fix: allow full-block state alignment with speculation by milesial · Pull Request #542 · ai-dynamo/aisimulate

milesial · 2026-10-21T07:10:16Z

Why and what changed

State-cache validation rejects speculation whenever prefix_match_unit is set, including full-block alignment that the native scheduler supports.

Allow speculation when prefix_match_unit == block_size. Keep rejection for sub-block matching. Apply the same rule in public prediction validation, runner materialization, and native engine validation. Update the cache, speculation, and feature-support documentation.

Review map

Risk level: medium.
Start with the three validation boundaries, then the state-cache regressions.
Public or serialized contract changed: existing fields accept full-block alignment with speculation. No fields or defaults change.
Compatibility or rollback concern: reverting rejects this newly accepted combination. Sub-block matching and offload restrictions remain in place.

Evidence

The public regression covers legacy nextn and canonical MTP configuration through serialization and native handoff. The native regression covers zero, full, and partial draft acceptance after a long decode. With four-token blocks and a 17-token prompt, the second request must reuse the 16-token checkpoint. Existing negative cases retain rejection of sub-block matching.

Fast CI, Full CI, and independent reviews are pending. This draft does not claim merge readiness.

Modeling or data provenance

Functional replay only. No timing tables, speculation cost model, acceptance defaults, or hardware accuracy claims change.

Tracking

PR #541 fixes the checkpoint boundary for prompts whose length is a multiple of the block size. These changes target main independently.

Signed-off-by: Alexandre Milesi <milesial@users.noreply.github.com>

coderabbitai · 2026-10-21T07:10:25Z

📝 Summary

Risk: Moderate. Human attention should focus on validation parity, full-block versus sub-block behavior, and regression coverage.

Changed behavior

Speculative decoding now allows prefix_match_unit == block_size.
Sub-block prefix matching remains rejected.
The rule applies to native validation, public prediction validation, and runner materialization.
Public and serialized configuration accepts this existing field combination. No fields or defaults change.
Offload restrictions remain unchanged.
Documentation now describes full-block alignment as supported.

Evidence supplied

Tests cover canonical MTP and legacy nextn configuration through validation, serialization, replay handoff, and native handoff.
Native tests cover zero, full, and partial draft acceptance after long decoding.
Tests cover reuse of a 16-token checkpoint for a 17-token prompt with four-token blocks.
Existing sub-block rejection cases remain covered.
The repository is clean after the supplied inspection.

Evidence still missing

Fast CI results.
Full CI results.
Independent review results.
Final merge-readiness confirmation.

Assessment

The reported changes align validation across the native and Python paths. The reported tests cover the accepted full-block case and the rejected sub-block case. Technical quality appears sound from the supplied evidence, but merge readiness remains unconfirmed while CI and independent reviews are pending.

Rust and Python validation now allow speculative decoding with state-cache prefix matching when the prefix unit equals the block size. Tests cover configuration handoff and cached-prefix reuse. Documentation clarifies that sub-block prefix matching remains unsupported.

Changes: Block-aligned state-cache speculation

Layer / File(s)	Summary
Validation and support boundaries `crates/core/src/engine/config.rs`, `python/aisimulate/src/aisimulate/config/engine.py`, `python/aisimulate/src/aisimulate/runner.py`, `docs/replay/engine/*`, `docs/replay/features.md`	Rust and Python validation now allows speculative decoding when `prefix_match_unit` is absent or equals `block_size`. Documentation identifies sub-block prefix matching as unsupported.
Configuration and cache reuse tests `crates/core/src/engine/config.rs`, `crates/core/src/engine/scheduler/vllm/core.rs`, `tests/test_state_cache_config.py`	Tests cover rejection of sub-block matching, aligned configuration handoff for canonical MTP and legacy `nextn`, and reuse of a 16-token cached prefix across acceptance-rate configurations.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 8ed13

Some inferred-state configurations advertised as supporting full-block speculation still fail at replay handoff. Align validation with the resolved block size before merging, or accept this bounded limitation.

Pre-merge checks: 6 · 2

Failed checks: 2 warnings

Check name	Status	Explanation	Resolution
Cross-Layer Contract		The public Python validation does not use the final resolved block size. `python/aisimulate/src/aisimulate/config/engine.py:1057-1059` compares `prefix_match_unit` with the user-requested `kv_cache.bl…	Validate speculative state-cache compatibility against the resolved physical block size, not only the requested `kv_cache.block_size`. Either defer this check until `_resolve_state_cache` resolves geometry or resolve the block size before `…
Review Evidence		The description includes useful boundary claims: it covers full-block alignment, zero/full/partial acceptance, and continued rejection of sub-block matching. It does not name exact commands or report …	Update the PR description with exact test and validation commands and their pass/fail results. Label each result as local, fake-runner, parity, or production evidence. Identify the tested commit for any CI result. Keep the negative and boun…

Passed checks: 6 passed

Check name	Status	Explanation
Title check		The title precisely states the behavioral change: speculation is allowed with full-block state alignment. The `fix:` prefix does not make the title vague.
Description check		The description covers the problem, behavior change, affected validation layers, risks, tests, compatibility, and related work. Exact test commands and results, CI results, and review commit identifie…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Modeling And Data Evidence		PASS. The PR changes validation logic, documentation, and functional regression tests. It does not add or refresh formulas, model/performance data, or predicted outputs. The tests provide reproducible…
Compatibility Boundaries		No compatibility-boundary failure is introduced. The PR changes validation behavior and documentation only; it does not change public field declarations, schemas, bindings, versions, or manifests. Pyt…

Full details: Description check

Explanation

The description covers the problem, behavior change, affected validation layers, risks, tests, compatibility, and related work. Exact test commands and results, CI results, and review commit identifiers are still pending, but the description is otherwise sufficiently complete and on topic.

Full details: Modeling And Data Evidence

Explanation

PASS. The PR changes validation logic, documentation, and functional regression tests. It does not add or refresh formulas, model/performance data, or predicted outputs. The tests provide reproducible checks for sub-block rejection and full-block acceptance, so the custom modeling/data-evidence failure condition is not triggered.

Full details: Compatibility Boundaries

Explanation

No compatibility-boundary failure is introduced. The PR changes validation behavior and documentation only; it does not change public field declarations, schemas, bindings, versions, or manifests. Python validation and runner materialization reject only sub-block prefix_match_unit, while Rust applies the same None or full-block_size rule. Existing serialization and native handoff tests cover legacy nextn, canonical MTP, and full-block reuse. python/aisimulate/pyproject.toml has no Dynamo dependency, and the unchanged release contract still defines one aisimulate wheel, one publishable aisimulate-core crate, and exactly two release artifacts.

Full details: Cross-Layer Contract

Explanation

The public Python validation does not use the final resolved block size. python/aisimulate/src/aisimulate/config/engine.py:1057-1059 compares prefix_match_unit with the user-requested kv_cache.block_size. The compiler later replaces that value with the estimator result in python/aisimulate/src/aisimulate/compiler.py:306-314, but it preserves prefix_match_unit in the native payload at compiler.py:665-669. Existing tests prove this change: Kimi K3 can request block_size=128 and resolve to 1536 (tests/test_state_cache_config.py:489-500), and the auto-state replay test emits block_size=1536 with prefix_match_unit=128 (tests/test_state_cache_config.py:686-694). Adding speculation to that input passes public validation because 128 equals the requested size, then fails in the runner or native validation because 128 is sub-block (runner.py:1939-1944; crates/core/src/engine/config.rs:807-810). The new positive tests cover only explicit manual geometry, so this inferred-geometry path is both stale across layers and untested.

Resolution

Validate speculative state-cache compatibility against the resolved physical block size, not only the requested kv_cache.block_size. Either defer this check until _resolve_state_cache resolves geometry or resolve the block size before _validate_prediction_state_cache; apply the same final geometry to disaggregated roles. Add CLI/public round-trip and engine-handoff tests for an inferred Kimi K3 configuration with prefix_match_unit equal to the requested but smaller than the resolved block, and verify that it is rejected. Add a positive inferred-geometry case where prefix_match_unit equals the resolved block size, for both canonical speculation and legacy nextn, if that combination is supported.

Full details: Review Evidence

Explanation

The description includes useful boundary claims: it covers full-block alignment, zero/full/partial acceptance, and continued rejection of sub-block matching. It does not name exact commands or report command results. It also does not distinguish local, fake-runner, parity, and production evidence. The repository policy requires these items (.coderabbit.yaml:291-297), and the PR template requests exact commands and CI commit identification (.github/pull_request_template.md:23-37). No inactive nested workflow is cited as hosted CI evidence, but the positive review-evidence requirements are incomplete.

Resolution

Update the PR description with exact test and validation commands and their pass/fail results. Label each result as local, fake-runner, parity, or production evidence. Identify the tested commit for any CI result. Keep the negative and boundary cases explicit, and do not present inactive nested workflows as hosted CI evidence.

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Actionable comments posted: 1

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Inline comments:
Review comments at @python/aisimulate/src/aisimulate/config/engine.py:
- Around line 1056-1063: Move the speculative-decoding equality validation from
the public validator into resolve_state_size, after state sizing resolves the
final block_size. There, validate prefix_match_unit against the resolved
block_size for divisibility and reject speculative or nextn decoding when it is
sub-block; remove the earlier worker.kv_cache check so authored sizing is not
validated prematurely.

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📥 Commits

Reviewing files that changed from the base of the PR and between a0a35e7 and 8ed13f4.

📒 Files selected for processing: 8

crates/core/src/engine/config.rs
crates/core/src/engine/scheduler/vllm/core.rs
docs/replay/engine/kv-cache.md
docs/replay/engine/speculation.md
docs/replay/features.md
python/aisimulate/src/aisimulate/config/engine.py
python/aisimulate/src/aisimulate/runner.py
tests/test_state_cache_config.py

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ai-dynamo/dynamo (manual)
ai-dynamo/aiconfigurator (manual)

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🧰 Additional context used

📚 Code guidelines: 4

crates/core/src/engine/scheduler/vllm/CLAUDE.md — auto-discovered
AGENTS.md — configured
REVIEW.md — configured
DEVELOPMENT.md — auto-discovered

📓 Path-based instructions: 9

Check unified CLI, Replay, Sweeper, and orchestration behavior together.