[Feature][Quant] Auto-detect quantization format from model files (#6645)

## Summary

- Add automatic quantization format detection, eliminating the need to
manually specify `--quantization` when serving quantized models.
- The detection inspects only lightweight JSON files
(`quant_model_description.json` and `config.json`) at engine
initialization time, with no `.safetensors` reads.
- User-explicit `--quantization` flags are always respected;
auto-detection only applies when the flag is omitted.

## Details

**Detection priority:**
1. `quant_model_description.json` exists → `quantization="ascend"`
(ModelSlim)
2. `config.json` contains `"quant_method": "compressed-tensors"` →
`quantization="compressed-tensors"` (LLM-Compressor)
3. Neither → default float behavior

**Technical approach:**
Hooked into `NPUPlatform.check_and_update_config()` to run detection
after `VllmConfig.__post_init__`. Since `quant_config` is already `None`
at that point, we explicitly recreate it via
`VllmConfig._get_quantization_config()` to trigger the full quantization
initialization pipeline.

## Files Changed

| File | Description |
|------|-------------|
| `vllm_ascend/quantization/utils.py` | Added
`detect_quantization_method()` and `maybe_auto_detect_quantization()` |
| `vllm_ascend/platform.py` | Integrated auto-detection in
`check_and_update_config()` |
| `vllm_ascend/quantization/modelslim_config.py` | Improved error
handling for weight loading |
- vLLM version: v0.15.0
- vLLM main:
d7e17aaacd

---------

Signed-off-by: SlightwindSec <slightwindsec@gmail.com>

tests/e2e/singlecard/test_quantization.py

@@ -49,6 +49,43 @@ def test_qwen3_w8a8_quant():
         name_1="vllm_quant_w8a8_outputs",
     )
 
+# fmt: off
+def test_qwen3_w8a8_quant_auto_detect():
+    """Test that ModelSlim quantization is auto-detected without --quantization.
+
+    Uses the same W8A8 model as test_qwen3_w8a8_quant but omits the
+    quantization parameter, verifying that the auto-detection in
+    maybe_auto_detect_quantization() picks up quant_model_description.json
+    and produces identical results.
+    """
+    max_tokens = 5
+    example_prompts = [
+        "vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs."
+    ]
+    vllm_target_outputs = [([
+        85, 4086, 44, 374, 264, 1550, 42747, 628, 323, 4938, 72816, 44378, 323,
+        13480, 4712, 369, 444, 10994, 82, 13, 1084, 374, 6188, 311, 387
+    ], 'vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs. It is designed to be'
+                            )]
+# fmt: on
+
+    with VllmRunner(
+            "vllm-ascend/Qwen3-0.6B-W8A8",
+            max_model_len=8192,
+            gpu_memory_utilization=0.7,
+            cudagraph_capture_sizes=[1, 2, 4, 8],
+    ) as vllm_model:
+        vllm_quant_auto_detect_outputs = vllm_model.generate_greedy(
+            example_prompts, max_tokens)
+
+    check_outputs_equal(
+        outputs_0_lst=vllm_target_outputs,
+        outputs_1_lst=vllm_quant_auto_detect_outputs,
+        name_0="vllm_target_outputs",
+        name_1="vllm_quant_auto_detect_outputs",
+    )
+
+
 # fmt: off
 def test_qwen3_dense_w8a16():
     max_tokens = 5

tests/ut/quantization/test_modelslim_config.py

@@ -1,3 +1,6 @@
+import json
+import os
+import tempfile
 from unittest.mock import MagicMock, patch
 
 from vllm.model_executor.layers.fused_moe import FusedMoE
@@ -6,7 +9,10 @@ from vllm.model_executor.layers.linear import LinearBase
 
 from tests.ut.base import TestBase
 from vllm_ascend.ops.linear import AscendUnquantizedLinearMethod
-from vllm_ascend.quantization.modelslim_config import AscendModelSlimConfig
+from vllm_ascend.quantization.modelslim_config import (
+    MODELSLIM_CONFIG_FILENAME,
+    AscendModelSlimConfig,
+)
 from vllm_ascend.utils import ASCEND_QUANTIZATION_METHOD, vllm_version_is
 
