Enforce that model is the first positional arg when --served-model-name is used (#34973)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/entrypoints/openai/test_cli_args.py

@@ -20,10 +20,22 @@ CHATML_JINJA_PATH = VLLM_PATH / "examples/template_chatml.jinja"
 assert CHATML_JINJA_PATH.exists()
 
 
+def _build_vllm_parsers():
+    vllm_parser = FlexibleArgumentParser()
+    subparsers = vllm_parser.add_subparsers()
+    serve_parser = subparsers.add_parser("serve")
+    make_arg_parser(serve_parser)
+    return {"vllm": vllm_parser, "vllm serve": serve_parser}
+
+
+@pytest.fixture
+def vllm_parser():
+    return _build_vllm_parsers()["vllm"]
+
+
 @pytest.fixture
 def serve_parser():
-    parser = FlexibleArgumentParser(description="vLLM's remote OpenAI server.")
-    return make_arg_parser(parser)
+    return _build_vllm_parsers()["vllm serve"]
 
 
 ### Test config parsing
@@ -241,3 +253,41 @@ def test_default_chat_template_kwargs_invalid_json(serve_parser):
         serve_parser.parse_args(
             args=["--default-chat-template-kwargs", "not valid json"]
         )
+
+
+@pytest.mark.parametrize(
+    "args, raises",
+    [
+        (["user/model"], None),
+        (["user/model", "--served-model-name", "model"], None),
+        (["--served-model-name", "model", "user/model"], ValueError),
+        (["--served-model-name", "model", "--config", "config.yaml"], None),
+        (["--served-model-name", "model", "--config", "config.yaml"], ValueError),
+    ],
+    ids=[
+        "model_tag_only",
+        "model_tag_with_served_model_name",
+        "served_model_name_before_model_tag",
+        "served_model_name_with_model_in_config",
+        "served_model_name_with_no_model_in_config",
+    ],
+)
+def test_served_model_name_parsing(tmp_path, vllm_parser, args, raises):
+    """Ensure that users don't misuse --served-model-name and end up with the default
+    model tag instead of the one they intended to serve."""
+    # Call the serve subparser
+    args.insert(0, "serve")
+    # Create a dummy config file if the test case includes it
+    if "config.yaml" in args:
+        # Create a dummy config file if the test case includes it
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text("model: user/model" if raises is None else "port: 8000")
+        args[args.index("config.yaml")] = config_path.as_posix()
+    # Do the parsing and check for expected exceptions or values
+    if raises is None:
+        parsed_args = vllm_parser.parse_args(args=args)
+        expected = "user/model"
+        assert parsed_args.model_tag == expected or parsed_args.model == expected
+    else:
+        with pytest.raises(raises):
+            vllm_parser.parse_args(args=args)