[Frontend] new online quantization frontend (#38138)

Signed-off-by: Vasiliy Kuznetsov <vasiliy@meta.com>

tests/quantization/test_online.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests online quantization."""
+
+import pytest
+import torch
+
+from tests.quantization.utils import (
+    _test_online_quant_peak_mem_impl,
+    is_quant_method_supported,
+)
+from vllm.model_executor.layers.linear import UnquantizedLinearMethod
+from vllm.model_executor.layers.quantization.online.fp8 import (
+    Fp8PerBlockOnlineLinearMethod,
+    Fp8PerBlockOnlineMoEMethod,
+    Fp8PerTensorOnlineLinearMethod,
+    Fp8PerTensorOnlineMoEMethod,
+)
+from vllm.platforms import current_platform
+
+
+@pytest.mark.skipif(
+    not is_quant_method_supported("fp8"),
+    reason="FP8 is not supported on this GPU type.",
+)
+@pytest.mark.parametrize(
+    "quant_scheme,online_quant_args,expected_linear_cls,expected_moe_cls",
+    [
+        # simple case - quantization='fp8_per_tensor'
+        (
+            "fp8_per_tensor",
+            None,
+            Fp8PerTensorOnlineLinearMethod,
+            Fp8PerTensorOnlineMoEMethod,
+        ),
+        # simple case - quantization='fp8_per_block'
+        (
+            "fp8_per_block",
+            None,
+            Fp8PerBlockOnlineLinearMethod,
+            Fp8PerBlockOnlineMoEMethod,
+        ),
+        # quantization='online with linear_scheme_override and
+        # moe_scheme_override
+        (
+            "online",
+            {
+                "linear_scheme_override": "fp8_per_block",
+                "moe_scheme_override": "fp8_per_tensor",
+            },
+            Fp8PerBlockOnlineLinearMethod,
+            Fp8PerTensorOnlineMoEMethod,
+        ),
+        # ignore with direct layer name
+        (
+            "fp8_per_tensor",
+            # qkv_proj is fused from q_proj/k_proj/v_proj, so currently the
+            # ignore regex must match the unfused shard names
+            # TODO(future PR): also make 're:.*qkv_proj.*' work
+            {"ignore": ["model.layers.1.self_attn.o_proj", "re:.*[qkv]_proj"]},
+            Fp8PerTensorOnlineLinearMethod,
+            Fp8PerTensorOnlineMoEMethod,
+        ),
+    ],
+)
+@pytest.mark.parametrize(
+    "use_rocm_aiter", [True, False] if current_platform.is_rocm() else [False]
+)
+def test_online_quantization(
+    vllm_runner,
+    quant_scheme: str,
+    online_quant_args: dict | None,
+    expected_linear_cls,
+    expected_moe_cls,
+    use_rocm_aiter: bool,
+    monkeypatch,
+) -> None:
+    """
+    Tests that online quantization frontend configuration works -
+    selecting quant schemes, overriding quant schemes by type, ignoring
+    layers.
+
+    Does not test performance, peak memory usage, etc.
+    """
+
+    if use_rocm_aiter:
+        monkeypatch.setenv("VLLM_ROCM_USE_AITER", "1")
+
+    # `LLM.apply_model` requires pickling a function.
+    monkeypatch.setenv("VLLM_ALLOW_INSECURE_SERIALIZATION", "1")
+
+    # a tiny model with both dense and MoE layers
+    model_name = "ibm-granite/granite-3.0-1b-a400m-base"
+
+    runner_kwargs = dict(
+        quantization=quant_scheme,
+        enforce_eager=True,
+    )
+    if online_quant_args is not None:
+        runner_kwargs["quantization_config"] = online_quant_args
+
+    with vllm_runner(
+        model_name,
+        **runner_kwargs,
+    ) as llm:
+
+        def check_model(model):
+            # checks further down in the test case are hardcoded for this
+            # model
+            assert model_name == "ibm-granite/granite-3.0-1b-a400m-base"
+
+            o_proj = model.model.layers[0].self_attn.o_proj
+            moe = model.model.layers[0].block_sparse_moe.experts
+
+            # o_proj and moe in layer 0 are always quantized (never ignored)
+            # because of how we craft the test case inputs
+            assert isinstance(o_proj.quant_method, expected_linear_cls)
+            if moe is not None:
+                assert isinstance(moe.quant_method, expected_moe_cls)
+
+            if current_platform.is_cuda():
+                assert o_proj.weight.dtype == torch.float8_e4m3fn
+            elif current_platform.is_rocm():
+                assert o_proj.weight.dtype == current_platform.fp8_dtype()
+            else:
+                pytest.skip("Only runs on CUDA and ROCm.")
+
+            # Verify ignored layers are unquantized.
+            if isinstance(online_quant_args, dict) and "ignore" in online_quant_args:
+                # only .*1.self_attn_o_proj is skipped
+                for layer_idx in range(len(model.model.layers)):
+                    o_proj = model.model.layers[layer_idx].self_attn.o_proj
+                    if layer_idx == 1:
+                        assert isinstance(o_proj.quant_method, UnquantizedLinearMethod)
+                    else:
+                        assert isinstance(o_proj.quant_method, expected_linear_cls)
+
+                # every .*self_attn.qkv_proj is skipped
+                for layer_idx in range(len(model.model.layers)):
+                    qkv_proj = model.model.layers[layer_idx].self_attn.qkv_proj
+                    assert isinstance(qkv_proj.quant_method, UnquantizedLinearMethod)
+
+        llm.apply_model(check_model)
+
+        outputs = llm.generate_greedy(["Hello my name is"], max_tokens=4)
+        print(outputs[0][1])
+
+
+@pytest.mark.skipif(
+    not is_quant_method_supported("fp8"),
+    reason="FP8 is not supported on this GPU type.",
+)
+def test_online_quant_peak_mem(
+    vllm_runner,
+    caplog_mp_spawn,
+    monkeypatch,
+) -> None:
+    _test_online_quant_peak_mem_impl(
+        "fp8_per_tensor", vllm_runner, caplog_mp_spawn, monkeypatch
+    )
+
+
+@pytest.mark.skipif(
+    not is_quant_method_supported("fp8"),
+    reason="FP8 is not supported on this GPU type.",
+)
+def test_online_quant_load_format_dummy(
+    vllm_runner,
+    monkeypatch,
+    caplog,
+) -> None:
+    with vllm_runner(
+        "ibm-granite/granite-3.0-1b-a400m-base",
+        quantization="fp8_per_tensor",
+        enforce_eager=True,
+        load_format="dummy",
+    ) as llm:
+        outputs = llm.generate_greedy(["The future of AI is"], max_tokens=4)
+        print(outputs[0][1])

tests/quantization/utils.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import logging
+
+import regex as re
+
 from vllm.model_executor.layers.quantization import get_quantization_config
 from vllm.platforms import current_platform
 
@@ -21,3 +25,74 @@ def is_quant_method_supported(quant_method: str) -> bool:
     min_capability = get_quantization_config(quant_method).get_min_capability()
 
     return capability.to_int() >= min_capability
+
+
+def _test_online_quant_peak_mem_impl(
+    quantization_arg_value,
+    vllm_runner,
+    caplog_mp_spawn,
+    monkeypatch,
+) -> None:
+    # Note: `allenai/OLMoE-1B-7B-0125-Instruct` was selected because:
+    # 1. it covers both Linear and MoE paths
+    # 2. it is already used by other tests in CI, so adding it here
+    #    does not increase disk space for CI runners
+    # I really wanted to use `ibm-granite/granite-3.0-1b-a400m-base`
+    # which I think is the smallest MoE model in vLLM (2.5 GiB bf16,
+    # 1.3 GiB fp8), but could not as adding one more model makes CI
+    # run out of disk space.
+    model_name = "allenai/OLMoE-1B-7B-0125-Instruct"
+
+    # Force spawn to ensure caplog_mp_spawn works consistently
+    # (it relies on VLLM_LOGGING_CONFIG_PATH which spawn reads but fork ignores)
+    monkeypatch.setenv("VLLM_WORKER_MULTIPROC_METHOD", "spawn")
+
+    with (
+        caplog_mp_spawn(logging.DEBUG) as log_holder,
+        vllm_runner(
+            model_name,
+            quantization=quantization_arg_value,
+            enforce_eager=True,
+        ) as llm,
+    ):
+        outputs = llm.generate_greedy(["The future of AI is"], max_tokens=4)
+        print(outputs[0][1])
+
+    log_text = log_holder.text
+
+    # Parse memory usage from captured logs
+    model_memory_gib = None
+    peak_memory_gib = None
+    for line in log_text.splitlines():
+        if model_memory_gib is None:
+            match = re.search(r"Model loading took ([\d.]+) GiB memory", line)
+            if match:
+                model_memory_gib = float(match.group(1))
+        if peak_memory_gib is None:
+            match = re.search(
+                r"Peak GPU memory after loading weights: ([\d.]+) GiB", line
+            )
+            if match:
+                peak_memory_gib = float(match.group(1))
+
+    assert model_memory_gib is not None, "Could not find model loading memory log"
+    assert peak_memory_gib is not None, "Could not find peak memory log"
+    print(f"GPU memory used after loading weights: {model_memory_gib} GiB")
+    print(f"Peak GPU memory usage while loading weights: {peak_memory_gib} GiB")
+
+    # model specific, allenai/OLMoE-1B-7B-0125-Instruct fp8 online quant
+    # uses 6.65 GiB for weight loading (bf16 checkpoint is ~12.89 GiB)
+    expected_model_memory_gib = 6.7
+
+    # for allenai/OLMoE-1B-7B-0125-Instruct the number we see today is 9.06
+    # GiB, which is 1.36x above model_memory_gib. A slightly higher number is
+    # expected as when we load and quantize weights in a streaming fashion we
+    # need to have individual weights in bf16 + fp8 alive at the same time.
+    expected_peak_memory_gib = expected_model_memory_gib * 1.4
+
+    assert model_memory_gib < expected_model_memory_gib, (
+        f"{model_memory_gib=} higher than {expected_model_memory_gib}"
+    )
+    assert peak_memory_gib < expected_peak_memory_gib, (
+        f"{peak_memory_gib=} higher than {expected_peak_memory_gib}"
+    )

vllm/config/model.py

@@ -21,6 +21,7 @@ from vllm.config.multimodal import (
     MultiModalConfig,
 )
 from vllm.config.pooler import PoolerConfig
+from vllm.config.quantization import OnlineQuantizationConfigArgs
 from vllm.config.scheduler import RunnerType
 from vllm.config.utils import config, getattr_iter
 from vllm.logger import init_logger
@@ -199,6 +200,10 @@ class ModelConfig:
     `quantization_config` attribute in the model config file. If that is
     `None`, we assume the model weights are not quantized and use `dtype` to
     determine the data type of the weights."""
+    quantization_config: dict[str, Any] | OnlineQuantizationConfigArgs | None = None
+    """Arguments for online quantization.
+    Auto-created when `quantization` equals to one of the string values of
+    the `OnlineQuantScheme` enum."""
     allow_deprecated_quantization: bool = False
     """Whether to allow deprecated quantization methods."""
     enforce_eager: bool = False

vllm/config/quantization.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from enum import Enum
+from typing import Any
+
+from pydantic import Field, field_validator
+
+from vllm.config.utils import config
+
+
+class OnlineQuantScheme(Enum):
+    """Supported online quantization schemes."""
+
+    # fp8, weights and activations scaled per-tensor
+    FP8_PER_TENSOR = "fp8_per_tensor"
+
+    # fp8, activations scaled in blocks of 1x128 elements, weights scaled in
+    # blocks of 128x128 elements (popularized by DeepSeek)
+    FP8_PER_BLOCK = "fp8_per_block"
+
+    # TODO(future PRs): add more online quant schemes here: mxfp8, etc
+
+
+@config
+class OnlineQuantizationConfigArgs:
+    """Configuration for online quantization.
+
+    Controls how ``OnlineQuantizationConfig`` is applied to a model.
+    At least one of ``global_scheme``, ``linear_scheme_override``, or
+    ``moe_scheme_override`` must be set.
+    """
+
+    global_scheme: OnlineQuantScheme | None = None
+    """Quantization scheme applied to every supported layer."""
+
+    linear_scheme_override: OnlineQuantScheme | None = None
+    """Quantization scheme override for ``LinearBase`` layers."""
+
+    moe_scheme_override: OnlineQuantScheme | None = None
+    """Quantization scheme override for ``FusedMoE`` layers."""
+
+    ignore: list[str] = Field(default_factory=list)
+    """Layers to skip quantization for. Supports exact names and regex
+    patterns with ``re:`` prefix (e.g. ``re:.*attn.*``), consistent with
+    compressed_tensors layer skipping."""
+
+    @field_validator(
+        "global_scheme", "linear_scheme_override", "moe_scheme_override", mode="before"
+    )
+    @classmethod
+    def _coerce_scheme(
+        cls, v: str | OnlineQuantScheme | None
+    ) -> OnlineQuantScheme | None:
+        if isinstance(v, str):
+            return OnlineQuantScheme(v)
+        return v
+
+
+def resolve_online_quant_config(
+    quantization: str | None,
+    quantization_config: dict[str, Any] | OnlineQuantizationConfigArgs | None,
+) -> OnlineQuantizationConfigArgs | None:
+    """Resolve online quant scheme shorthand into a quantization config.
+
+    If ``quantization`` is an online quant scheme (e.g. ``'fp8_per_tensor'``),
+    ensures ``quantization_config`` has a matching ``global_scheme`` and casts
+    it to :class:`OnlineQuantizationConfigArgs` if needed.
+    """
+    online_quant_values = {s.value for s in OnlineQuantScheme}
+    valid_quantization_values = online_quant_values | {"online"}
+    if quantization not in valid_quantization_values:
+        if quantization_config is not None:
+            raise ValueError(
+                f"quantization_config is only supported when quantization "
+                f"is one of {sorted(valid_quantization_values)}, "
+                f"got quantization={quantization!r}"
+            )
+        return None
+
+    if quantization in online_quant_values:
+        scheme = OnlineQuantScheme(quantization)
+
+        if quantization_config is None:
+            quantization_config = {
+                "global_scheme": scheme.value,
+            }
+        elif isinstance(quantization_config, OnlineQuantizationConfigArgs):
+            if quantization_config.global_scheme is None:
+                quantization_config.global_scheme = scheme
+            elif quantization_config.global_scheme != scheme:
+                raise ValueError(
+                    f"quantization={quantization!r} conflicts with "
+                    f"quantization_config.global_scheme="
+                    f"{quantization_config.global_scheme.value!r}. "
+                    f"These must match when both are specified."
+                )
+        elif isinstance(quantization_config, dict):
+            existing = quantization_config.get("global_scheme")
+            if existing is None:
+                quantization_config["global_scheme"] = scheme.value
+            else:
+                # Coerce to enum for comparison
+                existing_scheme = (
+                    OnlineQuantScheme(existing)
+                    if isinstance(existing, str)
+                    else existing
+                )
+                if existing_scheme != scheme:
+                    raise ValueError(
+                        f"quantization={quantization!r} conflicts "
+                        f"with quantization_config"
+                        f"['global_scheme']={existing!r}. "
+                        f"These must match when both are specified."
+                    )
+
+    # Cast dict to OnlineQuantizationConfigArgs
+    if isinstance(quantization_config, dict):
+        quantization_config = OnlineQuantizationConfigArgs(**quantization_config)
+
+    return quantization_config

vllm/config/vllm.py

@@ -1713,6 +1713,7 @@ class VllmConfig:
             f"dcp_comm_backend={self.parallel_config.dcp_comm_backend}, "  # noqa
             f"disable_custom_all_reduce={self.parallel_config.disable_custom_all_reduce}, "  # noqa
             f"quantization={self.model_config.quantization}, "
+            f"quantization_config={self.model_config.quantization_config}, "  # noqa
             f"enforce_eager={self.model_config.enforce_eager}, "
             f"enable_return_routed_experts={self.model_config.enable_return_routed_experts}, "  # noqa
             f"kv_cache_dtype={self.cache_config.cache_dtype}, "

vllm/engine/arg_utils.py

@@ -112,6 +112,7 @@ from vllm.v1.sample.logits_processor import LogitsProcessor
 from vllm.version import __version__ as VLLM_VERSION
 
 if TYPE_CHECKING:
+    from vllm.config.quantization import OnlineQuantizationConfigArgs
     from vllm.model_executor.layers.quantization import QuantizationMethods
     from vllm.model_executor.model_loader import LoadFormats
     from vllm.usage.usage_lib import UsageContext
@@ -483,6 +484,7 @@ class EngineArgs:
     hf_overrides: HfOverrides = get_field(ModelConfig, "hf_overrides")
     tokenizer_revision: str | None = ModelConfig.tokenizer_revision
     quantization: QuantizationMethods | str | None = ModelConfig.quantization
+    quantization_config: "dict[str, Any] | OnlineQuantizationConfigArgs | None" = None
     allow_deprecated_quantization: bool = ModelConfig.allow_deprecated_quantization
     enforce_eager: bool = ModelConfig.enforce_eager
     disable_custom_all_reduce: bool = ParallelConfig.disable_custom_all_reduce
@@ -661,6 +663,12 @@ class EngineArgs:
         if isinstance(self.ir_op_priority, dict):
             self.ir_op_priority = IrOpPriorityConfig(**self.ir_op_priority)
 
+        from vllm.config.quantization import resolve_online_quant_config
+
+        self.quantization_config = resolve_online_quant_config(
+            self.quantization, self.quantization_config
+        )
+
         # Setup plugins
         from vllm.plugins import load_general_plugins
 
@@ -1431,6 +1439,7 @@ class EngineArgs:
             tokenizer_revision=self.tokenizer_revision,
             max_model_len=self.max_model_len,
             quantization=self.quantization,
+            quantization_config=self.quantization_config,
             allow_deprecated_quantization=self.allow_deprecated_quantization,
             enforce_eager=self.enforce_eager,
             enable_return_routed_experts=self.enable_return_routed_experts,

vllm/entrypoints/llm.py

@@ -34,6 +34,9 @@ from vllm.config.model import (
     RunnerOption,
     TokenizerMode,
 )
+from vllm.config.quantization import (
+    OnlineQuantizationConfigArgs,
+)
 from vllm.distributed.weight_transfer.base import (
     WeightTransferInitRequest,
     WeightTransferUpdateRequest,
@@ -247,6 +250,9 @@ class LLM:
         attention_config: dict[str, Any] | AttentionConfig | None = None,
         kv_cache_memory_bytes: int | None = None,
         compilation_config: int | dict[str, Any] | CompilationConfig | None = None,
+        quantization_config: dict[str, Any]
+        | OnlineQuantizationConfigArgs
+        | None = None,
         logits_processors: list[str | type[LogitsProcessor]] | None = None,
         **kwargs: Any,
     ) -> None:
@@ -367,6 +373,7 @@ class LLM:
             profiler_config=profiler_config_instance,
             attention_config=attention_config_instance,
             compilation_config=compilation_config_instance,
+            quantization_config=quantization_config,
             logits_processors=logits_processors,
             **kwargs,
         )

vllm/model_executor/layers/quantization/__init__.py

@@ -33,6 +33,13 @@ QuantizationMethods = Literal[
     "mxfp8",
     "petit_nvfp4",
     "cpu_awq",
+    "online",
+    # Below are values of the OnlineQuantScheme enum, specified as strings to
+    # avoid circular import issues. This is here to provide a shortcut where
+    # the user can specify "LLM(..., quantization='fp8_per_tensor')" as
+    # shorthand for creating a more complicated online quant config object
+    "fp8_per_tensor",
+    "fp8_per_block",
 ]
 QUANTIZATION_METHODS: list[str] = list(get_args(QuantizationMethods))
 
@@ -103,6 +110,7 @@ def get_quantization_config(quantization: str) -> type[QuantizationConfig]:
         raise ValueError(f"Invalid quantization method: {quantization}")
 
     # lazy import to avoid triggering `torch.compile` too early
+    from vllm.config.quantization import OnlineQuantScheme
     from vllm.model_executor.layers.quantization.quark.quark import QuarkConfig
 
     from .awq import AWQConfig
@@ -129,6 +137,7 @@ def get_quantization_config(quantization: str) -> type[QuantizationConfig]:
     from .moe_wna16 import MoeWNA16Config
     from .mxfp4 import Mxfp4Config
     from .mxfp8 import Mxfp8Config
+    from .online.base import OnlineQuantizationConfig
     from .petit import PetitNvFp4Config
     from .torchao import TorchAOConfig
 
@@ -157,7 +166,20 @@ def get_quantization_config(quantization: str) -> type[QuantizationConfig]:
         "mxfp8": Mxfp8Config,
         "petit_nvfp4": PetitNvFp4Config,
         "cpu_awq": CPUAWQConfig,
+        "online": OnlineQuantizationConfig,
     }
+
+    # Below are values of the OnlineQuantScheme enum. This is here to provide
+    # a shortcut where the user can specify
+    # "LLM(..., quantization='fp8_per_tensor')" as shorthand for creating a
+    # more complicated online quant config object
+    for scheme in OnlineQuantScheme:
+        assert scheme.value not in method_to_config, (
+            f"Online quant scheme {scheme.value!r} conflicts with an "
+            f"existing quantization method"
+        )
+        method_to_config[scheme.value] = OnlineQuantizationConfig
+
     # Update the `method_to_config` with customized quantization methods.
     method_to_config.update(_CUSTOMIZED_METHOD_TO_QUANT_CONFIG)
 

vllm/model_executor/layers/quantization/fp8.py

@@ -497,6 +497,8 @@ class Fp8LinearMethod(LinearMethodBase):
         return self.fp8_linear.apply_weights(layer, x, bias)
 
 
+# TODO(future PR): remove this class in favor of
+# online/fp8.py::Fp8PerTensorOnlineLinearMethod
 class Fp8OnlineLinearMethod(Fp8LinearMethod):
     """Online version of Fp8LinearMethod which loads a full precision checkpoint
     and quantizes weights during loading."""
@@ -919,6 +921,8 @@ class Fp8MoEMethod(FusedMoEMethodBase):
         )
 
 
+# TODO(future PR): remove this class in favor of
+# online/fp8.py::Fp8PerTensorOnlineMoEMethod
 class Fp8OnlineMoEMethod(Fp8MoEMethod):
     """MoE method for online FP8 quantization.
     Supports loading quantized FP16/BF16 model checkpoints with dynamic

vllm/model_executor/layers/quantization/online/__init__.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project

vllm/model_executor/layers/quantization/online/base.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from typing import Any
+
+import torch
+
+from vllm.config.quantization import (
+    OnlineQuantizationConfigArgs,
+    OnlineQuantScheme,
+)
+from vllm.model_executor.layers.fused_moe import (
+    FusedMoE,
+)
+from vllm.model_executor.layers.fused_moe.unquantized_fused_moe_method import (
+    UnquantizedFusedMoEMethod,
+)
+from vllm.model_executor.layers.linear import (
+    LinearBase,
+    UnquantizedLinearMethod,
+)
+from vllm.model_executor.layers.quantization import QuantizationMethods
+from vllm.model_executor.layers.quantization.base_config import (
+    QuantizationConfig,
+    QuantizeMethodBase,
+)
+from vllm.model_executor.layers.quantization.compressed_tensors.utils import (
+    should_ignore_layer,
+)
+from vllm.model_executor.layers.quantization.online.fp8 import (
+    Fp8PerBlockOnlineLinearMethod,
+    Fp8PerBlockOnlineMoEMethod,
+    Fp8PerTensorOnlineLinearMethod,
+    Fp8PerTensorOnlineMoEMethod,
+)
+
+
+class OnlineQuantizationConfig(QuantizationConfig):
+    """Model-level config class for online quantization (quantize fp16/bf16 weights
+    during model loading, without requiring a pre-quantized checkpoint)."""
+
+    def __init__(
+        self,
+        args: OnlineQuantizationConfigArgs,
+    ) -> None:
+        super().__init__()
+        if (
+            args.global_scheme is None
+            and args.linear_scheme_override is None
+            and args.moe_scheme_override is None
+        ):
+            raise ValueError(
+                "OnlineQuantizationConfig requires at least one of "
+                "global_scheme, linear_scheme_override, or "
+                "moe_scheme_override to be set."
+            )
+        self.args = args
+        self.quant_scheme = args.global_scheme
+        self.ignored_layers: list[str] = args.ignore
+
+    @classmethod
+    def get_name(cls) -> QuantizationMethods:
+        return "online"
+
+    @classmethod
+    def get_supported_act_dtypes(cls) -> list[torch.dtype]:
+        return [torch.bfloat16, torch.half]
+
+    @classmethod
+    def get_min_capability(cls) -> int:
+        # Note: as more online quant schemes will be added, this
+        # value will become the minimum across all supported schemes.
+        return 75
+
+    @classmethod
+    def get_config_filenames(cls) -> list[str]:
+        return []
+
+    @classmethod
+    def from_config(cls, config: dict[str, Any]) -> "OnlineQuantizationConfig":
+        raise NotImplementedError(
+            "OnlineQuantizationConfig does not support loading from a "
+            "checkpoint config. Use quantization_config or "
+            "quantization='fp8_per_tensor'/'fp8_per_block' instead."
+        )
+
+    def get_quant_method(
+        self, layer: torch.nn.Module, prefix: str
+    ) -> "QuantizeMethodBase | None":
+        if isinstance(layer, LinearBase):
+            if should_ignore_layer(
+                prefix,
+                ignore=self.ignored_layers,
+                fused_mapping=self.packed_modules_mapping,
+            ):
+                return UnquantizedLinearMethod()
+
+            linear_scheme = self.args.linear_scheme_override or self.args.global_scheme
+            if linear_scheme == OnlineQuantScheme.FP8_PER_BLOCK:
+                return Fp8PerBlockOnlineLinearMethod()
+            else:
+                return Fp8PerTensorOnlineLinearMethod()
+        elif isinstance(layer, FusedMoE):
+            if should_ignore_layer(
+                prefix,
+                ignore=self.ignored_layers,
+                fused_mapping=self.packed_modules_mapping,
+            ):
+                return UnquantizedFusedMoEMethod(layer.moe_config)
+
+            moe_scheme = self.args.moe_scheme_override or self.args.global_scheme
+            if moe_scheme == OnlineQuantScheme.FP8_PER_BLOCK:
+                return Fp8PerBlockOnlineMoEMethod(layer=layer)
+            else:
+                return Fp8PerTensorOnlineMoEMethod(layer=layer)
+        return None

vllm/model_executor/model_loader/weight_utils.py

@@ -296,6 +296,13 @@ def get_quant_config(
         )
 
     if hf_quant_config is not None:
+        if model_config.quantization_config is not None:
+            raise ValueError(
+                "Setting `quantization_config` for online "
+                "quantization when the model checkpoint already "
+                "has a `quantization_config` is not supported"
+            )
+
         # For modelopt_mixed, config.json's quantization_config may or may
         # not contain the per-layer quantized_layers map.  Newer checkpoints
         # embed it directly; older ones keep it only in hf_quant_config.json.
@@ -319,6 +326,12 @@ def get_quant_config(
     quantization_config_file = hf_overrides.get("quantization_config_file", None)
     if quantization_config_file is not None:
         if hasattr(quant_cls, "from_config_file"):
+            if model_config.quantization_config is not None:
+                raise ValueError(
+                    "Setting `quantization_config` for online "
+                    "quantization when the model checkpoint already "
+                    "has a `quantization_config` is not supported"
+                )
             return quant_cls.from_config_file(quantization_config_file)
         else:
             raise NotImplementedError(
@@ -329,6 +342,12 @@ def get_quant_config(
     quantization_config_json = hf_overrides.get("quantization_config_dict_json", None)
     if quantization_config_json is not None:
         if hasattr(quant_cls, "from_config_dict_json"):
+            if model_config.quantization_config is not None:
+                raise ValueError(
+                    "Setting `quantization_config` for online "
+                    "quantization when the model checkpoint already "
+                    "has a `quantization_config` is not supported"
+                )
             return quant_cls.from_config_dict_json(quantization_config_json)
         else:
             raise NotImplementedError(
@@ -337,6 +356,19 @@ def get_quant_config(
                 f"{quant_cls}"
             )
 
+    # Online quantization doesn't read from checkpoint configs — it quantizes
+    # fp16/bf16 weights on the fly during loading.
+    if model_config.quantization_config is not None:
+        from vllm.config.quantization import OnlineQuantizationConfigArgs
+        from vllm.model_executor.layers.quantization.online.base import (
+            OnlineQuantizationConfig,
+        )
+
+        assert isinstance(
+            model_config.quantization_config, OnlineQuantizationConfigArgs
+        )
+        return OnlineQuantizationConfig(args=model_config.quantization_config)
+
     # Inflight BNB quantization
     if model_config.quantization == "bitsandbytes":
         return quant_cls.from_config({})