 if vllm_version_is("v0.15.0"):
@@ -54,7 +60,7 @@ class TestAscendModelSlimConfig(TestBase):
 
     def test_get_config_filenames(self):
         filenames = AscendModelSlimConfig.get_config_filenames()
-        self.assertEqual(filenames, ["quant_model_description.json"])
+        self.assertEqual(filenames, [])
 
     def test_from_config(self):
         config = AscendModelSlimConfig.from_config(self.sample_config)
@@ -162,5 +168,90 @@ class TestAscendModelSlimConfig(TestBase):
         with self.assertRaises(ValueError):
             config.is_layer_skipped_ascend("fused_layer", fused_mapping)
 
+    def test_init_with_none_config(self):
+        config = AscendModelSlimConfig(None)
+        self.assertEqual(config.quant_description, {})
+
+    def test_init_with_default_config(self):
+        config = AscendModelSlimConfig()
+        self.assertEqual(config.quant_description, {})
+
+    def test_maybe_update_config_already_populated(self):
+        # When quant_description is already populated, should be a no-op
+        self.assertTrue(len(self.ascend_config.quant_description) > 0)
+        self.ascend_config.maybe_update_config("/some/model/path")
+        # quant_description should remain unchanged
+        self.assertEqual(self.ascend_config.quant_description,
+                         self.sample_config)
+
+    def test_maybe_update_config_loads_from_file(self):
+        config = AscendModelSlimConfig()
+        self.assertEqual(config.quant_description, {})
+
+        quant_data = {"layer1.weight": "INT8", "layer2.weight": "FLOAT"}
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, MODELSLIM_CONFIG_FILENAME)
+            with open(config_path, "w") as f:
+                json.dump(quant_data, f)
+
+            config.maybe_update_config(tmpdir)
+
+        self.assertEqual(config.quant_description, quant_data)
+
+    def test_maybe_update_config_raises_when_file_missing(self):
+        config = AscendModelSlimConfig()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            with self.assertRaises(ValueError) as ctx:
+                config.maybe_update_config(tmpdir)
+
+            error_msg = str(ctx.exception)
+            self.assertIn("ModelSlim Quantization Config Not Found", error_msg)
+            self.assertIn(MODELSLIM_CONFIG_FILENAME, error_msg)
+
+    def test_maybe_update_config_raises_with_json_files_listed(self):
+        config = AscendModelSlimConfig()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            # Create a dummy json file that is NOT the config file
+            dummy_path = os.path.join(tmpdir, "config.json")
+            with open(dummy_path, "w") as f:
+                json.dump({"dummy": True}, f)
+
+            with self.assertRaises(ValueError) as ctx:
+                config.maybe_update_config(tmpdir)
+
+            error_msg = str(ctx.exception)
+            self.assertIn("config.json", error_msg)
+
+    def test_maybe_update_config_non_directory_raises(self):
+        config = AscendModelSlimConfig()
+
+        with self.assertRaises(ValueError) as ctx:
+            config.maybe_update_config("not_a_real_directory_path")
+
+        error_msg = str(ctx.exception)
+        self.assertIn("ModelSlim Quantization Config Not Found", error_msg)
+
+    def test_apply_extra_quant_adaptations_shared_head(self):
+        config = AscendModelSlimConfig()
+        config.quant_description = {
+            "model.layers.0.shared_head.weight": "INT8",
+        }
+        config._apply_extra_quant_adaptations()
+        self.assertIn("model.layers.0.weight", config.quant_description)
+        self.assertEqual(config.quant_description["model.layers.0.weight"],
+                         "INT8")
+
+    def test_apply_extra_quant_adaptations_weight_packed(self):
+        config = AscendModelSlimConfig()
+        config.quant_description = {
+            "model.layers.0.weight_packed": "INT8",
+        }
+        config._apply_extra_quant_adaptations()
+        self.assertIn("model.layers.0.weight", config.quant_description)
+        self.assertEqual(config.quant_description["model.layers.0.weight"],
+                         "INT8")
+
     def test_get_scaled_act_names(self):
         self.assertEqual(self.ascend_config.get_scaled_act_names(), [])

