feat: test for config validation and BC for new peft weight dtype

src/axolotl/kernels/lora.py

@@ -15,7 +15,7 @@ from torch import nn
 from torch.distributed.tensor import DTensor
 
 from .geglu import geglu_backward, geglu_forward
-from .quantize import dequantize
+from .quantize import dequantize_weight
 from .swiglu import swiglu_backward, swiglu_forward
 from .utils import torch_amp_custom_bwd, torch_amp_custom_fwd
 
@@ -46,6 +46,12 @@ def get_lora_parameters(
     W = base_layer.weight
     b = base_layer.bias
 
+    # Unwrap DTensor if FSDP2 left the weight wrapped -- DTensor does not proxy
+    # attribute access to the underlying tensor subclass, so torchao methods like
+    # .dequantize() or .get_original_weight() would not be visible.
+    if isinstance(W, DTensor):
+        W = W.full_tensor()
+
     if not hasattr(proj, "disable_adapters") or proj.disable_adapters or proj.merged:
         quant_state = getattr(W, "quant_state", None)
         return W, b, quant_state, None, None, None
@@ -86,6 +92,7 @@ def matmul_lora(
     B: torch.Tensor | None,
     s: float | None,
     out: torch.Tensor | None = None,
+    transpose: bool = True,
 ) -> torch.Tensor:
     """
     Efficient fused matmul + LoRA computation.
@@ -98,12 +105,15 @@ def matmul_lora(
         B: LoRA B matrix [out_features, rank]
         s: LoRA scaling factor
         out: Optional output tensor for inplace operations
+        transpose: If True (default), transpose W before matmul (forward path).
+            Set to False for backward paths where W is already in the correct layout.
 
     Returns:
         Result of X @ W + X @ A @ B
     """
     dtype = X.dtype
-    W = dequantize(W.t(), W_quant)
+    is_quantized = W_quant is not None or type(W) is not torch.Tensor
+    W = dequantize_weight(W, W_quant, transpose=transpose)
 
     reshape = False
     if X.dim() == 3:
@@ -112,7 +122,7 @@ def matmul_lora(
         reshape = True
 
     out = torch.matmul(X, W, out=out)
-    if W_quant is not None:
+    if is_quantized:
         del W
 
     if A is not None:
@@ -292,15 +302,16 @@ class LoRA_MLP(torch.autograd.Function):
         up = up.view(-1, up.shape[-1])
         dtype = X.dtype
 
-        # Down projection
+        # Down projection (backward: no transpose needed, W is already [out, in])
         grad_down = matmul_lora(
             grad_output,
-            down_weight.t(),
+            down_weight,
             None,
             down_quant,
             down_B,
             down_A,
             down_scale,
+            transpose=False,
         )
 
         # Activation backward
@@ -332,7 +343,7 @@ class LoRA_MLP(torch.autograd.Function):
 
         if dX is not None:
             # Up projection gradients
-            up_weight = dequantize(up_weight.t(), up_quant)
+            up_weight = dequantize_weight(up_weight, up_quant, transpose=True)
             if ctx.inplace:
                 dX = torch.matmul(grad_up, up_weight.t(), out=X)
             else:
@@ -344,7 +355,7 @@ class LoRA_MLP(torch.autograd.Function):
                 dX += grad_up @ up_B.to(dtype).t() @ (up_scale * up_A.to(dtype).t())
 
             # Gate projection gradients
-            gate_weight = dequantize(gate_weight, gate_quant)
+            gate_weight = dequantize_weight(gate_weight, gate_quant)
             dX += grad_gate @ gate_weight
             del gate_weight
 
@@ -631,7 +642,7 @@ class LoRA_QKV(torch.autograd.Function):
         out_buffer = X if ctx.inplace else None
 
