vllm.reasoning.basic_parsers ¶

class BaseThinkingReasoningParser(ReasoningParser):
    """
    Base class for reasoning parsers that use thinking tokens.

    This class provides common functionality for parsers that use start and end
    tokens to delimit reasoning content (
        e.g., <think>...</think>, <seed:think>...</seed:think>).

    Subclasses must implement the start and end tokens via abstract
    properties.
    """

    @property
    @abstractmethod
    def start_token(self) -> str:
        """The token that starts reasoning content."""
        raise NotImplementedError

    @property
    @abstractmethod
    def end_token(self) -> str:
        """The token that ends reasoning content."""
        raise NotImplementedError

    @property
    def reasoning_start_str(self) -> str:
        return self.start_token

    @property
    def reasoning_end_str(self) -> str:
        return self.end_token

    def __init__(self, tokenizer: TokenizerLike, *args, **kwargs):
        super().__init__(tokenizer, *args, **kwargs)

        if not self.model_tokenizer:
            raise ValueError(
                "The model tokenizer must be passed to the ReasoningParser "
                "constructor during construction."
            )

        if not self.start_token or not self.end_token:
            raise ValueError("start_token and end_token must be defined in subclasses")

        start_token_id = self.vocab.get(self.start_token)
        end_token_id = self.vocab.get(self.end_token)
        if start_token_id is None or end_token_id is None:
            raise RuntimeError(
                f"{self.__class__.__name__} reasoning parser could not locate "
                "think start/end tokens in the tokenizer!"
            )
        self.start_token_id: int = start_token_id
        self.end_token_id: int = end_token_id

    def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
        start_token_id = self.start_token_id
        end_token_id = self.end_token_id

        for i in range(len(input_ids) - 1, -1, -1):
            if input_ids[i] == start_token_id:
                return False
            if input_ids[i] == end_token_id:
                return True
        return False

    def is_reasoning_end_streaming(
        self, input_ids: Sequence[int], delta_ids: Iterable[int]
    ) -> bool:
        end_token_id = self.end_token_id
        return end_token_id in delta_ids

    def extract_content_ids(self, input_ids: list[int]) -> list[int]:
        """
        Extract the content after the end tokens
        """
        if self.end_token_id not in islice(input_ids, 0, max(0, len(input_ids) - 1)):
            return []
        else:
            return input_ids[input_ids.index(self.end_token_id) + 1 :]

    def extract_reasoning_streaming(
        self,
        previous_text: str,
        current_text: str,
        delta_text: str,
        previous_token_ids: Sequence[int],
        current_token_ids: Sequence[int],
        delta_token_ids: Sequence[int],
    ) -> DeltaMessage | None:
        """
        Extract reasoning content from a delta message.
        Handles streaming output where previous + delta = current.
        Uses token IDs for faster processing.
        """
        # Skip single special tokens
        if len(delta_token_ids) == 1 and (
            delta_token_ids[0] in [self.start_token_id, self.end_token_id]
        ):
            return None

        # Check if start token is present in previous or delta.
        # Keep compatibility with models that don't generate start tokens.
        if self.start_token_id in previous_token_ids:
            if self.end_token_id in delta_token_ids:
                # start token in previous, end token in delta,
                # extract reasoning content
                end_index = delta_text.find(self.end_token)
                reasoning = delta_text[:end_index]
                content = delta_text[end_index + len(self.end_token) :]
                return DeltaMessage(
                    reasoning=reasoning, content=content if content else None
                )
            elif self.end_token_id in previous_token_ids:
                # start token in previous, end token in previous,
                # reasoning content continues
                return DeltaMessage(content=delta_text)
            else:
                # start token in previous, no end token in previous or delta,
                # reasoning content continues
                return DeltaMessage(reasoning=delta_text)
        elif self.start_token_id in delta_token_ids:
            if self.end_token_id in delta_token_ids:
                # start token in delta, end token in delta,
                # extract reasoning content
                start_index = delta_text.find(self.start_token)
                end_index = delta_text.find(self.end_token)
                reasoning = delta_text[start_index + len(self.start_token) : end_index]
                content = delta_text[end_index + len(self.end_token) :]
                return DeltaMessage(
                    reasoning=reasoning, content=content if content else None
                )
            else:
                # start token in delta, no end token in delta,
                # reasoning content continues
                return DeltaMessage(reasoning=delta_text)
        else:
            # not find thinking start token
            return DeltaMessage(content=delta_text)

    def extract_reasoning(
        self, model_output: str, request: "ChatCompletionRequest | ResponsesRequest"
    ) -> tuple[str | None, str | None]:
        """
        Extract reasoning content from the model output.

        This is the base implementation that works for most models.
        Subclasses can override this method for specific behavior.
        """
        # Check if the start token is present in the model output, remove it
        # if it is present.
        model_output_parts = model_output.partition(self.start_token)
        model_output = (
            model_output_parts[2] if model_output_parts[1] else model_output_parts[0]
        )

        # For models that may not generate start token,
        # assume the reasoning content is always at the start.
        if self.end_token not in model_output:
            return model_output, None
        else:
            reasoning, _, content = model_output.partition(self.end_token)
            # If generation stops right after end-of-think, return null content
            final_content = content or None
            return reasoning, final_content

    def count_reasoning_tokens(self, token_ids: Sequence[int]) -> int:
        """Count tokens that fall within start/end thinking markers.

        Uses a depth counter so nested spans are handled safely and stray end
        tokens do not drive the counter negative.
        """
        count = 0
        depth = 0
        for token_id in token_ids:
            if token_id == self.start_token_id:
                depth += 1
                continue
            if token_id == self.end_token_id:
                if depth > 0:
                    depth -= 1
                continue
            if depth > 0:
                count += 1
        return count