fix(cache): distinguish stable prefix changes from history compaction - #213
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Tiny Sweeper reviewTiny Sweeper completed its review; deterministic results follow. State: Ready for maintainer review Review snapshot
Completeness: Complete What changedThe revision introduces `cacheable_system_prefix_end`, `mark_empty_frozen_prefix`, `prepend_system_message`, `promote_tools_after_zero_prefix_marker`, and `declared_system_prefix_len` functions. Modifies `refresh_prompt_cache_fingerprint`, `PromptCacheLayout::from_request`, `is_prefix_stable_against`, and `PromptCacheGuardMiddleware` to use canonical segment boundaries. Adds `prefix_message_count` to `TranscriptMeta` and updates session resume logic to respect frozen prefix length. Both `DynamicPromptMiddleware` and prompted structured-output paths now use `prepend_system_message`. Features
TestsNo supported feature-to-test mapping was produced. Test execution is not inferred. Findings
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How this fits togetherflowchart LR
n0["tempdir"]:::impacted
n1["transcript_history_conformance"]:::impacted
n2["...t_history_satisfies_the_conformance_suite"]:::impacted
n2 -->|calls| n0
n2 -->|calls| n1
n2 -->|tests| n1
classDef changed fill:#0d4429,stroke:#238636,color:#e6edf3
classDef impacted fill:#161b22,stroke:#6e7681,color:#c9d1d9
classDef flagged fill:#5a1e02,stroke:#d93f0b,color:#ffffff
classDef blocking fill:#67060c,stroke:#f85149,color:#ffffff
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe change persists frozen prompt-prefix counts in transcript metadata and restores them during session resume. The harness uses those boundaries to build prompt-cache layouts and compare requests. Middleware records layout events when the applicable prefix comparison detects a change. ChangesFrozen Prefixes and Prompt Cache
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~45 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant Session
participant Transcript
participant RunContext
participant AgentLoop
Session->>Transcript: Read prefix count and stored rows
Transcript-->>Session: Return metadata and rows
Session->>RunContext: Set frozen count for an all-System prefix
RunContext->>AgentLoop: Provide frozen System-prefix count
AgentLoop->>AgentLoop: Build cache layout from the declared boundary
Merge Risk: ⚪ Minimal · up to This change stores the frozen prompt-prefix boundary with each saved session and reuses it when a session resumes. As a result, compacted or rewritten history no longer counts as a change to the cacheable system and tool prefix. No outstanding defects were identified in the current changes, and the change appears ready to merge after normal checks. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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tinysweeper found nothing blocking. Approving.
$0.0056 · 192,161 in / 16,727 out · 8,739 cached (5%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 664 embedded
critique: $0.0031 · 109,693 in / 5,289 out · 4,282 cached (4%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0013 · 49,664 in / 1,356 out · 1,897 cached (4%) · gpt-5.6-luna
tests: $0.0006 · 17,595 in / 6,007 out · 1,536 cached (9%) · deepseek-v4-flash
description: $0.0003 · 9,645 in / 1,634 out · 1,024 cached (11%) · deepseek-v4-flash
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$0.0049 · 174,303 in / 18,416 out · 15,992 cached (9%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 707 embedded
critique: $0.0022 · 77,107 in / 3,777 out · 4,144 cached (5%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0019 · 67,571 in / 2,979 out · 3,656 cached (5%) · gpt-5.6-luna
tests: $0.0005 · 15,701 in / 7,280 out · 1,024 cached (7%) · deepseek-v4-flash
description: $0.0002 · 7,696 in / 1,575 out · 1,024 cached (13%) · deepseek-v4-flash
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Inline comments:
In `@crates/tinyagents-harness/src/middleware/library/context.rs`:
- Line 781: Update the stable-prefix fingerprinting used by from_request so
changes to cacheable message content are detected even when prompt_fingerprint
is unset and the segment ID is unchanged. Require a content fingerprint for
cacheable message segments or derive it from an explicit stable-message
boundary, excluding any compaction summary, so the comparison at
prev.has_same_stable_prefix_as(&layout) records a CacheLayoutEvent for changed
stable content.
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crates/tinyagents-harness/src/cache/layout.rscrates/tinyagents-harness/src/cache/types.rscrates/tinyagents-harness/src/middleware/README.mdcrates/tinyagents-harness/src/middleware/library/context.rscrates/tinyagents-harness/src/middleware/test.rscrates/tinyagents-harness/src/middleware/types.rs
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tinysweeper found nothing blocking. Approving.
$0.0296 · 1,017,049 in / 75,150 out · 51,395 cached (5%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,192 embedded
critique: $0.0164 · 558,738 in / 42,060 out · 30,643 cached (5%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0119 · 412,072 in / 19,167 out · 19,728 cached (5%) · gpt-5.6-luna
tests: $0.0006 · 21,152 in / 5,176 out · 0 cached (0%) · deepseek-v4-flash
description: $0.0004 · 12,563 in / 5,400 out · 1,024 cached (8%) · deepseek-v4-flash
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tinysweeper found nothing blocking. Approving.
