diff --git a/python/sglang/srt/layers/quantization/quark/quark.py b/python/sglang/srt/layers/quantization/quark/quark.py
index 57dc7f162..aaa26214c 100644
--- a/python/sglang/srt/layers/quantization/quark/quark.py
+++ b/python/sglang/srt/layers/quantization/quark/quark.py
@@ -295,10 +295,12 @@ class QuarkConfig(QuantizationConfig):
 
             supported = capability >= min_capability
             if error and not supported:
+                # Pass a single joined message; RuntimeError stringifies
+                # multiple positional args as a tuple repr.
                 raise RuntimeError(
-                    "Quantization scheme is not supported for ",
-                    f"the current GPU. Min capability: {min_capability}. ",
-                    f"Current capability: {capability}.",
+                    "Quantization scheme is not supported for "
+                    f"the current GPU. Min capability: {min_capability}. "
+                    f"Current capability: {capability}."
                 )
             return supported
         else:
diff --git a/test/registered/unit/layers/quantization/test_quark_config.py b/test/registered/unit/layers/quantization/test_quark_config.py
new file mode 100644
index 000000000..bd543e55e
--- /dev/null
+++ b/test/registered/unit/layers/quantization/test_quark_config.py
@@ -0,0 +1,87 @@
+"""Unit tests for QuarkConfig — CPU-only, no model loading."""
+
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+import unittest
+from unittest.mock import patch
+
+from sglang.srt.layers.quantization.quark.quark import QuarkConfig
+from sglang.test.test_utils import CustomTestCase
+
+_GET_CAP = "sglang.srt.layers.quantization.quark.quark.get_device_capability"
+
+
+def _bare_config() -> QuarkConfig:
+    """Skip __init__ — _check_scheme_supported reads no instance attributes."""
+    return QuarkConfig.__new__(QuarkConfig)
+
+
+class TestCheckSchemeSupportedError(CustomTestCase):
+    """Regression for `RuntimeError("a", "b", "c")` being passed three args.
+
+    Bug: `_check_scheme_supported` raised `RuntimeError` with three positional
+    string fragments. `RuntimeError.__str__` formats `self.args` as a tuple
+    when `len(args) != 1`, so the user saw
+        ('Quantization scheme is not supported for ', 'the current GPU…', 'Current capability: 70.')
+    instead of a sentence. Fix: pass one already-joined message.
+    """
+
+    def test_error_is_single_argument(self):
+        # The structural assertion that catches the bug regardless of wording.
+        with patch(_GET_CAP, return_value=(7, 0)):  # capability = 70 < 200
+            with self.assertRaises(RuntimeError) as ctx:
+                _bare_config()._check_scheme_supported(min_capability=200)
+        err = ctx.exception
+        self.assertEqual(
+            len(err.args),
+            1,
+            f"RuntimeError must carry a single joined message, got {err.args!r}",
+        )
+
+    def test_error_message_renders_as_sentence(self):
+        with patch(_GET_CAP, return_value=(7, 0)):
+            with self.assertRaises(RuntimeError) as ctx:
+                _bare_config()._check_scheme_supported(min_capability=200)
+        msg = str(ctx.exception)
+        # Tuple-repr leakage shows up as a leading '(' and quote-comma joins.
+        self.assertFalse(
+            msg.startswith("("),
+            f"error message starts with '(' (tuple repr leaked): {msg!r}",
+        )
+        self.assertNotIn(
+            "', '",
+            msg,
+            f"error message contains tuple-style fragment join: {msg!r}",
+        )
+
+    def test_error_message_content(self):
+        with patch(_GET_CAP, return_value=(7, 0)):
+            with self.assertRaises(RuntimeError) as ctx:
+                _bare_config()._check_scheme_supported(min_capability=200)
+        msg = str(ctx.exception)
+        self.assertIn("Quantization scheme is not supported", msg)
+        self.assertIn("Min capability: 200", msg)
+        self.assertIn("Current capability: 70", msg)
+
+    # ---- Guardrails: unchanged code paths ---------------------------------
+
+    def test_unsupported_returns_false_when_error_disabled(self):
+        with patch(_GET_CAP, return_value=(7, 0)):
+            ok = _bare_config()._check_scheme_supported(min_capability=200, error=False)
+        self.assertFalse(ok)
+
+    def test_supported_returns_true(self):
+        with patch(_GET_CAP, return_value=(8, 0)):  # capability = 80 >= 70
+            ok = _bare_config()._check_scheme_supported(min_capability=70)
+        self.assertTrue(ok)
+
+    def test_no_device_returns_false(self):
+        with patch(_GET_CAP, return_value=None):
+            ok = _bare_config()._check_scheme_supported(min_capability=70)
+        self.assertFalse(ok)
+
+
+if __name__ == "__main__":
+    unittest.main()