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>

vllm/utils/argparse_utils.py

@@ -184,13 +184,11 @@ class FlexibleArgumentParser(ArgumentParser):
         if args is None:
             args = sys.argv[1:]
 
-        # Check for --model in command line arguments first
         if args and args[0] == "serve":
+            # Check for --model in command line arguments first
             try:
                 model_idx = next(
-                    i
-                    for i, arg in enumerate(args)
-                    if arg == "--model" or arg.startswith("--model=")
+                    i for i, arg in enumerate(args) if re.match(r"^--model(=.+|$)", arg)
                 )
                 logger.warning(
                     "With `vllm serve`, you should provide the model as a "
@@ -219,6 +217,19 @@ class FlexibleArgumentParser(ArgumentParser):
                 ]
             except StopIteration:
                 pass
+            # Check for --served-model-name without a positional model argument
+            if (
+                len(args) > 1
+                and args[1].startswith("-")
+                and not any(re.match(r"^--config(=.+|$)", arg) for arg in args)
+                and any(
+                    re.match(r"^--served[-_]model[-_]name(=.+|$)", arg) for arg in args
+                )
+            ):
+                raise ValueError(
+                    "`model` should be provided as the first positional argument when "
+                    "using `vllm serve`. i.e. `vllm serve <model> --<arg> <value>`."
+                )
 
         if "--config" in args:
             args = self._pull_args_from_config(args)