fix: Handle tools without parameters in OpenAI frontend tool_choice

python/openai/openai_frontend/engine/utils/triton.py

@@ -612,6 +612,12 @@ def _get_guided_json_from_tool(
         if tool_name not in tools:
             raise ClientError(f"Tool '{tool_name}' has not been passed in `tools`.")
         tool = tools[tool_name]
+        # A function without "parameters" takes no arguments, so constrain
+        # the call to an empty JSON object.
+        if tool.parameters is None:
+            return json.dumps(
+                {"type": "object", "properties": {}, "additionalProperties": False}
+            )
         return tool.parameters.model_dump_json()
 
     return None

python/openai/tests/test_tool_calling.py

@@ -741,3 +741,27 @@ async def test_abandoned_completion_stream_cancels_backend_inference(self, clien
 
         _, status = self._stream(client, "cancellation-status", with_tools=False)
         assert status == "cancelled"
+
+
+# OpenAI allows a function without "parameters": it takes no arguments.
+@pytest.mark.parametrize(
+    "tool_choice",
+    [{"type": "function", "function": {"name": "get_current_time"}}, "required"],
+)
+def test_guided_json_for_tool_without_parameters(tool_choice):
+    from engine.utils.triton import _get_guided_json_from_tool
+    from schemas.openai import CreateChatCompletionRequest
+
+    request = CreateChatCompletionRequest(
+        model="test-model",
+        messages=[{"role": "user", "content": "What time is it?"}],
+        tools=[{"type": "function", "function": {"name": "get_current_time"}}],
+        tool_choice=tool_choice,
+    )
+
+    guided_json = _get_guided_json_from_tool(request)
+    assert json.loads(guided_json) == {
+        "type": "object",
+        "properties": {},
+        "additionalProperties": False,
+    }