fix(agent): correct session-prefix composition-order docs; scrub messagePrefix in snapshots

Two ds-review-bot findings:

The seam JSDoc claimed extending 'await next()' composes in
registration order — false for the append form: the waterfall unwinds
innermost-first, so appending places later-registered contributions
first. The canonical contribution is now documented as the PREPEND
'[mine, ...await next()]' (registration order on the wire), with the
append form's reverse-order behavior stated explicitly; the ordering
test now uses the canonical pattern in both listeners.

scrubRequestHeaders tokenized only system/tools, so a fixture recording
a composed session prefix would carry its raw text (workspace-specific
churn/leak). The scrubber now maps each header/delta messagePrefix
entry to a {{messagePrefix}} token — count stays a structural fact,
absence stays absent, the empty-array transition stays visible — with
normalize.spec coverage for the header, delta, absence, and odd-shape
paths.

docs/cordis-catalog/events.md

@@ -47,7 +47,7 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:455`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:460`](../../packages/core/agent/src/types.ts)
 
 ### `agent/pre-step` — serial
 
@@ -105,7 +105,7 @@ Waterfall: compose the SESSION PREFIX — request-only messages placed in front
 
 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.
 
-The seed is a frozen empty list; a contributing listener returns a NEW array extending `await next()` (`[...prefix, mine]` — never an in-place push), so contributions compose across plugins in registration order and compose deterministically for a fixed plugin set. Call `next()` to delegate, or return a list without it to short-circuit.
+The seed is a frozen empty list; a contributing listener returns a NEW array — never an in-place push. The canonical contribution is a PREPEND, `[mine, ...await next()]`: the waterfall unwinds innermost-first (the LAST-registered listener's `next()` resolves first), so prepending yields registration order on the wire, and every plugin using it composes deterministically. The append form `[...await next(), mine]` is legal but places a contribution AFTER every later-registered plugin's — reverse registration order when all contributors append. Call `next()` to delegate, or return a list without it to short-circuit.
 
 ```ts cordis-catalog
 'agent/session-prefix'(agent: Agent, prefix: Message[], signal: AbortSignal, next: () => Promise<Message[]>): Promise<Message[]>
@@ -113,7 +113,7 @@ The seed is a frozen empty list; a contributing listener returns a NEW array ext
 
 Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:420`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:425`](../../packages/core/agent/src/types.ts)
 
 ### `agent/session-start` — emit
 
@@ -149,7 +149,7 @@ Waterfall: post-process the assembled assistant Message before tool dispatch (va
 
 Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:430`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:435`](../../packages/core/agent/src/types.ts)
 
 ### `agent/turn-continuation` — waterfall
 
@@ -161,7 +161,7 @@ Waterfall: override the turn-continuation decision via a typed ContinuationDecis
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:443`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:448`](../../packages/core/agent/src/types.ts)
 
 ## `fs/*`
 

docs/event-producer-consumer.md

@@ -9,16 +9,16 @@ This matrix shows which packages dispatch each harness-owned event and which pac
 | --- | --- | --- | --- | --- |
 | `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) |
 | `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) |
-| `agent/error` | `emit` | [`packages/core/agent/src/types.ts:455`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
+| `agent/error` | `emit` | [`packages/core/agent/src/types.ts:460`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
 | `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) |
 | `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) |
 | `agent/queued` | `emit` | [`packages/core/agent/src/types.ts:290`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`emit`) | - |
 | `agent/request` | `waterfall` | [`packages/core/agent/src/types.ts:387`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
-| `agent/session-prefix` | `waterfall` | [`packages/core/agent/src/types.ts:420`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
+| `agent/session-prefix` | `waterfall` | [`packages/core/agent/src/types.ts:425`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
 | `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) |
 | `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) |
-| `agent/step-result` | `waterfall` | [`packages/core/agent/src/types.ts:430`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
+| `agent/step-result` | `waterfall` | [`packages/core/agent/src/types.ts:435`](../packages/core/agent/src/types.ts) | [`agent-loop`](../packages/core/agent-loop) (`waterfall`) | - |
-| `agent/turn-continuation` | `waterfall` | [`packages/core/agent/src/types.ts:443`](../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) |
+| `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) |
 | `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) |
 | `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) |
 | `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) |

packages/core/agent-loop/tests/interception.spec.ts

@@ -352,23 +352,24 @@ describe('agent/session-prefix', () => {
     expect(agent.session.deriveMessages()[0]).toEqual({ role: 'user', content: [{ type: 'text', text: 'go' }] })
   })
 
-  it('contributions compose across listeners in registration order', async () => {
+  it('the canonical prepend pattern composes contributions in registration order', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
+    // Both listeners use the canonical `[mine, ...await next()]` prepend: the
+    // waterfall unwinds innermost-first (the second listener's array is built
+    // first), so prepending puts the FIRST-registered contribution first.
     ctx.on('agent/session-prefix', async (_agent, _prefix, _signal, next): Promise<Message[]> => {
       return [{ role: 'user', content: [{ type: 'text', text: 'first' }] }, ...await next()]
     })
     ctx.on('agent/session-prefix', async (_agent, _prefix, _signal, next): Promise<Message[]> => {
-      return [...await next(), { role: 'user', content: [{ type: 'text', text: 'second' }] }]
+      return [{ role: 'user', content: [{ type: 'text', text: 'second' }] }, ...await next()]
     })
 
     send(agent, 'hi')
     await waitForIdle(ctx, agent)
 
-    // Registration order composes: the first listener runs last on the way
-    // out (waterfall), so its prepend lands first.
     const texts = adapter.requests[0]!.messages.map(m => m.content[0]?.type === 'text' ? m.content[0].text : '')
     expect(texts).toEqual(['first', 'second', 'hi'])
   })

packages/core/agent/src/types.ts

@@ -408,9 +408,14 @@ declare module 'cordis' {
      * durable `context/message` paid once and prefix-cached thereafter.
      *
      * The seed is a frozen empty list; a contributing listener returns a NEW
-     * array extending `await next()` (`[...prefix, mine]` — never an in-place
+     * array — never an in-place push. The canonical contribution is a
-     * push), so contributions compose across plugins in registration order
+     * PREPEND, `[mine, ...await next()]`: the waterfall unwinds
-     * and compose deterministically for a fixed plugin set. Call `next()` to
+     * innermost-first (the LAST-registered listener's `next()` resolves
+     * first), so prepending yields registration order on the wire, and every
+     * plugin using it composes deterministically. The append form
+     * `[...await next(), mine]` is legal but places a contribution AFTER
+     * every later-registered plugin's — reverse registration order when all
+     * contributors append. Call `next()` to
      * delegate, or return a list without it to short-circuit.
      * @param agent - the agent whose session prefix is being composed.
      * @param prefix - the frozen empty seed; return an extended replacement to contribute.

packages/support/acp-snapshot/src/normalize.ts

@@ -13,8 +13,9 @@
  * (deterministic — `seq = log.length`, part of the event-log contract).
  *
  * A separate, composable normalizer — {@link scrubRequestHeaders} — replaces
- * the bulky request-header CONTENT (the composed system prompt and the tool
+ * the bulky request-header CONTENT (the composed system prompt, the tool
- * schema list) with `{{system}}`/`{{tools}}` tokens. It is deliberately NOT
+ * schema list, and the session prefix) with
+ * `{{system}}`/`{{tools}}`/`{{messagePrefix}}` tokens. It is deliberately NOT
  * folded into {@link normalizeSessionLog}: each suite's one header-pinning
  * scenario compares that content verbatim, every other scenario composes the
  * scrub in (the `pinsHeader` flag on the scenario table, consumed by the suite
@@ -30,6 +31,7 @@ const SESSION_ID = '{{sessionId}}'
 const CWD = '{{cwd}}'
 const SYSTEM = '{{system}}'
 const TOOLS = '{{tools}}'
+const MESSAGE_PREFIX = '{{messagePrefix}}'
 
 /** A UUID v4 string, the shape `randomUUID()` produces for session ids. */
 const UUID_RE = /[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}/gi
@@ -135,15 +137,22 @@ export function normalizeSessionLog(rawLog: string, ctx: NormalizeContext): stri
 /**
  * Replace request-header CONTENT in a session JSONL with stable tokens,
  * keeping its structure: a `request/header` event's `data.header.system` →
- * `{{system}}` and `data.header.tools` → `{{tools}}`; a
+ * `{{system}}`, `data.header.tools` → `{{tools}}`, and
+ * `data.header.messagePrefix` → one `{{messagePrefix}}` token per message
+ * (the session prefix is model-visible bulk — an AGENTS digest, a skills
+ * catalog — so its COUNT stays a structural fact while its text never lands
+ * in a fixture); a
  * `request/header-delta` event keeps every structural fact — the system
  * delta's `keepStart`/`keepEnd` line positions and inserted-line COUNT (one
  * `{{system}}` token per inserted line), the tools delta's
- * added/removed/changed tool NAMES — and tokenizes only the bulk (prompt
+ * added/removed/changed tool NAMES, the prefix replacement's message COUNT —
- * text; each added/changed schema's fields other than `name` → `{{tools}}`),
+ * and tokenizes only the bulk (prompt
+ * text; each added/changed schema's fields other than `name` → `{{tools}}`;
+ * each replacement prefix message → `{{messagePrefix}}`),
  * so two different deltas still compare different.
- * Absent fields stay absent — WHETHER a header carried a system prompt or
+ * Absent fields stay absent — WHETHER a header carried a system prompt,
- * tools is behavior and stays visible; `config` and `reason` are small and
+ * tools, or a prefix is behavior and stays visible; `config` and `reason`
+ * are small and
  * stable, so they stay verbatim (a model swap churns every fixture by design
  * — it invalidates the recorded responses; a prompt/schema edit churns none —
  * replay never reads this content, see dsh-llm-replay).
@@ -166,9 +175,10 @@ export function scrubRequestHeaders(rawLog: string): string {
     if (record.type === 'request/header') {
       const header = data.header as Record<string, unknown> | null | undefined
       if (header === null || typeof header !== 'object') return line
-      if (!('system' in header) && !('tools' in header)) return line
+      if (!('system' in header) && !('tools' in header) && !('messagePrefix' in header)) return line
       if ('system' in header) header.system = SYSTEM
       if ('tools' in header) header.tools = TOOLS
+      if (Array.isArray(header.messagePrefix)) header.messagePrefix = header.messagePrefix.map(() => MESSAGE_PREFIX)
       return JSON.stringify(record)
     }
     if (record.type === 'request/header-delta') {
@@ -183,6 +193,10 @@ export function scrubRequestHeaders(rawLog: string): string {
         if (Array.isArray(tools.added)) { tools.added = tools.added.map(scrubToolSchema); touched = true }
         if (Array.isArray(tools.changed)) { tools.changed = tools.changed.map(scrubToolSchema); touched = true }
       }
+      if (Array.isArray(data.messagePrefix)) {
+        data.messagePrefix = data.messagePrefix.map(() => MESSAGE_PREFIX)
+        touched = true
+      }
       return touched ? JSON.stringify(record) : line
     }
     return line

packages/support/acp-snapshot/tests/normalize.spec.ts

@@ -155,6 +155,38 @@ describe('scrubRequestHeaders', () => {
     expect(toolsOnly).not.toContain('{{system}}')
   })
 
+  it('scrubs the header session prefix to one token per message, keeping the count', () => {
+    const ev = headerEvent({
+      config: { model: 'm' },
+      messagePrefix: [
+        { role: 'user', content: [{ type: 'text', text: 'workspace AGENTS digest' }] },
+        { role: 'user', content: [{ type: 'text', text: 'skills catalog' }] },
+      ],
+    })
+    const out = scrubRequestHeaders(`${headerLine}\n${ev}\n`)
+    expect(out).toContain('"messagePrefix":["{{messagePrefix}}","{{messagePrefix}}"]')
+    expect(out).not.toContain('AGENTS digest')
+    expect(out).not.toContain('skills catalog')
+    // Absence stays absent — a prefix-less header gains no token…
+    expect(scrubRequestHeaders(`${headerLine}\n${headerEvent({ system: 's' })}\n`)).not.toContain('{{messagePrefix}}')
+    // …and a non-array shape passes through untouched.
+    const odd = JSON.stringify({ type: 'request/header', seq: 4, time: 9, data: { header: { config: { model: 'm' }, messagePrefix: 'weird' }, reason: 'initial' } })
+    expect(scrubRequestHeaders(`${headerLine}\n${odd}\n`)).toContain('"messagePrefix":"weird"')
+  })
+
+  it('scrubs a header-delta prefix replacement to one token per message', () => {
+    const delta = JSON.stringify({
+      type: 'request/header-delta', seq: 8, time: 9,
+      data: { messagePrefix: [{ role: 'user', content: [{ type: 'text', text: 'leaked opener' }] }] },
+    })
+    const out = scrubRequestHeaders(`${headerLine}\n${delta}\n`)
+    expect(out).toContain('"messagePrefix":["{{messagePrefix}}"]')
+    expect(out).not.toContain('leaked opener')
+    // The empty-array transition-to-absence stays a structural fact.
+    const toNone = JSON.stringify({ type: 'request/header-delta', seq: 9, time: 9, data: { messagePrefix: [] } })
+    expect(scrubRequestHeaders(`${headerLine}\n${toNone}\n`)).toContain('"messagePrefix":[]')
+  })
+
   it('leaves a delta with no scrubbable payload byte-identical (config-only, or non-array shapes)', () => {
     const configOnly = JSON.stringify({ type: 'request/header-delta', seq: 8, time: 9, data: { config: { model: 'm2' } } })
     const oddShapes = JSON.stringify({ type: 'request/header-delta', seq: 9, time: 9, data: { system: { insert: 'not-an-array' }, tools: null } })