[Models] Step-3.5-Flash (#33523)

Signed-off-by: Jee Jee Li <pandaleefree@gmail.com>
Co-authored-by: i-zhangmingming <i-zhangmingming@stepfun.com>
Co-authored-by: xiewuxun <xiewuxun@stepfun.com>
Co-authored-by: zetaohong <i-hongzetao@stepfun.com>
Co-authored-by: Jee Jee Li <pandaleefree@gmail.com>

vllm/reasoning/step3p5_reasoning_parser.py

@@ -0,0 +1,153 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from collections.abc import Sequence
+
+from vllm.entrypoints.openai.chat_completion.protocol import (
+    ChatCompletionRequest,
+)
+from vllm.entrypoints.openai.engine.protocol import DeltaMessage
+from vllm.entrypoints.openai.responses.protocol import (
+    ResponsesRequest,
+)
+from vllm.reasoning.basic_parsers import BaseThinkingReasoningParser
+from vllm.tokenizers import TokenizerLike
+
+
+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 right before </think>
+      - the newline right after </think>
+    """
+
+    @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
+
+        # Used to delay the reasoning end detection.
+        # This is necessary to remove the newline appears immediately after </think>,
+        # which may cause the end detection to be delayed by one round.
+        self.end_offset = 1
+
+    def is_reasoning_end(self, input_ids: Sequence[int]) -> bool:
+        if self.end_token_id in input_ids and self.end_offset > 0:
+            self.end_offset -= 1
+            return False
+        return self.end_offset < 1
+
+    def is_reasoning_end_streaming(
+        self, input_ids: Sequence[int], delta_ids: Sequence[int]
+    ) -> bool:
+        if self.end_token_id in input_ids and self.end_offset > 0:
+            self.end_offset -= 1
+            return False
+        return self.end_offset < 1
+
+    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")
+        if content is not None:
+            content = content.removeprefix("\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:
+        # Drop the immediate newline that models often emit after </think>.
+        if previous_text.endswith(self.end_token) and delta_text:
+            if delta_text == "\n":
+                return None
+            elif delta_text.startswith("\n"):
+                remaining = delta_text.removeprefix("\n")
+                return DeltaMessage(content=remaining) if remaining else 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:
+            # No need to get into parser again to remove newline after </think>.
+            self.end_offset -= 1
+
+            # 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)