fix(agent-loop): compose the session prefix before pre-step; hand it to the pressure gate
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
@@ -70,10 +70,11 @@ forever:
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STEP loop:
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drain steering
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assemble system prompt and tool schemas
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agent/session-prefix (first step)
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agent/pre-step
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'step/start'
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snapshot the derived messages (the reconstruction boundary)
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agent/request (config only) -> agent/session-prefix (first request) -> log request/header -> llm/stream (frozen)
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agent/request (config only) -> log request/header -> llm/stream (frozen)
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'assistant/chunk'
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agent/step-result
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'assistant/message'
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@@ -47,21 +47,21 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
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Types: [Agent](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:460`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:472`](../../packages/core/agent/src/types.ts)
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### `agent/pre-step` — serial
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Awaited pre-step surface-mutation checkpoint, fired once per step AFTER `turn/start` (and after the prior step closed) but BEFORE this step's `step/start` — so anything a listener appends lands OUTSIDE the step, between `turn/start`/`step/end` and the upcoming `step/start`. `step` is the number of the step about to start. The loop awaits `ctx.serial('agent/pre-step', …)` after assembling the system prompt, then opens the step and derives the request history ONCE from whatever the surface now holds. This is where compaction belongs: it mutates the session surface in place (shadowing an older range with a summary node) with its log-only `compact/*` records cleanly outside any step, and the single subsequent derive reflects the mutation — so there is no double-derive and no listener can see (or be expected to act on) an assembled `messages` array that does not exist yet.
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Serial (awaited in registration order), not a waterfall: a listener mutates the surface as a side effect; there is nothing to transform, but the loop must wait for the mutation to complete before opening the step and deriving. Cordis `serial` bails early if a listener returns a bail value; this event is typed and documented as `void`, so listeners must not return a semantic veto value. `fullSystemPrompt` is the assembled prompt a listener needs to measure pressure (the system prompt counts toward the budget). `signal` cancels any in-flight work a listener starts (e.g. a summarization model call).
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Serial (awaited in registration order), not a waterfall: a listener mutates the surface as a side effect; there is nothing to transform, but the loop must wait for the mutation to complete before opening the step and deriving. Cordis `serial` bails early if a listener returns a bail value; this event is typed and documented as `void`, so listeners must not return a semantic veto value. `fullSystemPrompt` is the assembled prompt a listener needs to measure pressure (the system prompt counts toward the budget), and `sessionPrefix` is the instance's composed agent/session-prefix product for the same reason — every request carries it in front of the derived history, and it is composed BEFORE this seam fires precisely so a pressure gate counts the prefix the request will actually send (never a stale logged one). `signal` cancels any in-flight work a listener starts (e.g. a summarization model call).
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```ts cordis-catalog
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'agent/pre-step'(agent: Agent, turn: number, step: number, fullSystemPrompt: string, signal: AbortSignal): Promise<void> | void
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'agent/pre-step'(agent: Agent, turn: number, step: number, fullSystemPrompt: string, sessionPrefix: readonly Message[], signal: AbortSignal): Promise<void> | void
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```
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Types: [Agent](../core-data-structures/core.md)
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Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:350`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:357`](../../packages/core/agent/src/types.ts)
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### `agent/prompt-submit` — waterfall
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@@ -73,7 +73,7 @@ Waterfall: decide what happens to ONE drained queued message before it becomes a
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Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:363`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:370`](../../packages/core/agent/src/types.ts)
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### `agent/queued` — emit
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@@ -97,11 +97,11 @@ Waterfall: shape the step's call configuration — model switching, sampling ove
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Types: [Agent](../core-data-structures/core.md) · [LlmCallConfig](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:387`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:394`](../../packages/core/agent/src/types.ts)
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### `agent/session-prefix` — waterfall
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Waterfall: compose the SESSION PREFIX — request-only messages placed in front of the ENTIRE derived history (directly after the provider's system slot) on every request this loop instance sends. Fired ONCE per loop instance, lazily on its first request-building step; the composed result is deep-frozen, recorded as `EpochHeader.messagePrefix` on the instance's anchoring `'initial'`/`'resume'` header snapshot, and reused verbatim for every subsequent request — never recomputed mid-session, so the provider prefix cache holds by construction (a process restart or `ctx.agents.resume()` is a new instance: it recomposes, and any drift lands attributably on the `'resume'` snapshot).
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Waterfall: compose the SESSION PREFIX — request-only messages placed in front of the ENTIRE derived history (directly after the provider's system slot) on every request this loop instance sends. Fired ONCE per loop instance, lazily before its first step's agent/pre-step seam — BEFORE the pre-step so a token-pressure gate (compaction) counts the prefix this instance will actually send, never a previous instance's logged one. The composed result is deep-frozen, recorded as `EpochHeader.messagePrefix` on the instance's anchoring `'initial'`/`'resume'` header snapshot, and reused verbatim for every subsequent request — never recomputed mid-session, so the provider prefix cache holds by construction (a process restart or `ctx.agents.resume()` is a new instance: it recomposes, and any drift lands attributably on the `'resume'` snapshot). Composition runs outside the step, before the boundary snapshot: a composing listener's session append joins the CURRENT request's derived history.
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This is the home for session-stable openers the model must always see but that must NOT become durable history — a skills catalog, an AGENTS.md digest, a workspace baseline: `Session.deriveMessages()` never returns the prefix, and the header events are its only durable record, so the request stays reconstructable from the log. Content that CHANGES mid-session belongs in the append-only history channels instead — `agent.inject()`, a `tools/post-execute` decision's `additionalContext`, prompt-submit `additionalContext` — each a durable `context/message` paid once and prefix-cached thereafter.
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@@ -113,7 +113,7 @@ The seed is a frozen empty list; a contributing listener returns a NEW array —
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Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:425`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:437`](../../packages/core/agent/src/types.ts)
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### `agent/session-start` — emit
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@@ -149,7 +149,7 @@ Waterfall: post-process the assembled assistant Message before tool dispatch (va
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Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:435`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:447`](../../packages/core/agent/src/types.ts)
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### `agent/turn-continuation` — waterfall
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@@ -161,7 +161,7 @@ Waterfall: override the turn-continuation decision via a typed ContinuationDecis
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Types: [Agent](../core-data-structures/core.md)
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Source: [`packages/core/agent/src/types.ts:448`](../../packages/core/agent/src/types.ts)
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Source: [`packages/core/agent/src/types.ts:460`](../../packages/core/agent/src/types.ts)
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## `fs/*`
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@@ -98,11 +98,13 @@ Implementations MUST honor:
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- **Blocking**: no compaction begins while another is in progress for the same session. The recommended mechanism is the log-recorded lock — append `compact/start` before the slow work and `compact/end` after (even on failure) — so the lock is visible to replay and crash recovery.
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```ts cordis-catalog
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abstract compactIfNeeded( agent: CompactAgentContext, fullSystemPrompt: string, signal: AbortSignal, ): Promise<CompactionResult | null>
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abstract compactIfNeeded( agent: CompactAgentContext, fullSystemPrompt: string, sessionPrefix: readonly Message[], signal: AbortSignal, ): Promise<CompactionResult | null>
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abstract compactRegion( session: Session, start: number, end: number, agent: CompactAgentContext, signal?: AbortSignal, ): Promise<CompactionResult>
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```
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Source: [`packages/compact/compact/src/index.ts:63`](../../packages/compact/compact/src/index.ts)
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Types: [Message](../core-data-structures/core.md)
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Source: [`packages/compact/compact/src/index.ts:64`](../../packages/compact/compact/src/index.ts)
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## `ctx.fs` — `FileSystem` (abstract seam)
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@@ -9,16 +9,16 @@ This matrix shows which packages dispatch each harness-owned event and which pac
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| --- | --- | --- | --- | --- |
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| `agent/created` | `emit` | [`packages/core/agent/src/types.ts:265`](../packages/core/agent/src/types.ts) | [`agent`](../packages/core/agent) (`emit`) | [`stdio-agent`](../packages/ui/stdio-agent) |
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| `agent/disposed` | `emit` | [`packages/core/agent/src/types.ts:272`](../packages/core/agent/src/types.ts) | [`agent`](../packages/core/agent) (`emit`) | [`stdio-agent`](../packages/ui/stdio-agent) |
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| `agent/error` | `emit` | [`packages/core/agent/src/types.ts:460`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
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| `agent/pre-step` | `serial` | [`packages/core/agent/src/types.ts:350`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`serial`) | [`compact-basic`](../packages/compact/compact-basic) |
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| `agent/prompt-submit` | `waterfall` | [`packages/core/agent/src/types.ts:363`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`hooks-codex`](../packages/hooks/hooks-codex) |
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| `agent/error` | `emit` | [`packages/core/agent/src/types.ts:472`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
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| `agent/pre-step` | `serial` | [`packages/core/agent/src/types.ts:357`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`serial`) | [`compact-basic`](../packages/compact/compact-basic) |
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| `agent/prompt-submit` | `waterfall` | [`packages/core/agent/src/types.ts:370`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`hooks-codex`](../packages/hooks/hooks-codex) |
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| `agent/queued` | `emit` | [`packages/core/agent/src/types.ts:290`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
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| `agent/request` | `waterfall` | [`packages/core/agent/src/types.ts:387`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/session-prefix` | `waterfall` | [`packages/core/agent/src/types.ts:425`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/request` | `waterfall` | [`packages/core/agent/src/types.ts:394`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/session-prefix` | `waterfall` | [`packages/core/agent/src/types.ts:437`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/session-start` | `emit` | [`packages/core/agent/src/types.ts:305`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`hooks-codex`](../packages/hooks/hooks-codex) |
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| `agent/status` | `emit` | [`packages/core/agent/src/types.ts:281`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | [`acp`](../packages/ui/acp), [`invariants`](../packages/support/invariants), [`stdio-agent`](../packages/ui/stdio-agent) |
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| `agent/step-result` | `waterfall` | [`packages/core/agent/src/types.ts:435`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/turn-continuation` | `waterfall` | [`packages/core/agent/src/types.ts:448`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`hooks-codex`](../packages/hooks/hooks-codex) |
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| `agent/step-result` | `waterfall` | [`packages/core/agent/src/types.ts:447`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
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| `agent/turn-continuation` | `waterfall` | [`packages/core/agent/src/types.ts:460`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | [`hooks-claude`](../packages/hooks/hooks-claude), [`hooks-codex`](../packages/hooks/hooks-codex) |
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| `fs/edit-intent` | `waterfall` | [`packages/fs/fs/src/index.ts:123`](../packages/fs/fs/src/index.ts) | [`tool-fs`](../packages/fs/tool-fs) (`waterfall`) | [`fs-policy`](../packages/fs/fs-policy) |
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| `fs/observed` | `emit` | [`packages/fs/fs/src/index.ts:138`](../packages/fs/fs/src/index.ts) | [`tool-fs`](../packages/fs/tool-fs) (`emit`) | [`fs-policy`](../packages/fs/fs-policy) |
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| `fs/write-intent` | `waterfall` | [`packages/fs/fs/src/index.ts:109`](../packages/fs/fs/src/index.ts) | [`tool-fs`](../packages/fs/tool-fs) (`waterfall`) | [`fs-policy`](../packages/fs/fs-policy) |
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@@ -22,7 +22,7 @@ Prefix-cache stability is corollary #1, not the headline: an append-only log pro
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**The header.** The request's non-history half — `EpochHeader`: call config (`LlmCallConfig`: model + sampling scalars), rendered system prompt, assembled tool schemas, and the session prefix (`messagePrefix`, below) — is logged session state, in canonical form (empty system/tools/prefix ≡ absent). Two log-only, turn-enclosed events in dsh-session carry it: `request/header`, a full snapshot with reason `'initial' | 'resume' | 'fallback'`, and `request/header-delta`, an amendment (`SystemDelta`: a common-prefix/suffix line trim; `ToolsDelta`: name-keyed added/removed/changed; `config`: replaced whole; `messagePrefix`: replaced whole, an empty array encoding the transition to absence — an arm the loop never exercises in practice, kept for codec totality). The pure trio `foldRequestHeader` / `diffHeader` / `applyHeaderDelta` reconstructs; the live session tracks the fold with the same lazy cursor as the message cache. Snapshots anchor the fold where a fold needs anchors — conversation birth and process boundaries — and each loop instance appends one on its first request (`'initial'` when the log has none, `'resume'` otherwise, even when nothing changed: the boundary itself is a recorded fact, and cross-restart drift becomes attributable while an unchanged header resumes byte-identical). Deltas are an encoding optimization with a safety valve, never a correctness dependency: the writer verifies `applyHeaderDelta(prev, delta)` reproduces the new header exactly and records a `'fallback'` snapshot when the encoding cannot express a change (a pure tool reordering), so a well-formed log always folds.
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**The loop, transmission-stateless.** Per step: render assembly (every step — value comparison needs no change-signal discipline, and a section that varies per step surfaces as a *logged* header event per step instead of a silent bust) → `agent/pre-step` (compaction's surface mutations land before derivation) → **messages snapshot, then `step/start` appended as the next operation in the same synchronous frame** → seed the call config (first request of the instance: from `AgentOptions`, so explicit options always beat the logged baseline — fork model-overrides and resume reconfiguration stay correct; afterwards: from the folded header) → the `agent/request` waterfall, re-typed `(agent, turn, step, config: LlmCallConfig, next) → LlmCallConfig` — a frozen seed and a returned replacement are ALL a listener shapes; durable content flows through the log channels (`inject()`, steering, prompt-submit `additionalContext`, sections via `system-prompt/assemble`) — → on the instance's FIRST request only, the `agent/session-prefix` waterfall — request-ONLY messages fronting the entire derived history (a frozen empty seed, contributions returned as an extension of `next()`; the home for session-stable openers that must NOT become history — a skills catalog, an AGENTS.md digest), deep-frozen and cached on the instance so reuse is structural and the prefix cannot drift mid-session — → the header event the request owes the log, carrying the prefix as `messagePrefix` (no session event carries it, so the header is its only durable record; resume = a new instance = a recompose, anchored by its `'resume'` snapshot) → build `GenerateOptions` from `messagePrefix + snapshot` + header, deep-freeze (`deepFreeze` exempts the `AbortSignal`, the one live control channel — freezing one breaks `AbortController.abort()`), dispatch. The loop's per-instance bookkeeping is one boolean plus the cached prefix: whether this instance has logged its anchoring snapshot, and what it composed.
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**The loop, transmission-stateless.** Per step: render assembly (every step — value comparison needs no change-signal discipline, and a section that varies per step surfaces as a *logged* header event per step instead of a silent bust) → on the instance's FIRST step only, the `agent/session-prefix` waterfall — request-ONLY messages fronting the entire derived history (a frozen empty seed, contributions returned as an extension of `next()`; the home for session-stable openers that must NOT become history — a skills catalog, an AGENTS.md digest), deep-frozen and cached on the instance so reuse is structural and the prefix cannot drift mid-session — → `agent/pre-step`, carrying the composed prefix (compaction's surface mutations land before derivation, and its pressure gate counts the prefix this instance will actually send — never a previous instance's logged one, which could under-gate a resumed/forked instance whose contributor grew) → **messages snapshot, then `step/start` appended as the next operation in the same synchronous frame** → seed the call config (first request of the instance: from `AgentOptions`, so explicit options always beat the logged baseline — fork model-overrides and resume reconfiguration stay correct; afterwards: from the folded header) → the `agent/request` waterfall, re-typed `(agent, turn, step, config: LlmCallConfig, next) → LlmCallConfig` — a frozen seed and a returned replacement are ALL a listener shapes; durable content flows through the log channels (`inject()`, steering, prompt-submit `additionalContext`, sections via `system-prompt/assemble`) — → the header event the request owes the log, carrying the prefix as `messagePrefix` (no session event carries it, so the header is its only durable record; resume = a new instance = a recompose, anchored by its `'resume'` snapshot) → build `GenerateOptions` from `messagePrefix + snapshot` + header, deep-freeze (`deepFreeze` exempts the `AbortSignal`, the one live control channel — freezing one breaks `AbortController.abort()`), dispatch. The loop's per-instance bookkeeping is one boolean plus the cached prefix: whether this instance has logged its anchoring snapshot, and what it composed.
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**The reconstruction boundary is `step/start`, unconditionally.** A step's messages are the derivation over `events[0..stepStartSeq)`. Because the snapshot precedes the `step/start` append in the same synchronous frame, nothing can enter this request past the boundary: an `agent.inject()` from an `agent/request` listener (or any concurrent task, or a `session/event` listener firing on `step/start` itself) lands in the log after the boundary and joins the NEXT request. For waterfall-window appends this matches the prior loop (it also derived before its waterfall); for a synchronous `step/start` listener it is a deliberate change — such a listener could previously reach the current request — and `agent/pre-step` is the sanctioned seam for content that must affect the CURRENT request. A step's header for reconstruction is the fold after its own `request/header*` event (which sits between its `step/start` and first response event) or the fold carried forward.
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