diff --git a/tests/engine/test_arg_utils.py b/tests/engine/test_arg_utils.py
index 7902011519d9..052d5793c1b3 100644
--- a/tests/engine/test_arg_utils.py
+++ b/tests/engine/test_arg_utils.py
@@ -1,14 +1,118 @@
 # SPDX-License-Identifier: Apache-2.0
 
+import json
 from argparse import ArgumentError, ArgumentTypeError
+from contextlib import nullcontext
+from dataclasses import dataclass, field
+from typing import Literal, Optional
 
 import pytest
 
-from vllm.config import PoolerConfig
-from vllm.engine.arg_utils import EngineArgs, nullable_kvs
+from vllm.config import PoolerConfig, config
+from vllm.engine.arg_utils import (EngineArgs, contains_type, get_kwargs,
+                                   get_type, is_not_builtin, is_type,
+                                   nullable_kvs, optional_type)
 from vllm.utils import FlexibleArgumentParser
 
 
+@pytest.mark.parametrize(("type", "value", "expected"), [
+    (int, "42", 42),
+    (int, "None", None),
+    (float, "3.14", 3.14),
+    (float, "None", None),
+    (str, "Hello World!", "Hello World!"),
+    (str, "None", None),
+    (json.loads, '{"foo":1,"bar":2}', {
+        "foo": 1,
+        "bar": 2
+    }),
+    (json.loads, "foo=1,bar=2", {
+        "foo": 1,
+        "bar": 2
+    }),
+    (json.loads, "None", None),
+])
+def test_optional_type(type, value, expected):
+    optional_type_func = optional_type(type)
+    context = nullcontext()
+    if value == "foo=1,bar=2":
+        context = pytest.warns(DeprecationWarning)
+    with context:
+        assert optional_type_func(value) == expected
+
+
+@pytest.mark.parametrize(("type_hint", "type", "expected"), [
+    (int, int, True),
+    (int, float, False),
+    (list[int], list, True),
+    (list[int], tuple, False),
+    (Literal[0, 1], Literal, True),
+])
+def test_is_type(type_hint, type, expected):
+    assert is_type(type_hint, type) == expected
+
+
+@pytest.mark.parametrize(("type_hints", "type", "expected"), [
+    ({float, int}, int, True),
+    ({int, tuple[int]}, int, True),
+    ({int, tuple[int]}, float, False),
+    ({str, Literal["x", "y"]}, Literal, True),
+])
+def test_contains_type(type_hints, type, expected):
+    assert contains_type(type_hints, type) == expected
+
+
+@pytest.mark.parametrize(("type_hints", "type", "expected"), [
+    ({int, float}, int, int),
+    ({int, float}, str, None),
+    ({str, Literal["x", "y"]}, Literal, Literal["x", "y"]),
+])
+def test_get_type(type_hints, type, expected):
+    assert get_type(type_hints, type) == expected
+
+
+@config
+@dataclass
+class DummyConfigClass:
+    regular_bool: bool = True
+    """Regular bool with default True"""
+    optional_bool: Optional[bool] = None
+    """Optional bool with default None"""
+    optional_literal: Optional[Literal["x", "y"]] = None
+    """Optional literal with default None"""
+    tuple_n: tuple[int, ...] = field(default_factory=lambda: (1, 2, 3))
+    """Tuple with default (1, 2, 3)"""
+    tuple_2: tuple[int, int] = field(default_factory=lambda: (1, 2))
+    """Tuple with default (1, 2)"""
+    list_n: list[int] = field(default_factory=lambda: [1, 2, 3])
+    """List with default [1, 2, 3]"""
+
+
+@pytest.mark.parametrize(("type_hint", "expected"), [
+    (int, False),
+    (DummyConfigClass, True),
+])
+def test_is_not_builtin(type_hint, expected):
+    assert is_not_builtin(type_hint) == expected
+
+
+def test_get_kwargs():
+    kwargs = get_kwargs(DummyConfigClass)
+    print(kwargs)
+
+    # bools should not have their type set
+    assert kwargs["regular_bool"].get("type") is None
+    assert kwargs["optional_bool"].get("type") is None
+    # optional literals should have None as a choice
+    assert kwargs["optional_literal"]["choices"] == ["x", "y", "None"]
+    # tuples should have the correct nargs
+    assert kwargs["tuple_n"]["nargs"] == "+"
+    assert kwargs["tuple_2"]["nargs"] == 2
+    # lists should work
+    assert kwargs["list_n"]["type"] is int
+    assert kwargs["list_n"]["nargs"] == "+"
+
+
 @pytest.mark.parametrize(("arg", "expected"), [
     (None, dict()),
     ("image=16", {
diff --git a/vllm/engine/arg_utils.py b/vllm/engine/arg_utils.py
index 1c966469db00..5d735103fc03 100644
--- a/vllm/engine/arg_utils.py
+++ b/vllm/engine/arg_utils.py
@@ -11,7 +11,7 @@
                     TypeVar, Union, cast, get_args, get_origin)
 
