refactor: simplify session log representation

2026-07-13 23:56:10 +08:00
parent d060c0dd4f
commit 1123e946c0
64 changed files with 522 additions and 1032 deletions
--- a/packages/support/acp-snapshot/README.md
+++ b/packages/support/acp-snapshot/README.md
@@ -35,7 +35,7 @@ defineAcpSnapshotSuite({
 })
 ```

-A scenario booting a differently-composed tree sets its own `configPath` (an overlay whose basename still ends in `cordis.yml`, so the bin's replay swap finds the sibling `*cordis.snapshot.yml`) and, when that composition changes the request header, its own `headerClass` with its own pinning scenario — the acp-agent example's Code Mode scenarios are the template. Each pinning directory's generated `system-prompt.golden.md` is the reviewable snapshot of the normalized composed prompt; `session.jsonl` stores `"system":"{{system}}"` while retaining the complete tool list.
+A scenario booting a differently-composed tree sets its own `configPath` (an overlay whose basename still ends in `cordis.yml`, so the bin's replay swap finds the sibling `*cordis.snapshot.yml`) and, when that composition changes the request header, its own `headerClass` with its own pinning scenario — the acp-agent example's Code Mode scenarios are the template. Each pinning directory's generated `system-prompt.golden.md` is the reviewable snapshot of the normalized composed prompt; `session.jsonl` stores `"system":"{{system}}"` while retaining the complete tool list. A pin whose scenario legitimately changes its header mid-run declares `expectedHeaderChanges`; the Markdown snapshot then records each later full prompt under a `request/header change` marker.

 The example also ships a `cordis.snapshot.yml` replay overlay next to its `cordis.yml` (the bin swaps them under `DSH_SNAPSHOT=replay` — [single-source replay config RFC](../../../docs/rfc/implemented/testing/2026-07-04-single-source-acp-replay-config.md)); replay fixtures are served by [`dsh-llm-replay`](../llm-replay/README.md), which this package points at via the `DSH_SNAPSHOT_*` env vars it sets on the child. `pnpm run test:snapshot:record` calls the live LLM and rewrites the recorded scenarios' model fixtures; `pnpm run test:snapshot:refresh` stays keyless, runs the replay overlay, and rewrites stdout, comparable session-log goldens, and each pin's Markdown prompt snapshot from the committed model scripts. Fixture roles, record/replay/refresh semantics, and scenario-table fields are documented on `Scenario` and in the [snapshot RFC](../../../docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md).

--- a/packages/support/acp-snapshot/src/normalize.ts
+++ b/packages/support/acp-snapshot/src/normalize.ts
@@ -135,8 +135,8 @@ export function normalizeSessionLog(rawLog: string, ctx: NormalizeContext): stri
 }

 /**
- * Replace system-prompt content in request headers and header deltas with
- * `{{system}}` tokens while retaining field presence and delta structure.
+ * Replace system-prompt content in request headers with `{{system}}` tokens
+ * while retaining field presence.
 * Other header content stays verbatim, so a header-pinning fixture can keep
 * its complete tool schemas while every JSONL fixture omits the prompt text.
 * Lines without a system payload pass through byte-for-byte; the transform is
@@ -153,9 +153,9 @@ export function scrubSystemPrompts(rawLog: string): string {
 * Replace all bulky request-header content in a session JSONL with stable
 * tokens. This includes the system-prompt fields handled by
 * {@link scrubSystemPrompts}, tool schemas, and session-prefix messages. It
- * keeps system-delta line positions and arity, tool-delta names, prefix
- * message counts, field presence, config, and reason. Lines without content
- * to scrub pass through byte-for-byte, and the transform is idempotent.
+ * keeps prefix message counts, field presence, config, and reason. Lines
+ * without content to scrub pass through byte-for-byte, and the transform is
+ * idempotent.
 *
 * @param rawLog The raw session `.jsonl` content.
 * @returns The JSONL with all header bulk tokenized, other lines byte-identical.
@@ -184,33 +184,7 @@ function scrubHeaderContent(rawLog: string, scrubToolsAndPrefix: boolean): strin
      }
      return touched ? JSON.stringify(record) : line
    }
-    if (record.type === 'request/header-delta') {
-      let touched = false
-      const system = data.system as Record<string, unknown> | null | undefined
-      if (system !== null && typeof system === 'object' && Array.isArray(system.insert)) {
-        system.insert = system.insert.map(() => SYSTEM)
-        touched = true
-      }
-      const tools = data.tools as Record<string, unknown> | null | undefined
-      if (scrubToolsAndPrefix && tools !== null && typeof tools === 'object') {
-        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 (scrubToolsAndPrefix && Array.isArray(data.messagePrefix)) {
-        data.messagePrefix = data.messagePrefix.map(() => MESSAGE_PREFIX)
-        touched = true
-      }
-      return touched ? JSON.stringify(record) : line
-    }
    return line
  })
  return out.join('\n')
 }
-
-/** Tokenize one tool schema's bulk (description, parameters, anything else), keeping its identifying `name`. */
-function scrubToolSchema(tool: unknown): unknown {
-  if (tool === null || typeof tool !== 'object' || Array.isArray(tool)) return tool
-  const out: Record<string, unknown> = {}
-  for (const [k, v] of Object.entries(tool)) out[k] = k === 'name' ? v : TOOLS
-  return out
-}
--- a/packages/support/acp-snapshot/src/suite.ts
+++ b/packages/support/acp-snapshot/src/suite.ts
@@ -16,7 +16,7 @@
 * scenario stores the readable prompt in `system-prompt.golden.md` and keeps its full
 * tool schemas in `session.jsonl`, while every other fixture also scrubs tools
 * to `{{tools}}`. A per-run uniformity guard compares both artifacts against
- * every live header and forbids unrepresented header deltas (see the
+ * every live header and forbids unrepresented changed headers (see the
 * pinned-header RFC,
 * docs/rfc/implemented/testing/2026-07-06-pin-request-header-content-in-one-scenario.md).
 *
@@ -106,14 +106,11 @@ export interface Scenario {
   */
  pinsHeader?: boolean
  /**
-   * How many `request/header-delta` events this PINNING scenario's fixture
-   * legitimately carries (default 0). A recorded mid-run header change — a
-   * config-option switch rewriting a prompt section — is part of the pinned
-   * surface, with readable prompt text in Markdown; any OTHER count
-   * still fails, so fixture rot stays caught. Meaningless off the pin (the
-   * live uniformity guard keeps non-pinning scenarios delta-free).
+   * How many changed `request/header` snapshots this PINNING scenario's primary
+   * fixture legitimately carries (default 0). Their full prompt text is kept in
+   * the readable Markdown pin; any other count fails. Meaningless off the pin.
   */
-  expectedHeaderDeltas?: number
+  expectedHeaderChanges?: number
  /**
   * Which header-composition class this scenario belongs to. Scenarios that
   * boot the same config compose the same header; each class has exactly one
@@ -225,79 +222,46 @@ export function normalizedSystemPrompts(rawLog: string, ctx: NormalizeContext):
  })
 }

-/** One normalized system-prompt edit carried by a `request/header-delta`. */
-export interface SystemPromptDeltaSnapshot {
-  /** How many leading lines remain from the prior prompt. */
-  keepStart: number
-  /** How many trailing lines remain from the prior prompt. */
-  keepEnd: number
-  /** The normalized replacement lines inserted between the retained ranges. */
-  insert: string[]
-}
-
-/**
- * Extract normalized system-prompt edits from request-header deltas in log
- * order. Deltas without a well-formed system edit are omitted; their non-prompt
- * structure remains pinned in JSONL.
- *
- * @param rawLog The session `.jsonl` content to inspect.
- * @param ctx The volatile values of the run that produced it.
- * @returns The normalized system-prompt edits, in event order.
- */
-export function normalizedSystemPromptDeltas(rawLog: string, ctx: NormalizeContext): SystemPromptDeltaSnapshot[] {
-  return normalizeSessionLog(rawLog, ctx)
-    .split('\n')
-    .filter(line => line.trim().length > 0)
-    .map(line => JSON.parse(line) as { type?: unknown; data?: { system?: unknown } })
-    .filter(record => record.type === 'request/header-delta')
-    .flatMap((record) => {
-      const system = record.data?.system
-      if (system === null || typeof system !== 'object') return []
-      const { keepStart, keepEnd, insert } = system as { keepStart?: unknown; keepEnd?: unknown; insert?: unknown }
-      if (typeof keepStart !== 'number' || typeof keepEnd !== 'number' || !Array.isArray(insert)) return []
-      if (!insert.every(line => typeof line === 'string')) return []
-      return [{ keepStart, keepEnd, insert: insert }]
-    })
-}
-
 /**
 * Render a normalized prompt as a repository-friendly Markdown snapshot.
 * Prompt text is unchanged except that a missing terminal newline is added so
 * the committed file follows the repository newline contract.
 *
 * @param prompt The normalized system prompt.
- * @param deltas Normalized prompt edits to append as readable sections.
+ * @param changes Full normalized prompts from later changed-header snapshots.
