🚨 Give mixed-rank LoRAs without alpha keys their intended scale

src/diffusers/utils/peft_utils.py

@@ -156,6 +156,7 @@ def get_peft_kwargs(
     rank_pattern = {}
     alpha_pattern = {}
     r = lora_alpha = list(rank_dict.values())[0]
+    has_alphas = network_alpha_dict is not None and len(network_alpha_dict) > 0
 
     if len(set(rank_dict.values())) > 1:
         # get the rank occurring the most number of times
@@ -165,6 +166,13 @@ def get_peft_kwargs(
         rank_pattern = dict(filter(lambda x: x[1] != r, rank_dict.items()))
         rank_pattern = {k.split(".lora_B.")[0]: v for k, v in rank_pattern.items()}
 
+        if not has_alphas:
+            # No alpha data in the checkpoint: the diffusers/PEFT convention is
+            # `W_eff = W + lora_B @ lora_A`, i.e. alpha == rank per module (scale 1.0).
+            # Mirror the ranks into the alphas so every module keeps scale 1.0.
+            lora_alpha = r
+            alpha_pattern = dict(rank_pattern)
+
     if network_alpha_dict is not None and len(network_alpha_dict) > 0:
         if len(set(network_alpha_dict.values())) > 1:
             # get the alpha occurring the most number of times

tests/lora/test_peft_utils.py

@@ -0,0 +1,79 @@
+# Copyright 2026 HuggingFace Inc.
+#
+# 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.
+
+from diffusers.utils.peft_utils import get_peft_kwargs
+
+
+def _rank_dict(module_ranks):
+    return {f"{module}.lora_B.weight": rank for module, rank in module_ranks.items()}
+
+
+def _peft_state_dict(module_ranks):
+    state_dict = {}
+    for module in module_ranks:
+        state_dict[f"{module}.lora_A.weight"] = None
+        state_dict[f"{module}.lora_B.weight"] = None
+    return state_dict
+
+
+def _effective_scale(kwargs, module):
+    alpha = kwargs["alpha_pattern"].get(module, kwargs["lora_alpha"])
+    rank = kwargs["rank_pattern"].get(module, kwargs["r"])
+    return alpha / rank
+
+
+def test_mixed_ranks_without_alphas_apply_at_scale_one():
+    # An adapter with per-module ranks and no alpha keys means `W_eff = W + lora_B @ lora_A`,
+    # i.e. alpha == rank, so every module must come out at scale 1.0.
+    module_ranks = {"blocks.0.adaln": 16, "blocks.0.to_q": 64, "blocks.0.to_v": 64}
+    kwargs = get_peft_kwargs(_rank_dict(module_ranks), None, _peft_state_dict(module_ranks))
+    for module in module_ranks:
+        assert _effective_scale(kwargs, module) == 1.0
+
+
+def test_mixed_ranks_without_alphas_are_order_independent():
+    # The scale must not depend on which module happens to come first in the state dict.
+    module_ranks = {"blocks.0.to_q": 64, "blocks.0.to_v": 64, "blocks.0.adaln": 16}
+    reordered = dict(reversed(module_ranks.items()))
+    for ranks in (module_ranks, reordered):
+        kwargs = get_peft_kwargs(_rank_dict(ranks), None, _peft_state_dict(ranks))
+        for module in ranks:
+            assert _effective_scale(kwargs, module) == 1.0
+
+
+def test_uniform_rank_without_alphas_unchanged():
+    module_ranks = {"blocks.0.to_q": 32, "blocks.0.to_v": 32}
+    kwargs = get_peft_kwargs(_rank_dict(module_ranks), None, _peft_state_dict(module_ranks))
+    assert kwargs["r"] == kwargs["lora_alpha"] == 32
+    assert kwargs["rank_pattern"] == {}
+    assert kwargs["alpha_pattern"] == {}
+
+
+def test_mixed_ranks_with_uniform_alpha_keep_declared_alpha():
+    # A declared alpha must win over the alpha == rank convention: scale is alpha / rank per module.
+    module_ranks = {"blocks.0.adaln": 16, "blocks.0.to_q": 64}
+    network_alphas = {f"{module}.alpha": 32 for module in module_ranks}
+    kwargs = get_peft_kwargs(_rank_dict(module_ranks), network_alphas, _peft_state_dict(module_ranks))
+    assert kwargs["lora_alpha"] == 32
+    assert kwargs["alpha_pattern"] == {}
+    assert _effective_scale(kwargs, "blocks.0.adaln") == 2.0
+    assert _effective_scale(kwargs, "blocks.0.to_q") == 0.5
+
+
+def test_mixed_ranks_with_per_module_alphas_unchanged():
+    module_ranks = {"blocks.0.adaln": 16, "blocks.0.to_q": 64}
+    network_alphas = {"blocks.0.adaln.alpha": 16, "blocks.0.to_q.alpha": 64}
+    kwargs = get_peft_kwargs(_rank_dict(module_ranks), network_alphas, _peft_state_dict(module_ranks))
+    for module in module_ranks:
+        assert _effective_scale(kwargs, module) == 1.0