 import torch
-from typing_extensions import TypeIs
+from typing_extensions import TypeIs, deprecated
 
 import vllm.envs as envs
 from vllm import version
@@ -48,33 +48,29 @@
 TypeHintT = Union[type[T], object]
 
 
-def optional_arg(val: str, return_type: Callable[[str], T]) -> Optional[T]:
-    if val == "" or val == "None":
-        return None
-    try:
-        return return_type(val)
-    except ValueError as e:
-        raise argparse.ArgumentTypeError(
-            f"Value {val} cannot be converted to {return_type}.") from e
-
-
-def optional_str(val: str) -> Optional[str]:
-    return optional_arg(val, str)
-
-
-def optional_int(val: str) -> Optional[int]:
-    return optional_arg(val, int)
+def optional_type(
+        return_type: Callable[[str], T]) -> Callable[[str], Optional[T]]:
 
+    def _optional_type(val: str) -> Optional[T]:
+        if val == "" or val == "None":
+            return None
+        try:
+            if return_type is json.loads and not re.match("^{.*}$", val):
+                return cast(T, nullable_kvs(val))
+            return return_type(val)
+        except ValueError as e:
+            raise argparse.ArgumentTypeError(
+                f"Value {val} cannot be converted to {return_type}.") from e
 
-def optional_float(val: str) -> Optional[float]:
-    return optional_arg(val, float)
+    return _optional_type
 
 
-def nullable_kvs(val: str) -> Optional[dict[str, int]]:
-    """NOTE: This function is deprecated, args should be passed as JSON
-    strings instead.
-    
-    Parses a string containing comma separate key [str] to value [int]
+@deprecated(
+    "Passing a JSON argument as a string containing comma separated key=value "
+    "pairs is deprecated. This will be removed in v0.10.0. Please use a JSON "
+    "string instead.")
+def nullable_kvs(val: str) -> dict[str, int]:
+    """Parses a string containing comma separate key [str] to value [int]
     pairs into a dictionary.
 
     Args:
@@ -83,10 +79,7 @@ def nullable_kvs(val: str) -> Optional[dict[str, int]]:
     Returns:
         Dictionary with parsed values.
     """
-    if len(val) == 0:
-        return None
-
-    out_dict: Dict[str, int] = {}
+    out_dict: dict[str, int] = {}
     for item in val.split(","):
         kv_parts = [part.lower().strip() for part in item.split("=")]
         if len(kv_parts) != 2:
@@ -108,15 +101,103 @@ def nullable_kvs(val: str) -> Optional[dict[str, int]]:
     return out_dict
 
 
-def optional_dict(val: str) -> Optional[dict[str, int]]:
-    if re.match("^{.*}$", val):
-        return optional_arg(val, json.loads)
+def is_type(type_hint: TypeHint, type: TypeHintT) -> TypeIs[TypeHintT]:
+    """Check if the type hint is a specific type."""
+    return type_hint is type or get_origin(type_hint) is type
+
+
+def contains_type(type_hints: set[TypeHint], type: TypeHintT) -> bool:
+    """Check if the type hints contain a specific type."""
+    return any(is_type(type_hint, type) for type_hint in type_hints)
+
 
