[Bugfix] KimiK2ReasoningParser: guard against buffered end-token in streaming by JasonKeyiL · Pull Request #41068 · vllm-project/vllm

JasonKeyiL · 2026-04-28T00:04:14Z

Summary

KimiK2ReasoningParser.extract_reasoning_streaming() calls delta_text.find(token) after checking token_id in delta_token_ids, but does not guard against find() returning -1.

When stop sequences are configured, output_text_buffer_length > 0 causes token IDs to arrive in delta_token_ids before the corresponding text is flushed into delta_text. This makes find() return -1 and silently corrupts the reasoning/content split:

# Without fix — </think> buffered, delta_text='extra'
end_index = delta_text.find("</think>")  # returns -1
reasoning = delta_text[:-1]             # 'extr'  ← drops last char
content   = delta_text[7:]              # ''

The fix adds a text-presence guard before each find() call and returns None (wait for next flush) when the token string is not yet visible — matching the pattern from PR #39044 (which fixed BaseThinkingReasoningParser, DeepSeekR1ReasoningParser, etc.) and PR #40352 (which fixed Step3ReasoningParser). KimiK2ReasoningParser was not included in either.

Two paths are fixed:

</think> end token (line 215)
<|tool_calls_section_begin|> section start token (line 223)

Why not a duplicate

PR fix(reasoning): prevent streaming end-token desync in base and other parsers #39044 fixes BaseThinkingReasoningParser / DeepSeekR1 / Ernie45 / Step3p5 — KimiK2ReasoningParser is not in that list.
PR [Bugfix][Reasoning] Handle buffered Step3 end-token deltas #40352 fixes Step3ReasoningParser — separate parser.
No other open or merged PR addresses KimiK2ReasoningParser's find() buffering issue.

Test plan

New unit tests that use a mock tokenizer (no GPU / model download needed):

.venv/bin/python -m pytest tests/reasoning/test_kimi_k2_reasoning_parser.py::test_streaming_end_token_id_buffered tests/reasoning/test_kimi_k2_reasoning_parser.py::test_streaming_tool_section_id_buffered -v

Both pass after the fix; both fail on main (returning a corrupted DeltaMessage instead of None).

AI assistance (Claude) was used to identify the bug and draft this fix. The submitter has reviewed every changed line and understands the fix end-to-end.

claude

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

github-actions · 2026-04-28T00:04:24Z

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mergify · 2026-04-28T00:04:58Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @JasonKeyiL.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

gemini-code-assist

Code Review

This pull request resolves a bug in KimiK2ReasoningParser where token IDs arriving before their text caused incorrect parsing. The fix introduces checks to wait for the text and includes regression tests. In MiniMaxM2Parser, keyword arguments are now passed to the reasoning parser, but feedback suggests also passing them to the base class constructor for consistency.

I am having trouble creating individual review comments. Click here to see my feedback.

vllm/parser/minimax_m2_parser.py (49)

The **kwargs are being passed to the underlying MiniMaxM2ReasoningParser but are swallowed for the base DelegatingParser class. While the current base class implementation might not use them, it is better practice to pass *args and **kwargs up to ensure that any base class logic (such as parameter tracking or future updates) correctly receives the request parameters.

        super().__init__(tokenizer, **kwargs)