tests/ut/quantization/test_quant_utils.py

@@ -0,0 +1,182 @@
+import json
+import logging
+import os
+import tempfile
+from unittest.mock import MagicMock, patch
+
+from tests.ut.base import TestBase
+from vllm_ascend.quantization.modelslim_config import MODELSLIM_CONFIG_FILENAME
+from vllm_ascend.quantization.utils import (
+    detect_quantization_method,
+    maybe_auto_detect_quantization,
+)
+from vllm_ascend.utils import ASCEND_QUANTIZATION_METHOD, COMPRESSED_TENSORS_METHOD
+
+
+class TestDetectQuantizationMethod(TestBase):
+
+    def test_returns_none_for_non_directory(self):
+        result = detect_quantization_method("/non/existent/path")
+        self.assertIsNone(result)
+
+    def test_detects_modelslim(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, MODELSLIM_CONFIG_FILENAME)
+            with open(config_path, "w") as f:
+                json.dump({"layer.weight": "INT8"}, f)
+
+            result = detect_quantization_method(tmpdir)
+            self.assertEqual(result, ASCEND_QUANTIZATION_METHOD)
+
+    def test_detects_compressed_tensors(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, "config.json")
+            with open(config_path, "w") as f:
+                json.dump({
+                    "quantization_config": {
+                        "quant_method": "compressed-tensors"
+                    }
+                }, f)
+
+            result = detect_quantization_method(tmpdir)
+            self.assertEqual(result, COMPRESSED_TENSORS_METHOD)
+
+    def test_returns_none_for_no_quant(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            result = detect_quantization_method(tmpdir)
+            self.assertIsNone(result)
+
+    def test_returns_none_for_non_compressed_tensors_quant_method(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, "config.json")
+            with open(config_path, "w") as f:
+                json.dump({
+                    "quantization_config": {
+                        "quant_method": "gptq"
+                    }
+                }, f)
+
+            result = detect_quantization_method(tmpdir)
+            self.assertIsNone(result)
+
+    def test_returns_none_for_config_without_quant_config(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, "config.json")
+            with open(config_path, "w") as f:
+                json.dump({"model_type": "llama"}, f)
+
+            result = detect_quantization_method(tmpdir)
+            self.assertIsNone(result)
+
+    def test_returns_none_for_malformed_config_json(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            config_path = os.path.join(tmpdir, "config.json")
+            with open(config_path, "w") as f:
+                f.write("not valid json{{{")
+
+            result = detect_quantization_method(tmpdir)
+            self.assertIsNone(result)
+
+    def test_modelslim_takes_priority_over_compressed_tensors(self):
+        """When both ModelSlim config and compressed-tensors config exist,
+        ModelSlim should take priority."""
+        with tempfile.TemporaryDirectory() as tmpdir:
+            # Create ModelSlim config
+            modelslim_path = os.path.join(tmpdir, MODELSLIM_CONFIG_FILENAME)
+            with open(modelslim_path, "w") as f:
+                json.dump({"layer.weight": "INT8"}, f)
+
+            # Create compressed-tensors config
+            config_path = os.path.join(tmpdir, "config.json")
+            with open(config_path, "w") as f:
+                json.dump({
+                    "quantization_config": {
+                        "quant_method": "compressed-tensors"
+                    }
+                }, f)
+
+            result = detect_quantization_method(tmpdir)
+            self.assertEqual(result, ASCEND_QUANTIZATION_METHOD)
+
+
+class TestMaybeAutoDetectQuantization(TestBase):
+
+    def _make_vllm_config(self, model_path="/fake/model", quantization=None):
+        vllm_config = MagicMock()
+        vllm_config.model_config.model = model_path
+        vllm_config.model_config.quantization = quantization
+        return vllm_config
+
+    @patch("vllm_ascend.quantization.utils.detect_quantization_method",
+           return_value=None)
+    def test_no_detection_does_nothing(self, mock_detect):
+        vllm_config = self._make_vllm_config()
+        maybe_auto_detect_quantization(vllm_config)
+        # quantization should remain unchanged
+        self.assertIsNone(vllm_config.model_config.quantization)
+
+    @patch("vllm_ascend.quantization.utils.detect_quantization_method",
+           return_value=ASCEND_QUANTIZATION_METHOD)
+    def test_user_specified_same_method_no_change(self, mock_detect):
+        vllm_config = self._make_vllm_config(
+            quantization=ASCEND_QUANTIZATION_METHOD)
+        maybe_auto_detect_quantization(vllm_config)
+        self.assertEqual(vllm_config.model_config.quantization,
+                         ASCEND_QUANTIZATION_METHOD)
+
+    @patch("vllm.config.VllmConfig._get_quantization_config",
+           return_value=MagicMock())
+    @patch("vllm_ascend.quantization.utils.detect_quantization_method",
+           return_value=ASCEND_QUANTIZATION_METHOD)
+    def test_auto_detect_sets_quantization_and_logs_info(
+            self, mock_detect, mock_get_quant_config):
+        """When no --quantization is specified but ModelSlim config is found,
+        the method should auto-set quantization and emit an INFO log."""
+        vllm_config = self._make_vllm_config(
+            model_path="/fake/quant_model", quantization=None)
+
+        with self.assertLogs("vllm_ascend.quantization.utils",
+                             level=logging.INFO) as cm:
+            maybe_auto_detect_quantization(vllm_config)
+
+        self.assertEqual(vllm_config.model_config.quantization,
+                         ASCEND_QUANTIZATION_METHOD)
+        log_output = "\n".join(cm.output)
+        self.assertIn("Auto-detected quantization method", log_output)
+        self.assertIn(ASCEND_QUANTIZATION_METHOD, log_output)
+        self.assertIn("/fake/quant_model", log_output)
+
+    @patch("vllm_ascend.quantization.utils.detect_quantization_method",
+           return_value=ASCEND_QUANTIZATION_METHOD)
+    def test_user_mismatch_logs_warning(self, mock_detect):
+        """When user specifies a different method than auto-detected,
+        a WARNING should be emitted and user's choice should be respected."""
+        vllm_config = self._make_vllm_config(
+            model_path="/fake/quant_model",
+            quantization=COMPRESSED_TENSORS_METHOD)
+
+        with self.assertLogs("vllm_ascend.quantization.utils",
+                             level=logging.WARNING) as cm:
+            maybe_auto_detect_quantization(vllm_config)
+
+        # User's choice is respected
+        self.assertEqual(vllm_config.model_config.quantization,
+                         COMPRESSED_TENSORS_METHOD)
+        log_output = "\n".join(cm.output)
+        self.assertIn("Auto-detected quantization method", log_output)
+        self.assertIn(ASCEND_QUANTIZATION_METHOD, log_output)
+        self.assertIn(COMPRESSED_TENSORS_METHOD, log_output)
+
+    @patch("vllm_ascend.quantization.utils.detect_quantization_method",
+           return_value=None)
+    def test_no_detection_emits_no_log(self, mock_detect):
+        """When no quantization is detected, no log should be emitted."""
+        vllm_config = self._make_vllm_config(quantization=None)
+        logger_name = "vllm_ascend.quantization.utils"
+
+        with self.assertRaises(AssertionError):
+            # assertLogs raises AssertionError when no logs are emitted
+            with self.assertLogs(logger_name, level=logging.DEBUG):
+                maybe_auto_detect_quantization(vllm_config)
+
+        self.assertIsNone(vllm_config.model_config.quantization)

