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.
A new waterfall near request construction lets plugins contribute
request-ONLY messages framing the derived history: RequestMessages
{ before, after } with a frozen empty seed, fired inside the open step
after the agent/request config waterfall, so the step/start boundary
snapshot and its same-sync-frame invariant are untouched. The request
becomes messagePrefix + boundary snapshot + messageSuffix.
Contributions never enter session history — deriveMessages() is
unchanged — so the request header is their durable record:
EpochHeader gains messagePrefix/messageSuffix (canonical absence for
empty arrays), request/header-delta replaces either array whole with
an empty array encoding the transition back to absence, and the
dev-mode reconstruction cross-check now expects the folded header's
framing around the boundary derivation.
This is the seam for per-request advisory context that must be
model-visible now without becoming durable history (a skills catalog,
an environment reminder), keeping the base system prompt
workspace-independent and provider prefix caches stable. The docs
carry the channel cost model: session-frozen content belongs in
before, low-frequency change notices belong in durable history via
inject() (paid once, prefix-cached thereafter), and after is reserved
for small frequently-refreshed state snapshots re-paid on every
request they ride. No shipped producer yet, so ACP snapshot fixtures
are byte-identical.
The loop is now transmission-stateless; a request is a pure function of
(session log, this step's rendered assembly, current AgentOptions):
- The reconstruction boundary is step/start: the messages snapshot is
taken in the same synchronous frame immediately before the step/start
append, so the request's messages are exactly the derivation over
events[0..stepStartSeq) — an inject() from an agent/request listener
(or any concurrent task) lands after the boundary and joins the NEXT
request. This changes behavior for a synchronous step/start
session/event listener that appends content (master derived after the
append, so such a listener could reach the current request):
agent/pre-step is the sanctioned seam for current-request content.
- agent/request is re-typed to config-only: (agent, turn, step,
config: LlmCallConfig, next) → LlmCallConfig. The frozen seed comes
from AgentOptions on a loop instance's first request (explicit options
beat the logged baseline — fork overrides and resume reconfiguration
stay correct) and from the log's folded header afterwards; listeners
return a replacement to switch. Content shaping through the request is
no longer expressible — model-visible content flows through the log
channels.
- recordRequestHeader appends whatever header event the request owes the
log before dispatch: an 'initial'/'resume' snapshot anchoring each
loop instance, a round-trip-verified delta on change, a 'fallback'
snapshot when the encoding cannot express it. Session.requestHeader()
is the log's incrementally-folded baseline.
- Requests are deep-frozen before dispatch (deepFreeze exempts the
AbortSignal — freezing one breaks AbortController.abort() outright);
frozen + sessionId is the loop-built marker the dev invariant keys on.
Ported from #162 and re-anchored on the log: the append-extension /
frozen-end-to-end / compaction-resend / prompt-change property tests,
plus new specs for the boundary semantics, resume anchoring, and the
end-to-end theorem (every recorded request rebuilds byte-equal from the
log alone). Live cache-hit e2e (request-cache.e2e.ts) verified against
the real DeepSeek API. Snapshot goldens intentionally stale until the
single re-record after the compact/summary envelope lands.
Every conversation request's non-content half (system prompt, tool
schemas, call config — the EpochHeader) is now recorded in the session
log: a 'request/header' full snapshot (reason 'initial' | 'resume' |
'fallback') anchors the fold at conversation birth and process
boundaries, and 'request/header-delta' events (system line-trim,
name-keyed tools delta, whole config) encode mid-run changes. The pure
trio — foldRequestHeader / diffHeader / applyHeaderDelta — reconstructs
the header any request was built under from the log alone; the writer
contract round-trip-verifies every delta with a 'fallback' snapshot
when the encoding cannot express a change (pure tool reordering), so a
well-formed log always folds cleanly. Canonical absence: empty system
and empty tools normalize to absent fields, matching request builds.
Persistence and cordis catalogs regenerated; SessionEventMap paste and
EpochHeader added to the core-data-structures session page.
Tianyi Cui
Yichen Jiang