sampling_params.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""Sampling parameters for text generation."""

import copy
import json
from dataclasses import field
from enum import Enum, IntEnum
from functools import cached_property
from typing import Annotated, Any

import msgspec
from pydantic.dataclasses import dataclass

from vllm.config import ModelConfig, SpeculativeConfig, StructuredOutputsConfig
from vllm.exceptions import VLLMValidationError
from vllm.logger import init_logger
from vllm.tokenizers import TokenizerLike
from vllm.v1.serial_utils import PydanticMsgspecMixin

logger = init_logger(__name__)

_SAMPLING_EPS = 1e-5
_MAX_TEMP = 1e-2


class SamplingType(IntEnum):
    GREEDY = 0
    RANDOM = 1
    RANDOM_SEED = 2


# maybe make msgspec?
@dataclass
class StructuredOutputsParams:
    # One of these fields will be used to build a logit processor.
    json: str | dict | None = None
    regex: str | None = None
    choice: list[str] | None = None
    grammar: str | None = None
    json_object: bool | None = None
    # These are other options that can be set.
    disable_fallback: bool = False
    disable_any_whitespace: bool = False
    disable_additional_properties: bool = False
    whitespace_pattern: str | None = None
    structural_tag: str | None = None

    _backend: str | None = field(default=None, init=False)
    """CAUTION: Should only be set by Processor._validate_structured_output"""
    _backend_was_auto: bool = field(default=False, init=False)
    """CAUTION: Should only be set by Processor._validate_structured_output"""

    def __post_init__(self):
        """Validate that some fields are mutually exclusive."""
        count = sum(
            [
                self.json is not None,
                self.regex is not None,
                self.choice is not None,
                self.grammar is not None,
                self.json_object is not None,
                self.structural_tag is not None,
            ]
        )
        if count > 1:
            raise ValueError(
                "You can only use one kind of structured outputs constraint "
                f"but multiple are specified: {self.__dict__}"
            )
        if count < 1:
            raise ValueError(
                "You must use one kind of structured outputs constraint "
                f"but none are specified: {self.__dict__}"
            )

    def all_constraints_none(self) -> bool:
        """
        Returns True if all structured-output constraint fields are None.
        """
        return all(
            getattr(self, field) is None
            for field in (
                "json",
                "regex",
                "choice",
                "grammar",
                "json_object",
                "structural_tag",
            )
        )

    def all_non_structural_tag_constraints_none(self) -> bool:
        """
        Returns True if all structured-output constraint fields are None.
        """
        return all(
            getattr(self, field) is None
            for field in (
                "json",
                "regex",
                "choice",
                "grammar",
                "json_object",
            )
        )


class RequestOutputKind(Enum):
    # Return entire output so far in every RequestOutput
    CUMULATIVE = 0
    # Return only deltas in each RequestOutput
    DELTA = 1
    # Do not return intermediate RequestOutput
    FINAL_ONLY = 2


class SamplingParams(
    PydanticMsgspecMixin,
    msgspec.Struct,
    omit_defaults=True,  # type: ignore[call-arg]
    # required for @cached_property.
    dict=True,
):  # type: ignore[call-arg]
    """Sampling parameters for text generation.

    Overall, we follow the sampling parameters from the OpenAI text completion
    API (https://platform.openai.com/docs/api-reference/completions/create).
    In addition, we support beam search, which is not supported by OpenAI.
    """

    n: int = 1
    """Number of outputs to return for the given prompt request.

    NOTE:
        `AsyncLLM` streams outputs by default. When `n > 1`, all `n` outputs
        are generated and streamed cumulatively per request. To see all `n`
        outputs upon completion, use `output_kind=RequestOutputKind.FINAL_ONLY`
        in `SamplingParams`."""
    presence_penalty: float = 0.0
    """Penalizes new tokens based on whether they appear in the generated text
    so far. Values > 0 encourage the model to use new tokens, while values < 0
    encourage the model to repeat tokens."""
    frequency_penalty: float = 0.0
    """Penalizes new tokens based on their frequency in the generated text so
    far. Values > 0 encourage the model to use new tokens, while values < 0
    encourage the model to repeat tokens."""
    repetition_penalty: float = 1.0
    """Penalizes new tokens based on whether they appear in the prompt and the
    generated text so far. Values > 1 encourage the model to use new tokens,
    while values < 1 encourage the model to repeat tokens."""
    temperature: float = 1.0
    """Controls the randomness of the sampling. Lower values make the model
    more deterministic, while higher values make the model more random. Zero
    means greedy sampling."""
    top_p: float = 1.0
    """Controls the cumulative probability of the top tokens to consider. Must
    be in (0, 1]. Set to 1 to consider all tokens."""
    top_k: int = 0
    """Controls the number of top tokens to consider. Set to 0 (or -1) to
    consider all tokens."""
    min_p: float = 0.0
    """Represents the minimum probability for a token to be considered,
    relative to the probability of the most likely token. Must be in [0, 1].
    Set to 0 to disable this."""
    seed: int | None = None
    """Random seed to use for the generation."""
    stop: str | list[str] | None = None
    """String(s) that stop the generation when they are generated. The returned
    output will not contain the stop strings."""
    stop_token_ids: list[int] | None = None
    """Token IDs that stop the generation when they are generated. The returned
    output will contain the stop tokens unless the stop tokens are special
    tokens."""
    ignore_eos: bool = False
    """Whether to ignore the EOS token and continue generating
    tokens after the EOS token is generated."""
    max_tokens: int | None = 16
    """Maximum number of tokens to generate per output sequence."""
    min_tokens: int = 0
    """Minimum number of tokens to generate per output sequence before EOS or
    `stop_token_ids` can be generated"""
    logprobs: int | None = None
    """Number of log probabilities to return per output token. When set to
    `None`, no probability is returned. If set to a non-`None` value, the
    result includes the log probabilities of the specified number of most
    likely tokens, as well as the chosen tokens. Note that the implementation
    follows the OpenAI API: The API will always return the log probability of
    the sampled token, so there may be up to `logprobs+1` elements in the
    response. When set to -1, return all `vocab_size` log probabilities."""
    prompt_logprobs: int | None = None
    """Number of log probabilities to return per prompt token.
    When set to -1, return all `vocab_size` log probabilities."""
    flat_logprobs: bool = False
    """Whether to return logprobs in flatten format (i.e. FlatLogprob)
    for better performance.
    NOTE: GC costs of FlatLogprobs is significantly smaller than
    list[dict[int, Logprob]]. After enabled, PromptLogprobs and
    SampleLogprobs would populated as FlatLogprobs."""
    # NOTE: This parameter is only exposed at the engine level for now.
    # It is not exposed in the OpenAI API server, as the OpenAI API does
    # not support returning only a list of token IDs.
    detokenize: bool = True
    """Whether to detokenize the output."""
    skip_special_tokens: bool = True
    """Whether to skip special tokens in the output."""
    spaces_between_special_tokens: bool = True
    """Whether to add spaces between special tokens in the output."""
    include_stop_str_in_output: bool = False
    """Whether to include the stop strings in output text."""
    truncate_prompt_tokens: Annotated[int, msgspec.Meta(ge=-1)] | None = None
    """If set to -1, will use the truncation size supported by the model. If
    set to an integer k, will use only the last k tokens from the prompt
    (i.e., left truncation). If set to `None`, truncation is disabled."""
    output_kind: RequestOutputKind = RequestOutputKind.CUMULATIVE
    skip_clone: bool = False
    """Internal flag indicating that this SamplingParams instance is safe to
    reuse without cloning. When True, clone() will return self without
    performing a deep copy. This should only be set when the params object
    is guaranteed to be dedicated to a single request and won't be modified
    in ways that would affect other uses."""

    # The below fields are not supposed to be used as an input.
    # They are set in post_init.
    output_text_buffer_length: int = 0
    _eos_token_id: int | None = None
    _all_stop_token_ids: set[int] = msgspec.field(default_factory=set)

    # Fields used to construct logits processors
    structured_outputs: StructuredOutputsParams | None = None
    """Parameters for configuring structured outputs."""
    logit_bias: dict[int, float] | None = None
    """If provided, the engine will construct a logits processor that applies
    these logit biases."""
    allowed_token_ids: list[int] | None = None
    """If provided, the engine will construct a logits processor which only
    retains scores for the given token ids."""
    extra_args: dict[str, Any] | None = None
    """Arbitrary additional args, that can be used by custom sampling
    implementations, plugins, etc. Not used by any in-tree sampling
    implementations."""

    # Fields used for bad words
    bad_words: list[str] | None = None
    """Words that are not allowed to be generated. More precisely, only the
    last token of a corresponding token sequence is not allowed when the next
    generated token can complete the sequence."""
    _bad_words_token_ids: list[list[int]] | None = None

    skip_reading_prefix_cache: bool | None = None

    @staticmethod
    def from_optional(
        n: int | None = 1,
        presence_penalty: float | None = 0.0,
        frequency_penalty: float | None = 0.0,
        repetition_penalty: float | None = 1.0,
        temperature: float | None = 1.0,
        top_p: float | None = 1.0,
        top_k: int = 0,
        min_p: float = 0.0,
        seed: int | None = None,
        stop: str | list[str] | None = None,
        stop_token_ids: list[int] | None = None,
        bad_words: list[str] | None = None,
        include_stop_str_in_output: bool = False,
        ignore_eos: bool = False,
        max_tokens: int | None = 16,
        min_tokens: int = 0,
        logprobs: int | None = None,
        prompt_logprobs: int | None = None,
        detokenize: bool = True,
        skip_special_tokens: bool = True,
        spaces_between_special_tokens: bool = True,
        truncate_prompt_tokens: Annotated[int, msgspec.Meta(ge=-1)] | None = None,
        output_kind: RequestOutputKind = RequestOutputKind.CUMULATIVE,
        structured_outputs: StructuredOutputsParams | None = None,
        logit_bias: dict[int, float] | dict[str, float] | None = None,
        allowed_token_ids: list[int] | None = None,
        extra_args: dict[str, Any] | None = None,
        skip_clone: bool = False,
    ) -> "SamplingParams":
        if logit_bias is not None:
            # Convert token_id to integer
            # Clamp the bias between -100 and 100 per OpenAI API spec
            logit_bias = {
                int(token): min(100.0, max(-100.0, bias))
                for token, bias in logit_bias.items()
            }

        return SamplingParams(
            n=1 if n is None else n,
            presence_penalty=0.0 if presence_penalty is None else presence_penalty,
            frequency_penalty=0.0 if frequency_penalty is None else frequency_penalty,
            repetition_penalty=1.0
            if repetition_penalty is None
            else repetition_penalty,
            temperature=1.0 if temperature is None else temperature,
            top_p=1.0 if top_p is None else top_p,
            top_k=top_k,
            min_p=min_p,
            seed=seed,
            stop=stop,
            stop_token_ids=stop_token_ids,
            bad_words=bad_words,
            include_stop_str_in_output=include_stop_str_in_output,
            ignore_eos=ignore_eos,
            max_tokens=max_tokens,
            min_tokens=min_tokens,
            logprobs=logprobs,
            prompt_logprobs=prompt_logprobs,
            detokenize=detokenize,
            skip_special_tokens=skip_special_tokens,
            spaces_between_special_tokens=spaces_between_special_tokens,
            truncate_prompt_tokens=truncate_prompt_tokens,
            output_kind=output_kind,
            structured_outputs=structured_outputs,
            logit_bias=logit_bias,
            allowed_token_ids=allowed_token_ids,
            extra_args=extra_args,
            skip_clone=skip_clone,
        )

    def __post_init__(self) -> None:
        if 0 < self.temperature < _MAX_TEMP:
            logger.warning(
                "temperature %s is less than %s, which may cause numerical "
                "errors nan or inf in tensors. We have maxed it out to %s.",
                self.temperature,
                _MAX_TEMP,
                _MAX_TEMP,
            )
            self.temperature = max(self.temperature, _MAX_TEMP)

        if self.seed == -1:
            self.seed = None

        if self.stop is None:
            self.stop = []
        elif isinstance(self.stop, str):
            self.stop = [self.stop]

        if self.stop_token_ids is None:
            self.stop_token_ids = []

        if self.bad_words is None:
            self.bad_words = []

        if self.logprobs is True:
            self.logprobs = 1

        if self.prompt_logprobs is True:
            self.prompt_logprobs = 1

        # Number of characters to hold back for stop string evaluation
        # until sequence is finished.
        if self.stop and not self.include_stop_str_in_output:
            self.output_text_buffer_length = max(len(s) for s in self.stop) - 1

        self._verify_args()

        if self.temperature < _SAMPLING_EPS:
            # Zero temperature means greedy sampling.
            self.top_p = 1.0
            self.top_k = 0
            self.min_p = 0.0
            self._verify_greedy_sampling()

        # eos_token_id is added to this by the engine
        self._all_stop_token_ids.update(self.stop_token_ids)

        if self.skip_reading_prefix_cache is None:
            # If prefix caching is enabled,
            # the output of prompt logprobs may less than n_prompt_tokens,
            # we need to skip reading cache at this request.
            self.skip_reading_prefix_cache = self.prompt_logprobs is not None

    def _verify_args(self) -> None:
        if not isinstance(self.n, int):
            raise ValueError(f"n must be an int, but is of type {type(self.n)}")
        if self.n < 1:
            raise ValueError(f"n must be at least 1, got {self.n}.")
        if not -2.0 <= self.presence_penalty <= 2.0:
            raise ValueError(
                f"presence_penalty must be in [-2, 2], got {self.presence_penalty}."
            )
        if not -2.0 <= self.frequency_penalty <= 2.0:
            raise ValueError(
                f"frequency_penalty must be in [-2, 2], got {self.frequency_penalty}."
            )
        if self.repetition_penalty <= 0.0:
            raise ValueError(
                "repetition_penalty must be greater than zero, got "
                f"{self.repetition_penalty}."
            )
        if self.temperature < 0.0:
            raise VLLMValidationError(
                f"temperature must be non-negative, got {self.temperature}.",
                parameter="temperature",
                value=self.temperature,
            )
        if not 0.0 < self.top_p <= 1.0:
            raise VLLMValidationError(
                f"top_p must be in (0, 1], got {self.top_p}.",
                parameter="top_p",
                value=self.top_p,
            )
        # quietly accept -1 as disabled, but prefer 0
        if self.top_k < -1:
            raise ValueError(
                f"top_k must be 0 (disable), or at least 1, got {self.top_k}."
            )
        if not isinstance(self.top_k, int):
            raise TypeError(
                f"top_k must be an integer, got {type(self.top_k).__name__}"
            )
        if not 0.0 <= self.min_p <= 1.0:
            raise ValueError(f"min_p must be in [0, 1], got {self.min_p}.")
        if self.max_tokens is not None and self.max_tokens < 1:
            raise VLLMValidationError(
                f"max_tokens must be at least 1, got {self.max_tokens}.",
                parameter="max_tokens",
                value=self.max_tokens,
            )
        if self.min_tokens < 0:
            raise ValueError(
                f"min_tokens must be greater than or equal to 0, got {self.min_tokens}."
            )
        if self.max_tokens is not None and self.min_tokens > self.max_tokens:
            raise ValueError(
                f"min_tokens must be less than or equal to "
                f"max_tokens={self.max_tokens}, got {self.min_tokens}."
            )
        if self.logprobs is not None and self.logprobs != -1 and self.logprobs < 0:
            raise VLLMValidationError(
                f"logprobs must be non-negative or -1, got {self.logprobs}.",
                parameter="logprobs",
                value=self.logprobs,
            )
        if (
            self.prompt_logprobs is not None
            and self.prompt_logprobs != -1
            and self.prompt_logprobs < 0
        ):
            raise VLLMValidationError(
                f"prompt_logprobs must be non-negative or -1, got "
                f"{self.prompt_logprobs}.",
                parameter="prompt_logprobs",
                value=self.prompt_logprobs,
            )
        if self.truncate_prompt_tokens is not None and (
            self.truncate_prompt_tokens == 0 or self.truncate_prompt_tokens < -1
        ):
            raise VLLMValidationError(
                f"truncate_prompt_tokens must be an integer >= 1 or -1, "
                f"got {self.truncate_prompt_tokens}",
                parameter="truncate_prompt_tokens",
                value=self.truncate_prompt_tokens,
            )
        assert isinstance(self.stop_token_ids, list)
        if not all(isinstance(st_id, int) for st_id in self.stop_token_ids):
            raise ValueError(
                f"stop_token_ids must contain only integers, got {self.stop_token_ids}."
            )
        assert isinstance(self.stop, list)
        if any(not stop_str for stop_str in self.stop):
            raise ValueError("stop cannot contain an empty string.")
        if self.stop and not self.detokenize:
            raise ValueError(
                "stop strings are only supported when detokenize is True. "
                "Set detokenize=True to use stop."
            )

    def _verify_greedy_sampling(self) -> None:
        if self.n > 1:
            raise ValueError(f"n must be 1 when using greedy sampling, got {self.n}.")

    def update_from_generation_config(
        self,
        generation_config: dict[str, Any],
        eos_token_id: int | None = None,
    ) -> None:
        """Update if there are non-default values from generation_config"""
        if not self.ignore_eos:
            self._eos_token_id = eos_token_id

        if eos_token_id is not None:
            # Add the eos token id into the sampling_params to support
            # min_tokens processing.
            self._all_stop_token_ids.add(eos_token_id)

        # Update eos_token_id for generation
        if (eos_ids := generation_config.get("eos_token_id")) is not None:
            # it can be either int or list of int
            eos_ids = {eos_ids} if isinstance(eos_ids, int) else set(eos_ids)
            if eos_token_id is not None:
                # We don't need to include the primary eos_token_id in
                # stop_token_ids since it's handled separately for stopping
                # purposes.
                eos_ids.discard(eos_token_id)
            if eos_ids:
                self._all_stop_token_ids.update(eos_ids)
                if not self.ignore_eos:
                    eos_ids.update(self.stop_token_ids)
                    self.stop_token_ids = list(eos_ids)

    def update_from_tokenizer(self, tokenizer: TokenizerLike) -> None:
        if not self.bad_words:
            return
        self._bad_words_token_ids = []
        for bad_word in self.bad_words:
            # To prohibit words both at the beginning
            # and in the middle of text
            # (related to add_prefix_space tokenizer parameter)
            for add_prefix_space in [False, True]:
                prefix = " " if add_prefix_space else ""
                prompt = prefix + bad_word.lstrip()
                prompt_token_ids = tokenizer.encode(
                    text=prompt, add_special_tokens=False
                )

                # If no space at the beginning
                # or if prefix space produces a new word token
                if (not add_prefix_space) or (
                    add_prefix_space
                    and prompt_token_ids[0] != self._bad_words_token_ids[-1][0]
                    and len(prompt_token_ids) == len(self._bad_words_token_ids[-1])
                ):
                    self._bad_words_token_ids.append(prompt_token_ids)

        invalid_token_ids = [
            token_id
            for bad_words_token_ids in self._bad_words_token_ids
            for token_id in bad_words_token_ids
            if token_id < 0 or token_id > tokenizer.max_token_id
        ]
        if len(invalid_token_ids) > 0:
            raise VLLMValidationError(
                f"The model vocabulary size is {tokenizer.max_token_id + 1},"
                f" but the following tokens"
                f" were specified as bad: {invalid_token_ids}."
                f" All token id values should be integers satisfying:"
                f" 0 <= token_id <= {tokenizer.max_token_id}.",
                parameter="bad_words",
                value=self.bad_words,
            )

    @cached_property
    def sampling_type(self) -> SamplingType:
        if self.temperature < _SAMPLING_EPS:
            return SamplingType.GREEDY
        if self.seed is not None:
            return SamplingType.RANDOM_SEED
        return SamplingType.RANDOM

    @property
    def eos_token_id(self) -> int | None:
        return self._eos_token_id

    @property
    def all_stop_token_ids(self) -> set[int]:
        return self._all_stop_token_ids

    @property
    def bad_words_token_ids(self) -> list[list[int]] | None:
        # For internal use only. Backward compatibility not guaranteed
        return self._bad_words_token_ids

    def clone(self) -> "SamplingParams":
        """If skip_clone is True, uses shallow copy instead of deep copy."""
        if self.skip_clone:
            return copy.copy(self)

        return copy.deepcopy(self)

    def verify(
        self,
        model_config: ModelConfig,
        speculative_config: SpeculativeConfig | None,
        structured_outputs_config: StructuredOutputsConfig | None,
        tokenizer: TokenizerLike | None,
    ) -> None:
        self._validate_logprobs(model_config)
        self._validate_logit_bias(model_config)
        self._validate_logits_processors(model_config)
        self._validate_allowed_token_ids(tokenizer)
        self._validate_spec_decode(speculative_config)
        self._validate_structured_outputs(structured_outputs_config, tokenizer)

    def _validate_logprobs(self, model_config: ModelConfig) -> None:
        max_logprobs = model_config.max_logprobs
        if max_logprobs == -1:
            max_logprobs = model_config.get_vocab_size()

        # Validate sample logprobs.
        if num_logprobs := self.logprobs:
            if num_logprobs == -1:
                num_logprobs = model_config.get_vocab_size()
            if num_logprobs > max_logprobs:
                raise VLLMValidationError(
                    f"Requested sample logprobs of {num_logprobs}, "
                    f"which is greater than max allowed: {max_logprobs}",
                    parameter="logprobs",
                    value=num_logprobs,
                )

        # Validate prompt logprobs.
        if num_prompt_logprobs := self.prompt_logprobs:
            if num_prompt_logprobs == -1:
                num_prompt_logprobs = model_config.get_vocab_size()
            if num_prompt_logprobs > max_logprobs:
                raise VLLMValidationError(
                    f"Requested prompt logprobs of {num_prompt_logprobs}, "
                    f"which is greater than max allowed: {max_logprobs}",
                    parameter="prompt_logprobs",
                    value=num_prompt_logprobs,
                )

    def _validate_logit_bias(self, model_config: ModelConfig) -> None:
        """Validate logit_bias token IDs are within vocabulary range."""
        if not self.logit_bias:
            return

        vocab_size = model_config.get_vocab_size()
        invalid_token_ids = [
            token_id
            for token_id in self.logit_bias
            if token_id < 0 or token_id >= vocab_size
        ]

        if invalid_token_ids:
            raise VLLMValidationError(
                f"token_id(s) {invalid_token_ids} in logit_bias contain "
                f"out-of-vocab token ids. Vocabulary size: {vocab_size}",
                parameter="logit_bias",
                value=invalid_token_ids,
            )

    def _validate_logits_processors(self, model_config: ModelConfig) -> None:
        from vllm.v1.sample.logits_processor import (
            validate_logits_processors_parameters,
        )

        validate_logits_processors_parameters(model_config.logits_processors, self)

    def _validate_allowed_token_ids(self, tokenizer: TokenizerLike | None) -> None:
        allowed_token_ids = self.allowed_token_ids
        if allowed_token_ids is None:
            return

        if len(allowed_token_ids) == 0:
            raise VLLMValidationError(
                "allowed_token_ids is not None and empty!",
                parameter="allowed_token_ids",
                value=allowed_token_ids,
            )

        if tokenizer is not None:
            vocab_size = len(tokenizer)
            invalid_token_ids = [
                token_id
                for token_id in allowed_token_ids
                if token_id < 0 or token_id >= vocab_size
            ]
            if invalid_token_ids:
                raise VLLMValidationError(
                    "allowed_token_ids contains out-of-vocab token id!",
                    parameter="allowed_token_ids",
                    value=invalid_token_ids,
                )

    def _validate_spec_decode(
        self,
        speculative_config: SpeculativeConfig | None,
    ) -> None:
        if speculative_config is None:
            return

        # Some sampling parameters are not yet compatible with spec decoding.
        if self.min_tokens > 1 or self.min_p > _SAMPLING_EPS or self.logit_bias:
            raise ValueError(
                "The min_tokens, min_p, and logit_bias sampling parameters "
                "are not yet supported with speculative decoding."
            )

    def _validate_structured_outputs(
        self,
        structured_outputs_config: StructuredOutputsConfig | None,
        tokenizer: TokenizerLike | None,
    ) -> None:
        if structured_outputs_config is None or self.structured_outputs is None:
            return

        if tokenizer is None:
            raise ValueError(
                "Structured outputs requires a tokenizer so it can't be used with 'skip_tokenizer_init'"  # noqa: E501
            )

        backend = structured_outputs_config.backend
        if _backend := self.structured_outputs._backend:
            # Request-level backend selection is not supported.
            # The values may differ if `params` is reused and was set
            # to a specific backend based on `auto` behavior in a previous
            # request. We remember that it was set as a result of `auto`
            # using the `_backend_was_auto` field set in the params.
            if backend != _backend and not (
                backend == "auto" and self.structured_outputs._backend_was_auto
            ):
                raise ValueError(
                    "Request-level structured output backend selection is not "
                    f"supported. The request specified '{_backend}', but vLLM "
                    f"was initialised with '{backend}'. This error can be "
                    "resolved by removing '_backend' from the request."
                )
        else:
            self.structured_outputs._backend = backend

        # Request content validation
        if (
            isinstance(self.structured_outputs.choice, list)
            and not self.structured_outputs.choice
        ):
            # It is invalid for choice to be an empty list
            raise ValueError(
                f"Choice '{self.structured_outputs.choice}' cannot be an empty list"  # noqa: E501
            )
        # Reject empty string grammar early to avoid engine-side crashes
        if (
            isinstance(self.structured_outputs.grammar, str)
            and self.structured_outputs.grammar.strip() == ""
        ):
            raise ValueError("structured_outputs.grammar cannot be an empty string")

        from vllm.tokenizers.mistral import MistralTokenizer
        from vllm.v1.structured_output.backend_guidance import (
            has_guidance_unsupported_json_features,
            validate_guidance_grammar,
        )
        from vllm.v1.structured_output.backend_lm_format_enforcer import (
            validate_structured_output_request_lm_format_enforcer,
        )
        from vllm.v1.structured_output.backend_outlines import (
            validate_structured_output_request_outlines,
        )
        from vllm.v1.structured_output.backend_xgrammar import validate_xgrammar_grammar

        if backend.startswith("xgrammar"):
            # xgrammar with no fallback
            validate_xgrammar_grammar(self)
        elif backend.startswith("guidance"):
            # TODO: ideally we would have the LLTokenizer here as Lark syntax
            # allows <|special_token|> and similar, see
            # https://github.com/guidance-ai/llguidance/blob/main/docs/syntax.md#special-tokens
            # Without tokenizer these are disallowed in grammars.
            if isinstance(tokenizer, MistralTokenizer):
                raise ValueError(
                    "Mistral tokenizer is not supported for the 'guidance' "
                    "structured output backend. Please use ['xgrammar', 'outlines'] "
                    "backends or tokenizer_mode='hf' instead."
                )
            validate_guidance_grammar(self, tokenizer=None)
        elif backend == "outlines":
            # outlines backend
            validate_structured_output_request_outlines(self)
        elif backend == "lm-format-enforcer":
            # lm format enforcer backend
            if isinstance(tokenizer, MistralTokenizer):
                raise ValueError(
                    "Mistral tokenizer is not supported for the 'lm-format-enforcer' "
                    "structured output backend. Please use ['xgrammar', 'outlines'] "
                    "backends or tokenizer_mode='hf' instead."
                )
            validate_structured_output_request_lm_format_enforcer(self)
        else:
            # NOTE: backend must be "auto" here, because we have
            # checked supported_backends above.
            # In this mode, we set opinionated defaults based on what we think
            # will satisfy the most use cases without having to worry about
            # this setting. We include fallback behavior here, but not with any
            # other setting where a specific backend was specified.
            try:
                validate_xgrammar_grammar(self)
                self.structured_outputs._backend = "xgrammar"
            except ValueError:
                # The request either failed validation
                # or includes some jsonschema feature(s) that
                # are not supported in xgrammar.

                # Check if schema has features unsupported by guidance
                so_params = self.structured_outputs
                skip_guidance = False
                if so_params.json:
                    if isinstance(so_params.json, str):
                        schema = json.loads(so_params.json)
                    else:
                        schema = so_params.json
                    skip_guidance = has_guidance_unsupported_json_features(schema)

                if isinstance(tokenizer, MistralTokenizer) or skip_guidance:
                    # Fall back to outlines if the tokenizer is Mistral
                    # or if schema contains features unsupported by guidance
                    validate_structured_output_request_outlines(self)
                    self.structured_outputs._backend = "outlines"
                else:
                    # Fall back to guidance by default.
                    validate_guidance_grammar(self, tokenizer=None)
                    self.structured_outputs._backend = "guidance"
            # Remember that this backend was set automatically
            self.structured_outputs._backend_was_auto = True

        # Run post-init validation. This is also important to ensure subsequent
        # roundtrip serialization/deserialization won't fail.
        self.structured_outputs.__post_init__()

    def __repr__(self) -> str:
        return (
            f"SamplingParams(n={self.n}, "
            f"presence_penalty={self.presence_penalty}, "
            f"frequency_penalty={self.frequency_penalty}, "
            f"repetition_penalty={self.repetition_penalty}, "
            f"temperature={self.temperature}, "
            f"top_p={self.top_p}, "
            f"top_k={self.top_k}, "
            f"min_p={self.min_p}, "
            f"seed={self.seed}, "
            f"stop={self.stop}, "
            f"stop_token_ids={self.stop_token_ids}, "
            f"bad_words={self.bad_words}, "
            f"include_stop_str_in_output={self.include_stop_str_in_output}, "
            f"ignore_eos={self.ignore_eos}, "
            f"max_tokens={self.max_tokens}, "
            f"min_tokens={self.min_tokens}, "
            f"logprobs={self.logprobs}, "
            f"prompt_logprobs={self.prompt_logprobs}, "
            f"skip_special_tokens={self.skip_special_tokens}, "
            "spaces_between_special_tokens="
            f"{self.spaces_between_special_tokens}, "
            f"truncate_prompt_tokens={self.truncate_prompt_tokens}, "
            f"structured_outputs={self.structured_outputs}, "
            f"extra_args={self.extra_args})"
        )


class BeamSearchParams(
    msgspec.Struct,
    omit_defaults=True,  # type: ignore[call-arg]
    # required for @cached_property.
    dict=True,
):  # type: ignore[call-arg]
    """Beam search parameters for text generation."""

    beam_width: int
    max_tokens: int
    ignore_eos: bool = False
    temperature: float = 0.0
    length_penalty: float = 1.0
    include_stop_str_in_output: bool = False