When stop sequences are configured, vLLM enables an output_text_buffer that delays text emission so suffix-matching against the stop strings can be performed without leaking partial matches. As a side effect, the reasoning end-token id (</think>) can arrive in delta_token_ids ahead of its rendered text — the text flushes over the next 1-N chunks. Two related defects follow. In KimiK2ReasoningParser.extract_reasoning_streaming, the unguarded delta_text.find(self._end_token) returns -1 on the boundary chunk and the slice over-truncates reasoning while seeding content with whatever chars happened to be in delta_text. After the boundary chunk, DelegatingParser.parse_delta sets state.reasoning_ended=True and routes subsequent chunks through the tool branch, which has no awareness of the still-buffered end-token text. The buffered </think> then flushes verbatim into delta.content over the next 1-N chunks. This bypasses the reasoning parser entirely and is therefore not caught by patches confined to that class. Reproduced on a real serving deployment with max_tokens=512, temperature=0, stop=['<|im_end|>', '\n\nUser:'], single-turn request: 8/8 streamed responses leaked the literal '</think>' into delta.content. Per-delta capture showed the boundary as delta[N]: reasoning='T' (last reasoning delta) delta[N+1]: content=' ' delta[N+2]: content='</thin' (buffered </think> flushing) delta[N+3]: content='k> ' delta[N+4]: content='I will...' (real content begins) Fix: * KimiK2ReasoningParser: when the end-token id is in delta_token_ids but the literal is not in delta_text, return None and let the buffered text flush. Same guard for the implicit <|tool_calls_section_begin|> reasoning end. * DelegatingParser.parse_delta: add a post-boundary skip phase. When the parser returned None on the boundary handoff (i.e. the end-token text is still buffered), accumulate phase-B delta_text until the end-token literal appears, then strip everything up through it and forward the remainder to the tool parser. Three new StreamState fields gate the mechanism so it remains a no-op on streams where the parser already split delta_text in a single chunk and on streams without a reasoning_end_str. The skip applies to any ReasoningParser exposing a non-empty reasoning_end_str, so DeepSeek-R1, Step3, Step3p5, Ernie45 and similar parsers benefit without further changes. Tests added in tests/reasoning/test_kimi_k2_reasoning_parser.py: * test_streaming_end_token_id_arrives_before_text — parser-level guard for </think>. * test_streaming_tool_section_id_arrives_before_text — same guard for the implicit tool-section reasoning end. * test_delegating_parser_does_not_leak_buffered_end_token_text — end-to-end through DelegatingParser, verifies no </think> leak and post-boundary content survives. * test_delegating_parser_disarmed_when_parser_splits_in_one_chunk — the skip mechanism stays disarmed when delta_text already contains </think> on the boundary chunk; would over-truncate otherwise. * test_delegating_parser_handles_torch_tensor_token_ids — exercises the same flow with delta_token_ids constructed as torch tensors and converted via .tolist(), mirroring the engine code path. Verification: * uv venv + uv pip install per AGENTS.md, then PYTHONPATH=. python -m pytest --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v → 13 passed in 6.08s. * Reverted both implementation files to HEAD~1 (pre-fix), reran the five new tests → all five fail with AssertionError: "Leak: '</think>' in content" (or AttributeError on the disarm test referencing the new StreamState fields). * pre-commit run on the three changed files: ruff-check, ruff-format, typos, mypy, SPDX headers all pass. * Production serving deployment: 8/8 leaks pre-fix → 0/25 leaks post-fix (10 direct vLLM port-forward + 15 via service mesh, on the previously-100%-leaking prompt and stop config). Non-streaming remained leak-free (structurally unaffected). This work is AI-assisted. The change was authored end-to-end by Claude under human direction; the human submitter has reviewed every changed line and confirms the analysis, the diff, and the test results above. No open PR was found addressing the same DelegatingParser-level leak (`gh pr list --repo vllm-project/vllm --search '41067 in:body'` returns only vllm-project#41068, which patches only the parser-side guard and does not address the buffered-flush race in the tool-branch handoff). Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

… in streaming When stop sequences set output_text_buffer_length > 0, token IDs arrive in delta_token_ids before their text is flushed into delta_text. Without a guard, find() returns -1 and the reasoning/content split is silently corrupted. Add text-presence checks before both find() calls in extract_reasoning_streaming: - </think> end token path (line 215) - <|tool_calls_section_begin|> section start path (line 223) Return None (wait for flush) when the token ID is present but the text is not, matching the fix pattern from PR vllm-project#39044 (BaseThinkingReasoningParser / DeepSeekR1) and PR vllm-project#40352 (Step3ReasoningParser). Fixes vllm-project#41067 Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Keyi Li <likey6688@gmail.com>

JasonKeyiL · 2026-04-30T01:56:25Z

Hi @sfeng33 , would you be mind take a look when you have a moment?
much appreciate it!

