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refactor: simplify session log representation

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This commit is contained in:
Tianyi Cui
2026-07-13 23:56:10 +08:00
parent d060c0dd4f
commit 1123e946c0
64 changed files with 522 additions and 1032 deletions
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37
docs/core-data-structures/session.md
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@@ -64,25 +64,14 @@ interface SessionEventMap {
* Full snapshot of the {@link EpochHeader} the NEXT request is built under,
* with the {@link RequestHeaderReason} it was recorded whole. Appended by
* the loop inside the step, before dispatch, on a loop instance's first
* request-building step (`'initial'`/`'resume'`) or when a delta failed its
* round-trip guard (`'fallback'`); always records what the request actually
* used, post-`agent/request`. Anchors the header fold: reconstruction reads
* the latest snapshot and applies the deltas after it. NOT a
* request-building step (`'initial'`/`'resume'`) or when a later request's
* header changes (`'change'`); always records what the request actually
* used, post-`agent/request`. Reconstruction reads the latest snapshot. NOT a
* {@link SurfaceEventType}: it produces no LLM message — it is the request
* envelope, logged so every request is a pure function of the session log
* (the reconstructability RFC).
*/
'request/header': { header: EpochHeader; reason: RequestHeaderReason }
/**
* Amendment to the folded {@link EpochHeader}: system line-trim, name-keyed
* tools delta, whole replacement config, or whole replacement session
* prefix (an EMPTY array encodes the transition to "none"). The
* writer verifies `applyHeaderDelta(previous, delta)` reproduces the new
* header exactly and falls back to a `'fallback'` `request/header` snapshot
* when it cannot, so a logged delta ALWAYS round-trips. NOT a
* {@link SurfaceEventType}.
*/
'request/header-delta': { system?: SystemDelta; tools?: ToolsDelta; config?: LlmCallConfig; messagePrefix?: Message[] }
}
```
@@ -97,9 +86,9 @@ export interface TodoItem {
}
```
### The request header events: `request/header` and `request/header-delta`
### The request header event: `request/header`
The request envelope — the `EpochHeader` (call config + rendered system prompt + assembled tool schemas + the session prefix) — is logged session state, so every conversation request is a pure function of the log (the reconstructability RFC). A `request/header` snapshot (reason `'initial' | 'resume' | 'fallback'`) anchors the fold at conversation birth, process boundaries, and delta-encoding fallbacks; `request/header-delta` events amend it mid-run. `foldRequestHeader(events)` reconstructs the header any request was built under; the writer round-trip-verifies every delta before logging it, so a well-formed log always folds. Neither is a `SurfaceEventType` — they produce no LLM message.
The request envelope — the `EpochHeader` (call config + rendered system prompt + assembled tool schemas + the session prefix) — is logged session state, so every conversation request is a pure function of the log (the reconstructability RFC). A full `request/header` snapshot with reason `'initial'` or `'resume'` records each loop-instance boundary; a later changed request records another full snapshot with reason `'change'`. `foldRequestHeader(events)` reconstructs the header by selecting the latest snapshot. The event is not a `SurfaceEventType`: it produces no LLM message.
```ts type-equiv
export interface EpochHeader {
@@ -120,7 +109,7 @@ export interface EpochHeader {
}
```
Canonical form: an empty system prompt, an empty tool list, and an empty session prefix are ABSENT fields, matching how requests are built. `messagePrefix` is the durable record of the `agent/session-prefix` waterfall's product (the request is `messagePrefix + derived history`); composed once per loop instance and anchored by that instance's snapshot, so the loop never produces a prefix delta in practice — the delta arm (whole-array replacement, an empty array encoding the transition back to absence) exists for codec totality. The other delta payloads (`SystemDelta` — a common-prefix/suffix line trim; `ToolsDelta` — name-keyed added/removed/changed) live beside the events in [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts).
Canonical form: an empty system prompt, an empty tool list, and an empty session prefix are absent fields, matching how requests are built. `messagePrefix` is the durable record of the `agent/session-prefix` waterfall's product (the request is `messagePrefix + derived history`); it is composed once per loop instance and included in every full snapshot that instance records. Legacy v0 logs containing the removed `request/header-delta` format are rejected at seed and persistence-load boundaries rather than replayed incompletely.
## `SessionEvent<T>` — one log entry
@@ -152,7 +141,7 @@ type SessionEvent<T extends SessionEventType = SessionEventType> = {
## Surface types
The five message-producing types (`SurfaceEventType` — `user/message`, `assistant/message`, `tool/result`, `context/message`, `steering/message`) carry surface metadata declaring how they join the derived surface linked list. See the [session surface RFC](../rfc/implemented/architecture/2026-06-18-session-surface.md).
The five message-producing types (`SurfaceEventType` — `user/message`, `assistant/message`, `tool/result`, `context/message`, `steering/message`) carry surface metadata declaring how they join the ordered derived surface. See the [session surface RFC](../rfc/implemented/architecture/2026-06-18-session-surface.md).
### `SurfaceEventType` — the message-producing subset of event types
@@ -173,7 +162,7 @@ export type SurfaceOp =
| { op: 'replace'; start: number; end: number }
```
`'append'` is the normal tail-append path. `replace` shadows surface nodes from `start` through `end` inclusive (both must be valid surface node seqs; `start === end` replaces a single node) and inserts the new node in their place.
`'append'` is the normal tail-append path. `replace` shadows surface entries from `start` through `end` inclusive (both must be valid surface seqs; `start === end` replaces a single entry) and inserts the new event in their place.
### `SurfaceIntent` — the parameter to `session.append()`
@@ -186,16 +175,6 @@ export interface SurfaceIntent {
Required for `SurfaceEventType` events — every message-producing event must declare how it joins the surface, the sole source of derived history. Non-surface types reject it at compile time.
### `SurfaceNode` — a node in the surface linked list
```ts type-equiv
export interface SurfaceNode {
seq: number
prev: number | null
next: number | null
}
```
## Derived history: `deriveMessages()` and `deriveEventMessage()`
`Session.deriveMessages()` projects the event log into the `Message[]` the model sees — cached (each surface node projected once, when first seen; a surface rewrite rebuilds) and frozen (a fresh array per call over shared, deep-frozen messages, so mutating logged history through a projection is unrepresentable). `deriveEventMessage(event)` is the per-node pure function the fold applies — public so external reconstructors and the dev invariant project a log prefix with exactly the same rules and cannot disagree with the cache. The projection rules: