speculative.py

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

import ast
from typing import TYPE_CHECKING, Any, Literal, get_args

from pydantic import Field, SkipValidation, model_validator
from pydantic.dataclasses import dataclass
from typing_extensions import Self

from vllm.config import LoadConfig
from vllm.config.model import ModelConfig
from vllm.config.parallel import ParallelConfig
from vllm.config.utils import config
from vllm.logger import init_logger
from vllm.transformers_utils.config import get_hf_text_config
from vllm.utils.hashing import safe_hash
from vllm.utils.import_utils import LazyLoader, has_arctic_inference

if TYPE_CHECKING:
    from transformers import PretrainedConfig

    import vllm.model_executor.layers.quantization as me_quant
else:
    PretrainedConfig = Any

    me_quant = LazyLoader(
        "model_executor", globals(), "vllm.model_executor.layers.quantization"
    )

logger = init_logger(__name__)

MTPModelTypes = Literal[
    "deepseek_mtp",
    "mimo_mtp",
    "glm4_moe_mtp",
    "glm4_moe_lite_mtp",
    "ernie_mtp",
    "exaone_moe_mtp",
    "qwen3_next_mtp",
    "qwen3_5_mtp",
    "longcat_flash_mtp",
    "mtp",
    "pangu_ultra_moe_mtp",
    "step3p5_mtp",
]
EagleModelTypes = Literal["eagle", "eagle3", MTPModelTypes]
SpeculativeMethod = Literal[
    "ngram",
    "medusa",
    "mlp_speculator",
    "draft_model",
    "suffix",
    EagleModelTypes,
]


@config
@dataclass
class SpeculativeConfig:
    """Configuration for speculative decoding."""

    enforce_eager: bool | None = None
    """Override the default enforce_eager from model_config"""
    # General speculative decoding control
    num_speculative_tokens: int = Field(default=None, gt=0)
    """The number of speculative tokens, if provided. It will default to the
    number in the draft model config if present, otherwise, it is required."""
    model: str | None = None
    """The name of the draft model, eagle head, or additional weights, if
    provided."""
    method: SpeculativeMethod | None = None
    """The name of the speculative method to use. If users provide and set the
    `model` param, the speculative method type will be detected automatically
    if possible, if `model` param is not provided, the method name must be
    provided.

    If using `ngram` method, the related configuration `prompt_lookup_max` and
    `prompt_lookup_min` should be considered."""
    enable_multi_layers_mtp: bool = False
    """If set to True, the MTP method will run multiple layers of MTP
    speculator. If set to False, it will run only one layer of MTP speculator.
    This is only effective when the method is set to `mtp`."""
    draft_tensor_parallel_size: int | None = Field(default=None, ge=1)
    """The degree of the tensor parallelism for the draft model. Can only be 1
    or the same as the target model's tensor parallel size."""
    tensor_parallel_size: int | None = None
    """Users should pass "draft_tensor_parallel_size". This parameter's purpose is to
    warn users when they mistakenly provide the wrong argument."""

    # Draft model configuration
    quantization: me_quant.QuantizationMethods | None = None
    """Quantization method that was used to quantize the draft model weights.
    If `None`, we assume the model weights are not quantized. Note that it only
    takes effect when using the draft model-based speculative method."""
    max_model_len: int | None = Field(default=None, ge=1)
    """The maximum model length of the draft model. Used when testing the
    ability to skip speculation for some sequences."""
    revision: str | None = None
    """The specific model version to use for the draft model. It can be a
    branch name, a tag name, or a commit id. If unspecified, will use the
    default version."""
    code_revision: str | None = None
    """The specific revision to use for the draft model code on Hugging Face
    Hub. It can be a branch name, a tag name, or a commit id. If unspecified,
    will use the default version."""

