[Bugfix] Fix and add tests for GptOss reasoning parser (#28000)

Signed-off-by: Benjamin Chislett <bchislett@nvidia.com>

tests/reasoning/test_gptoss_reasoning_parser.py

@@ -0,0 +1,127 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+from transformers import AutoTokenizer
+
+from vllm.reasoning import ReasoningParser
+from vllm.reasoning.gptoss_reasoning_parser import GptOssReasoningParser
+
+REASONING_MODEL_NAME = "openai/gpt-oss-120b"
+
+
+@pytest.fixture(scope="module")
+def gpt_oss_tokenizer():
+    return AutoTokenizer.from_pretrained(REASONING_MODEL_NAME)
+
+
+USER_MESSAGE_START = "<|start|>user<|message|>"
+REASONING_SECTION_START = "<|end|><|start|>assistant<|channel|>analysis<|message|>"
+ASSISTANT_CONTENT_START_PREFIX = "<|end|><|start|>assistant<|channel|>final"
+ASSISTANT_CONTENT_START_SUFFIX = "<|message|>"
+ASSISTANT_CONTENT_START = (
+    ASSISTANT_CONTENT_START_PREFIX + ASSISTANT_CONTENT_START_SUFFIX
+)
+
+BASIC_CONTENT = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START
+    + "This is the rest",
+    "is_reasoning_end": True,
+}
+
+BASIC_REASONING_ONLY = {
+    "output": REASONING_SECTION_START + "This is reasoning" + "<|end|>",
+    "is_reasoning_end": False,
+}
+BASIC_NO_REASONING_NO_ASSISTANT = {
+    "output": USER_MESSAGE_START + "This is a user message",
+    "is_reasoning_end": False,
+}
+
+# Edge-case where the model omits the assistant tag entirely.
+BASIC_NO_REASONING_ASSISTANT = {
+    "output": USER_MESSAGE_START + "This is a user message<|end|><|channel|>final",
+    "is_reasoning_end": True,
+}
+
+COMPLEX_CONTENT_INCOMPLETE_PREFIX_ONLY = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_PREFIX,
+    "is_reasoning_end": False,
+}
+
+COMPLEX_CONTENT_SUFFIX_ONLY = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_SUFFIX,
+    "is_reasoning_end": False,
+}
+
+COMPLEX_CONTENT_1_NO_SUFFIX = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_PREFIX
+    + "<|constrain|> JSON ",
+    "is_reasoning_end": False,
+}
+
+COMPLEX_CONTENT_1 = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_PREFIX
+    + "<|constrain|> JSON "
+    + ASSISTANT_CONTENT_START_SUFFIX,
+    "is_reasoning_end": True,
+}
+
+COMPLEX_CONTENT_1_WITH_CONTENT = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_PREFIX
+    + "<|constrain|> JSON "
+    + ASSISTANT_CONTENT_START_SUFFIX
+    + "This is the rest",
+    "is_reasoning_end": True,
+}
+
+COMPLEX_CONTENT_2 = {
+    "output": REASONING_SECTION_START
+    + "This is reasoning"
+    + ASSISTANT_CONTENT_START_PREFIX
+    + "<|constrain|>ReplyAction "
+    + ASSISTANT_CONTENT_START_SUFFIX
+    + "This is the rest",
+    "is_reasoning_end": True,
+}
+
+TEST_CASES = [
+    BASIC_CONTENT,
+    BASIC_REASONING_ONLY,
+    COMPLEX_CONTENT_INCOMPLETE_PREFIX_ONLY,
+    COMPLEX_CONTENT_SUFFIX_ONLY,
+    COMPLEX_CONTENT_1_NO_SUFFIX,
+    COMPLEX_CONTENT_1,
+    COMPLEX_CONTENT_1_WITH_CONTENT,
+    COMPLEX_CONTENT_2,
+]
+
+
+@pytest.mark.parametrize(
+    "output, is_reasoning_end",
+    [(t["output"], t["is_reasoning_end"]) for t in TEST_CASES],
+)
+def test_gptoss_is_reasoning_end(
+    output,
+    is_reasoning_end,
+    gpt_oss_tokenizer,
+):
+    output = gpt_oss_tokenizer.tokenize(output)
+    parser: ReasoningParser = GptOssReasoningParser(gpt_oss_tokenizer)
+
+    # Test is_reasoning_end
+    output_ids = gpt_oss_tokenizer.convert_tokens_to_ids(output)
+    actual_is_reasoning_end = parser.is_reasoning_end(output_ids)
+    assert is_reasoning_end == actual_is_reasoning_end

vllm/reasoning/gptoss_reasoning_parser.py

@@ -67,18 +67,35 @@ class GptOssReasoningParser(ReasoningParser):
 
     def __init__(self, tokenizer: PreTrainedTokenizerBase, *args, **kwargs):
         super().__init__(tokenizer, *args, **kwargs)
-        self.reasoning_end_token_ids = self.model_tokenizer.encode(
-            "<|start|>assistant<|channel|>final<|message|>"
+        # The model can output some special tokens between "final" and "<|message|>"
+        # So we need to look for both sequences to determine the end of reasoning.
+        self.reasoning_end_token_ids_prefix = self.model_tokenizer.encode(
+            "<|channel|>final"
         )
+        self.reasoning_end_token_ids_suffix = self.model_tokenizer.encode("<|message|>")
+        self.reasoning_max_num_between_tokens = 20
 
     def is_reasoning_end(self, input_ids: list[int]) -> bool:
-        end_token_ids = self.reasoning_end_token_ids
-        assert len(end_token_ids) > 0, "reasoning_end_token_ids is empty"
+        end_token_ids_prefix = self.reasoning_end_token_ids_prefix
+        end_token_ids_suffix = self.reasoning_end_token_ids_suffix
+        assert len(end_token_ids_prefix) > 0, "reasoning_end_token_ids_prefix is empty"
+        assert len(end_token_ids_suffix) > 0, "reasoning_end_token_ids_suffix is empty"
         # Check if the end sequence is present in the input_ids.
         # We search from the end of input_ids to find the last match.
-        for i in range(len(input_ids) - len(end_token_ids), -1, -1):
-            if input_ids[i : i + len(end_token_ids)] == end_token_ids:
-                return True
+        for i in range(len(input_ids) - len(end_token_ids_prefix), -1, -1):
+            if input_ids[i : i + len(end_token_ids_prefix)] == end_token_ids_prefix:
+                # We have found the prefix, now we look for the suffix after the prefix.
+                suffix_start = i + len(end_token_ids_prefix)
+                for j in range(
+                    suffix_start, len(input_ids) - len(end_token_ids_suffix) + 1
+                ):
+                    if j - suffix_start >= self.reasoning_max_num_between_tokens:
+                        break
+                    if (
+                        input_ids[j : j + len(end_token_ids_suffix)]
+                        == end_token_ids_suffix
+                    ):
+                        return True
         return False
 
     def extract_content_ids(self, input_ids: list[int]) -> list[int]: