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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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8
packages/core/agent-loop/README.md
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@@ -59,10 +59,10 @@ forever:
boundary = session.deriveMessages() ⟵ reconstruction boundary: same sync frame,
session('step/start') strictly before step/start
config = waterfall agent/request ⟵ frozen seed; return a replacement to switch
reqMsgs = waterfall agent/request-advice ⟵ request-only before/after messages; recorded
on the header, never session history
prefix ??= waterfall agent/session-prefix ⟵ once per instance (first request): frozen
session prefix; on the header, never history
session('request/header'[-delta]) ⟵ the header event this request owes the log
stream llm.stream(freeze({header..., messages: before+boundary+after})) → session('assistant/chunk')
stream llm.stream(freeze({header..., messages: prefix+boundary})) → session('assistant/chunk')
message = waterfall agent/step-result
session('assistant/message')
each tool-call: session('tool/call')
@@ -86,7 +86,7 @@ Cancellation: `agent.cancel()` is the single public stop primitive — it clears
### What is NOT here
Everything that goes beyond "call the model, run the tools, repeat" belongs to plugins listening on the event taxonomy:
- Hooks: `agent/session-start`, `agent/prompt-submit`, `agent/pre-step`, `agent/request`, `agent/request-advice`, `agent/step-result`, `tools/pre-execute`, `tools/post-execute`, `agent/turn-continuation`
- Hooks: `agent/session-start`, `agent/prompt-submit`, `agent/pre-step`, `agent/request`, `agent/session-prefix`, `agent/step-result`, `tools/pre-execute`, `tools/post-execute`, `agent/turn-continuation`
- Compaction: `agent/pre-step`
- Sandbox, permission, plan mode: `tools/pre-execute` (deny/ask gate), `tools/post-execute`
- Sub-agents: implemented outside the loop as `ctx.subagents` providers; in-process providers use `ctx.agents.create()` and owned `AgentHandle` teardown, while child streaming/progress and background/poll collection remain deferred.