[Bugfix] Reject empty tools array with HTTP 400 (#39780)

Signed-off-by: Jigang Zhou <zjg0907008@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

tests/tool_parsers/test_ernie45_moe_tool_parser.py

@@ -328,7 +328,7 @@ def test_extract_tool_calls_streaming_incremental(
     expected_content,
 ):
     """Verify the Ernie45 Parser streaming behavior by verifying each chunk is as expected."""  # noqa: E501
-    request = ChatCompletionRequest(model=MODEL, messages=[], tools=[])
+    request = ChatCompletionRequest(model=MODEL, messages=[])
 
     tool_calls_dict = {}
     for delta_message in stream_delta_message_generator(

tests/tool_parsers/test_xlam_tool_parser.py

@@ -484,7 +484,7 @@ def test_extract_tool_calls_streaming_incremental(
     expected_content,
 ):
     """Verify the XLAM Parser streaming behavior by verifying each chunk is as expected."""  # noqa: E501
-    request = ChatCompletionRequest(model=MODEL, messages=[], tools=[])
+    request = ChatCompletionRequest(model=MODEL, messages=[])
 
     chunks = []
     for delta_message in stream_delta_message_generator(

tests/tool_use/test_chat_completion_request_validations.py

@@ -26,15 +26,15 @@ def test_chat_completion_request_with_no_tools():
     )
     assert request.tool_choice == "none"
 
-    # tools key present but empty
-    request = ChatCompletionRequest.model_validate(
-        {
-            "messages": [{"role": "user", "content": "Hello"}],
-            "model": "facebook/opt-125m",
-            "tools": [],
-        }
-    )
-    assert request.tool_choice == "none"
+    # tools key present but empty -- should be rejected
+    with pytest.raises(ValueError, match="must not be an empty array"):
+        ChatCompletionRequest.model_validate(
+            {
+                "messages": [{"role": "user", "content": "Hello"}],
+                "model": "facebook/opt-125m",
+                "tools": [],
+            }
+        )
 
 
 @pytest.mark.parametrize("tool_choice", ["auto", "required"])

vllm/entrypoints/openai/chat_completion/protocol.py

@@ -681,6 +681,18 @@ class ChatCompletionRequest(OpenAIBaseModel):
     @model_validator(mode="before")
     @classmethod
     def check_tool_usage(cls, data):
+        if isinstance(data, ValueError):
+            raise data
+        if not isinstance(data, dict):
+            return data
+
+        # Reject empty tools array, matching OpenAI API behavior
+        if data.get("tools") == []:
+            raise ValueError(
+                "`tools` must not be an empty array. "
+                "Either provide at least one tool or omit the field entirely."
+            )
+
         # if "tool_choice" is not specified but tools are provided,
         # default to "auto" tool_choice
         if "tool_choice" not in data and data.get("tools"):
@@ -707,18 +719,6 @@ class ChatCompletionRequest(OpenAIBaseModel):
                     "are supported."
                 )
 
-            # if tool_choice is "required" but the "tools" list is empty,
-            # override the data to behave like "none" to align with
-            # OpenAI’s behavior.
-            if (
-                data["tool_choice"] == "required"
-                and isinstance(data["tools"], list)
-                and len(data["tools"]) == 0
-            ):
-                data["tool_choice"] = "none"
-                del data["tools"]
-                return data
-
             # ensure that if "tool_choice" is specified as an object,
             # it matches a valid tool
             correct_usage_message = (