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vllm.reasoning.step3p5_reasoning_parser

Classes:

Step3p5ReasoningParser

Bases: BaseThinkingReasoningParser

Reasoning parser for Step3p5 model.

Step3p5 uses the ... format, but it tends to emit an extra newline immediately before and/or after the token. This parser trims the newline before in extract_reasoning*.

Reasoning end detection doesn't wait for an extra token after so structured output can constrain the first post-think token.

Source code in vllm/reasoning/step3p5_reasoning_parser.py 
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class Step3p5ReasoningParser(BaseThinkingReasoningParser):
    """
    Reasoning parser for Step3p5 model.

    Step3p5 uses the <think>...</think> format, but it tends to emit an extra
    newline immediately before and/or after the </think> token. This parser
    trims the newline before </think> in extract_reasoning*.

    Reasoning end detection doesn't wait for an extra token after </think> so
    structured output can constrain the first post-think token.
    """

    @property
    def start_token(self) -> str:
        return "<think>"

    @property
    def end_token(self) -> str:
        return "</think>"

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

        # Used to hold a trailing "\n" from reasoning content so we can decide
        # whether it is immediately before </think>.
        self._pending_reasoning_newline = False

    def extract_reasoning(
        self,
        model_output: str,
        request: "ChatCompletionRequest | ResponsesRequest",
    ) -> tuple[str | None, str | None]:
        reasoning, content = super().extract_reasoning(model_output, request)
        if reasoning is not None:
            reasoning = reasoning.removesuffix("\n")
        return reasoning or None, content or None

    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:
        ret = super().extract_reasoning_streaming(
            previous_text,
            current_text,
            delta_text,
            previous_token_ids,
            current_token_ids,
            delta_token_ids,
        )

        if ret is None:
            return None

        # Compatibility path for models that don't generate the start token:
        # treat everything before </think> as reasoning and everything after
        # as content.
        if (
            self.start_token_id not in previous_token_ids
            and self.start_token_id not in delta_token_ids
        ):
            if self.end_token_id in delta_token_ids:
                end_index = delta_text.find(self.end_token)
                reasoning = delta_text[:end_index]
                content = delta_text[end_index + len(self.end_token) :]
                ret = DeltaMessage(reasoning=reasoning, content=content or None)
            elif self.end_token_id in previous_token_ids:
                ret = DeltaMessage(content=delta_text)
            else:
                ret = DeltaMessage(reasoning=delta_text)

        reasoning_to_output = ret.reasoning
        content_to_output = ret.content

        # Reasoning: handle the newline immediately before </think>.
        if reasoning_to_output is not None:
            if self._pending_reasoning_newline:
                reasoning_to_output = "\n" + reasoning_to_output
                self._pending_reasoning_newline = False

            if reasoning_to_output.endswith("\n"):
                reasoning_to_output = reasoning_to_output.removesuffix("\n")
                if self.end_token in delta_text:
                    # Trailing "\n" is right before </think>, drop it.
                    self._pending_reasoning_newline = False
                else:
                    # Hold the trailing "\n" until we know whether </think> follows.
                    self._pending_reasoning_newline = True

        # Content: handle the newline immediately after </think>.
        if content_to_output is not None:
            # If we have content, reasoning must have ended.
            self._pending_reasoning_newline = False

            if self.end_token in delta_text and content_to_output.startswith("\n"):
                content_to_output = content_to_output.removeprefix("\n")

        reasoning_to_output = reasoning_to_output or None
        content_to_output = content_to_output or None
        if reasoning_to_output is None and content_to_output is None:
            return None

        return DeltaMessage(reasoning=reasoning_to_output, content=content_to_output)