Stacks on top of vllm-project#41068 (vendored at this branch's base): that PR fixes the parser-side off-by-one when the end-token id arrives ahead of its rendered text in delta_text, but a second leak survives downstream. After the parser returns None on the boundary chunk, DelegatingParser.parse_delta sets state.reasoning_ended = True and routes the next chunks through the tool branch — which has no awareness of the still-buffered end-token text and emits it verbatim as delta.content over the next 1..N chunks. With vllm-project#41068 alone applied, 8/8 streamed responses on a real Kimi-K2.5 deployment leaked the literal '</think>' into delta.content (max_tokens=512, temperature=0, stop=['<|im_end|>', '\n\nUser:'], single-turn). Add three StreamState fields and a small post-boundary skip block in DelegatingParser.parse_delta. When the parser returns None on the boundary handoff (i.e. the end-token text is still buffered) the skip arms; subsequent chunks accumulate delta_text until the end-token literal becomes visible, are stripped through it, and the remainder is forwarded to the tool parser. The skip stays disarmed when the parser already split delta_text in the same chunk (no buffered tail follows). The mechanism applies to any ReasoningParser exposing a non-empty reasoning_end_str, so DeepSeek-R1, Step3, Step3p5, Ernie45 and similar parsers benefit without further changes. Tests added in tests/reasoning/test_kimi_k2_reasoning_parser.py: - test_delegating_parser_strips_buffered_end_token (parametrized over list and torch tensor.tolist() input shapes — engine call site produces token_ids via as_list(output.token_ids) on a torch tensor) - test_delegating_parser_skip_disarmed_when_parser_splits_in_one_chunk Verification: uv venv --python 3.12 && uv pip install … PYTHONPATH=. .venv/bin/python -m pytest \ --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v # → 13 passed in 4.40s (10 pre-existing + 3 new from this commit) # Reverted vllm/parser/abstract_parser.py to PR vllm-project#41068 HEAD, reran # the three new tests → all three fail (AssertionError on the literal # </think> appearing in content, AttributeError on the disarm test # referencing the new StreamState fields). pre-commit run --files vllm/parser/abstract_parser.py \ tests/reasoning/test_kimi_k2_reasoning_parser.py # → ruff-check, ruff-format, typos, mypy, SPDX headers all pass Production rollout (Kimi-K2.5-INT4 P/D disagg, vLLM nightly fe9c3d6 + vllm-project#41068's parser fix + this commit, applied as a runtime overlay): 8/8 leaks pre-fix → 0/25 leaks post-fix (10 direct vLLM port-forward + 15 via service mesh, on the previously-100%- leaking prompt and stop config). Output integrity hand-checked: reasoning ends cleanly at the natural stop point, content begins immediately with the model's actual answer, no missing chars. This work is AI-assisted. The change was authored end-to-end by Claude under human direction; the human submitter has reviewed every changed line and confirms the analysis, the diff, and the test results above. `gh pr list --repo vllm-project/vllm --search '41067 in:body'` returns only vllm-project#41068 (which patches the parser-side guard) — no open PR addresses the DelegatingParser-level leak this commit fixes. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

marcostephan · 2026-04-30T07:27:37Z

Hey, confirming this fix is needed. We hit the exact same bug on a Kimi-K2.5-INT4 disaggregated deployment running the upstream nightly (vllm-openai:cu130-nightly-fe9c3d6c5f66). 8/8 streamed responses leaked the literal </think> into delta.content on max_tokens=512, temperature=0, stop=['<|im_end|>', '\n\nUser:'], single-turn requests.

I spent a day digging into this with Claude. One thing worth surfacing: this PR's guard is necessary, but on its own doesn't fully fix the leak. We still got 8/8 leaks with just this PR's logic applied as an overlay on our serving image.

The second-stage leak is in DelegatingParser.parse_delta: after extract_reasoning_streaming returns None on the boundary chunk (which is exactly what your new guard makes it do), state.reasoning_ended flips True and subsequent chunks go through the tool branch. The buffered </think> then flushes a few chars at a time while reasoning is "ended", and the tool branch doesn't know those chars are the lagging end-token, so it emits them as delta.content. Per-delta capture from a real request:

delta[N]:   reasoning='T'         (last reasoning delta; parser returns None)
delta[N+1]: content=' '
delta[N+2]: content='</thin'      <- buffered </think> flushing
delta[N+3]: content='k> '
delta[N+4]: content='I will...'

The follow-up patch is a small post-boundary skip in DelegatingParser.parse_delta: 3 new StreamState fields plus a buffer-and-strip block. With both fixes applied, 0/25 leaks across our production verification on the prompt that previously leaked 100%. Same logic should also help DeepSeek-R1, Step3, Step3p5, Ernie45 and any other parser with a non-empty reasoning_end_str.

Cleanest would probably be me opening a PR into your branch with the additional fix so it all lands as one upstream PR. Otherwise this can land as-is and I'll send a follow-up against main, or we roll both into a fresh PR with you as co-author. Whatever works for you.

Branch with the follow-up is here for reference: marcostephan#1 (+174 lines on top of your branch: the skip block plus 3 new tests parametrized over list and torch.tensor.tolist() token-id shapes).

Happy either way. Let me know what you prefer.

Stacks on top of vllm-project#41068. PR vllm-project#41068 fixes the parser-side off-by-one when the </think> token id arrives ahead of its rendered text in delta_text. A second leak survives downstream: after the parser returns None on the boundary chunk, DelegatingParser.parse_delta sets state.reasoning_ended = True and routes the next chunks through the tool branch — which has no awareness of the still-buffered end-token text and emits it verbatim as delta.content over the next 1..N chunks. Empirically: 8/8 streamed responses leaked the literal '</think>' into delta.content on a real Kimi-K2.5 deployment with PR vllm-project#41068's parser change applied alone. Per-delta capture from a real request: delta[N]: reasoning='T' (last reasoning delta; parser returns None) delta[N+1]: content=' ' delta[N+2]: content='</thin' <- buffered </think> flushing delta[N+3]: content='k> ' delta[N+4]: content='I will...' Add four StreamState fields (end_token_skip_buffer, end_token_skip_armed, end_token_skip_done, end_token_skip_literal) plus a small post-boundary skip block in DelegatingParser.parse_delta and a helper _pending_end_token_literal that returns whichever terminator's text is still buffered. On boundary handoff: * The helper checks both </think> and (for parsers that expose it) <|tool_calls_section_begin|>. It returns the literal whose token id is in delta_token_ids but whose text has not yet flushed into delta_text, or None when the boundary text was fully visible. * If the helper returns a literal, the skip arms with that literal stored in state and the boundary chunk's delta_text seeded into the buffer (so a partial prefix of the literal that already flushed isn't lost). * If None, the skip stays disarmed (no buffered tail follows). While armed, subsequent chunks accumulate delta_text into the buffer. When the stored literal becomes visible, the buffer is split: * For </think>, the literal is consumed (downstream content begins after it). * For <|tool_calls_section_begin|>, the literal is preserved in the cleaned text — KimiK2ToolParser uses it to identify the section boundary, matching the immediate-text path in KimiK2ReasoningParser.extract_reasoning_streaming where 'content = delta_text[tool_index:]' keeps the literal. The tool-section branch is opt-in via getattr so it stays a no-op for parsers that don't expose those attributes (DeepSeek-R1, Step3p5, Ernie45, etc. all still benefit from the </think> half). Tests added in tests/reasoning/test_kimi_k2_reasoning_parser.py: * test_delegating_parser_strips_buffered_end_token — primary scenario exercised end-to-end through DelegatingParser.parse_delta. * test_delegating_parser_skip_disarmed_when_parser_splits_in_one_chunk — when </think> id and text arrive together, the skip stays disarmed; otherwise it would over-truncate post-boundary content. * test_delegating_parser_boundary_at_end_of_delta_text — when </think> lands at the very end of delta_text with no content after, the skip must still stay disarmed (buffered tail won't appear). * test_delegating_parser_handles_buffered_tool_section_end — when reasoning ends implicitly via <|tool_calls_section_begin|> under buffering, the skip waits for the section literal (not </think>), preserves it in cleaned text, and routes the in-section payload to the tool parser instead of leaking it as content. * test_delegating_parser_recovers_partial_end_token_in_boundary_chunk — the boundary chunk's delta_text can end mid-literal (e.g. '</th' on the boundary chunk, 'ink>' on the next). The buffer must be seeded with the boundary chunk's delta_text so the partial prefix isn't lost. All five rely on a small _drive helper, a Chunk dataclass, and a wrapped_kimi_parser fixture that composes KimiK2ReasoningParser + KimiK2ToolParser via _WrappedParser. Verification: uv venv --python 3.12 && uv pip install … PYTHONPATH=. .venv/bin/python -m pytest \ --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v # → 15 passed in 6.07s (10 pre-existing + 5 new from this commit) # Reverted vllm/parser/abstract_parser.py to PR vllm-project#41068's HEAD; reran # the new tests: # → 5 failed (each on the literal leaking into content or the # post-boundary content being suppressed) pre-commit run --files vllm/parser/abstract_parser.py \ tests/reasoning/test_kimi_k2_reasoning_parser.py # → ruff-check, ruff-format, typos, mypy, SPDX headers all pass Production rollout: Kimi-K2.5-INT4 P/D disagg deployment, vLLM v0.20.0 + PR vllm-project#41068's parser fix + this commit applied as a runtime overlay. 8/8 leaks pre-fix → 0/25 leaks post-fix (10 direct vLLM port-forward, 15 via service mesh) on the previously-100%-leaking prompt and stop config. Output integrity hand-checked: reasoning ends cleanly at the natural stop point, content begins immediately with the model's actual answer. Duplicate-work check (gh pr list --search '41067 in:body' --state open) returns only PR vllm-project#41068, which patches the parser-side guard. No open PR addresses the DelegatingParser-level leak this commit fixes. This work is AI-assisted. The change was authored end-to-end by Claude under human direction; the human submitter has reviewed every changed line and confirms the analysis, the diff, and the test results above. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

JasonKeyiL · 2026-04-30T17:25:54Z

Hi @marcostephan, option 1 works great for me,
feel free to open PR into this branch with additional fix, I'd happy to review and merge it in!

JasonKeyiL · 2026-05-01T18:32:48Z

Hi @sfeng33,
this pr has been opened for few days, could we approve and merge this first? and have @marcostephan create separate pr against main for the additional fix he identified.
thanks!
cc: @marcostephan

JasonKeyiL · 2026-05-03T20:31:59Z

Hi @aarnphm @chaunceyjiang @bbrowning , could you take a look? thanks!

marcostephan · 2026-05-04T07:57:15Z

@JasonKeyiL Opened a pr in your fork. Also happy to create another one upstream if it gets merged before

ehfd · 2026-05-04T09:31:19Z

Awesome! We could merge the fork PR before upstream.

sfeng33

Thank you for the fix!

JasonKeyiL · 2026-05-04T17:43:10Z

Thank you! @sfeng33

JasonKeyiL · 2026-05-04T17:44:02Z

Hi @marcostephan, looks like its getting merged, can you create one upstream?

…al end-token Follow-up to the previous commit on this branch, addressing review feedback from gemini-code-assist on PR vllm-project#41717. When the boundary chunk's delta_text contains reasoning content immediately followed by a partial `</think>` literal (e.g. `"final_thought</th"` with the end-token id in delta_token_ids), the Kimi parser returns None per vllm-project#41068 (literal not yet visible) and the entire `"final_thought</th"` lands in `end_token_skip_buffer`. On the next chunk that completes the literal, the prior split cleaned = state.end_token_skip_buffer[content_start:] discarded everything before the literal — silently dropping the `"final_thought"` prefix from the response. Fix: capture the pre-literal slice into `recovered_reasoning_prefix` during skip resolution and merge it into `delta_message.reasoning` after the tool branch runs (the tool branch returns a fresh DeltaMessage, so the merge has to happen post-branch to avoid being overwritten). Test added: test_delegating_parser_recovers_reasoning_prefix_buffered_with_partial_end_token - Boundary chunk: `"final_thought</th"` + end-token id. - Verifies `final_thought` ends up in delta.reasoning, not dropped or leaked into delta.content. - Confirmed failing on the prior commit (drops `final_thought`), passing here. Verification: PYTHONPATH=. .venv/bin/python -m pytest \ --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v # 16 passed in 6.55s (15 prior + 1 new) pre-commit run --files vllm/parser/abstract_parser.py \ tests/reasoning/test_kimi_k2_reasoning_parser.py # ruff, format, typos, mypy, SPDX all pass Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

…treaming (vllm-project#41068) Signed-off-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Flora Feng <4florafeng@gmail.com>