$0.0185 · 659,666 in / 42,322 out · 56,030 cached (8%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,158 embedded
critique: $0.0093 · 326,532 in / 21,546 out · 30,974 cached (9%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0077 · 286,039 in / 10,683 out · 23,008 cached (8%) · gpt-5.6-luna
tests: $0.0005 · 21,549 in / 3,649 out · 1,024 cached (5%) · deepseek-v4-flash
description: $0.0004 · 12,992 in / 5,484 out · 1,024 cached (8%) · deepseek-v4-flash
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Inline comments:
In `@crates/tinyagents-runtime/src/session.rs`:
- Around line 242-246: Update the stored boundary calculation in the cold-resume
path to count only leading root rows that are system messages and equal the
corresponding decoded head rows. Keep the existing boundary resolution behavior;
use the matching count so replacement-prefix rows are not drained or frozen into
PrefixSnapshot.
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crates/tinyagents-harness/src/agent_loop/run_loop.rscrates/tinyagents-harness/src/agent_loop/test.rscrates/tinyagents-harness/src/cache/layout.rscrates/tinyagents-harness/src/cache/mod.rscrates/tinyagents-harness/src/cache/test.rscrates/tinyagents-harness/src/cache/types.rscrates/tinyagents-harness/src/middleware/README.mdcrates/tinyagents-harness/src/middleware/library/context.rscrates/tinyagents-harness/src/middleware/library/observe.rscrates/tinyagents-harness/src/middleware/test.rscrates/tinyagents-harness/src/middleware/types.rscrates/tinyagents-harness/src/prompt/mod.rscrates/tinyagents-runtime/src/session.rscrates/tinyagents-runtime/src/test.rs
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tinysweeper found nothing blocking. Approving.
$0.1002 · 641,024 in / 49,395 out · 64,042 cached (10%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,224 embedded
critique: $0.0691 · 332,336 in / 18,650 out · 26,455 cached (8%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0294 · 237,666 in / 13,538 out · 12,499 cached (5%) · gpt-5.6-luna
tests: $0.0007 · 30,130 in / 6,319 out · 3,072 cached (10%) · deepseek-v4-flash
description: $0.0006 · 21,589 in / 6,961 out · 3,072 cached (14%) · deepseek-v4-flash
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The previously-blocking findings are resolved. Clearing the changes request.
$0.0192 · 688,394 in / 49,686 out · 70,542 cached (10%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,206 embedded
critique: $0.0112 · 396,335 in / 21,305 out · 26,469 cached (7%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0059 · 187,504 in / 13,014 out · 10,793 cached (6%) · gpt-5.6-luna
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description: $0.0007 · 32,076 in / 3,620 out · 1,024 cached (3%) · deepseek-v4-flash
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tinysweeper found nothing blocking. Approving.
$0.0159 · 582,999 in / 39,185 out · 72,202 cached (12%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,212 embedded
critique: $0.0076 · 268,891 in / 13,457 out · 20,294 cached (8%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0063 · 204,626 in / 14,339 out · 16,068 cached (8%) · gpt-5.6-luna
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description: $0.0007 · 33,766 in / 4,110 out · 2,048 cached (6%) · deepseek-v4-flash
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tinyagents/crates/tinyagents-harness/src/cache/layout.rs
Lines 228 to 230 in 9b73b3f
Exclude history compaction from prefix-change events
For two canonical layouts whose declared system/tool prefix is identical but whose history was compacted, has_same_stable_prefix_as correctly returns true while is_prefix_stable_against returns false because the full message streams are no longer compatible. Using the latter here makes the public CacheLayoutEvent::new and under_policy APIs report changed_prefix and even a policy violation for a history-only rewrite, despite the cacheable prefix remaining reusable; the middleware avoids this false event only through its separate pre-check. Base these event fields on the stable-prefix comparison for canonical layouts, retaining the conservative history comparison for unmapped layouts.
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tinysweeper found nothing blocking. Approving.
$0.0079 · 274,746 in / 30,530 out · 20,145 cached (7%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,216 embedded
critique: $0.0025 · 78,426 in / 7,727 out · 9,154 cached (12%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0030 · 88,982 in / 8,599 out · 8,943 cached (10%) · gpt-5.6-luna
tests: $0.0009 · 41,068 in / 3,688 out · 1,024 cached (2%) · deepseek-v4-flash
description: $0.0008 · 32,121 in / 6,808 out · 1,024 cached (3%) · deepseek-v4-flash
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tinysweeper found nothing blocking. Approving.
$0.0173 · 592,571 in / 65,640 out · 102,010 cached (17%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,215 embedded
critique: $0.0085 · 279,415 in / 25,223 out · 38,308 cached (14%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0066 · 194,142 in / 18,503 out · 14,550 cached (7%) · gpt-5.6-luna
tests: $0.0008 · 45,725 in / 9,554 out · 26,624 cached (58%) · deepseek-v4-flash
description: $0.0007 · 36,885 in / 8,619 out · 22,528 cached (61%) · deepseek-v4-flash
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tinysweeper found nothing blocking. Approving.
$0.0078 · 279,061 in / 36,192 out · 55,734 cached (20%) · ladder/vectors, gpt-5.6-luna, deepseek-v4-flash · 1,228 embedded
critique: $0.0019 · 54,961 in / 10,689 out · 7,298 cached (13%) · gpt-5.6-luna, deepseek-v4-flash
security: $0.0034 · 105,438 in / 8,763 out · 7,476 cached (7%) · gpt-5.6-luna
tests: $0.0013 · 44,761 in / 7,740 out · 1,024 cached (2%) · deepseek-v4-flash
description: $0.0008 · 35,751 in / 5,515 out · 2,048 cached (6%) · deepseek-v4-flash
| /// this type exists to catch. | ||
| pub fn is_prefix_stable_against(&self, other: &PromptCacheLayout) -> bool { | ||
| if self.prefix_ids != other.prefix_ids || self.fingerprint != other.fingerprint { | ||
| if !self.has_same_stable_prefix_as(other) { |
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Ignore volatile history for canonical prefix comparisons
declared_system_prefix_len establishes an exact boundary for canonical layouts, but is_prefix_stable_against still compares the entire message digest stream after has_same_stable_prefix_as succeeds. Consequently, changing or replacing a volatile summary after the declared stable system prefix makes is_prefix_stable_against return false, even though the canonical prefix and its content are unchanged. The new compaction test demonstrates this inconsistency by expecting has_same_stable_prefix_as to be true while is_prefix_stable_against is false. Use the canonical boundary to skip history comparison in this method; retain the history check only for layouts without a mapped boundary.
[RULE] incorrect-prefix-comparison ·
| } else { | ||
| (&other.message_digests, &self.message_digests) | ||
| }; | ||
| longer.starts_with(shorter.as_slice()) |
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Compare the whole history for unannotated layouts
When declared_system_prefix_len is None, the layout has no reliable mapping from segments to message boundaries, so a tail extension cannot be known to be outside the cacheable material. This prefix comparison nevertheless reports the layouts as compatible whenever one complete message-digest stream starts with the other, allowing an unannotated request with appended or otherwise changed history to be treated as preserving the stable prefix. Use whole-history equality for the unannotated fallback, and reserve tail-extension logic for layouts with an explicit canonical message boundary.
[RULE] cache-prefix-comparison ·
1 participant
Summary
The cache guard treated a compacted conversation history as a change to the cacheable system/tool prefix. A history rewrite can lose the cached tail while leaving the leading prefix reusable, so this produced misleading prefix-change warnings during long turns.
The guard now compares the actual messages named by a canonical cacheable layout, so a stale builder fingerprint cannot hide a rewritten system instruction. Layouts without a known message boundary check byte-prefix preservation across the full message stream; tool schemas are included independently. Dispatch retains the declared prefix boundary when a System compaction summary follows it, keeping the provider routing key stable. Each committed generation records its frozen-prefix count in optional transcript metadata; a cold resume uses the head generation's count, with sealed-root recovery for older transcripts.
API Or Behavior Changes
PromptCacheLayout::has_same_stable_prefix_as.PromptCacheGuardMiddleware::layout_eventsnow records stable segment changes for canonical layouts, not history-only rewrites. Layouts without a known boundary retain full-message byte-prefix detection. Full prompt stability remains available throughis_prefix_stable_against.TranscriptMeta.prefix_message_countis an optional, backward-compatible_metafield written on new turns. Older transcripts omit it and use the legacy recovery path.Supports tinyhumansai/openhuman#6618. The companion OpenHuman PR #6633 limits its preliminary fingerprint to the session's frozen tiers and advances the vendored gitlink.
Tests
cargo fmt --all --check(OpenHuman workspace, including this submodule)cargo test -p tinyagents-harness -p tinyagents-runtime -p tinyagents-session --lib(1,345 harness, 60 runtime, and 161 session tests passed)cargo test -p tinyagents-integration-tests --test wave2_cache_layout(11 passed)cargo test -p tinyagents-integration-tests --test session_conformance(4 passed) and--test feature_session_transcript(1 passed)cargo clippy --workspace --all-targets -- -D warningscargo clippy --workspace --all-targets --all-features -- -D warningscargo build --all-targets(not run locally)cargo build --all-targets --all-features(not run locally)cargo test(focused tests run locally)cargo test --all-features(not run locally)Documentation
Updated the harness middleware and session/runtime READMEs to document cache boundaries and persisted prefix counts.