tests/ut/test_platform.py

@@ -116,13 +116,14 @@ class TestNPUPlatform(TestBase):
         self.assertIsNone(self.platform.inference_mode())
         mock_inference_mode.assert_called_once()
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.utils.update_aclgraph_sizes")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("os.environ", {})
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
     def test_check_and_update_config_basic_config_update(
-        self, mock_init_recompute, mock_soc_version, mock_update_acl, mock_init_ascend
+        self, mock_init_recompute, mock_soc_version, mock_update_acl, mock_init_ascend, mock_auto_detect
     ):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
@@ -146,11 +147,12 @@ class TestNPUPlatform(TestBase):
 
         mock_init_ascend.assert_called_once_with(vllm_config)
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
     def test_check_and_update_config_no_model_config_warning(
-        self, mock_init_recompute, mock_init_ascend, mock_soc_version
+        self, mock_init_recompute, mock_init_ascend, mock_soc_version, mock_auto_detect
     ):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
@@ -172,10 +174,11 @@ class TestNPUPlatform(TestBase):
 
         self.assertTrue("Model config is missing" in cm.output[0])
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
-    def test_check_and_update_config_enforce_eager_mode(self, mock_init_recompute, mock_init_ascend, mock_soc_version):
+    def test_check_and_update_config_enforce_eager_mode(self, mock_init_recompute, mock_init_ascend, mock_soc_version, mock_auto_detect):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
         vllm_config.model_config.enforce_eager = True
@@ -206,11 +209,12 @@ class TestNPUPlatform(TestBase):
             CUDAGraphMode.NONE,
         )
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
     def test_check_and_update_config_unsupported_compilation_level(
-        self, mock_init_recompute, mock_init_ascend, mock_soc_version
+        self, mock_init_recompute, mock_init_ascend, mock_soc_version, mock_auto_detect
     ):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
@@ -244,9 +248,10 @@ class TestNPUPlatform(TestBase):
             )
 
     @pytest.mark.skip("Revert me when vllm support setting cudagraph_mode on oot platform")
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
-    def test_check_and_update_config_unsupported_cudagraph_mode(self, mock_init_ascend, mock_soc_version):
+    def test_check_and_update_config_unsupported_cudagraph_mode(self, mock_init_ascend, mock_soc_version, mock_auto_detect):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
         vllm_config.model_config.enforce_eager = False
@@ -268,11 +273,12 @@ class TestNPUPlatform(TestBase):
                 CUDAGraphMode.NONE,
             )
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
     def test_check_and_update_config_cache_config_block_size(
-        self, mock_init_recompute, mock_init_ascend, mock_soc_version
+        self, mock_init_recompute, mock_init_ascend, mock_soc_version, mock_auto_detect
     ):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
@@ -292,11 +298,12 @@ class TestNPUPlatform(TestBase):
 
         self.assertEqual(vllm_config.cache_config.block_size, 128)
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType.A3)
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
     def test_check_and_update_config_v1_worker_class_selection(
-        self, mock_init_recompute, mock_init_ascend, mock_soc_version
+        self, mock_init_recompute, mock_init_ascend, mock_soc_version, mock_auto_detect
     ):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
@@ -327,10 +334,11 @@ class TestNPUPlatform(TestBase):
             "vllm_ascend.xlite.xlite_worker.XliteWorker",
         )
 
+    @patch("vllm_ascend.quantization.utils.maybe_auto_detect_quantization")
     @patch("vllm_ascend.ascend_config.init_ascend_config")
     @patch("vllm_ascend.utils.get_ascend_device_type", return_value=AscendDeviceType._310P)
     @patch("vllm_ascend.core.recompute_scheduler.RecomputeSchedulerConfig.initialize_from_config")
-    def test_check_and_update_config_310p_no_custom_ops(self, mock_init_recompute, mock_soc_version, mock_init_ascend):
+    def test_check_and_update_config_310p_no_custom_ops(self, mock_init_recompute, mock_soc_version, mock_init_ascend, mock_auto_detect):
         mock_init_ascend.return_value = TestNPUPlatform.mock_vllm_ascend_config()
         vllm_config = TestNPUPlatform.mock_vllm_config()
         vllm_config.compilation_config.custom_ops = []

vllm_ascend/platform.py

@@ -171,6 +171,11 @@ class NPUPlatform(Platform):
 