         # Q path
-        q_weight_t = dequantize(q_weight, q_quant)
+        q_weight_t = dequantize_weight(q_weight, q_quant)
         grad_X = torch.mm(q_grad, q_weight_t, out=out_buffer)
         del q_weight
         del q_weight_t
@@ -639,7 +650,7 @@ class LoRA_QKV(torch.autograd.Function):
             grad_X.addmm_(q_grad, torch.mm(B_q_scaled, A_q_scaled))
 
         # K path
-        k_weight_t = dequantize(k_weight, k_quant)
+        k_weight_t = dequantize_weight(k_weight, k_quant)
         grad_X.addmm_(k_grad, k_weight_t)
         del k_weight
         del k_weight_t
@@ -647,7 +658,7 @@ class LoRA_QKV(torch.autograd.Function):
             grad_X.addmm_(k_grad, torch.mm(B_k_scaled, A_k_scaled))
 
         # V path
-        v_weight_t = dequantize(v_weight, v_quant)
+        v_weight_t = dequantize_weight(v_weight, v_quant)
         grad_X.addmm_(v_grad, v_weight_t)
         del v_weight
         del v_weight_t
@@ -810,7 +821,7 @@ class LoRA_O(torch.autograd.Function):
         d_B = s * A @ dY_X
 
         # Get derivative for dX
-        W = dequantize(W.t(), W_quant)
+        W = dequantize_weight(W, W_quant, transpose=True)
         dX = dY @ W.t()
         del W
 

src/axolotl/kernels/quantize.py

@@ -146,3 +146,43 @@ def dequantize(
     # Handle transposed data
     is_transposed: bool = W.shape[0] == 1
     return out.t() if is_transposed else out
+
+
+def dequantize_weight(
+    W: torch.Tensor,
+    quant_state: QuantState | list | None = None,
+    transpose: bool = False,
+) -> torch.Tensor:
+    """Unified dequantization for both torchao and bnb quantized weights.
+
+    For torchao tensor subclasses (AffineQuantizedTensor, NF4Tensor), dequantizes
+    using the appropriate instance method. For bnb Params4bit, delegates to the
+    optimized CUDA kernel in ``dequantize``.
+
+    Args:
+        W: Quantized weight tensor ``[out_features, in_features]``.
+        quant_state: bnb ``QuantState`` (None for torchao / unquantized).
+        transpose: If True, return ``[in_features, out_features]``.
+
+    Returns:
+        Dequantized float tensor, optionally transposed.
+    """
+    # torchao path: tensor subclass with embedded quantization state
+    if quant_state is None and type(W) is not torch.Tensor:
+        result = None
+        # NF4Tensor (check first — NF4Tensor.dequantize is a static method)
+        if hasattr(W, "get_original_weight"):
+            result = W.get_original_weight()
+        else:
+            # AffineQuantizedTensor (INT4, etc.)
+            try:
+                result = W.dequantize()
+            except (TypeError, RuntimeError):
+                pass
+        if result is not None:
+            return result.t() if transpose else result
+
+    # bnb path: transpose input before the CUDA kernel (existing convention)
+    if transpose:
+        return dequantize(W.t(), quant_state)
+    return dequantize(W, quant_state)

src/axolotl/loaders/adapter.py

@@ -23,6 +23,7 @@ from axolotl.loaders.utils import get_linear_embedding_layers
 from axolotl.telemetry.errors import send_errors
 from axolotl.utils.dict import DictDefault
 from axolotl.utils.logging import get_logger
+from axolotl.utils.schemas.enums import TorchAOQuantDType
 
 LOG = get_logger(__name__)
 
@@ -134,11 +135,13 @@ def load_lora(
 
     rank = int(os.environ.get("LOCAL_RANK", 0))
 
+    is_torchao = cfg.peft and cfg.peft.backend == "torchao"
     if (
         cfg.fsdp_config
         and cfg.adapter
         and cfg.fsdp_config.cpu_ram_efficient_loading
         and rank != 0
+        and not is_torchao
     ):
         setup_quantized_meta_for_peft(model)
 
@@ -146,6 +149,15 @@ def load_lora(
     if cfg.peft_autocast_adapter_dtype is not None:
         model_kwargs["autocast_adapter_dtype"] = cfg.peft_autocast_adapter_dtype
 
+    # Patch PEFT's torchao dispatch before any model creation/loading.
+    # Must happen before both get_peft_model and PeftModel.from_pretrained,
+    # as both trigger LoRA layer dispatch that would fail for INT4/NF4 weights.
+    # INT8 is natively supported by PEFT's TorchaoLoraLinear, so skip the patch.
+    if is_torchao and cfg.peft.weight_dtype != TorchAOQuantDType.int8:
+        from axolotl.monkeypatch.peft.utils import patch_peft_torchao_dispatch
+
+        patch_peft_torchao_dispatch()
+
     if cfg.lora_model_dir:
         LOG.debug("Loading pretrained PEFT - LoRA")
         if cfg.lora_on_cpu:
@@ -172,6 +184,7 @@ def load_lora(
         and cfg.adapter
         and cfg.fsdp_config.cpu_ram_efficient_loading
         and rank != 0
+        and not is_torchao
     ):
         setup_quantized_peft_meta_for_training(model)
 

src/axolotl/loaders/model.py

@@ -158,6 +158,15 @@ class ModelLoader:
         """Property that determines if FSDP with QLoRA is enabled."""
         return self.is_fsdp_enabled and self.cfg.adapter == "qlora"
 
+    @property
+    def is_torchao_qlora(self):
+        """Property that determines if torchao backend is used for QLoRA."""
+        return (
+            self.cfg.adapter == "qlora"
+            and self.cfg.peft
+            and self.cfg.peft.backend == "torchao"
+        )
+
     @send_errors
     def load(self) -> tuple[PreTrainedModel | PeftModelForCausalLM, PeftConfig | None]:
         """Load and prepare the model with all configurations and patches.
@@ -491,8 +500,9 @@ class ModelLoader:
 
         # FSDP requires control over device placement, so don't set device_map when FSDP is enabled
         if self.is_fsdp_enabled:
-            # For QLoRA + FSDP, we still need to set device_map to "auto" for proper initialization
+            # For QLoRA + FSDP with bnb, we still need to set device_map for proper initialization
-            if self.is_qlora_and_fsdp_enabled:
+            # torchao tensors work natively with FSDP2, no device_map override needed
+            if self.is_qlora_and_fsdp_enabled and not self.is_torchao_qlora:
                 self.model_kwargs["device_map"] = {
                     "": int(os.environ.get("LOCAL_RANK", 0))
                 }
@@ -561,6 +571,44 @@ class ModelLoader:
                 self.model_kwargs["quantization_config"] = BitsAndBytesConfig(
                     **self.model_config.quantization_config
                 )
+        elif (
+            self.cfg.adapter == "qlora"
+            and self.cfg.peft
+            and self.cfg.peft.backend == "torchao"
+            and not self.cfg.merge_lora
+        ):
+            from transformers import TorchAoConfig
+
+            from axolotl.utils.schemas.enums import TorchAOQuantDType
+
+            weight_dtype = self.cfg.peft.weight_dtype
+            if weight_dtype == TorchAOQuantDType.int4:
+                group_size = self.cfg.peft.group_size or 128
+                self.model_kwargs["quantization_config"] = TorchAoConfig(
+                    quant_type="int4_weight_only",
+                    group_size=group_size,
+                )
+            elif weight_dtype == TorchAOQuantDType.int8:
+                group_size = self.cfg.peft.group_size or 128
+                self.model_kwargs["quantization_config"] = TorchAoConfig(
+                    quant_type="int8_weight_only",
+                    group_size=group_size,
+                )
+            elif weight_dtype == TorchAOQuantDType.nf4:
+                from torchao.dtypes._nf4tensor_api import NF4WeightOnlyConfig
+
+                block_size = self.cfg.peft.group_size or 64
+                self.model_kwargs["quantization_config"] = TorchAoConfig(
+                    quant_type=NF4WeightOnlyConfig(
+                        block_size=block_size,
+                        scaler_block_size=256,
+                    ),
+                )
+            else:
+                raise ValueError(
+                    f"Unsupported torchao weight_dtype for QLoRA: {weight_dtype}. "
+                    "Supported: int4, int8, nf4"
+                )
         elif self.cfg.adapter == "qlora" and self.cfg.load_in_4bit:
             bnb_config = {
                 "load_in_4bit": True,
@@ -860,6 +908,10 @@ class ModelLoader:
             # Make sure everything is in the same dtype
             skip_prepare_model_for_kbit_training = True
 
