[Bugfix][Frontend] Reject Cohere REQUIRED tool choice without tools (#61825)

Signed-off-by: Kevin <jia__kuan@163.com>

tests/entrypoints/cohere/test_api_router.py

@@ -598,6 +598,21 @@ class TestCohereErrorEnvelope:
         assert "message" in body
         assert isinstance(body["message"], str) and body["message"]
 
+    @pytest.mark.parametrize("path", ["/cohere/v2/chat", "/cohere/v2/chat/render"])
+    def test_required_tool_choice_without_tools_returns_400(self, path):
+        """Reject an impossible tool choice before reaching either handler."""
+        app = _build_app_with_vllm_handlers(handler=None)
+        with TestClient(app) as client:
+            response = client.post(
+                path,
+                json={**_minimal_request_body(), "tool_choice": "REQUIRED"},
+            )
+        assert response.status_code == HTTPStatus.BAD_REQUEST
+        body = response.json()
+        assert "error" not in body
+        assert "REQUIRED" in body["message"]
+        assert "tools" in body["message"]
+
     def test_http_error_body_is_cohere_shaped(self):
         # A raised ``HTTPException`` from anywhere in the request cycle
         # is routed through ``http_exception_handler`` (producing the

tests/entrypoints/cohere/test_protocol.py

@@ -95,6 +95,27 @@ class TestCohereChatV2Request:
                 tool_choice="ANY",  # not REQUIRED/NONE
             )
 
+    @pytest.mark.parametrize("tool_fields", [{}, {"tools": None}, {"tools": []}])
+    def test_required_tool_choice_requires_tools(self, tool_fields):
+        with pytest.raises(VLLMValidationError, match="REQUIRED.*tools") as exc_info:
+            CohereChatV2Request(
+                model="m",
+                messages=[{"role": "user", "content": "hi"}],
+                tool_choice="REQUIRED",
+                **tool_fields,
+            )
+        assert exc_info.value.parameter == "tool_choice"
+
+    @pytest.mark.parametrize("tool_choice", [None, "NONE"])
+    def test_nonrequired_tool_choice_allows_empty_tools(self, tool_choice):
+        request = CohereChatV2Request(
+            model="m",
+            messages=[{"role": "user", "content": "hi"}],
+            tool_choice=tool_choice,
+            tools=[],
+        )
+        assert request.tool_choice == tool_choice
+
     def test_stop_sequences_bounded_by_env_limit(self):
         messages = [{"role": "user", "content": "hi"}]
         limit = envs.VLLM_MAX_STOP_STRINGS

vllm/entrypoints/cohere/protocol.py

@@ -44,7 +44,7 @@ from cohere.types import (
     ToolV2,
     UserChatMessageV2,
 )
-from pydantic import BaseModel, Field, field_validator
+from pydantic import BaseModel, Field, field_validator, model_validator
 
 import vllm.envs as envs
 from vllm.exceptions import VLLMValidationError
@@ -211,6 +211,16 @@ class CohereChatV2Request(BaseModel):
             )
         return v
 
+    @model_validator(mode="after")
+    def _validate_tool_choice(self) -> CohereChatV2Request:
+        if self.tool_choice == "REQUIRED" and not self.tools:
+            raise VLLMValidationError(
+                'tool_choice="REQUIRED" requires at least one tool in tools.',
+                parameter="tool_choice",
+                value=self.tool_choice,
+            )
+        return self
+
     @field_validator("messages", mode="before")
     @classmethod
     def _normalize_message_roles(cls, v: Any) -> Any: