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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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29
packages/core/session/tests/request-header.spec.ts
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@@ -107,38 +107,37 @@ describe('diffHeader / applyHeaderDelta', () => {
})
})
describe('request-only messages (messagePrefix / messageSuffix)', () => {
it('canonicalHeader normalizes empty arrays to absent fields', () => {
expect(canonicalHeader({ config: CONFIG, messagePrefix: [], messageSuffix: [] })).toEqual({ config: CONFIG })
const full = canonicalHeader({ config: CONFIG, messagePrefix: [msg('p')], messageSuffix: [msg('s')] })
describe('the session prefix (messagePrefix)', () => {
it('canonicalHeader normalizes an empty prefix to an absent field', () => {
expect(canonicalHeader({ config: CONFIG, messagePrefix: [] })).toEqual({ config: CONFIG })
const full = canonicalHeader({ config: CONFIG, messagePrefix: [msg('p')] })
expect(full.messagePrefix).toEqual([msg('p')])
expect(full.messageSuffix).toEqual([msg('s')])
})
it('headerEquals treats absence and empty as one representation, content differences as unequal', () => {
expect(headerEquals(canonicalHeader({ config: CONFIG }), { config: CONFIG, messagePrefix: [] })).toBe(true)
expect(headerEquals({ config: CONFIG, messagePrefix: [msg('a')] }, { config: CONFIG, messagePrefix: [msg('b')] })).toBe(false)
expect(headerEquals({ config: CONFIG, messageSuffix: [msg('a')] }, { config: CONFIG })).toBe(false)
expect(headerEquals({ config: CONFIG, messagePrefix: [msg('a')] }, { config: CONFIG })).toBe(false)
})
it('replaces a changed prefix whole and leaves an untouched suffix alone', () => {
const prev = canonicalHeader({ config: CONFIG, messagePrefix: [msg('old')], messageSuffix: [msg('keep')] })
const next = canonicalHeader({ config: CONFIG, messagePrefix: [msg('new'), msg('more')], messageSuffix: [msg('keep')] })
it('replaces a changed prefix whole and leaves untouched parts alone', () => {
const prev = canonicalHeader({ config: CONFIG, system: 'keep', messagePrefix: [msg('old')] })
const next = canonicalHeader({ config: CONFIG, system: 'keep', messagePrefix: [msg('new'), msg('more')] })
const delta = roundTrip(prev, next)
expect(delta).toEqual({ messagePrefix: [msg('new'), msg('more')] })
})
it('round-trips framing gained from a bare header and lost back to one (empty array encodes absence)', () => {
it('round-trips a prefix gained from a bare header and lost back to one (empty array encodes absence)', () => {
const none = canonicalHeader({ config: CONFIG })
const some = canonicalHeader({ config: CONFIG, messagePrefix: [msg('p')], messageSuffix: [msg('s')] })
const some = canonicalHeader({ config: CONFIG, messagePrefix: [msg('p')] })
const gained = roundTrip(none, some)
expect(gained).toEqual({ messagePrefix: [msg('p')], messageSuffix: [msg('s')] })
expect(gained).toEqual({ messagePrefix: [msg('p')] })
const lost = roundTrip(some, none)
expect(lost).toEqual({ messagePrefix: [], messageSuffix: [] })
expect(lost).toEqual({ messagePrefix: [] })
})
it('folds framing deltas over the log like any other header amendment', () => {
const session = new Session(SessionId('fold-framing'))
it('folds prefix deltas over the log like any other header amendment', () => {
const session = new Session(SessionId('fold-prefix'))
session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
const first = canonicalHeader({ config: CONFIG, messagePrefix: [msg('catalog v1')] })
session.append('request/header', { header: first, reason: 'initial' })