+        # torchao quantized models don't use Params4bit and don't need kbit preparation
+        if self.is_torchao_qlora:
+            skip_prepare_model_for_kbit_training = True
+
         if (
             not skip_prepare_model_for_kbit_training
             and self.cfg.adapter in ["lora", "qlora"]

src/axolotl/loaders/patch_manager.py

@@ -348,10 +348,12 @@ class PatchManager:
 
     def _apply_fsdp2_bnb_patches(self):
         """Apply FSDP2 BNB patches."""
+        is_torchao = self.cfg.peft and self.cfg.peft.backend == "torchao"
         if (
             self.cfg.fsdp_config
             and str(self.cfg.fsdp_version) == "2"
             and self.cfg.adapter == "qlora"
+            and not is_torchao
         ):
             from axolotl.monkeypatch.fsdp2_qlora import (
                 apply_init_sharded_param_patch,

src/axolotl/monkeypatch/peft/utils.py

@@ -78,3 +78,30 @@ def patch_peft_prep_code():
     axolotl.loaders.model.prepare_model_for_kbit_training = (
         fixed_prepare_model_for_kbit_training
     )
+
+
+def patch_peft_torchao_dispatch():
+    """Skip PEFT's TorchaoLoraLinear for non-INT8 torchao weights.
+
+    PEFT's dispatch_torchao() matches AffineQuantizedTensor but then errors in
+    _check_dtype_supported() because it only allows INT8. Our LoRA kernels handle
+    dequantization explicitly, so we bypass PEFT's torchao dispatch entirely and
+    let it fall back to standard Linear LoRA layers.
+    """
+    try:
+        from peft.tuners.lora import torchao as peft_torchao
+    except ImportError:
+        LOG.warning("Could not import peft.tuners.lora.torchao for patching")
+        return
+
+    if getattr(peft_torchao, "_axolotl_patched", False):
+        return
+
+    def patched_dispatch(target, adapter_name, lora_config, **kwargs):
+        # Return None so PEFT falls back to standard Linear LoRA layers.
+        # Our LoRA kernels handle torchao dequantization explicitly.
+        return None
+
+    peft_torchao.dispatch_torchao = patched_dispatch
+    peft_torchao._axolotl_patched = True
+    LOG.info("Patched PEFT dispatch_torchao to skip TorchaoLoraLinear")

src/axolotl/utils/schemas/enums.py

@@ -8,6 +8,7 @@ import torch
 class TorchAOQuantDType(Enum):
     int4 = torch.int4
     int8 = torch.int8
+    nf4 = "nf4"
     float8_e4m3fn = torch.float8_e4m3fn
     nvfp4 = "nvfp4"
 
@@ -16,6 +17,8 @@ class TorchAOQuantDType(Enum):
             return TorchAOQuantDType.int4
         if str == "int8":
             return TorchAOQuantDType.int8
+        if str == "nf4":
+            return TorchAOQuantDType.nf4
         if str in ["float8_e4m3fn", "fp8", "float8"]:
             return TorchAOQuantDType.float8_e4m3fn
         if str == "nvfp4":

src/axolotl/utils/schemas/peft.py

@@ -1,9 +1,12 @@
 """Pydantic models for PEFT-related configuration"""
 
