[Bugfix] Fix --config arg expansion called from api_server.py (#23944)

Signed-off-by: Jean-Francois Dube <dubejf+gh@gmail.com>
Co-authored-by: Jean-Francois Dube <dubejf+gh@gmail.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

tests/entrypoints/openai/test_cli_args.py

@@ -27,6 +27,28 @@ def serve_parser():
     return make_arg_parser(parser)
 
 
+### Test config parsing
+def test_config_arg_parsing(serve_parser, cli_config_file):
+    args = serve_parser.parse_args([])
+    assert args.port == 8000
+    args = serve_parser.parse_args(['--config', cli_config_file])
+    assert args.port == 12312
+    args = serve_parser.parse_args([
+        '--config',
+        cli_config_file,
+        '--port',
+        '9000',
+    ])
+    assert args.port == 9000
+    args = serve_parser.parse_args([
+        '--port',
+        '9000',
+        '--config',
+        cli_config_file,
+    ])
+    assert args.port == 9000
+
+
 ### Tests for LoRA module parsing
 def test_valid_key_value_format(serve_parser):
     # Test old format: name=path

vllm/utils/__init__.py

@@ -1976,13 +1976,16 @@ class FlexibleArgumentParser(ArgumentParser):
 
         config_args = self.load_config_file(file_path)
 
-        # 0th index is for {serve,chat,complete}
+        # 0th index might be the sub command {serve,chat,complete,...}
         # 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 args[0].startswith('-'):
+            # No sub command (e.g., api_server entry point)
+            args = config_args + args[0:index] + args[index + 2:]
+        elif args[0] == "serve":
             model_in_cli = len(args) > 1 and not args[1].startswith('-')
             model_in_config = any(arg == '--model' for arg in config_args)