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fix(agent-loop): compose the session prefix before pre-step; hand it to the pressure gate

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ds-review-bot critical (follow-up): on the first step of a resumed or
seeded/forked instance, auto-compaction ran before runStep composed
this instance's prefix, so the gate read the PREVIOUS instance's logged
prefix from the header fold — a contributor that grew across
resume/fork (skills added, AGENTS.md grown: exactly the
environment-dependent case) could under-gate and ship an over-window
first request.

The loop now composes agent/session-prefix before the instance's first
agent/pre-step (still once per instance; runStep just reads the cache),
and agent/pre-step carries the composed prefix to its listeners.
CompactService.compactIfNeeded gains the sessionPrefix parameter;
BasicCompactService.estimatePressure gates on the handed value — the
header-fold read is gone, so the estimate is exact at every step
including a resumed/forked instance's first. Composition moving before
the boundary snapshot also means a composing listener's session append
now joins the CURRENT request (documented on the seam).

New coverage: composition precedes pre-step and the seam receives the
composed prefix; cancel and disposal landing inside the composition
window drop the step cleanly; the compact gate test hands the prefix
directly.
This commit is contained in:
Yichen Jiang
2026-07-08 20:30:10 +08:00
parent e6bd6cc9be
commit 765052a7d1
16 changed files with 244 additions and 107 deletions
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34
packages/compact/compact-basic/src/index.ts
View File

@@ -183,11 +183,11 @@ export class BasicCompactService extends CompactService {
// log-only `compact/*` records and the replacement node cleanly outside a
// step, so a crash mid-compaction leaves an inert orphan the turn-repair
// closes — never a half-open step.
ctx.on('agent/pre-step', async (agent: Agent, _turn: number, _step: number, fullSystemPrompt: string, signal: AbortSignal) => {
ctx.on('agent/pre-step', async (agent: Agent, _turn: number, _step: number, fullSystemPrompt: string, sessionPrefix: readonly Message[], signal: AbortSignal) => {
try {
const result = await this.compactIfNeeded(agent, fullSystemPrompt, signal)
const result = await this.compactIfNeeded(agent, fullSystemPrompt, sessionPrefix, signal)
if (result) {
const after = this.estimatePressure(agent.session, fullSystemPrompt)
const after = this.estimatePressure(agent.session, fullSystemPrompt, sessionPrefix)
ctx.logger.info(
`compaction: shadowed ${result.shadowedSeqs.length} surface nodes ` +
`(seqs ${result.shadowedRange.start}-${result.shadowedRange.end}, ` +
@@ -360,7 +360,7 @@ export class BasicCompactService extends CompactService {
/**
* The sole token-pressure gate: estimate the NEXT request's pressure — the
* logged session prefix + the surface-derived history + the system prompt
* session prefix + the surface-derived history + the system prompt
* ({@link estimatePressure}) — and if it exceeds the threshold
* (`contextWindow * thresholdRatio`), compact
* the oldest surface nodes outside the `retainTokens` budget. The auto-
@@ -369,7 +369,11 @@ export class BasicCompactService extends CompactService {
* carries it in front of the history (`EpochHeader.messagePrefix`) even
* though it is not derived history — omitting it would under-estimate by
* exactly the prefix and let a deployment at the window edge skip
* compaction, then ship an over-window request. Compaction itself can only
* compaction, then ship an over-window request. The loop composes the
* prefix BEFORE the pre-step seam and hands it through, so the gate sees
* this instance's actual prefix (never a previous instance's logged one —
* a resumed/forked instance whose contributor grew is gated on the grown
* value from its very first step). Compaction itself can only
* shrink HISTORY: a prefix that alone approaches the window is a
* configuration error no compactor fixes.
*
@@ -395,13 +399,14 @@ export class BasicCompactService extends CompactService {
override async compactIfNeeded(
agent: Agent,
fullSystemPrompt: string,
sessionPrefix: readonly Message[],
signal: AbortSignal,
): Promise<CompactionResult | null> {
const session = agent.session
const threshold = Math.floor(this.config.contextWindow * this.config.thresholdRatio)
let result: CompactionResult | null = null
for (let attempt = 0; attempt <= this.config.compactionRetries; attempt++) {
const totalTokens = this.estimatePressure(session, fullSystemPrompt)
const totalTokens = this.estimatePressure(session, fullSystemPrompt, sessionPrefix)
if (totalTokens < threshold) return result
const range = this._compactableRange(session)
@@ -415,7 +420,7 @@ export class BasicCompactService extends CompactService {
result = await this.compactRegion(session, range.start, range.end, agent, signal)
}
const totalTokens = this.estimatePressure(session, fullSystemPrompt)
const totalTokens = this.estimatePressure(session, fullSystemPrompt, sessionPrefix)
if (totalTokens < threshold) return result
throw new Error(
@@ -425,19 +430,16 @@ export class BasicCompactService extends CompactService {
}
/**
* Estimated token pressure of the NEXT request: the logged session prefix
* (`EpochHeader.messagePrefix` from the header fold — request-only messages
* the loop sends in front of the derived history), the derived history, and
* the system prompt. The fold is exact from the loop instance's second
* request on (and from a resumed instance's first — the previous instance
* logged its prefix); it is absent only before a fresh session's first
* request, where the history is a single prompt and compaction is moot.
* Estimated token pressure of the NEXT request: the session prefix
* (`EpochHeader.messagePrefix` — request-only messages the loop sends in
* front of the derived history, composed before the pre-step seam and
* handed to the gate), the derived history, and the system prompt.
* @param session - the session whose next request is being estimated.
* @param fullSystemPrompt - the assembled system prompt (counts toward pressure).
* @param sessionPrefix - the instance's composed session prefix (counts toward pressure).
* @returns the estimated token total the next request will carry.
*/
estimatePressure(session: Session, fullSystemPrompt: string): number {
const sessionPrefix = session.requestHeader()?.messagePrefix ?? []
estimatePressure(session: Session, fullSystemPrompt: string, sessionPrefix: readonly Message[]): number {
return this.estimateTokens([...sessionPrefix, ...session.deriveMessages()], fullSystemPrompt)
}