-from typing import Any
+from typing import Any, Literal
 
 from pydantic import BaseModel, Field, field_validator, model_validator
 
+from axolotl.utils.schemas.enums import TorchAOQuantDType
+from axolotl.utils.schemas.quantization import validate_ao_dtype
+
 
 class LoftQConfig(BaseModel):
     """LoftQ configuration subset"""
@@ -15,7 +18,7 @@ class LoftQConfig(BaseModel):
 
 
 class PeftConfig(BaseModel):
-    """peftq configuration subset"""
+    """PEFT configuration subset"""
 
     loftq_config: LoftQConfig | None = Field(
         default=None,
@@ -23,6 +26,29 @@ class PeftConfig(BaseModel):
             "description": "Configuration options for loftq initialization for LoRA"
         },
     )
+    backend: Literal["bnb", "torchao"] | None = Field(
+        default=None,
+        json_schema_extra={
+            "description": "Quantization backend for QLoRA. 'bnb' for bitsandbytes (default), 'torchao' for torchao."
+        },
+    )
+    weight_dtype: TorchAOQuantDType | None = Field(
+        default=None,
+        json_schema_extra={
+            "description": "Weight quantization dtype (int4, int8, or nf4). Also used with bnb backend to auto-configure quantization."
+        },
+    )
+    group_size: int | None = Field(
+        default=None,
+        json_schema_extra={
+            "description": "Group size for quantization. Defaults to 128 for int4, 64 for nf4."
+        },
+    )
+
+    @field_validator("weight_dtype", mode="before")
+    @classmethod
+    def validate_weight_dtype(cls, v):
+        return validate_ao_dtype(v)
 
 
 class LoraConfig(BaseModel):
@@ -156,6 +182,56 @@ class LoraConfig(BaseModel):
 
     merge_lora: bool | None = None
 
+    @model_validator(mode="before")
+    @classmethod
+    def auto_detect_qlora(cls, data):
+        """Auto-set adapter type and quantization flags from peft config.
+
+        When peft.backend and peft.weight_dtype are set, this infers the correct
+        adapter type and internal flags (load_in_4bit, load_in_8bit) so users
+        don't need to set them manually.
+        """
+        peft = data.get("peft")
+        if not isinstance(peft, dict):
+            return data
+
+        backend = peft.get("backend")
+        weight_dtype = peft.get("weight_dtype")
+
+        # Validate: weight_dtype requires backend
+        if weight_dtype and not backend:
+            raise ValueError(
+                "peft.backend is required when peft.weight_dtype is set. "
+                "Use 'torchao' or 'bnb'."
+            )
+
+        if not weight_dtype:
+            return data
+
+        adapter = data.get("adapter")
+
+        if backend == "torchao":
+            # torchao: any quantized weight_dtype means qlora
+            if adapter == "lora":
+                data["adapter"] = "qlora"
+
+        elif backend == "bnb":
+            if weight_dtype == "nf4":
+                # bnb nf4 = qlora with load_in_4bit
+                if adapter == "lora":
+                    data["adapter"] = "qlora"
+                data.setdefault("load_in_4bit", True)
+            elif weight_dtype == "int8":
+                # bnb int8 = lora with load_in_8bit
+                data.setdefault("load_in_8bit", True)
+            else:
+                raise ValueError(
+                    f"peft.weight_dtype '{weight_dtype}' is not supported with bnb backend. "
+                    "Supported: nf4, int8."
+                )
+
+        return data
+
     @model_validator(mode="before")
     @classmethod
     def validate_adapter(cls, data):
@@ -173,6 +249,8 @@ class LoraConfig(BaseModel):
     @model_validator(mode="after")
     def validate_qlora(self):
         if self.adapter == "qlora":
+            is_torchao = self.peft and self.peft.backend == "torchao"
+
             if self.merge_lora:
                 # can't merge qlora if loaded in 8bit or 4bit
                 if self.load_in_8bit:
@@ -184,7 +262,20 @@ class LoraConfig(BaseModel):
                 if self.load_in_4bit:
                     raise ValueError("Can't merge qlora if loaded in 4bit")
 
+            elif is_torchao:
+                # torchao backend: validate torchao-specific requirements
+                if self.load_in_4bit or self.load_in_8bit:
+                    raise ValueError(
+                        "load_in_4bit/load_in_8bit are for bitsandbytes. "
+                        "With peft.backend: torchao, quantization is handled by torchao."
+                    )
+                if not self.peft.weight_dtype:
+                    raise ValueError(
+                        "peft.weight_dtype is required when peft.backend is 'torchao'"
+                    )
+
             else:
+                # Default bnb path
                 if self.load_in_8bit:
                     raise ValueError("Can't load qlora in 8bit")
 

src/axolotl/utils/schemas/quantization.py

@@ -16,6 +16,8 @@ def validate_ao_dtype(v: Any) -> TorchAOQuantDType | None:
         return TorchAOQuantDType.int4
     if v == "int8":
         return TorchAOQuantDType.int8
+    if v == "nf4":
+        return TorchAOQuantDType.nf4
     if v in ["float8_e4m3fn", "fp8", "float8"]:
         return TorchAOQuantDType.float8_e4m3fn
     if v == "nvfp4":

tests/e2e/kernels/test_quantize.py

@@ -3,7 +3,7 @@
 import torch
 from bitsandbytes.functional import QuantState
 
-from axolotl.kernels.quantize import dequantize
+from axolotl.kernels.quantize import dequantize, dequantize_weight
 
 
 def test_dequantize_null_state():
@@ -100,3 +100,18 @@ def test_dequantize_output_tensor():
 
     result = dequantize(W, quant_state, out=out)
     assert result is out
+
+
+def test_dequantize_weight_plain_tensor():
+    """Test that dequantize_weight passes through unquantized tensors unchanged"""
+    W = torch.randn(32, 64)
+    result = dequantize_weight(W, quant_state=None, transpose=False)
+    assert torch.equal(result, W)
+
+
+def test_dequantize_weight_plain_tensor_transpose():
+    """Test that dequantize_weight transposes unquantized tensors"""
+    W = torch.randn(32, 64)
+    result = dequantize_weight(W, quant_state=None, transpose=True)
+    assert result.shape == (64, 32)
+    assert torch.equal(result, W.t())

tests/utils/lora/test_config_validation_lora.py

@@ -3,6 +3,14 @@ import pytest
 from axolotl.utils.config import validate_config
 from axolotl.utils.dict import DictDefault
 
+BASE_CFG = {
+    "datasets": [{"path": "dummy_dataset", "type": "alpaca"}],
+    "micro_batch_size": 1,
+    "gradient_accumulation_steps": 1,
+    "learning_rate": 1e-5,
+    "base_model": "dummy_model",
+}
+
 
