "user", "content": "hi"}],
+                "tools": tools,
+                "tool_choice": {"type": "function", "function": {"name": "lookup"}},
+                "stream": False,
+            },
+        )
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(mock_start.call_args.kwargs["tool_choice"], {"type": "function", "name": "lookup"})
+
     @patch("chatmock.routes_openai.start_upstream_request")
     def test_chat_completions_honors_debug_model_override(self, mock_start) -> None:
         app = create_app(debug_model="gpt-5.4", model_sync=False)