diff --git a/tests/test_utils.py b/tests/test_utils.py
index 0b674ea6a85c1..8203b5d2f960d 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -7,7 +7,8 @@
 
 import pytest
 
-from vllm.utils import deprecate_kwargs, get_open_port, merge_async_iterators
+from vllm.utils import (FlexibleArgumentParser, deprecate_kwargs,
+                        get_open_port, merge_async_iterators)
 
 from .utils import error_on_warning
 
@@ -130,3 +131,61 @@ def test_get_open_port():
             with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s3:
                 s3.bind(("localhost", get_open_port()))
     os.environ.pop("VLLM_PORT")
+
+
+# Tests for FlexibleArgumentParser
+@pytest.fixture
+def parser():
+    parser = FlexibleArgumentParser()
+    parser.add_argument('--image-input-type',
+                        choices=['pixel_values', 'image_features'])
+    parser.add_argument('--model-name')
+    parser.add_argument('--batch-size', type=int)
+    parser.add_argument('--enable-feature', action='store_true')
+    return parser
+
+
+def test_underscore_to_dash(parser):
+    args = parser.parse_args(['--image_input_type', 'pixel_values'])
+    assert args.image_input_type == 'pixel_values'
+
+
+def test_mixed_usage(parser):
+    args = parser.parse_args([
+        '--image_input_type', 'image_features', '--model-name',
+        'facebook/opt-125m'
+    ])
+    assert args.image_input_type == 'image_features'
+    assert args.model_name == 'facebook/opt-125m'
+
+
+def test_with_equals_sign(parser):
+    args = parser.parse_args(
+        ['--image_input_type=pixel_values', '--model-name=facebook/opt-125m'])
+    assert args.image_input_type == 'pixel_values'
+    assert args.model_name == 'facebook/opt-125m'
+
+
+def test_with_int_value(parser):
+    args = parser.parse_args(['--batch_size', '32'])
+    assert args.batch_size == 32
+    args = parser.parse_args(['--batch-size', '32'])
+    assert args.batch_size == 32
+
+
+def test_with_bool_flag(parser):
+    args = parser.parse_args(['--enable_feature'])
+    assert args.enable_feature is True
+    args = parser.parse_args(['--enable-feature'])
+    assert args.enable_feature is True
+
+
+def test_invalid_choice(parser):
+    with pytest.raises(SystemExit):
+        parser.parse_args(['--image_input_type', 'invalid_choice'])
+
+
+def test_missing_required_argument(parser):
+    parser.add_argument('--required-arg', required=True)
+    with pytest.raises(SystemExit):
+        parser.parse_args([])