 class TestLoRAConfigValidation:
     """Test suite for LoRA/QLoRA configuration validation"""
@@ -149,3 +157,195 @@ class TestLoRAConfigValidation:
         result = validate_config(valid_config)
         assert result["lora_qkv_kernel"] is True
         assert result["trust_remote_code"] is None
+
+
+class TestTorchaoQLoRAConfigValidation:
+    """Test suite for torchao QLoRA auto-detection and validation"""
+
+    # --- Auto-detection: torchao ---
+
+    @pytest.mark.parametrize("weight_dtype", ["int4", "int8", "nf4"])
+    def test_torchao_auto_detect_from_lora(self, weight_dtype):
+        """adapter: lora + peft.backend: torchao auto-upgrades to qlora"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "peft": {"backend": "torchao", "weight_dtype": weight_dtype},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+        assert result["peft"]["backend"] == "torchao"
+
+    def test_torchao_explicit_qlora(self):
+        """adapter: qlora + peft.backend: torchao works directly"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "peft": {"backend": "torchao", "weight_dtype": "int4"},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+
+    # --- Auto-detection: bnb ---
+
+    def test_bnb_nf4_auto_detect_from_lora(self):
+        """adapter: lora + peft.backend: bnb + weight_dtype: nf4 → qlora + load_in_4bit"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "peft": {"backend": "bnb", "weight_dtype": "nf4"},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+        assert result["load_in_4bit"] is True
+
+    def test_bnb_int8_auto_detect_from_lora(self):
+        """adapter: lora + peft.backend: bnb + weight_dtype: int8 → lora + load_in_8bit"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "peft": {"backend": "bnb", "weight_dtype": "int8"},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "lora"
+        assert result["load_in_8bit"] is True
+
+    def test_bnb_nf4_explicit_qlora_auto_sets_load_in_4bit(self):
+        """adapter: qlora + peft.backend: bnb + weight_dtype: nf4 auto-sets load_in_4bit"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "peft": {"backend": "bnb", "weight_dtype": "nf4"},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+        assert result["load_in_4bit"] is True
+
+    # --- Backward compat ---
+
+    def test_old_style_qlora_unchanged(self):
+        """Old-style adapter: qlora + load_in_4bit: true still works"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "load_in_4bit": True,
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+        assert result["load_in_4bit"] is True
+
+    def test_old_style_lora_8bit_unchanged(self):
+        """Old-style adapter: lora + load_in_8bit: true still works"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "load_in_8bit": True,
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "lora"
+        assert result["load_in_8bit"] is True
+
+    def test_plain_lora_unchanged(self):
+        """adapter: lora without peft block stays as lora"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "lora"
+
+    # --- Validation errors ---
+
+    def test_torchao_with_load_in_4bit_errors(self):
+        """peft.backend: torchao + load_in_4bit is a conflict"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "load_in_4bit": True,
+                "peft": {"backend": "torchao", "weight_dtype": "int4"},
+                **BASE_CFG,
+            }
+        )
+        with pytest.raises(ValueError, match="load_in_4bit.*bitsandbytes"):
+            validate_config(cfg)
+
+    def test_torchao_with_load_in_8bit_errors(self):
+        """peft.backend: torchao + load_in_8bit is a conflict"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "load_in_8bit": True,
+                "peft": {"backend": "torchao", "weight_dtype": "int4"},
+                **BASE_CFG,
+            }
+        )
+        with pytest.raises(ValueError, match="load_in_4bit.*bitsandbytes"):
+            validate_config(cfg)
+
+    def test_torchao_without_weight_dtype_errors(self):
+        """peft.backend: torchao without weight_dtype errors"""
+        cfg = DictDefault(
+            {
+                "adapter": "qlora",
+                "peft": {"backend": "torchao"},
+                **BASE_CFG,
+            }
+        )
+        with pytest.raises(ValueError, match="peft.weight_dtype is required"):
+            validate_config(cfg)
+
+    def test_weight_dtype_without_backend_errors(self):
+        """peft.weight_dtype without peft.backend errors"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "peft": {"weight_dtype": "int4"},
+                **BASE_CFG,
+            }
+        )
+        with pytest.raises(ValueError, match="peft.backend is required"):
+            validate_config(cfg)
+
+    def test_bnb_unsupported_weight_dtype_errors(self):
+        """peft.backend: bnb + unsupported weight_dtype errors"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "peft": {"backend": "bnb", "weight_dtype": "int4"},
+                **BASE_CFG,
+            }
+        )
+        with pytest.raises(ValueError, match="not supported with bnb"):
+            validate_config(cfg)
+
+    # --- Redundant flags don't conflict ---
+
+    def test_bnb_nf4_with_explicit_load_in_4bit(self):
+        """peft.backend: bnb + weight_dtype: nf4 + load_in_4bit: true is fine (redundant)"""
+        cfg = DictDefault(
+            {
+                "adapter": "lora",
+                "load_in_4bit": True,
+                "peft": {"backend": "bnb", "weight_dtype": "nf4"},
+                **BASE_CFG,
+            }
+        )
+        result = validate_config(cfg)
+        assert result["adapter"] == "qlora"
+        assert result["load_in_4bit"] is True