peft/pr_3111/en/package_reference/lora_variant_kasa.md

KaSA

This is a variant of LoRA and therefore everything that is possible with LoRA is valid for this method except otherwise stated on this page.

KaSA (Knowledge-aware Singular-value Adaptation) is a LoRA variant that uses the singular value decomposition of the base weight to filter out task-irrelevant knowledge and parametrizes the update with learnable singular values. It changes vanilla LoRA in two ways:

1. Knowledge-based SVD truncation of the frozen base weight. At initialization, the base weight W is SVD-factored and its r smallest ("noisy"/long-tail) singular components are discarded, leaving the rank-(k - r) approximation as the new frozen base (k = min(in_features, out_features)). The trainable branch then re-learns in the discarded residual subspace.
2. Knowledge-aware singular-value adaptation. The trainable update is parametrized in SVD form with a learnable diagonal of singular values inserted between the LoRA factors: ΔW = scaling * B @ diag(ΔΣ) @ A, where ΔΣ (lora_diag) is a learnable r-vector and the only new parameter per layer.

In PEFT, KaSA is configured as a LoRA variant through the kasa_config argument on LoraConfig:

from peft import KasaConfig, LoraConfig

config = LoraConfig(
    target_modules=["q_proj", "v_proj"],
    kasa_config=KasaConfig(beta=1e-4, gamma=1e-3),
)

The paper additionally trains with two auxiliary regularizers: an L2 penalty on the learnable singular values (weighted by beta) and an orthogonal regularization on the adapter factors (weighted by gamma), which softly enforces the semi-orthogonality assumed by the SVD parametrization. These cannot be injected automatically by PEFT, so during training you must add them to the task loss by calling LoraModel._get_kasa_loss() on the underlying LoraModel:

task_loss = ...  # standard loss returned by your model
kasa_loss = model._get_kasa_loss()  # 0.0 if KaSA is not used
total_loss = task_loss + kasa_loss

For detailed usage, see these instructions.

Caveats

• KaSA is currently supported on standard LoRA linear layers only, and not with fan_in_fan_out=True layers (e.g. transformers Conv1D).
• KaSA adapters cannot be combined with non-KaSA adapters on the same model, since the base-weight truncation would change the base weights under the other adapters' feet. Multiple KaSA adapters are allowed.
• convert_to_lora is not supported: the KaSA update depends on lora_diag and on the truncated base weight, neither of which is representable in a vanilla LoRA adapter.
• The SVD truncation of the base weight is destructive: adding a KaSA adapter permanently changes the layer's frozen weight. Disabling or unloading the adapter does not restore the original base weight, and merge followed by unmerge round-trips to the truncated weight, not the original one. This is inherent to the method. Keep the original checkpoint if you need to recover the unmodified base model.
• Loading a trained KaSA adapter with PeftModel.from_pretrained re-applies the same truncation to the freshly loaded base weight, so saving and reloading is consistent.