refactor: simplify session log representation

2026-07-13 23:56:10 +08:00
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+# RFC: Simplify session-log representation
+
+Status: implemented
+
+## Problem
+
+The session log maintains two representations that cost more machinery than their consumers require: a pseudo-linked surface and custom request-header deltas.
+
+`SurfaceManager` stores the same order in an array, a seq map, and mutable `prev`/`next` links. Production never reads `prev`; compact's sole `next` read is the successor of an array position. Replacement already uses `indexOf`, so the links do not make its dominant operation constant-time. A seq array with linear replacement lookup has the same asymptotic replacement cost and one representation to validate.
+
+The request-header subsystem implements a custom system/tool delta codec and transmission-decision layer even though its contract says deltas are an encoding optimization, not a reconstructability requirement. Retaining the initial/resume full snapshot at each loop-instance boundary, then writing a canonical full `request/header` whenever that instance's assembled header changes, preserves replay while deleting `SystemDelta`, `ToolsDelta`, round-trip fallback, and the durable `request/header-delta` variant. Codec-only vocabulary disappears with the codec, not because its individual arms were invalid.
+
+This proposal deliberately retains append and replacement `sourceEventSeqs`, crash-repair provenance, and all `SessionStartSource` variants: implemented RFCs give those fields an audit/interception role that zero current readers does not overturn.
+
+## Decision
+
+`SurfaceManager.nodes` is a `readonly number[]` of event sequences; the public `SurfaceNode` shape, node links, and seq-to-node map are removed. The internal replace-generation signal remains. Tool-pairing balance and compaction use array values and indices for successor and replacement ranges.
+
+Request headers use canonical full snapshots only. Initial and resume anchors remain full snapshots even when unchanged; an in-instance change appends another full `request/header` with reason `change`. The delta event, codec types, diff/apply helpers, and codec-only `fallback` reason are removed. Request reconstruction selects the latest snapshot.
+
+`SESSION_FORMAT_VERSION` remains pinned at `0`, so seed and persistence-load validation explicitly reject an old v0 log containing `request/header-delta`. There is no compatibility fold or migration. JSONL and SQLite tests pin this fail-loud boundary, and the ACP snapshot harness represents legitimate mid-session changes as full pinned headers and full readable prompts.
+
+## Alternatives considered
+
+**Keep linked nodes and compact deltas for possible scale.** Links could help a future cursor API, and deltas can reduce logs when large tool schemas change by a small amount. No shipped cursor uses the links, while full snapshots trade disk size for substantially simpler correctness. If header volume proves material, compression or a measured canonical-delta scheme can be designed around real traces.
+
+## Verification
+
+Unit coverage pins ordered-surface append/replace behavior, tool pairing, compaction, full-header folding/logging, request reconstruction, and dev invariants. Seed validation plus JSONL and SQLite load tests reject the legacy event before replay. The keyless ACP suite exercises record, refresh, replay, changed-header pinning, and the sandbox mode-switch fixture in the new shape.
+
+## Consequences
+
+Full headers increase log volume, and linear replacement lookup could be slower on very large surfaces. Replacements were already linear because the prior implementation called `indexOf`; benchmarks are deferred until real traces show the simpler array is a bottleneck. The format version remains `0`, so explicit legacy-event rejection is a permanent part of the pre-release format boundary. In return, surface order and request-header state each have one representation, deleting link maintenance, maps, codec arms, round-trip fallback, and delta-aware snapshot normalization.