[Bugfix][Spec Decode] Keep EAGLE cache registration on the partial-hash-hit path

[mispa-ms]

Purpose

HybridKVCacheCoordinator.cache_blocks decides once how far a request may be
registered in the prefix-cache hash map. With fine-grained partial hash hits that
bound is the raw token count, because a hit no longer has to land on a
scheduler_block_size boundary.

#50062 rewrote the EAGLE branch to re-derive its own bound from
num_finalized_computed_tokens with an unconditional

// self.scheduler_block_size * self.scheduler_block_size

so the rounding comes back on the one path that had removed it. Registration is
then capped at floor(n / scheduler_block_size) * scheduler_block_size + manager.block_size instead of n, and everything between the last aligned
boundary and the tail stops being registered — (n % scheduler_block_size) - manager.block_size tokens per call, which is most of a segment whenever the
group's own block is much smaller than the scheduler block. Only EAGLE-family
groups take that branch, so a model without speculative decoding never sees it.

This routes both bounds through one helper, _align_cacheable, so the exemption
cannot be lost again by re-deriving it in a second place.

Reachability. enable_partial_hash_hits requires a Mamba "align" group
with block_size > hash_block_size, prefix caching, and dcp_world_size == 1.
mamba_cache_mode is a cache_config knob rather than a model property, so any
hybrid Mamba model served with --mamba-cache-mode align and an EAGLE-family
drafter reaches this path; the remaining condition — every group's manager
supporting fine-grained lookup — holds automatically for the
full-attention + Mamba pair.
A stock dcp_world_size == 1 hybrid model with EAGLE-family speculative decoding
is affected as written. We run that path under DCP > 1 through a local carry,
which is why we hit it on every run.

Out of scope, flagged for a reviewer. The soundness guard
scheduler_block_size >= num_prefill_lookahead is justified in its comment by
"hits land on scheduler-block boundaries", which holds only when
enable_partial_hash_hits is off — _cache_hit_alignment_tokens returns
hash_block_size when it is on. That guard predates this change and only matters
for multi-module MTP on a hybrid Mamba model, a combination we do not run and
cannot measure, so it is left alone rather than adjusted on a guess.

Not a duplicate

No open PR addresses this. The nearest neighbours, checked before opening:

• [2/N][Core] support partial prefix cache hit for hybrid model #46384 — the partial prefix-cache hit feature itself
• [RFC]: Partial Cache Hits for Hybrid Models #45702 — its design RFC
• [RFC]: Lookahead-aware prefix-cache hashing for EAGLE-style draft models #50438 — RFC: lookahead-aware prefix-cache hashing for EAGLE-style drafters,
  the closest in subject but a proposal for different hashing, not this bug
• [Bug]: Stale partial prefix-cache hash resurrected into the cache after full-block promotion #49125 — a different bug in the same area (stale partial hash revival)

AI assistance

This change was developed with AI assistance (Claude). I reviewed every changed
line, ran the tests below myself, and can defend the change end to end.

Test Plan

pytest tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py        -k eagle_group_registers_unaligned_tail
pytest tests/v1/core/prefix_cache/
pytest tests/v1/core/test_prefix_caching.py -k clamped_to_lcm
pytest tests/v1/core/          # before/after, to check nothing else moves

End to end: Kimi-K3 MXFP4 (hybrid, 24 MLA + 69 KDA layers), 8x B300, TP8, DSpark
speculative decoding with num_speculative_tokens=7, agentic replay workload,
60 minutes per point.

Test Result

The new test fails on the parent commit (acb0f1dcdb) and passes with the fix.
The failure is the arithmetic above: the EAGLE group registers
floor(22/8)*8 + 2 = 18 tokens where it should register 22.

>       assert recorded == [num_tokens] * len(coordinator.single_type_managers)
E       assert [18, 22] == [22, 22]
E         At index 0 diff: 18 != 22

1 failed, 24 deselected

tests/v1/core/prefix_cache/                      25 passed
tests/v1/core/test_prefix_caching.py -k clamped_to_lcm    1 passed

tests/v1/core/ in full, same command on both trees: +1 passed, −1 failed,
no other test changes state. The one that moves is the new test. My sandbox
cannot import the compiled extensions, so a large fixed set of tests in that
directory errors out identically on both trees and is not informative here;
the targeted suites above are the ones that run clean.

End to end, one 60-minute run per cell:

concurrency	before #50062	with #50062	with this fix
8	5,146 tok/s/GPU	4,767 (−7.4%)	5,183 (+0.7%)
16	7,669 tok/s/GPU	6,611 (−13.8%)	7,654 (−0.2%)

GPU prefix-cache hit rate at concurrency 16: 86.3% → 77.4% → 86.2%. The
non-speculative arm of the same ladder is unaffected throughout (13 points from
concurrency 1 to 78, all within −1.3% to +1.8%). The regressed pair was repeated
and the two runs agreed to within 0.3%.

Effect on output

The change only widens which already-computed tokens may be registered in the
hash map; it does not alter what is computed, what a hit returns, or the
correctness conditions #50062 added (num_reprefillable_tokens still excludes
re-prefillable tail tokens, and the EAGLE last-block drop is untouched). Output
is bit-identical in the sense that no sampling path changes; the measured effect
is throughput and prefix-hit rate only. No accuracy evaluation was run, and none
is claimed.

For scale on this model: TOKENSPEED_MLA sets block_size=32 and the attention
block is forced to 1536 to cover the mamba page. The unregistered part of a
prefix tail is (n % 1536) - 32, so up to 1,503 tokens go unregistered per
call — 1,472 when n is a multiple of 32.

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[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.

[TheEpicDolphin]

Thanks for the fix!

[github-actions]

✅ @mispa-ms, CI is now available for this PR.

• /ci run starts a CI build.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
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[TheEpicDolphin]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84039 for commit b13757b467e7.

[njhill]

Thanks @mispa-ms, just have some minor cosmetic suggestions

[mispa-ms]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84043 for commit 8aa47ecd359d.

[mispa-ms]

/ci retry

[github-actions]

✅ Queued 1 failed job(s) for retry in Buildkite CI #84043.

[mispa-ms]

CI is green on this PR except multi-modal-models-standard-4-other-plus-whisper, which looks like a pre-existing main breakage rather than anything from this change:

• It is failing right now on unrelated PRs [Bugfix][DSv4] Revert adaptive C128A metadata packing #51318, [Bugfix] Pick the DeepSeek V4 eager cudagraph region per model runner #52401 and [Bugfix] Temporarily disable FA4 head-dim 256 #52050 as well.
• It passed on builds from ~12:00 UTC today ([Bugfix][Multimodal] Keep Gemma 4 video frame counts on CPU #52441) and fails on every build from ~20:00 UTC onwards. A /ci retry on this same commit reproduced it, so it is deterministic rather than flaky.
• Everything this change actually touches is green: v1-core-plus-kv-plus-metrics, v1-spec-decode, language-models-tests-hybrid-1/-2, hybrid-ssm-nixlconnector-pd-prefix-cache-test, and the AMD equivalents.