…treaming (vllm-project#41068) Signed-off-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Flora Feng <4florafeng@gmail.com> Co-authored-by: hongbolv <33214277+hongbolv@users.noreply.github.com>

…treaming (vllm-project#41068) Signed-off-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Flora Feng <4florafeng@gmail.com> Signed-off-by: Ifta Khairul Alam Adil <ikaadil007@gmail.com>

…treaming (vllm-project#41068) Signed-off-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Keyi Li <likey6688@gmail.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Flora Feng <4florafeng@gmail.com>

Stacks on top of vllm-project#41068. PR vllm-project#41068 fixes the parser-side off-by-one when the </think> token id arrives ahead of its rendered text in delta_text. A second leak survives downstream: after the parser returns None on the boundary chunk, DelegatingParser.parse_delta sets state.reasoning_ended = True and routes the next chunks through the tool branch — which has no awareness of the still-buffered end-token text and emits it verbatim as delta.content over the next 1..N chunks. Empirically: 8/8 streamed responses leaked the literal '</think>' into delta.content on a real Kimi-K2.5 deployment with PR vllm-project#41068's parser change applied alone. Per-delta capture from a real request: delta[N]: reasoning='T' (last reasoning delta; parser returns None) delta[N+1]: content=' ' delta[N+2]: content='</thin' <- buffered </think> flushing delta[N+3]: content='k> ' delta[N+4]: content='I will...' Add four StreamState fields (end_token_skip_buffer, end_token_skip_armed, end_token_skip_done, end_token_skip_literal) plus a small post-boundary skip block in DelegatingParser.parse_delta and a helper _pending_end_token_literal that returns whichever terminator's text is still buffered. On boundary handoff: * The helper checks both </think> and (for parsers that expose it) <|tool_calls_section_begin|>. It returns the literal whose token id is in delta_token_ids but whose text has not yet flushed into delta_text, or None when the boundary text was fully visible. * If the helper returns a literal, the skip arms with that literal stored in state and the boundary chunk's delta_text seeded into the buffer (so a partial prefix of the literal that already flushed isn't lost). * If None, the skip stays disarmed (no buffered tail follows). While armed, subsequent chunks accumulate delta_text into the buffer. When the stored literal becomes visible, the buffer is split: * For </think>, the literal is consumed (downstream content begins after it). * For <|tool_calls_section_begin|>, the literal is preserved in the cleaned text — KimiK2ToolParser uses it to identify the section boundary, matching the immediate-text path in KimiK2ReasoningParser.extract_reasoning_streaming where 'content = delta_text[tool_index:]' keeps the literal. The tool-section branch is opt-in via getattr so it stays a no-op for parsers that don't expose those attributes (DeepSeek-R1, Step3p5, Ernie45, etc. all still benefit from the </think> half). Tests added in tests/reasoning/test_kimi_k2_reasoning_parser.py: * test_delegating_parser_strips_buffered_end_token — primary scenario exercised end-to-end through DelegatingParser.parse_delta. * test_delegating_parser_skip_disarmed_when_parser_splits_in_one_chunk — when </think> id and text arrive together, the skip stays disarmed; otherwise it would over-truncate post-boundary content. * test_delegating_parser_boundary_at_end_of_delta_text — when </think> lands at the very end of delta_text with no content after, the skip must still stay disarmed (buffered tail won't appear). * test_delegating_parser_handles_buffered_tool_section_end — when reasoning ends implicitly via <|tool_calls_section_begin|> under buffering, the skip waits for the section literal (not </think>), preserves it in cleaned text, and routes the in-section payload to the tool parser instead of leaking it as content. * test_delegating_parser_recovers_partial_end_token_in_boundary_chunk — the boundary chunk's delta_text can end mid-literal (e.g. '</th' on the boundary chunk, 'ink>' on the next). The buffer must be seeded with the boundary chunk's delta_text so the partial prefix isn't lost. All five rely on a small _drive helper, a Chunk dataclass, and a wrapped_kimi_parser fixture that composes KimiK2ReasoningParser + KimiK2ToolParser via _WrappedParser. Verification: uv venv --python 3.12 && uv pip install … PYTHONPATH=. .venv/bin/python -m pytest \ --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v # → 15 passed in 6.07s (10 pre-existing + 5 new from this commit) # Reverted vllm/parser/abstract_parser.py to PR vllm-project#41068's HEAD; reran # the new tests: # → 5 failed (each on the literal leaking into content or the # post-boundary content being suppressed) pre-commit run --files vllm/parser/abstract_parser.py \ tests/reasoning/test_kimi_k2_reasoning_parser.py # → ruff-check, ruff-format, typos, mypy, SPDX headers all pass Production rollout: Kimi-K2.5-INT4 P/D disagg deployment, vLLM v0.20.0 + PR vllm-project#41068's parser fix + this commit applied as a runtime overlay. 8/8 leaks pre-fix → 0/25 leaks post-fix (10 direct vLLM port-forward, 15 via service mesh) on the previously-100%-leaking prompt and stop config. Output integrity hand-checked: reasoning ends cleanly at the natural stop point, content begins immediately with the model's actual answer. Duplicate-work check (gh pr list --search '41067 in:body' --state open) returns only PR vllm-project#41068, which patches the parser-side guard. No open PR addresses the DelegatingParser-level leak this commit fixes. This work is AI-assisted. The change was authored end-to-end by Claude under human direction; the human submitter has reviewed every changed line and confirms the analysis, the diff, and the test results above. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

