[Bugfix][Frontend] Fix openai CLI arg `middleware` (#21220)

Signed-off-by: 22quinn <33176974+22quinn@users.noreply.github.com>

tests/entrypoints/openai/test_cli_args.py

@@ -153,3 +153,13 @@ def test_chat_template_validation_for_sad_paths(serve_parser):
     args = serve_parser.parse_args(args=["--chat-template", "does/not/exist"])
     with pytest.raises(ValueError):
         validate_parsed_serve_args(args)
+
+
+@pytest.mark.parametrize(
+    "cli_args, expected_middleware",
+    [(["--middleware", "middleware1", "--middleware", "middleware2"
+       ], ["middleware1", "middleware2"]), ([], [])])
+def test_middleware(serve_parser, cli_args, expected_middleware):
+    """Ensure multiple middleware args are parsed properly"""
+    args = serve_parser.parse_args(args=cli_args)
+    assert args.middleware == expected_middleware

vllm/entrypoints/openai/cli_args.py

@@ -215,6 +215,10 @@ schema. Example: `[{"type": "text", "text": "Hello world!"}]`"""
 
         # Special case: Middleware needs append action
         frontend_kwargs["middleware"]["action"] = "append"
+        frontend_kwargs["middleware"]["type"] = str
+        if "nargs" in frontend_kwargs["middleware"]:
+            del frontend_kwargs["middleware"]["nargs"]
+        frontend_kwargs["middleware"]["default"] = []
 
         # Special case: Tool call parser shows built-in options.
         valid_tool_parsers = list(ToolParserManager.tool_parsers.keys())