-    logger.warning(
-        "Failed to parse JSON string. Attempting to parse as "
-        "comma-separated key=value pairs. This will be deprecated in a "
-        "future release.")
-    return nullable_kvs(val)
+def get_type(type_hints: set[TypeHint], type: TypeHintT) -> TypeHintT:
+    """Get the specific type from the type hints."""
+    return next((th for th in type_hints if is_type(th, type)), None)
+
+
+def is_not_builtin(type_hint: TypeHint) -> bool:
+    """Check if the class is not a built-in type."""
+    return type_hint.__module__ != "builtins"
+
+
+def get_kwargs(cls: ConfigType) -> dict[str, Any]:
+    cls_docs = get_attr_docs(cls)
+    kwargs = {}
+    for field in fields(cls):
+        # Get the default value of the field
+        default = field.default
+        if field.default_factory is not MISSING:
+            default = field.default_factory()
+
+        # Get the help text for the field
+        name = field.name
+        help = cls_docs[name]
+        # Escape % for argparse
+        help = help.replace("%", "%%")
+
+        # Initialise the kwargs dictionary for the field
+        kwargs[name] = {"default": default, "help": help}
+
+        # Get the set of possible types for the field
+        type_hints: set[TypeHint] = set()
+        if get_origin(field.type) is Union:
+            type_hints.update(get_args(field.type))
+        else:
+            type_hints.add(field.type)
+
+        # Set other kwargs based on the type hints
+        if contains_type(type_hints, bool):
+            # Creates --no-<name> and --<name> flags
+            kwargs[name]["action"] = argparse.BooleanOptionalAction
+        elif contains_type(type_hints, Literal):
+            # Creates choices from Literal arguments
+            type_hint = get_type(type_hints, Literal)
+            choices = sorted(get_args(type_hint))
+            kwargs[name]["choices"] = choices
+            choice_type = type(choices[0])
+            assert all(type(c) is choice_type for c in choices), (
+                "All choices must be of the same type. "
+                f"Got {choices} with types {[type(c) for c in choices]}")
+            kwargs[name]["type"] = choice_type
+        elif contains_type(type_hints, tuple):
+            type_hint = get_type(type_hints, tuple)
+            types = get_args(type_hint)
+            tuple_type = types[0]
+            assert all(t is tuple_type for t in types if t is not Ellipsis), (
+                "All non-Ellipsis tuple elements must be of the same "
+                f"type. Got {types}.")
+            kwargs[name]["type"] = tuple_type
+            kwargs[name]["nargs"] = "+" if Ellipsis in types else len(types)
+        elif contains_type(type_hints, list):
+            type_hint = get_type(type_hints, list)
+            types = get_args(type_hint)
+            assert len(types) == 1, (
+                "List type must have exactly one type. Got "
+                f"{type_hint} with types {types}")
+            kwargs[name]["type"] = types[0]
+            kwargs[name]["nargs"] = "+"
+        elif contains_type(type_hints, int):
+            kwargs[name]["type"] = int
+        elif contains_type(type_hints, float):
+            kwargs[name]["type"] = float
+        elif contains_type(type_hints, dict):
+            # Dict arguments will always be optional
+            kwargs[name]["type"] = optional_type(json.loads)
+        elif (contains_type(type_hints, str)
+              or any(is_not_builtin(th) for th in type_hints)):
+            kwargs[name]["type"] = str
+        else:
+            raise ValueError(
+                f"Unsupported type {type_hints} for argument {name}.")
+
+        # If None is in type_hints, make the argument optional.
+        # But not if it's a bool, argparse will handle this better.
+        if type(None) in type_hints and not contains_type(type_hints, bool):
+            kwargs[name]["type"] = optional_type(kwargs[name]["type"])
+            if kwargs[name].get("choices"):
+                kwargs[name]["choices"].append("None")
+    return kwargs
 
 
 @dataclass
@@ -279,100 +360,6 @@ def __post_init__(self):
     def add_cli_args(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
         """Shared CLI arguments for vLLM engine."""
 
-        def is_type_in_union(cls: TypeHint, type: TypeHint) -> bool:
-            """Check if the class is a type in a union type."""
-            is_union = get_origin(cls) is Union
-            type_in_union = type in [get_origin(a) or a for a in get_args(cls)]
-            return is_union and type_in_union
-
-        def get_type_from_union(cls: TypeHint, type: TypeHintT) -> TypeHintT:
-            """Get the type in a union type."""
-            for arg in get_args(cls):
-                if (get_origin(arg) or arg) is type:
-                    return arg
-            raise ValueError(f"Type {type} not found in union type {cls}.")
-
-        def is_optional(cls: TypeHint) -> TypeIs[Union[Any, None]]:
-            """Check if the class is an optional type."""
-            return is_type_in_union(cls, type(None))
-
-        def can_be_type(cls: TypeHint, type: TypeHintT) -> TypeIs[TypeHintT]:
-            """Check if the class can be of type."""
-            return cls is type or get_origin(cls) is type or is_type_in_union(
-                cls, type)
-
-        def is_custom_type(cls: TypeHint) -> bool:
-            """Check if the class is a custom type."""
-            return cls.__module__ != "builtins"
-
-        def get_kwargs(cls: ConfigType) -> dict[str, Any]:
-            cls_docs = get_attr_docs(cls)
-            kwargs = {}
-            for field in fields(cls):
-                # Get the default value of the field
-                default = field.default
-                if field.default_factory is not MISSING:
-                    default = field.default_factory()
-
-                # Get the help text for the field
-                name = field.name
-                help = cls_docs[name]
-                # Escape % for argparse
-                help = help.replace("%", "%%")
-
-                # Initialise the kwargs dictionary for the field
-                kwargs[name] = {"default": default, "help": help}
-
-                # Make note of if the field is optional and get the actual
-                # type of the field if it is
-                optional = is_optional(field.type)
-                field_type = get_args(
-                    field.type)[0] if optional else field.type
-
-                # Set type, action and choices for the field depending on the
-                # type of the field
-                if can_be_type(field_type, bool):
-                    # Creates --no-<name> and --<name> flags
-                    kwargs[name]["action"] = argparse.BooleanOptionalAction
-                    kwargs[name]["type"] = bool
-                elif can_be_type(field_type, Literal):
-                    # Creates choices from Literal arguments
-                    if is_type_in_union(field_type, Literal):
-                        field_type = get_type_from_union(field_type, Literal)
-                    choices = get_args(field_type)
-                    kwargs[name]["choices"] = choices
-                    choice_type = type(choices[0])
-                    assert all(type(c) is choice_type for c in choices), (
-                        "All choices must be of the same type. "
-                        f"Got {choices} with types {[type(c) for c in choices]}"
-                    )
-                    kwargs[name]["type"] = choice_type
-                elif can_be_type(field_type, tuple):
-                    if is_type_in_union(field_type, tuple):
-                        field_type = get_type_from_union(field_type, tuple)
-                    dtypes = get_args(field_type)
-                    dtype = dtypes[0]
-                    assert all(
-                        d is dtype for d in dtypes if d is not Ellipsis
-                    ), ("All non-Ellipsis tuple elements must be of the same "
-                        f"type. Got {dtypes}.")
-                    kwargs[name]["type"] = dtype
-                    kwargs[name]["nargs"] = "+"
-                elif can_be_type(field_type, int):
-                    kwargs[name]["type"] = optional_int if optional else int
-                elif can_be_type(field_type, float):
-                    kwargs[name][
-                        "type"] = optional_float if optional else float
-                elif can_be_type(field_type, dict):
-                    kwargs[name]["type"] = optional_dict
-                elif (can_be_type(field_type, str)
-                      or is_custom_type(field_type)):
-                    kwargs[name]["type"] = optional_str if optional else str
-                else:
-                    raise ValueError(
-                        f"Unsupported type {field.type} for argument {name}. ")
-            return kwargs
-
         # Model arguments
         parser.add_argument(
             '--model',
@@ -390,13 +377,13 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
             'which task to use.')
         parser.add_argument(
             '--tokenizer',
-            type=optional_str,
+            type=optional_type(str),
             default=EngineArgs.tokenizer,
             help='Name or path of the huggingface tokenizer to use. '
             'If unspecified, model name or path will be used.')
         parser.add_argument(
             "--hf-config-path",
-            type=optional_str,
+            type=optional_type(str),
             default=EngineArgs.hf_config_path,
             help='Name or path of the huggingface config to use. '
             'If unspecified, model name or path will be used.')
@@ -408,21 +395,21 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
             'the input. The generated output will contain token ids.')
         parser.add_argument(
             '--revision',
-            type=optional_str,
+            type=optional_type(str),
             default=None,
             help='The specific model version to use. It can be a branch '
             'name, a tag name, or a commit id. If unspecified, will use '
             'the default version.')
         parser.add_argument(
             '--code-revision',
-            type=optional_str,
+            type=optional_type(str),
             default=None,
             help='The specific revision to use for the model code on '
             'Hugging Face Hub. It can be a branch name, a tag name, or a '
             'commit id. If unspecified, will use the default version.')
         parser.add_argument(
             '--tokenizer-revision',
-            type=optional_str,
+            type=optional_type(str),
             default=None,
             help='Revision of the huggingface tokenizer to use. '
             'It can be a branch name, a tag name, or a commit id. '
@@ -513,7 +500,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
 
