diff --git a/invokeai/backend/model_manager/load/load_default.py b/invokeai/backend/model_manager/load/load_default.py
index 542a6774d5a..040b55cb6ec 100644
--- a/invokeai/backend/model_manager/load/load_default.py
+++ b/invokeai/backend/model_manager/load/load_default.py
@@ -218,14 +218,18 @@ def _apply_fp8_layerwise_casting(
             if first_param is not None:
                 compute_dtype = first_param.dtype
 
-        from diffusers.models.modeling_utils import ModelMixin
-
-        if isinstance(model, ModelMixin):
-            model.enable_layerwise_casting(
-                storage_dtype=storage_dtype,
-                compute_dtype=compute_dtype,
-            )
-        elif isinstance(model, torch.nn.Module):
+        # We use our own hook-based path for every nn.Module — including diffusers ModelMixin —
+        # rather than `model.enable_layerwise_casting()`. Diffusers' LayerwiseCastingHook installs
+        # an instance-level `forward` attribute that captures the original `Linear.forward` in a
+        # closure. `ModelCache.put()` later runs `apply_custom_layers_to_model`, which constructs a
+        # new `CustomLinear` sharing the original Linear's `__dict__` — so the diffusers wrapper
+        # carries over and routes calls back to the captured original forward, silently bypassing
+        # `CustomLinear.forward` and its `cast_to_device` autocast. With partial loading (e.g. FLUX.2
+        # Klein 9B) some weights stay on CPU, the diffusers pre_forward only casts dtype, and
+        # `F.linear` then sees input on cuda and weight on cpu. Our `register_forward_pre_hook` /
+        # `register_forward_hook` path fires around `nn.Module._call_impl` without replacing
+        # `forward`, so `CustomLinear.forward` is still reached.
+        if isinstance(model, torch.nn.Module):
             self._apply_fp8_to_nn_module(model, storage_dtype=storage_dtype, compute_dtype=compute_dtype)
         else:
             return model
diff --git a/tests/backend/model_manager/load/test_load_default_fp8.py b/tests/backend/model_manager/load/test_load_default_fp8.py
index 4a4491c6a07..97a1cfb2e30 100644
--- a/tests/backend/model_manager/load/test_load_default_fp8.py
+++ b/tests/backend/model_manager/load/test_load_default_fp8.py
@@ -323,3 +323,49 @@ def tracked_patch_branch(self, input):
         "FP8-wrapped forward did not reach CustomLinear.forward — LoRA/ControlLoRA patches "
         "would be silently bypassed on FP8 checkpoint models."
     )
+
+
+def test_apply_fp8_layerwise_casting_uses_hook_path_for_model_mixin():
+    """Regression test for the FLUX.2 Klein 9B partial-load device-mismatch crash.
+
+    Diffusers' `enable_layerwise_casting()` registers a `LayerwiseCastingHook` whose
+    `pre_forward` only casts dtype (not device) and whose hook system replaces
+    `Linear.forward` with a wrapper that calls the *original* `Linear.forward` captured
+    before the hook was installed. `ModelCache.put()` later wraps Linear as CustomLinear
+    sharing `__dict__`, so the diffusers wrapper is carried into the new CustomLinear and
+    routes calls to the captured original Linear.forward — bypassing
+    `CustomLinear.forward`'s `cast_to_device`. On partial load (some weights on CPU,
+    input on cuda), this raises a device-mismatch error.
+
+    The fix routes ModelMixin through `_apply_fp8_to_nn_module` (hook-based,
+    `forward`-preserving). This test asserts that path is taken even when the model
+    inherits from ModelMixin.
+    """
+    from diffusers.models.modeling_utils import ModelMixin
+
+    class _FakeModelMixin(ModelMixin):
+        # ModelMixin requires a config_name class attribute and a config dict for serialization.
+        # We never serialize, so we only need to satisfy isinstance() checks.
+        config_name = "config.json"
+
+        def __init__(self):
+            super().__init__()
+            self.linear = torch.nn.Linear(4, 4, bias=False)
+
+        def forward(self, x):
+            return self.linear(x)
+
+    loader = _make_loader(device="cuda")
+    config = _make_config(ModelType.Main, fp8=True)
+
+    model = _FakeModelMixin()
+
+    with (
+        patch.object(ModelLoader, "_should_use_fp8", return_value=True),
+        patch.object(ModelLoader, "_apply_fp8_to_nn_module") as mock_to_nn,
+        patch.object(_FakeModelMixin, "enable_layerwise_casting") as mock_enable,
+    ):
+        loader._apply_fp8_layerwise_casting(model, config)
+
+    mock_to_nn.assert_called_once()
+    mock_enable.assert_not_called()