[ker-zhao]

Overview

Refs #63149 (P1 "Establish coverage across cache layouts"), claimed there on
2026-10-23 as the CPU-level part. Test-only: adds
tests/v1/core/test_kv_cache_events_layouts.py with CPU assertions for the
cache layouts that have no event assertions today — MLA, GDN/recurrent, and
dense partial hits where hash_block_size < block_size.

Claims

• MLA groups emit kv_cache_spec_kind == "mla_attention" with a hash chain,
  token span and parent that match the reusable prefix — on store, on reuse, and
  on a kv_cache_report_mode="full" republish.
• GDN and Mamba2 groups both emit kv_cache_spec_kind == "mamba" with one
  checkpoint hash covering the reusable span; the event shape is identical for
  both mamba_type values.
• With hash_block_size < block_size, a prompt ending inside a cache block
  emits a tail BlockStored at hash granularity whose hash, parent and token
  span are exactly what a sibling resumes from. The companion case shows an
  attention tail past the last recurrent checkpoint is not a reusable hybrid
  prefix.

The assertions are phrased semantically so they can serve as the expected-event
spec for the GPU matrix that @qwert123q proposed in the tracker.

Validation

.venv/bin/python -m pytest tests/v1/core/test_kv_cache_events_layouts.py -q  # 6 passed in 2.2s
.venv/bin/python -m pytest tests/v1/core/test_prefix_caching.py -q           # 174 passed in 57s
.venv/bin/python -m pytest tests/v1/core/prefix_cache/ -q                    # 206 passed in 6:04
.venv/bin/pre-commit run --files tests/v1/core/test_kv_cache_events_layouts.py

No model/GPU evaluation applies: this changes no production code and no output.
CI already covers it — .buildkite/test_areas/misc.yaml runs
pytest -m 'cpu_test' v1/core on cpu-small, and tests/v1 is in that step's
source_file_dependencies, so no Buildkite change is needed.

Details

Why these cases. Event tests in test_prefix_caching.py cover
FullAttentionSpec and sliding window; partial-block events are covered only at
the BlockPool.cache_partial_block primitive level, not through the manager.
Nothing asserts the emitted kind or hash semantics for MLA, a recurrent group,
or a sub-block hit.

Why this is not a duplicate.

• Fix sparse BlockStored event token/hash mapping #44488 changes the wire shape of sparse events. These tests avoid that shape:
  the zero-hash chunk a recurrent group emits is left unasserted and only the
  dense group's chunk event is checked.
• [Bugfix][kv-events] Making partial blocks emit all tail hashes #62195 adds report_all_hash_boundaries (opt-in, default False). These
  tests never pass it and assert the tail via block_hashes[-1], so they hold
  either way.
• [Core][KV Events] Emit skipped ancestry for partial cache events #58121 adds skipped-ancestry fields to partial events; not exercised here —
  the parent asserted in the dense partial case is a published full-block
  boundary.
• [KV Events] Coalesce consecutive BlockRemoved events #63390 coalesces consecutive BlockRemoved events; nothing here asserts
  removal counts or total event counts.
• test(prefix cache): pin two mamba "align" + EAGLE prefix-cache defects #52371 pins Mamba-align + EAGLE defects as xfail; different feature axis,
  and there is no EAGLE in this change.

Not covered (stated in the module docstring): zero-hash recurrent chunk
events (#44488), off-grid single-chunk prefills, and DCP > 1.

AI assistance. AI assistance was used: the change was drafted with an AI
coding agent (Claude Code) and reviewed line by line by the submitter, who ran
the commands above.

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Pull Request Checklist

• I used vLLM's /pr-checklist skill. (Mandatory for agents, optional for humans).

• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

• Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.

• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

🤖 Generated with Claude Code

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