[Bugfix][V1] Register accepted spec-decode tokens before terminal cleanup

[kamb-code]

Purpose

Follow-up to the local-GPU-cache investigation on #52771:
#52771 (comment)

A terminal speculative-verification response can accept draft tokens after the
last normal prefix-cache registration. If that same response finishes the
request, cleanup releases its blocks before those newly finalized boundaries
are registered. A repeated or extended prompt then recomputes KV that was
successfully produced.

The test-only 2026-09-12 main snapshot control reproduces the loss:

Case	main with tests only	This PR
Ordinary K=3 speculative decode	32 cached tokens	48
Emulated multi-module-MTP, synchronous	0 cached tokens	16
Emulated multi-module-MTP, asynchronous	0 cached tokens	16

The MTP safety boundary remains at 0 and the unwritten final sampled-token case
remains at 32, so the change does not publish speculative or trimmed work.

Fix

Immediately before freeing a request that both entered the stop path as
RUNNING and actually finished, register this finalized prefix through the
existing cache coordinator:

min(
    max(0, request.num_computed_tokens - request.num_in_flight_tokens),
    request.num_tokens,
)

• Subtracting num_in_flight_tokens excludes work belonging to a later async
  schedule.
• Capping at request.num_tokens excludes verifier output trimmed by the stop
  condition.
• The existing coordinator still applies its MTP re-prefill fence.

The new terminal call also exercises an existing hybrid-Mamba retry path. With
async run-ahead, a retained partial boundary may already have moved to a durable
copy-on-write block while the request-table state block remains mutable. The
Mamba guard prevents that settled boundary from being reattached to mutable
state while work is in flight, while preserving first publication beyond the
current computed frontier. Its ablation fails the copy-on-write ownership test;
the scheduler and Mamba hunks are independently load-bearing.

This PR covers local/core automatic prefix-cache registration. It does not
claim that a newly registered terminal Mamba boundary is propagated through an
external KV connector.

Not a duplicate

On 2026-09-12 I reread the originating #52771 thread and repeated the
duplicate search against current open work. No other open PR implements this
generic lifecycle fix.

• [Hybrid KV Cache] Retain finalized Mamba decode checkpoints for prefix caching #50551 predates this PR and independently retains a finalized decode
  checkpoint for sparse-retention Mamba align mode. It computes the same
  settled-token bound, and deserves explicit credit for that Mamba-specific
  lifecycle. Its feature is opt-in, recurrent-only, and currently excludes
  hidden-state speculative decoding; it does not register the ordinary
  attention/ngram or emulated-MTP terminal cases reproduced here.
• [V1] Fix speculative decoding mean acceptance length for terminal steps #56137 and fix(metrics): Accurately track emitted tokens for speculative decoding #56195 are newer concurrent work in the terminal
  speculative-output path. They correct acceptance metrics after EOS/stop
  truncation but do not call cache_blocks or change KV-cache state.
• [Feature][Core] Prefix checkpoint coordination and same-step pairing in V1 scheduler #55875 adds application-directed Mamba checkpoint coordination and same-step
  producer/consumer pairing. It does not implement automatic accepted-token
  registration on terminal speculative output.
• [Core][Spec Decode] Add lookahead-aware prefix cache hashing for EAGLE-style draft models #50897 publishes output-time blocks only under its successor-aware EAGLE hash
  protocol when drafter KV is reported materialized; it does not repair the
  generic registration path exercised here, including ordinary ngram
  speculation.
• [Core][Bugfix] Roll back eager prefix-cache registration on uncommitted free #43569 tracks rollback and commit of eager registrations. Its normal finish
  path remains unchanged and it does not register accepted terminal draft
  tokens.
• [Bugfix][V1] Restore hybrid GDN prefix-cache hits under MTP spec decoding #52244 changes the write-side prompt-tail/replay boundary for hybrid GDN
  under MTP; it does not add terminal accepted-token registration.
• [Bugfix] Route every speculation-capable row through the speculative path so recurrent state stays consistent #56531 corrects worker-side recurrent-state selection on zero-draft
  speculative steps; it does not change scheduler KV-cache publication.
• [Bugfix][Spec Decode][Mooncake] Fix MooncakeStore cache misses from EAGLE/DSpark-drop-induced Mamba/a… #56615 pairs external MooncakeStore attention and recurrent boundary writes;
  it does not change local/core terminal registration.
• Recent hybrid-Mamba work changes which recurrent boundaries are retained or
  retired, not the missing terminal registration. The exact 2026-09-12
  negative-control snapshot below still reproduces both production defects.