     @classmethod
     def check_and_update_config(cls, vllm_config: VllmConfig) -> None:
+        from vllm_ascend.quantization.utils import maybe_auto_detect_quantization
+
+        if vllm_config.model_config is not None:
+            maybe_auto_detect_quantization(vllm_config)
+
         # initialize ascend config from vllm additional_config
         cls._fix_incompatible_config(vllm_config)
         ascend_config = init_ascend_config(vllm_config)

vllm_ascend/quantization/modelslim_config.py

@@ -21,6 +21,9 @@ This module provides the AscendModelSlimConfig class for parsing quantization
 configs generated by the ModelSlim tool, along with model-specific mappings.
 """
 
+import glob
+import json
+import os
 from collections.abc import Mapping
 from types import MappingProxyType
 from typing import Any, Optional
@@ -39,6 +42,9 @@ from vllm_ascend.utils import ASCEND_QUANTIZATION_METHOD
 
 from .methods import get_scheme_class
 
+# The config filename that ModelSlim generates after quantizing a model.
+MODELSLIM_CONFIG_FILENAME = "quant_model_description.json"
+
 logger = init_logger(__name__)
 
 # key: model_type
@@ -310,9 +316,9 @@ class AscendModelSlimConfig(QuantizationConfig):
     quantized using the ModelSlim tool.
     """
 
-    def __init__(self, quant_config: dict[str, Any]):
+    def __init__(self, quant_config: dict[str, Any] | None = None):
         super().__init__()
-        self.quant_description = quant_config
+        self.quant_description = quant_config if quant_config is not None else {}
         # TODO(whx): remove this adaptation after adding "shared_head"
         # to prefix of DeepSeekShareHead in vLLM.
         extra_quant_dict = {}
@@ -342,7 +348,12 @@ class AscendModelSlimConfig(QuantizationConfig):
 
     @classmethod
     def get_config_filenames(cls) -> list[str]:
-        return ["quant_model_description.json"]
+        # Return empty list so that vllm's get_quant_config() skips the
+        # file-based lookup (which raises an unfriendly "Cannot find the
+        # config file for ascend" error when the model is not quantized).
+        # Instead, the config file is loaded in maybe_update_config(),
+        # which can provide a user-friendly error message.
+        return []
 
     @classmethod
     def from_config(cls, config: dict[str, Any]) -> "AscendModelSlimConfig":
@@ -456,5 +467,98 @@ class AscendModelSlimConfig(QuantizationConfig):
         assert is_skipped is not None
         return is_skipped
 
+    def maybe_update_config(self, model_name: str) -> None:
+        """Load the ModelSlim quantization config from model directory.
+
+        This method is called by vllm after get_quant_config() returns
+        successfully. Since we return an empty list from get_config_filenames()
+        to bypass vllm's built-in file lookup, we do the actual config loading
+        here and provide user-friendly error messages when the config is missing.
+
+        Args:
+            model_name: Path to the model directory or model name.
+        """
+        # If quant_description is already populated (e.g. from from_config()),
+        # there is nothing to do.
+        if self.quant_description:
+            return
+
+        # Try to find and load the ModelSlim config file
+        if os.path.isdir(model_name):
+            config_path = os.path.join(model_name, MODELSLIM_CONFIG_FILENAME)
+            if os.path.isfile(config_path):
+                with open(config_path) as f:
+                    self.quant_description = json.load(f)
+                self._apply_extra_quant_adaptations()
+                return
+
+            # Check if there are any json files at all to help diagnose
+            json_files = glob.glob(os.path.join(model_name, "*.json"))
+            json_names = [os.path.basename(f) for f in json_files]
+        else:
+            json_names = []
+
+        # Config file not found - raise a friendly error message
+        raise ValueError(
+            "\n"
+            + "=" * 80
+            + "\n"
+            + "ERROR: ModelSlim Quantization Config Not Found\n"
+            + "=" * 80
+            + "\n"
+            + "\n"
+            + f"You have enabled '--quantization {ASCEND_QUANTIZATION_METHOD}' "
+            + "(ModelSlim quantization),\n"
+            + f"but the model at '{model_name}' does not contain the required\n"
+            + f"quantization config file ('{MODELSLIM_CONFIG_FILENAME}').\n"
+            + "\n"
+            + "This usually means the model weights are NOT quantized by "
+            + "ModelSlim.\n"
+            + "\n"
+            + "Please choose one of the following solutions:\n"
+            + "\n"
+            + "  Solution 1: Remove the quantization option "
+            + "(for float/unquantized models)\n"
+            + "  "
+            + "-" * 58
+            + "\n"
+            + f"    Remove '--quantization {ASCEND_QUANTIZATION_METHOD}' from "
+            + "your command if you want to\n"
+            + "    run the model with the original (float) weights.\n"
+            + "\n"
+            + "    Example:\n"
+            + f"      vllm serve {model_name}\n"
+            + "\n"
+            + "  Solution 2: Quantize your model weights with ModelSlim first\n"
+            + "  "
+            + "-" * 58
+            + "\n"
+            + "    Use the ModelSlim tool to quantize your model weights "
+            + "before deployment.\n"
+            + "    After quantization, the model directory should contain "
+            + f"'{MODELSLIM_CONFIG_FILENAME}'.\n"
+            + "    For more information, please refer to:\n"
+            + "    https://gitee.com/ascend/msit/tree/master/msmodelslim\n"
+            + "\n"
+            + (f"  (Found JSON files in model directory: {json_names})\n" if json_names else "")
+            + "=" * 80
+        )
+
+    def _apply_extra_quant_adaptations(self) -> None:
+        """Apply extra adaptations to the quant_description dict.
+
+        This handles known key transformations such as shared_head and
+        weight_packed mappings.
+        """
+        extra_quant_dict = {}
+        for k in self.quant_description:
+            if "shared_head" in k:
+                new_k = k.replace(".shared_head.", ".")
+                extra_quant_dict[new_k] = self.quant_description[k]
+            if "weight_packed" in k:
+                new_k = k.replace("weight_packed", "weight")
+                extra_quant_dict[new_k] = self.quant_description[k]
+        self.quant_description.update(extra_quant_dict)
+
     def get_scaled_act_names(self) -> list[str]:
         return []