 * @returns Markdown snapshot text ending in a newline.
 */
 export function formatSystemPromptSnapshot(
  prompt: string,
-  deltas: readonly SystemPromptDeltaSnapshot[] = [],
+  changes: readonly string[] = [],
 ): string {
  let snapshot = prompt.endsWith('\n') ? prompt : `${prompt}\n`
-  for (const [index, delta] of deltas.entries()) {
-    snapshot += `\n<!-- request/header-delta ${index + 1}: keepStart=${delta.keepStart}, keepEnd=${delta.keepEnd} -->\n\n`
-    const insert = delta.insert.join('\n')
-    snapshot += insert.endsWith('\n') ? insert : `${insert}\n`
+  for (const [index, change] of changes.entries()) {
+    snapshot += `\n<!-- request/header change ${index + 1} -->\n\n`
+    snapshot += change.endsWith('\n') ? change : `${change}\n`
  }
  return snapshot
 }

-/** Return the initial-prompt portion of a possibly delta-bearing snapshot. */
+/** Return the initial-prompt portion of a possibly multi-header snapshot. */
 function initialSystemPromptSnapshot(snapshot: string): string {
-  const marker = snapshot.indexOf('\n<!-- request/header-delta ')
+  const marker = snapshot.indexOf('\n<!-- request/header change ')
  return marker < 0 ? snapshot : snapshot.slice(0, marker)
 }

 /**
- * Count the `request/header-delta` events in a session JSONL.
+ * Count changed `request/header` snapshots in a session JSONL.
 *
 * @param rawLog The session `.jsonl` content.
- * @returns How many `request/header-delta` events the log carries.
+ * @returns How many headers carry reason `change`.
 */
-export function headerDeltaCount(rawLog: string): number {
+export function headerChangeCount(rawLog: string): number {
  return rawLog.split('\n')
    .filter(line => line.trim().length > 0)
-    .filter(line => (JSON.parse(line) as { type?: unknown }).type === 'request/header-delta')
+    .filter((line) => {
+      const record = JSON.parse(line) as { type?: unknown; data?: { reason?: unknown } }
+      return record.type === 'request/header' && record.data?.reason === 'change'
+    })
    .length
 }

@@ -490,18 +454,10 @@ export function defineAcpSnapshotSuite(options: SnapshotSuiteOptions): void {
            ))
          }
          if (scenario.pinsHeader === true) {
-            const prompts = result.sessionLogs.flatMap(log => normalizedSystemPrompts(log.content, ctx))
-            expect(prompts.length, `${mode} produced no system prompt to snapshot`).toBeGreaterThan(0)
-            const initialSnapshot = formatSystemPromptSnapshot(prompts[0] as string)
-            for (const prompt of prompts) {
-              expect(formatSystemPromptSnapshot(prompt), 'the pinning run produced divergent system prompts')
-                .toEqual(initialSnapshot)
-            }
            const primary = result.sessionLogs[0] as HarvestedLog
-            const snapshot = formatSystemPromptSnapshot(
-              prompts[0] as string,
-              normalizedSystemPromptDeltas(primary.content, ctx),
-            )
+            const prompts = normalizedSystemPrompts(primary.content, ctx)
+            expect(prompts.length, `${mode} produced no system prompt to snapshot`).toBeGreaterThan(0)
+            const snapshot = formatSystemPromptSnapshot(prompts[0] as string, prompts.slice(1))
            await writeFile(join(dir, SYSTEM_PROMPT_SNAPSHOT), snapshot)
          }
        }
@@ -534,9 +490,8 @@ export function defineAcpSnapshotSuite(options: SnapshotSuiteOptions): void {

        // Header-uniformity guard: every live header in a class must equal the
        // class pin split across its JSONL header (system token + real tools)
-        // and readable Markdown prompt. A pinning scenario may carry its
-        // declared header deltas; their prompt edits live in the Markdown
-        // golden while JSONL retains the tokenized edit structure.
+        // and readable Markdown prompt. A pinning scenario may carry declared
+        // changed-header snapshots; each full prompt lives in that Markdown.
