[Feature] specify model in config.yaml (#15798)

Signed-off-by: weizeng <weizeng@roblox.com>

vllm/utils.py

@@ -1241,6 +1241,16 @@ class FlexibleArgumentParser(argparse.ArgumentParser):
         if args is None:
             args = sys.argv[1:]
 
+        # Check for --model in command line arguments first
+        if args and args[0] == "serve":
+            model_in_cli_args = any(arg == '--model' for arg in args)
+
+            if model_in_cli_args:
+                raise ValueError(
+                    "With `vllm serve`, you should provide the model as a "
+                    "positional argument or in a config file instead of via "
+                    "the `--model` option.")
+
         if '--config' in args:
             args = self._pull_args_from_config(args)
 
@@ -1324,19 +1334,29 @@ class FlexibleArgumentParser(argparse.ArgumentParser):
         config_args = self._load_config_file(file_path)
 
         # 0th index is for {serve,chat,complete}
-        # followed by model_tag (only for serve)
+        # optionally followed by model_tag (only for serve)
         # followed by config args
         # followed by rest of cli args.
         # maintaining this order will enforce the precedence
         # of cli > config > defaults
         if args[0] == "serve":
-            if index == 1:
+            model_in_cli = len(args) > 1 and not args[1].startswith('-')
+            model_in_config = any(arg == '--model' for arg in config_args)
+
+            if not model_in_cli and not model_in_config:
                 raise ValueError(
-                    "No model_tag specified! Please check your command-line"
-                    " arguments.")
-            args = [args[0]] + [
-                args[1]
-            ] + config_args + args[2:index] + args[index + 2:]
+                    "No model specified! Please specify model either "
+                    "as a positional argument or in a config file.")
+
+            if model_in_cli:
+                # Model specified as positional arg, keep CLI version
+                args = [args[0]] + [
+                    args[1]
+                ] + config_args + args[2:index] + args[index + 2:]
+            else:
+                # No model in CLI, use config if available
+                args = [args[0]
+                        ] + config_args + args[1:index] + args[index + 2:]
         else:
             args = [args[0]] + config_args + args[1:index] + args[index + 2:]
 
@@ -1354,9 +1374,7 @@ class FlexibleArgumentParser(argparse.ArgumentParser):
                 '--port': '12323',
                 '--tensor-parallel-size': '4'
             ]
-
         """
-
         extension: str = file_path.split('.')[-1]
         if extension not in ('yaml', 'yml'):
             raise ValueError(