fix(title): omit unsupported structured output

agent/title_generator.py

@@ -105,6 +105,22 @@
     },
 }
 
+
+def _title_request_extra_body(
+    response_format_supported: Optional[bool],
+) -> dict[str, Any]:
+    """Return structured-output request fields when the route supports them.
+
+    Unknown capability is intentionally treated like unsupported capability:
+    title parsing already has JSON/prose fallbacks, while an optimistic
+    response_format parameter can make an otherwise valid auxiliary request
+    fail at the provider boundary.
+    """
+    if response_format_supported is True:
+        return {"response_format": _TITLE_RESPONSE_FORMAT}
+    return {}
+
+
 # Control-tag wrappers that surround machine-authored content inside what is
 # nominally a "user" message. Titling from these is what produces a session
 # named after a slash command or an injected reminder rather than the user's
@@ -336,6 +352,7 @@ def generate_title(
     failure_callback: Optional[FailureCallback] = None,
     main_runtime: dict = None,
     runtime_validator: Optional[RuntimeValidator] = None,
+    response_format_supported: Optional[bool] = None,
 ) -> Optional[str]:
     """Generate a session title from the user's opening message.
 
@@ -392,17 +409,20 @@ def generate_title(
     ]
 
     try:
-        response = call_llm(
-            task="title_generation",
-            messages=messages,
+        request_extra_body = _title_request_extra_body(response_format_supported)
+        request_kwargs = {
+            "task": "title_generation",
+            "messages": messages,
             # A title is a handful of tokens. The old 500-token ceiling let a
             # chatty model burn seconds generating prose we then threw away.
-            max_tokens=64,
-            temperature=0.3,
-            timeout=timeout,
-            main_runtime=main_runtime,
-            extra_body={"response_format": _TITLE_RESPONSE_FORMAT},
-        )
+            "max_tokens": 64,
+            "temperature": 0.3,
+            "timeout": timeout,
+            "main_runtime": main_runtime,
+        }
+        if request_extra_body:
+            request_kwargs["extra_body"] = request_extra_body
+        response = call_llm(**request_kwargs)
         content = response.choices[0].message.content or ""
         return _clean_title(_extract_title_text(content))
     except Exception as e:

tests/agent/test_title_generator.py

@@ -46,6 +46,54 @@ def mock_call_llm(**kwargs):
         assert captured_kwargs["task"] == "title_generation"
         assert captured_kwargs["timeout"] is None
 
+    @pytest.mark.parametrize("capability", [None, False])
+    def test_title_generation_omits_response_format_without_capability(self, capability):
+        captured_kwargs = {}
+
+        def mock_call_llm(**kwargs):
+            captured_kwargs.update(kwargs)
+            response = MagicMock()
+            response.choices = [MagicMock()]
+            response.choices[0].message.content = '{"title": "Fix login"}'
+            return response
+
+        with patch("agent.title_generator.call_llm", side_effect=mock_call_llm):
+            assert generate_title(
+                "fix login", response_format_supported=capability
+            ) == "Fix login"
+
+        assert "extra_body" not in captured_kwargs
+        assert "response_format" not in captured_kwargs
+
+    def test_title_generation_sends_response_format_when_capability_supported(self):
+        captured_kwargs = {}
+
+        def mock_call_llm(**kwargs):
+            captured_kwargs.update(kwargs)
+            response = MagicMock()
+            response.choices = [MagicMock()]
+            response.choices[0].message.content = '{"title": "Fix login"}'
+            return response
+
+        with patch("agent.title_generator.call_llm", side_effect=mock_call_llm):
+            assert generate_title(
+                "fix login", response_format_supported=True
+            ) == "Fix login"
+
+        response_format = captured_kwargs["extra_body"]["response_format"]
+        assert response_format["type"] == "json_schema"
+        assert response_format["json_schema"]["name"] == "session_title"
+        assert response_format["json_schema"]["schema"]["required"] == ["title"]
+
+    def test_title_generation_parses_plain_prose_without_response_format(self):
+        response = MagicMock()
+        response.choices = [MagicMock()]
+        response.choices[0].message.content = "Fix login flow on mobile"
+
+        with patch("agent.title_generator.call_llm", return_value=response):
+            assert generate_title(
+                "fix login", response_format_supported=None
+            ) == "Fix login flow on mobile"
 
 
     def test_strips_think_blocks(self):