…al end-token Follow-up to the previous commit on this branch, addressing review feedback from gemini-code-assist on PR vllm-project#41717. When the boundary chunk's delta_text contains reasoning content immediately followed by a partial `</think>` literal (e.g. `"final_thought</th"` with the end-token id in delta_token_ids), the Kimi parser returns None per vllm-project#41068 (literal not yet visible) and the entire `"final_thought</th"` lands in `end_token_skip_buffer`. On the next chunk that completes the literal, the prior split cleaned = state.end_token_skip_buffer[content_start:] discarded everything before the literal — silently dropping the `"final_thought"` prefix from the response. Fix: capture the pre-literal slice into `recovered_reasoning_prefix` during skip resolution and merge it into `delta_message.reasoning` after the tool branch runs (the tool branch returns a fresh DeltaMessage, so the merge has to happen post-branch to avoid being overwritten). Test added: test_delegating_parser_recovers_reasoning_prefix_buffered_with_partial_end_token - Boundary chunk: `"final_thought</th"` + end-token id. - Verifies `final_thought` ends up in delta.reasoning, not dropped or leaked into delta.content. - Confirmed failing on the prior commit (drops `final_thought`), passing here. Verification: PYTHONPATH=. .venv/bin/python -m pytest \ --confcutdir=tests/reasoning -p no:cacheprovider \ tests/reasoning/test_kimi_k2_reasoning_parser.py -v # 16 passed in 6.55s (15 prior + 1 new) pre-commit run --files vllm/parser/abstract_parser.py \ tests/reasoning/test_kimi_k2_reasoning_parser.py # ruff, format, typos, mypy, SPDX all pass Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marco Stephan <marco@magic.dev>

JasonKeyiL requested review from aarnphm, bbrowning, chaunceyjiang and sfeng33 as code owners April 28, 2026 00:04

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marcostephan mentioned this pull request Apr 29, 2026

[Bugfix] DelegatingParser: strip buffered end-token text from phase B (stacks on #41068) marcostephan/vllm#1

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JasonKeyiL force-pushed the fix/kimi-k2-streaming-end-token-buffer branch from 8a304a4 to 093c83a Compare April 30, 2026 01:51

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Merge branch 'main' into fix/kimi-k2-streaming-end-token-buffer

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marcostephan mentioned this pull request May 4, 2026

Fix/kimi reasoning parser streaming buffered end token JasonKeyiL/vllm#1

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Merge branch 'main' into fix/kimi-k2-streaming-end-token-buffer

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sfeng33 merged commit 712ad02 into vllm-project:main May 4, 2026
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liulanze mentioned this pull request May 5, 2026

[Bugfix] Guard against buffered token IDs in BaseThinkingReasoningParser streaming #41698

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marcostephan mentioned this pull request May 5, 2026

[Bugfix] DelegatingParser: strip buffered end-token text in phase B (follow-up to #41068) #41717

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