vllm_ascend/quantization/utils.py

@@ -0,0 +1,147 @@
+#
+# Copyright (c) 2025 Huawei Technologies Co., Ltd. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# This file is a part of the vllm-ascend project.
+#
+
+import json
+import os
+
+from vllm.logger import init_logger
+
+from vllm_ascend.quantization.modelslim_config import MODELSLIM_CONFIG_FILENAME
+from vllm_ascend.utils import ASCEND_QUANTIZATION_METHOD, COMPRESSED_TENSORS_METHOD
+
+logger = init_logger(__name__)
+
+
+def detect_quantization_method(model_path: str) -> str | None:
+    """Auto-detect the quantization method from model directory files.
+
+    This function performs a lightweight check (JSON files and file existence
+    only — no .safetensors or .bin inspection) to determine which quantization
+    method was used to produce the weights in *model_path*.
+
+    Detection priority:
+        1. **ModelSlim (Ascend)** – ``quant_model_description.json`` exists
+           in the model directory.
+        2. **LLM-Compressor (compressed-tensors)** – ``config.json`` contains
+           a ``quantization_config`` section with
+           ``"quant_method": "compressed-tensors"``.
+        3. **None** – neither condition is met; the caller should fall back to
+           the default (float) behaviour.
+
+    Args:
+        model_path: Path to the local model directory.
+
+    Returns:
+        ``"ascend"`` for ModelSlim models,
+        ``"compressed-tensors"`` for LLM-Compressor models,
+        or ``None`` if no quantization signature is found.
+    """
+    if not os.path.isdir(model_path):
+        return None
+
+    # Case 1: ModelSlim — look for quant_model_description.json
+    modelslim_config_path = os.path.join(model_path, MODELSLIM_CONFIG_FILENAME)
+    if os.path.isfile(modelslim_config_path):
+        return ASCEND_QUANTIZATION_METHOD
+
+    # Case 2: LLM-Compressor — look for compressed-tensors in config.json
+    config_json_path = os.path.join(model_path, "config.json")
+    if os.path.isfile(config_json_path):
+        try:
+            with open(config_json_path) as f:
+                config = json.load(f)
+            quant_cfg = config.get("quantization_config")
+            if isinstance(quant_cfg, dict):
+                quant_method = quant_cfg.get("quant_method", "")
+                if quant_method == COMPRESSED_TENSORS_METHOD:
+                    return COMPRESSED_TENSORS_METHOD
+        except (json.JSONDecodeError, OSError):
+            # Malformed or unreadable config.json — skip silently.
+            pass
+
+    # Case 3: No quantization signature found.
+    return None
+
+
+def maybe_auto_detect_quantization(vllm_config) -> None:
+    """Auto-detect and apply the quantization method on *vllm_config*.
+
+    This should be called during engine initialisation (from
+    ``NPUPlatform.check_and_update_config``) **after** ``VllmConfig`` has been
+    created but **before** heavy weights are loaded.
+
+    Because ``check_and_update_config`` runs *after*
+    ``VllmConfig.__post_init__`` has already evaluated
+    ``_get_quantization_config`` (which returned ``None`` when
+    ``model_config.quantization`` was not set), we must:
+
+    1. Set ``model_config.quantization`` to the detected value.
+    2. Recreate ``vllm_config.quant_config`` so that the quantization
+       pipeline (``get_quant_config`` → ``QuantizationConfig`` →
+       ``get_quant_method`` for every layer) is properly initialised.
+
+    Rules:
+        * If the user explicitly set ``--quantization``, that value is
+          respected.  A warning is emitted when the detected method differs.
+        * If no ``--quantization`` was given, the detected method (if any) is
+          applied automatically.
+
+    Args:
+        vllm_config: A ``vllm.config.VllmConfig`` instance (mutable).
+    """
+    model_config = vllm_config.model_config
+    model_path = model_config.model
+    user_quant = model_config.quantization
+    detected = detect_quantization_method(model_path)
+
+    if detected is None:
+        # No quantization signature found — nothing to do.
+        return
+
+    if user_quant is not None:
+        # User explicitly specified a quantization method.
+        if user_quant != detected:
+            logger.warning(
+                "Auto-detected quantization method '%s' from model "
+                "files at '%s', but user explicitly specified "
+                "'--quantization %s'. Respecting the user-specified "
+                "value. If you encounter errors during model loading, "
+                "consider using '--quantization %s' instead.",
+                detected,
+                model_path,
+                user_quant,
+                detected,
+            )
+        return
+
+    # No user-specified quantization — apply auto-detected value.
+    model_config.quantization = detected
+    logger.info(
+        "Auto-detected quantization method '%s' from model files "
+        "at '%s'. To override, pass '--quantization <method>' explicitly.",
+        detected,
+        model_path,
+    )
+
+    # Recreate quant_config on VllmConfig.  The original __post_init__
+    # already ran _get_quantization_config(), but at that point
+    # model_config.quantization was None so it returned None.  Now that
+    # we've set it, we need to build the actual QuantizationConfig so the
+    # downstream model-loading code can use it.
+    from vllm.config import VllmConfig as _VllmConfig
+
+    vllm_config.quant_config = _VllmConfig._get_quantization_config(model_config, vllm_config.load_config)