         parser.add_argument(
             '--logits-processor-pattern',
-            type=optional_str,
+            type=optional_type(str),
             default=None,
             help='Optional regex pattern specifying valid logits processor '
             'qualified names that can be passed with the `logits_processors` '
@@ -612,7 +599,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
         # Quantization settings.
         parser.add_argument('--quantization',
                             '-q',
-                            type=optional_str,
+                            type=optional_type(str),
                             choices=[*QUANTIZATION_METHODS, None],
                             default=EngineArgs.quantization,
                             help='Method used to quantize the weights. If '
@@ -921,7 +908,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
             'class without changing the existing functions.')
         parser.add_argument(
             "--generation-config",
-            type=optional_str,
+            type=optional_type(str),
             default="auto",
             help="The folder path to the generation config. "
             "Defaults to 'auto', the generation config will be loaded from "
diff --git a/vllm/entrypoints/openai/cli_args.py b/vllm/entrypoints/openai/cli_args.py
index af546c3032af..b3824013f055 100644
--- a/vllm/entrypoints/openai/cli_args.py
+++ b/vllm/entrypoints/openai/cli_args.py
@@ -11,7 +11,7 @@
 from collections.abc import Sequence
 from typing import Optional, Union, get_args
 
-from vllm.engine.arg_utils import AsyncEngineArgs, optional_str
+from vllm.engine.arg_utils import AsyncEngineArgs, optional_type
 from vllm.entrypoints.chat_utils import (ChatTemplateContentFormatOption,
                                          validate_chat_template)
 from vllm.entrypoints.openai.serving_models import (LoRAModulePath,
@@ -79,7 +79,7 @@ def __call__(
 
 def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
     parser.add_argument("--host",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="Host name.")
     parser.add_argument("--port", type=int, default=8000, help="Port number.")
@@ -108,13 +108,13 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
                         default=["*"],
                         help="Allowed headers.")
     parser.add_argument("--api-key",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="If provided, the server will require this key "
                         "to be presented in the header.")
     parser.add_argument(
         "--lora-modules",
-        type=optional_str,
+        type=optional_type(str),
         default=None,
         nargs='+',
         action=LoRAParserAction,
@@ -126,14 +126,14 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
         "\"base_model_name\": \"id\"}``")
     parser.add_argument(
         "--prompt-adapters",
-        type=optional_str,
+        type=optional_type(str),
         default=None,
         nargs='+',
         action=PromptAdapterParserAction,
         help="Prompt adapter configurations in the format name=path. "
         "Multiple adapters can be specified.")
     parser.add_argument("--chat-template",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="The file path to the chat template, "
                         "or the template in single-line form "
@@ -151,20 +151,20 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
         'similar to OpenAI schema. '
         'Example: ``[{"type": "text", "text": "Hello world!"}]``')
     parser.add_argument("--response-role",
-                        type=optional_str,
+                        type=optional_type(str),
                         default="assistant",
                         help="The role name to return if "
                         "``request.add_generation_prompt=true``.")
     parser.add_argument("--ssl-keyfile",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="The file path to the SSL key file.")
     parser.add_argument("--ssl-certfile",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="The file path to the SSL cert file.")
     parser.add_argument("--ssl-ca-certs",
-                        type=optional_str,
+                        type=optional_type(str),
                         default=None,
                         help="The CA certificates file.")
     parser.add_argument(
@@ -180,13 +180,13 @@ def make_arg_parser(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
     )
     parser.add_argument(
         "--root-path",
-        type=optional_str,
+        type=optional_type(str),
         default=None,
         help="FastAPI root_path when app is behind a path based routing proxy."
     )
     parser.add_argument(
         "--middleware",
-        type=optional_str,
+        type=optional_type(str),
         action="append",
         default=[],
         help="Additional ASGI middleware to apply to the app. "
diff --git a/vllm/entrypoints/openai/run_batch.py b/vllm/entrypoints/openai/run_batch.py
index 3ffa5a32c173..fccf459f17dc 100644
--- a/vllm/entrypoints/openai/run_batch.py
+++ b/vllm/entrypoints/openai/run_batch.py
@@ -12,7 +12,7 @@
 from prometheus_client import start_http_server
 from tqdm import tqdm
 
-from vllm.engine.arg_utils import AsyncEngineArgs, optional_str
+from vllm.engine.arg_utils import AsyncEngineArgs, optional_type
 from vllm.engine.async_llm_engine import AsyncLLMEngine
 from vllm.entrypoints.logger import RequestLogger, logger
 # yapf: disable
@@ -61,7 +61,7 @@ def parse_args():
         "to the output URL.",
     )
     parser.add_argument("--response-role",
-                        type=optional_str,
+                        type=optional_type(str),
                         default="assistant",
                         help="The role name to return if "
                         "`request.add_generation_prompt=True`.")