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refactor(agent): replace the per-step advice seam with agent/session-prefix

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Review discussion converged on the industry shape (Claude Code caches
user context per conversation; Codex separates initial context from
diffs; Kimi appends at continuation boundaries to protect prompt
caching): stable openers belong in a compose-once prefix, mid-session
changes belong in append-only history — not in a per-request slot.

agent/session-prefix fires ONCE per loop instance, lazily on its first
request-building step: the composed Message[] is deep-frozen, cached on
the transmission bookkeeping, recorded as EpochHeader.messagePrefix on
the anchoring 'initial'/'resume' snapshot, and reused verbatim for
every request the instance sends — prefix stability is structural, not
a producer discipline, and a resume recomposes with attributable drift.
The request is messagePrefix + boundary snapshot.

The per-step RequestAdvice/RequestAdviceContext surface and the
messageSuffix header field are dropped: the tail slot had no consumer,
and every current update pattern (new AGENTS.md discovered, memory
update, skills change) routes through the existing append-only history
channels — inject(), tools/post-execute additionalContext,
prompt-submit additionalContext — each paid once and prefix-cached
thereafter. The messagePrefix delta arm stays for codec totality; the
loop never produces one in practice.
This commit is contained in:
Yichen Jiang
2026-07-08 15:44:30 +08:00
parent 2cbdeb0872
commit ea4c10d753
21 changed files with 260 additions and 380 deletions
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21
docs/core-data-structures/session.md
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@@ -75,14 +75,14 @@ interface SessionEventMap {
'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 request-only
* message arrays (an EMPTY array encodes the transition to "none"). The
* 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[]; messageSuffix?: Message[] }
'request/header-delta': { system?: SystemDelta; tools?: ToolsDelta; config?: LlmCallConfig; messagePrefix?: Message[] }
}
```
@@ -99,7 +99,7 @@ export interface TodoItem {
### The request header events: `request/header` and `request/header-delta`
The request envelope — the `EpochHeader` (call config + rendered system prompt + assembled tool schemas + request-only messages) — 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 `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.
```ts type-equiv
export interface EpochHeader {
@@ -110,18 +110,17 @@ export interface EpochHeader {
/** Assembled tool schemas; absent for a tool-less request. */
tools?: ToolSchema[]
/**
* Request-only messages sent BEFORE the derived history (the
* `agent/request-advice` waterfall's `before` contributions). Not session
* history — `deriveMessages()` never returns them — so the header is their
* only durable record; absent when the request carried none.
* The session prefix: request-only messages sent BEFORE the entire derived
* history (the `agent/session-prefix` waterfall's product, composed once
* per loop instance and reused for every request it sends). Not session
* history — `deriveMessages()` never returns it — so the header is its
* only durable record; absent when the instance composed none.
*/
messagePrefix?: Message[]
/** Request-only messages sent AFTER the derived history; absent when none. */
messageSuffix?: Message[]
}
```
Canonical form: an empty system prompt, an empty tool list, and empty request-only message arrays are ABSENT fields, matching how requests are built. `messagePrefix`/`messageSuffix` are the durable record of the `agent/request-advice` waterfall's contributions (the request is `messagePrefix + derived history + messageSuffix`); their deltas replace the array whole, an empty array encoding the transition back to absence. 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`); 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).
## `SessionEvent<T>` — one log entry