Test Plan

Candidate publication head: fc99cc53652a304834fe9d0deabfa1de369f5742,
parent 22f6e4eccb674b534f62810c66838317169b6b98.

VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest \
  tests/v1/core/test_scheduler.py::test_finished_spec_decode_registers_accepted_tokens \
  tests/v1/core/test_async_scheduler.py::test_finished_request_does_not_cache_in_flight_tokens \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py::test_hybrid_mamba_partial_tail_owner_uses_cow_on_continue \
  -q

VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest \
  tests/v1/core/prefix_cache/test_mamba_eagle_resume_checkpoint.py \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py \
  tests/v1/core/test_async_scheduler.py -q

VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest \
  tests/v1/core/test_mamba_align_chunk_split.py \
  tests/v1/core/test_prefix_caching.py -q

.venv/bin/pre-commit run --files \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py \
  tests/v1/core/test_async_scheduler.py \
  tests/v1/core/test_scheduler.py \
  vllm/v1/core/sched/scheduler.py \
  vllm/v1/core/single_type_kv_cache_manager.py

Test Result

• Focused regression and safety cases: 7 passed.
• Same tests on the test-only 22f6e4eccb67 control: 4 failed, 3 passed.
  The failures are ordinary terminal publication (32 != 48), synchronous and
  asynchronous emulated-MTP publication (0 != 16), and Mamba copy-on-write
  ownership. The three safety cases pass.
• Mamba/EAGLE, partial-prefix, and async-scheduler group: 79 passed.
• Mamba align chunk-split and prefix-caching group: 196 passed.
• All applicable changed-file pre-commit hooks and git diff --check passed.

Model evaluation

On 2026-09-10, one controlled A/B run per arm on an NVIDIA RTX A6000 used
Qwen/Qwen3-0.6B at revision
c1899de289a04d12100db370d81485cdf75e47ca, fp16, PyTorch
2.13.0+cu130, eager synchronous V1 execution, block size 16, APC, ngram
speculation with K=3, and greedy sampling.

The exact-source baseline 9e257065601 reused 32 tokens. The patched pod head
38be1bf93a2 (the same patch as local 4c4c4076e4f on that baseline) reused
48. Both arms accepted all three draft tokens in one speculative step and
returned identical token IDs and text for both requests.

Both arms loaded the same compatible precompiled CUDA extension artifacts from
08b3e67b669; their Python source came from the exact heads above, and this PR
changes only Python. This was n=1 per arm on one A6000 pod. It validates the
ordinary full-attention ngram reuse boundary, not throughput or
hybrid/Mamba/real-MTP behavior.

Disclosure

AI assistance (Claude and OpenAI Codex) was used for analysis, test development,
and drafting. I reviewed and understand every changed line and ran the commands
reported above in my workspace.

[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.

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🚀

[kamb-code]

Rebased onto the 2026-09-10 main snapshot (86aca661916), including #54713;
the refreshed head is
aea0c7bdfae.

Fresh controls on that head: the focused cases pass 7/7, while the test-only
main control fails exactly four publication/ownership cases and passes the
three safety cases. The adjacent suites pass 78/78 and 147/147, and all
applicable changed-file pre-commit hooks pass. A fresh A6000 A/B (n=1 per
arm, ordinary full-attention ngram) on the pinned pre-rebase source reproduced
32 cached tokens on main and 48 with this patch, with identical outputs and
draft acceptance. It is a correctness-boundary check, not a throughput or
hybrid/MTP result. I also refreshed the body and duplicate check against the
current open-PR landscape. Ready for review whenever convenient.

(AI-assisted drafting; I reviewed the final diff and ran the reported checks
locally.)

[mergify]

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

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

[kamb-code]

Rebased onto the 2026-09-12 main snapshot (22f6e4eccb67); the refreshed
head is fc99cc53652a. The only conflict was the async-scheduler test import:
the resolution retains both #53781's mock_kv helper and this PR's
EOS_TOKEN_ID; the production patch is unchanged.

Fresh controls on that head: the focused cases pass 7/7, while the exact-main
test-only control fails exactly four regression cases and passes the three
safety cases. The adjacent groups pass 79/79 and 196/196, and all applicable
changed-file pre-commit hooks pass. I also repeated the duplicate search and
updated the body with the current adjacent work. Ready for review whenever
convenient.

(AI-assisted drafting; I reviewed the final diff and ran the reported checks
locally.)