diff --git a/src/diffusers/quantizers/quantization_config.py b/src/diffusers/quantizers/quantization_config.py
index c3d829fde8cf..1a45a31cd750 100644
--- a/src/diffusers/quantizers/quantization_config.py
+++ b/src/diffusers/quantizers/quantization_config.py
@@ -476,8 +476,56 @@ def post_init(self):
         from torchao.quantization.quant_api import AOBaseConfig
 
         if not isinstance(self.quant_type, AOBaseConfig):
+            if isinstance(self.quant_type, str):
+                raise TypeError(self._build_string_quant_type_error(self.quant_type))
             raise TypeError(f"quant_type must be an AOBaseConfig instance, got {type(self.quant_type).__name__}")
 
+    @staticmethod
+    def _build_string_quant_type_error(quant_type: str) -> str:
+        """Build a migration-guidance error for legacy string ``quant_type`` values.
+
+        Older diffusers releases accepted lowercase strings such as ``"int8_weight_only"`` or ``"float8dq_e4m3_row"``.
+        That path was removed (see PR #13291) in favour of passing an ``AOBaseConfig`` subclass instance from
+        ``torchao.quantization`` directly. Users on torchao >= 0.16 also hit this because the legacy lowercase
+        factories were removed upstream. This helper surfaces the rename so users can self-migrate.
+        """
+        # Map common legacy strings to their torchao Config-class replacements. We deliberately
+        # do not auto-instantiate the Config; the new API exposes options (granularity, dtype,
+        # version, ...) the legacy strings hard-coded, and silent defaults would be surprising.
+        legacy_to_config = {
+            "int4wo": "Int4WeightOnlyConfig",
+            "int4_weight_only": "Int4WeightOnlyConfig",
+            "int8wo": "Int8WeightOnlyConfig",
+            "int8_weight_only": "Int8WeightOnlyConfig",
+            "int8dq": "Int8DynamicActivationInt8WeightConfig",
+            "int8_dynamic_activation_int8_weight": "Int8DynamicActivationInt8WeightConfig",
+            "float8wo": "Float8WeightOnlyConfig",
+            "float8wo_e4m3": "Float8WeightOnlyConfig",
+            "float8wo_e5m2": "Float8WeightOnlyConfig",
+            "float8_weight_only": "Float8WeightOnlyConfig",
+            "float8dq": "Float8DynamicActivationFloat8WeightConfig",
+            "float8dq_e4m3": "Float8DynamicActivationFloat8WeightConfig",
+            "float8dq_e4m3_row": "Float8DynamicActivationFloat8WeightConfig",
+            "float8dq_e4m3_tensor": "Float8DynamicActivationFloat8WeightConfig",
+            "float8_dynamic_activation_float8_weight": "Float8DynamicActivationFloat8WeightConfig",
+            "float8_static_activation_float8_weight": "Float8StaticActivationFloat8WeightConfig",
+        }
+        suggestion = legacy_to_config.get(quant_type)
+        message = (
+            f"TorchAoConfig no longer accepts string quant_type values (got {quant_type!r}); "
+            "pass an AOBaseConfig instance from torchao.quantization instead."
+        )
+        if suggestion is not None:
+            message += (
+                f" For {quant_type!r}, use "
+                f"`from torchao.quantization import {suggestion}; TorchAoConfig({suggestion}())`."
+            )
+        message += (
+            " See https://huggingface.co/docs/diffusers/main/en/quantization/torchao for "
+            "the full list of supported AOBaseConfig classes."
+        )
+        return message
+
     def to_dict(self):
         """Convert configuration to a dictionary."""
         d = super().to_dict()
diff --git a/tests/quantization/torchao/test_torchao.py b/tests/quantization/torchao/test_torchao.py
index 8a811cfc1c73..842b3c9e10d7 100644
--- a/tests/quantization/torchao/test_torchao.py
+++ b/tests/quantization/torchao/test_torchao.py
@@ -112,6 +112,31 @@ def test_post_init_check(self):
         with self.assertRaises(TypeError):
             _ = TorchAoConfig(42)
 
+    def test_string_quant_type_error_includes_migration_hint(self):
+        """
+        Passing a legacy string quant_type should raise TypeError and the message should name the
+        replacement AOBaseConfig class so users on torchao >= 0.16 (where the legacy lowercase
+        factories were removed) can self-migrate. See issues #13286 and #13266.
+        """
+        legacy_to_config = {
+            "int8_weight_only": "Int8WeightOnlyConfig",
+            "int8wo": "Int8WeightOnlyConfig",
+            "float8_weight_only": "Float8WeightOnlyConfig",
+            "float8dq_e4m3_row": "Float8DynamicActivationFloat8WeightConfig",
+            "float8_dynamic_activation_float8_weight": "Float8DynamicActivationFloat8WeightConfig",
+        }
+        for legacy, config_name in legacy_to_config.items():
+            with self.assertRaises(TypeError) as cm:
+                TorchAoConfig(legacy)
+            message = str(cm.exception)
+            self.assertIn(repr(legacy), message)
+            self.assertIn(config_name, message)
+
+        # Strings without a known mapping should still raise TypeError and point at the docs.
+        with self.assertRaises(TypeError) as cm:
+            TorchAoConfig("not_a_real_quant_type")
+        self.assertIn("AOBaseConfig", str(cm.exception))
+
     def test_repr(self):
         """
         Check that there is no error in the repr