fix(session): make compaction snapshots crash-safe - #599
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Kaguya-19 merged 2 commits intoSep 18, 2026
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Persist replacement context in a single flushed JSONL boundary record and replay only validated complete snapshots. Retain original context for legacy or invalid boundaries, and separate truncated tails before resumed writes. Cover snapshot replay before turn completion, invalid snapshots, fsync, mutation isolation, append failures, and every byte truncation including UTF-8 boundaries. Co-authored-by: WUU Agent <305930189+wuu-agent[bot]@users.noreply.github.com>
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Problem
Compaction persists a boundary followed by individual replacement messages. A crash after the boundary but before all replacement messages are written can cause replay to discard original history and recover only partial context. Even complete replacement records are skipped until the active turn has a turn_result. A truncated JSONL tail can also absorb the next append after restart.
Changes
Compatibility
Legacy boundaries cannot prove replacement completeness, so they conservatively restore original history. This may require compaction again after resume. Original transcript records remain on disk.
Validation
On Node.js 22.23.2, all 119 related tests passed: 57 session/compaction-engine/Web replay tests and 62 Agent loop/session/subagent/turn-environment tests. Full TypeScript typecheck, npm run build, and git diff --check also passed.
Regression coverage includes replay before turn completion, invalid and successive snapshots, multimedia/tool content, snapshot immutability, fsync, append failures, and every byte truncation of a snapshot record including UTF-8 boundaries followed by resumed writes.
The full repository suite and real-provider E2E were not run. The branch starts from the fork origin/main; this PR contains one fix commit affecting seven files.