    # Advanced control
    disable_by_batch_size: int | None = Field(default=None, ge=2)
    """Disable speculative decoding for new incoming requests when the number
    of enqueued requests is larger than this value, if provided."""
    disable_padded_drafter_batch: bool = False
    """Disable input padding for speculative decoding. If set to True,
    speculative input batches can contain sequences of different lengths,
    which may only be supported by certain attention backends. This currently
    only affects the EAGLE method of speculation."""

    use_local_argmax_reduction: bool = False
    """Use vocab-parallel local argmax instead of all-gathering full logits
    for draft token generation. Reduces communication from O(vocab_size) to
    O(2 * tp_size) per token. Only applies to greedy draft selection in
    non-tree speculation."""
    # Ngram proposer configuration
    prompt_lookup_max: int | None = Field(default=None, ge=1)
    """Maximum size of ngram token window when using Ngram proposer, required
    when method is set to ngram."""
    prompt_lookup_min: int | None = Field(default=None, ge=1)
    """Minimum size of ngram token window when using Ngram proposer, if
    provided. Defaults to 1."""

    speculative_token_tree: str | None = None
    """Specifies the tree structure for speculative token generation.
    """
    parallel_drafting: bool = False
    """Enable parallel drafting, where all speculative tokens are generated
    in parallel rather than sequentially. This can improve performance but
    requires the speculative model be trained to support parallel drafting.
    Only compatible with EAGLE and draft model methods."""

    # required configuration params passed from engine
    target_model_config: SkipValidation[ModelConfig] = None  # type: ignore
    """The configuration of the target model."""
    target_parallel_config: SkipValidation[ParallelConfig] = None  # type: ignore
    """The parallel configuration for the target model."""

    # params generated in the post-init stage
    draft_model_config: SkipValidation[ModelConfig] = None  # type: ignore
    """The configuration of the draft model initialized internal."""
    draft_parallel_config: SkipValidation[ParallelConfig] = None  # type: ignore
    """The parallel configuration for the draft model initialized internal."""

    # Suffix decoding configuration
    suffix_decoding_max_tree_depth: int = 24
    """The maximum depth of the suffix decoding global and prompt trees. The
    tree depth limits the sum of the prefix match and speculation lengths."""

    suffix_decoding_max_cached_requests: int = 10000
    """The maximum number of requests to cache in the global suffix tree. If
    exceeded, will trigger eviction in FIFO order. If set to 0, the global
    suffix tree is disabled and past responses are not cached (prompt trees
    are still used)."""

    suffix_decoding_max_spec_factor: float = 1.0
    """The maximum spec factor for suffix decoding. The spec factor controls
    speculation lengths based on the prefix match length: max_spec_tokens =
    max_spec_factor * prefix_match_length."""

    suffix_decoding_min_token_prob: float = 0.1
    """The minimum token probability for suffix decoding. Will only speculate
    tokens with estimated probability (based on frequency counts) greater than
    or equal to this value."""

    draft_load_config: LoadConfig | None = None
    """Load config for the draft model. If not specified, will use the load
    config from the target model."""

    def compute_hash(self) -> str:
        """
        WARNING: Whenever a new field is added to this config,
        ensure that it is included in the factors list if
        it affects the computation graph.

        Provide a hash that uniquely identifies all the configs
        that affect the structure of the computation
        graph from input ids/embeddings to the final hidden states,
        excluding anything before input ids/embeddings and after
        the final hidden states.
        """
        factors: list[Any] = []
        # Eagle3 affects the computation graph because it returns intermediate
        # hidden states in addition to the final hidden state.
        factors.append(self.method == "eagle3")
        hash_str = safe_hash(str(factors).encode(), usedforsecurity=False).hexdigest()
        return hash_str

    @staticmethod
    def hf_config_override(hf_config: PretrainedConfig) -> PretrainedConfig:
        initial_architecture = hf_config.architectures[0]
        if hf_config.model_type in ("deepseek_v3", "deepseek_v32", "glm_moe_dsa"):
            hf_config.model_type = "deepseek_mtp"
        if hf_config.model_type == "deepseek_mtp":
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["DeepSeekMTPModel"]}
            )
        if hf_config.model_type in ("pangu_ultra_moe"):
            hf_config.model_type = "pangu_ultra_moe_mtp"
        if hf_config.model_type == "pangu_ultra_moe_mtp":
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["OpenPanguMTPModel"]}
            )

        if hf_config.architectures[0] == "MiMoForCausalLM":
            hf_config.model_type = "mimo_mtp"
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {
                    "num_hidden_layers": 0,
                    "n_predict": n_predict,
                    "architectures": ["MiMoMTPModel"],
                }
            )

        if hf_config.architectures[0] == "Glm4MoeForCausalLM":
            hf_config.model_type = "glm4_moe_mtp"
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {
                    "n_predict": n_predict,
                    "architectures": ["Glm4MoeMTPModel"],
                }
            )

        if hf_config.architectures[0] == "Glm4MoeLiteForCausalLM":
            hf_config.model_type = "glm4_moe_lite_mtp"
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {
                    "num_hidden_layers": 0,
                    "n_predict": n_predict,
                    "architectures": ["Glm4MoeLiteMTPModel"],
                }
            )

        if hf_config.model_type == "ernie4_5_moe":
            hf_config.model_type = "ernie_mtp"
        if hf_config.model_type == "ernie_mtp":
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["ErnieMTPModel"]}
            )

        if hf_config.model_type == "qwen3_next":
            hf_config.model_type = "qwen3_next_mtp"
        if hf_config.model_type == "qwen3_next_mtp":
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["Qwen3NextMTP"]}
            )

        if hf_config.model_type == "exaone_moe":
            hf_config.model_type = "exaone_moe_mtp"
        if hf_config.model_type == "exaone_moe_mtp":
            n_predict = getattr(hf_config, "num_nextn_predict_layers", None)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["ExaoneMoeMTP"]}
            )

        if hf_config.model_type in ("qwen3_5", "qwen3_5_moe"):
            is_moe = hf_config.model_type == "qwen3_5_moe"
            hf_config.model_type = "qwen3_5_mtp"
            n_predict = getattr(hf_config, "mtp_num_hidden_layers", None)
            hf_config.update(
                {
                    "n_predict": n_predict,
                    "architectures": ["Qwen3_5MoeMTP" if is_moe else "Qwen3_5MTP"],
                }
            )
        if hf_config.model_type == "longcat_flash":
            hf_config.model_type = "longcat_flash_mtp"
            n_predict = getattr(hf_config, "num_nextn_predict_layers", 1)
            hf_config.update(
                {"n_predict": n_predict, "architectures": ["LongCatFlashMTPModel"]}
            )

        if hf_config.model_type == "step3p5":
            hf_config.model_type = "step3p5_mtp"
            n_predict = getattr(hf_config, "num_nextn_predict_layers", 1)
            hf_config.update({"n_predict": n_predict, "architectures": ["Step3p5MTP"]})

        if initial_architecture == "MistralLarge3ForCausalLM":
            hf_config.update({"architectures": ["EagleMistralLarge3ForCausalLM"]})

        return hf_config

    def __post_init__(self):
        # Note: "method" is a new parameter that helps to extend the
        # configuration of non-model-based proposers, and the "model" parameter
        # will be used to set the draft model, eagle head, or additional weight
        # when needed. If users do not specify "method", the speculative method
        # will be detected automatically if possible. If the speculative method
        # can not be detected, it will be considered as the "draft_model" by
        # default.

        if self.method in get_args(MTPModelTypes) and self.method != "mtp":
            logger.warning(
                "method `%s` is deprecated and replaced with mtp.", self.method
            )
            self.method = "mtp"

        if self.model is None and self.num_speculative_tokens is not None:
            if self.method == "mtp":
                if self.target_model_config is None:
                    raise ValueError("target_model_config must be present for mtp")
                if self.target_model_config.hf_text_config.model_type == "deepseek_v32":
                    # FIXME(luccafong): cudgraph with v32 MTP is not supported,
                    # remove this when the issue is fixed.
                    self.enforce_eager = True
                # use the draft model from the same model:
                self.model = self.target_model_config.model
                # Align the quantization of draft model for cases such as
                # --quantization fp8 with a bf16 checkpoint.
                if not self.quantization:
                    self.quantization = self.target_model_config.quantization
            elif self.method in ("ngram", "[ngram]"):
                self.model = "ngram"
            elif self.method == "suffix":
                self.model = "suffix"
            else:
                raise ValueError(
                    "num_speculative_tokens was provided but without speculative model."
                )

        # Automatically configure the method for ngram when "model" is used
        # instead of "method"
        if self.method is None and (
            self.model is not None and self.model in ("ngram", "[ngram]")
        ):
            self.method = "ngram"

        if self.method in ("ngram", "[ngram]"):
            # Unified to "ngram" internally
            self.method = "ngram"
            # Set default values if not provided
            if self.prompt_lookup_min is None and self.prompt_lookup_max is None:
                # TODO(woosuk): Tune these values. They are arbitrarily chosen.
                self.prompt_lookup_min = 5
                self.prompt_lookup_max = 5
            elif self.prompt_lookup_min is None:
                if self.prompt_lookup_max is None:
                    raise ValueError(
                        "Either prompt_lookup_max or prompt_lookup_min must be "
                        "provided when using the ngram method."
                    )
                self.prompt_lookup_min = self.prompt_lookup_max
            elif self.prompt_lookup_max is None:
                if self.prompt_lookup_min is None:
                    raise ValueError(
                        "Either prompt_lookup_max or prompt_lookup_min must be "
                        "provided when using the ngram method."
                    )
                self.prompt_lookup_max = self.prompt_lookup_min

            # Validate values
            if self.prompt_lookup_min > self.prompt_lookup_max:
                raise ValueError(
                    f"prompt_lookup_min={self.prompt_lookup_min} must "
                    f"be <= prompt_lookup_max={self.prompt_lookup_max}"
                )

            # TODO: current we still need extract vocab_size from target model
            # config, in future, we may try refactor it out, and set
            # draft related config as None here.
            self.draft_model_config = self.target_model_config
            self.draft_parallel_config = self.target_parallel_config
        elif self.method == "suffix":
            self._validate_suffix_decoding()
        else:
            self.prompt_lookup_max = 0
            self.prompt_lookup_min = 0

            if self.model is not None:
                self.draft_model_config = ModelConfig(
                    model=self.model,
                    runner="draft",
                    tokenizer=self.target_model_config.tokenizer,
                    tokenizer_mode=self.target_model_config.tokenizer_mode,
                    trust_remote_code=self.target_model_config.trust_remote_code,
                    allowed_local_media_path=self.target_model_config.allowed_local_media_path,
                    allowed_media_domains=self.target_model_config.allowed_media_domains,
                    dtype=self.target_model_config.dtype,
                    seed=self.target_model_config.seed,
                    revision=self.revision,
                    code_revision=self.code_revision,
                    tokenizer_revision=self.target_model_config.tokenizer_revision,
                    spec_target_max_model_len=self.target_model_config.max_model_len,
                    quantization=self.quantization,
                    enforce_eager=self.target_model_config.enforce_eager,
                    max_logprobs=self.target_model_config.max_logprobs,
                    hf_overrides=SpeculativeConfig.hf_config_override,
                    config_format=self.target_model_config.config_format,
                )

                # Automatically detect the method
                if self.method in ("eagle", "eagle3"):
                    pass
                # examples:
                # yuhuili/EAGLE-LLaMA3-Instruct-8B
                # yuhuili/EAGLE3-LLaMA3.1-Instruct-8B
                # AngelSlim/Qwen3-8B_eagle3
                elif "eagle-" in self.draft_model_config.model.lower():
                    self.method = "eagle"
                elif "eagle3" in self.draft_model_config.model.lower():
                    self.method = "eagle3"
                elif self.draft_model_config.hf_config.model_type == "medusa":
                    self.method = "medusa"
                elif self.draft_model_config.hf_config.model_type == "mlp_speculator":
                    self.method = "mlp_speculator"
                elif self.draft_model_config.hf_config.model_type in get_args(
                    MTPModelTypes
                ):
                    self.method = "mtp"
                    # if self.num_speculative_tokens > 1:
                    if (
                        self.enable_multi_layers_mtp is False
                        and self.num_speculative_tokens > 1
                    ):
                        logger.warning(
                            "Enabling num_speculative_tokens > 1 will run"
                            "multiple times of forward on same MTP layer"
                            ",which may result in lower acceptance rate"
                        )
                elif self.draft_model_config.hf_config.model_type in (
                    "longcat_flash_mtp"
                ):
                    self.method = "longcat_flash_mtp"
                    if self.num_speculative_tokens > 1:
                        logger.warning(
                            "LongCat MTP models only have "
                            "one layer. Might need some code changes "
                            "to support multiple layers."
                        )
                elif self.method == "draft_model":
                    pass
                else:
                    raise NotImplementedError(
                        f"Unsupported speculative method: '{self.method}'"
                    )

                # Replace hf_config for EAGLE draft_model
                if self.method in ("eagle", "eagle3"):
                    from vllm.transformers_utils.configs import SpeculatorsConfig
                    from vllm.transformers_utils.configs.eagle import EAGLEConfig

                    if isinstance(
                        self.draft_model_config.hf_config,
                        (EAGLEConfig, SpeculatorsConfig),
                    ):
                        pass
                    else:
                        eagle_config = EAGLEConfig(
                            self.draft_model_config.hf_config,
                            method=self.method,
                            model_type="eagle",
                        )
                        # EAGLEConfig primarily updates architectures, so update
                        # all architectures-related fields in draft_model_config
                        self.draft_model_config.hf_config = eagle_config
                        self.draft_model_config.hf_text_config = get_hf_text_config(
                            self.draft_model_config.hf_config
                        )
                        self.draft_model_config.model_arch_config = (
                            self.draft_model_config.get_model_arch_config()
                        )
                        model_info, arch = (
                            self.draft_model_config.registry.inspect_model_cls(
                                self.draft_model_config.architectures,
                                self.draft_model_config,
                            )
                        )
                        self.draft_model_config._model_info = model_info
                        self.draft_model_config._architecture = arch

                if self.num_speculative_tokens is not None and hasattr(
                    self.draft_model_config.hf_config, "num_lookahead_tokens"
                ):
                    self.draft_model_config.hf_config.num_lookahead_tokens = (
                        self.num_speculative_tokens
                    )

                n_predict = getattr(
                    self.draft_model_config.hf_config, "n_predict", None
                )
                if n_predict is not None:
                    if self.num_speculative_tokens is None:
                        # Default to max value defined in draft model config.
                        self.num_speculative_tokens = n_predict
                    elif (
                        self.method == "mtp"
                        and self.enable_multi_layers_mtp
                        and self.num_speculative_tokens > n_predict
                    ):
                        logger.warning_once(
                            "For multi_layer_eagle, num_speculative_tokens "
                            "is greater than the layer_num, adjusting to "
                            "layer_num"
                        )
                        self.num_speculative_tokens = n_predict
                    elif (
                        self.num_speculative_tokens > n_predict
                        and self.num_speculative_tokens % n_predict != 0
                    ):
                        # Ensure divisibility for MTP module reuse.
                        raise ValueError(
                            f"num_speculative_tokens:{self.num_speculative_tokens}"
                            f" must be divisible by {n_predict=}"
                        )

                if self.speculative_token_tree is None:
                    # Generate chain of tokens.
                    self.speculative_token_tree = str(
                        [(i + 1) * (0,) for i in range(self.num_speculative_tokens)]
                    )
                else:
                    # Sort the token tree breadth-first.
                    tree_choices = ast.literal_eval(self.speculative_token_tree)
                    self.speculative_token_tree = str(
                        sorted(tree_choices, key=lambda t: (len(t), t))
                    )

                self.draft_tensor_parallel_size = (
                    SpeculativeConfig._verify_and_get_draft_tp(
                        self.target_parallel_config,
                        self.draft_tensor_parallel_size,
                        self.draft_model_config.hf_config,
                    )
                )

                self.draft_model_config.max_model_len = (
                    SpeculativeConfig._maybe_override_draft_max_model_len(
                        self.max_model_len,
                        self.draft_model_config.max_model_len,
                        self.target_model_config.max_model_len,
                    )
                )

                self.draft_parallel_config = (
                    SpeculativeConfig.create_draft_parallel_config(
                        self.target_parallel_config, self.draft_tensor_parallel_size
                    )
                )
        return self

    def _validate_suffix_decoding(self):
        if not has_arctic_inference():
            raise ImportError(
                "Arctic Inference is required for suffix decoding. "
                "Install via `pip install arctic-inference==0.1.1`."
            )
        if self.num_speculative_tokens is None:
            # Suffix decoding decides the actual number of speculative tokens
            # dynamically and treats num_speculative_tokens as a maximum limit.
            self.num_speculative_tokens = self.suffix_decoding_max_tree_depth
            logger.warning(
                "Defaulted num_speculative_tokens to %s for suffix decoding.",
                self.num_speculative_tokens,
            )
        # Validate values
        if self.suffix_decoding_max_tree_depth < 1:
            raise ValueError(
                f"suffix_decoding_max_tree_depth="
                f"{self.suffix_decoding_max_tree_depth} must be >= 1"
            )
        if self.suffix_decoding_max_cached_requests < 0:
            raise ValueError(
                f"suffix_decoding_max_cached_requests="
                f"{self.suffix_decoding_max_cached_requests} must be >= 0"
            )
        if self.suffix_decoding_max_spec_factor < 0:
            raise ValueError(
                f"suffix_decoding_max_spec_factor="
                f"{self.suffix_decoding_max_spec_factor} must be >= 0"
            )
        if not 0 <= self.suffix_decoding_min_token_prob <= 1:
            raise ValueError(
                f"suffix_decoding_min_token_prob="
                f"{self.suffix_decoding_min_token_prob} must be in [0, 1]"
            )

    @staticmethod
    def _maybe_override_draft_max_model_len(
        speculative_max_model_len: int | None,
        draft_max_model_len: int,
        target_max_model_len: int,
    ) -> int:
        """Determine the max sequence len for the draft model. This is usually
        the draft_max_model_len, but may be the target_max_model_len if it is
        less than the draft_max_model_len, or may be speculative_max_model_len
        if it is specified.

        This is necessary so that sequences do not exceed the capacity of the
        draft model or the target model.

        speculative_max_model_len is mainly used for testing that sequences can
        skip speculation.
        """

        if speculative_max_model_len is not None:
            if speculative_max_model_len > draft_max_model_len:
                raise ValueError(
                    f"{speculative_max_model_len=} cannot be "
                    f"larger than {draft_max_model_len=}"
                )

            if speculative_max_model_len > target_max_model_len:
                raise ValueError(
                    f"{speculative_max_model_len=} cannot be "
                    f"larger than {target_max_model_len=}"
                )

            return speculative_max_model_len

        return min(
            draft_max_model_len,
            target_max_model_len,
        )

    @staticmethod
    def _verify_and_get_draft_tp(
        target_parallel_config: ParallelConfig,
        speculative_draft_tensor_parallel_size: int | None,
        draft_hf_config: PretrainedConfig,
    ) -> int:
        """
        Verifies and adjusts the tensor parallel size for a draft model
        specified using speculative_draft_tensor_parallel_size.
        """
        # If speculative_draft_tensor_parallel_size is unset then set it
        # appropriately else verify that it is set correctly.
        if speculative_draft_tensor_parallel_size is None:
            if draft_hf_config.model_type == "mlp_speculator":
                speculative_draft_tensor_parallel_size = 1
                if target_parallel_config.tensor_parallel_size > 1:
                    logger.warning(
                        "%s cannot currently be run with tp>1; "
                        "setting speculative_draft_tensor_parallel_size=1",
                        draft_hf_config.model_type,
                    )
            else:
                speculative_draft_tensor_parallel_size = (
                    target_parallel_config.tensor_parallel_size
                )
        elif speculative_draft_tensor_parallel_size not in (
            1,
            target_parallel_config.tensor_parallel_size,
        ):
            raise ValueError(
                f"{speculative_draft_tensor_parallel_size=} cannot be "
                f"other value than 1 or target model tensor_parallel_size"
            )
        return speculative_draft_tensor_parallel_size

    @staticmethod
    def create_draft_parallel_config(
        target_parallel_config: ParallelConfig,
        speculative_draft_tensor_parallel_size: int,
    ) -> ParallelConfig:
        """Create a parallel config for use by the draft worker.

        This is mostly a copy of the target parallel config, except the tp_size.
        """
        draft_parallel_config = ParallelConfig(
            pipeline_parallel_size=target_parallel_config.pipeline_parallel_size,
            tensor_parallel_size=speculative_draft_tensor_parallel_size,
            distributed_executor_backend=target_parallel_config.distributed_executor_backend,
            max_parallel_loading_workers=target_parallel_config.max_parallel_loading_workers,
            disable_custom_all_reduce=target_parallel_config.disable_custom_all_reduce,
            ray_workers_use_nsight=target_parallel_config.ray_workers_use_nsight,
            placement_group=target_parallel_config.placement_group,
        )

        return draft_parallel_config

    @model_validator(mode="after")
    def _verify_args(self) -> Self:
        if self.tensor_parallel_size is not None:
            raise ValueError(
                "'tensor_parallel_size' is not a valid argument in the "
                "speculative_config. Please pass 'draft_tensor_parallel_size' instead."
            )

        if self.num_speculative_tokens is None:
            raise ValueError(
                "num_speculative_tokens must be provided with "
                "speculative model unless the draft model config contains an "
                "n_predict parameter."
            )

        if self.num_speculative_tokens <= 0:
            raise ValueError(
                "Expected num_speculative_tokens to be greater "
                f"than zero ({self.num_speculative_tokens})."
            )

        if self.draft_model_config:
            self.draft_model_config.verify_with_parallel_config(
                self.draft_parallel_config
            )

        if self.disable_by_batch_size is not None and self.disable_by_batch_size < 2:
            raise ValueError(
                "Expect the batch size threshold of disabling "
                "speculative decoding is > 1, but got "
                f"{self.disable_by_batch_size=}"
            )

        eagle3_target_supported = ["llama", "qwen", "minicpm", "gpt_oss"]
        if (
            self.method == "eagle3"
            and self.target_model_config
            and not any(
                supported_model in self.target_model_config.hf_text_config.model_type
                for supported_model in eagle3_target_supported
            )
        ):
            raise ValueError(
                f"Eagle3 is only supported for {eagle3_target_supported} models. "  # noqa: E501
                f"Got {self.target_model_config.hf_text_config.model_type=}"
            )
        self.verify_equal_vocab_size_if_draft_model()
        return self

    def verify_equal_vocab_size_if_draft_model(self):
        if (
            self.method == "draft_model"
            and self.target_model_config is not None
            and self.draft_model_config is not None
        ):
            target_vocab_size = self.target_model_config.get_vocab_size()
            draft_vocab_size = self.draft_model_config.get_vocab_size()
            if target_vocab_size != draft_vocab_size:
                raise ValueError(
                    f"Target and draft model should have the same vocabulary size. "
                    f"Target model vocab_size={target_vocab_size}. "
                    f"Draft model vocab_size={draft_vocab_size}. "
                    f"Using models with different tokenizers can cause out-of-bounds "
                    f"errors during speculative decoding."
                )

    @property
    def max_num_new_slots_for_drafting(self) -> int:
        """
        Calculate the maximum number of new slots that might be added to the batch
        when drafting.
        """
        slots_per_req = 0  # for serial non-draft-model methods, no change needed
        if self.parallel_drafting:
            # For parallel drafting, we need one new slot per 'masked' token
            slots_per_req = self.num_speculative_tokens - 1
        if self.uses_draft_model():
            # For draft model-based speculation, we need one new slot per request
            # Since we do not slice the draft tokens
            slots_per_req += 1
        return slots_per_req

    def use_eagle(self) -> bool:
        return self.method in ("eagle", "eagle3", "mtp")

    def uses_draft_model(self) -> bool:
        return self.method == "draft_model"

    def uses_extract_hidden_states(self) -> bool:
        return self.method == "extract_hidden_states"

    def __repr__(self) -> str:
        method = self.method
        model = None if method in ("ngram", "suffix") else self.draft_model_config.model
        num_spec_tokens = self.num_speculative_tokens
        return f"SpeculativeConfig({method=}, {model=}, {num_spec_tokens=})"