        /* v8 ignore next -- construction guarantees the pin exists; a miss would fail the one-header assertion loudly. */
        const pinningScenario = pinningByClass.get(classOf(scenario)) ?? scenario
        const pinningDir = join(snapshotsDir, pinningScenario.name)
@@ -544,29 +499,32 @@ export function defineAcpSnapshotSuite(options: SnapshotSuiteOptions): void {
        const pinned = normalizedHeaders(pinnedFixture, fixtureContext(pinnedFixture))
        const promptSnapshot = await readFile(join(pinningDir, SYSTEM_PROMPT_SNAPSHOT), 'utf8')
        const initialPromptSnapshot = initialSystemPromptSnapshot(promptSnapshot)
-        expect(pinned.length, `the pinning fixture (${pinningScenario.name}) must carry exactly one request/header`)
-          .toBe(1)
+        expect(pinned.length, `the pinning fixture (${pinningScenario.name}) has an unexpected request/header count`)
+          .toBe(1 + (pinningScenario.expectedHeaderChanges ?? 0))
        for (const [logIndex, log] of result.sessionLogs.entries()) {
-          const expectedDeltas = scenario.pinsHeader === true && logIndex === 0
-            ? scenario.expectedHeaderDeltas ?? 0
+          const expectedChanges = scenario.pinsHeader === true && logIndex === 0
+            ? scenario.expectedHeaderChanges ?? 0
            : 0
-          expect(headerDeltaCount(log.content), `session ${log.id}: request/header-delta count`)
-            .toBe(expectedDeltas)
+          expect(headerChangeCount(log.content), `session ${log.id}: changed request/header count`)
+            .toBe(expectedChanges)
          const headers = normalizedHeaders(scrubSystemPrompts(log.content), ctx)
          const prompts = normalizedSystemPrompts(log.content, ctx)
          expect(prompts.length, `session ${log.id}: every request/header must carry a string system prompt`)
            .toBe(headers.length)
          for (const [k, header] of headers.entries()) {
+            const expected = expectedChanges > 0 ? pinned[k] : pinned[0]
            expect(header, `session ${log.id}: request/header #${k + 1} diverged from the pinned (${pinningScenario.name}) header`)
-              .toEqual(pinned[0])
-            expect(formatSystemPromptSnapshot(prompts[k] as string), `session ${log.id}: initial system prompt #${k + 1} diverged from ${pinningScenario.name}/${SYSTEM_PROMPT_SNAPSHOT}`)
-              .toEqual(initialPromptSnapshot)
+              .toEqual(expected)
+            if (expectedChanges === 0) {
+              expect(formatSystemPromptSnapshot(prompts[k] as string), `session ${log.id}: initial system prompt #${k + 1} diverged from ${pinningScenario.name}/${SYSTEM_PROMPT_SNAPSHOT}`)
+                .toEqual(initialPromptSnapshot)
+            }
          }
          if (scenario.pinsHeader === true && logIndex === 0) {
            expect(formatSystemPromptSnapshot(
              prompts[0] as string,
-              normalizedSystemPromptDeltas(log.content, ctx),
-            ), `session ${log.id}: system-prompt deltas diverged from ${pinningScenario.name}/${SYSTEM_PROMPT_SNAPSHOT}`)
+              prompts.slice(1),
+            ), `session ${log.id}: changed system prompts diverged from ${pinningScenario.name}/${SYSTEM_PROMPT_SNAPSHOT}`)
              .toEqual(promptSnapshot)
          }
        }
@@ -632,22 +590,22 @@ export function defineAcpSnapshotSuite(options: SnapshotSuiteOptions): void {
      }
    })

-    it('every pinning fixture carries one request/header, one readable prompt, and its declared deltas', async () => {
+    it('every pinning fixture carries one anchor, one readable prompt artifact, and its declared changes', async () => {
      // The live uniformity guard runs only in NON-pinning scenarios, so a
      // class made of just its pinning scenario would otherwise accept a
-      // re-recorded pin with several headers or an undeclared mid-run
-      // header-delta — shapes the pin design cannot represent. Assert the
+      // re-recorded pin with undeclared mid-run header changes. Assert the
      // committed pins directly; a scenario whose arc legitimately rewrites
-      // a prompt section declares the exact count via expectedHeaderDeltas.
+      // a prompt section declares the exact count via expectedHeaderChanges.
      for (const scenario of pinningByClass.values()) {
        const fixture = await readFile(join(snapshotsDir, scenario.name, 'session.jsonl'), 'utf8')
        const headers = normalizedHeaders(fixture, fixtureContext(fixture))
        const promptSnapshot = await readFile(join(snapshotsDir, scenario.name, SYSTEM_PROMPT_SNAPSHOT), 'utf8')
-        expect(headers.length, `${scenario.name}: a pinning fixture must carry exactly one request/header`).toBe(1)
+        expect(headers.length, `${scenario.name}: unexpected request/header count`)
+          .toBe(1 + (scenario.expectedHeaderChanges ?? 0))
        expect(promptSnapshot.length, `${scenario.name}/${SYSTEM_PROMPT_SNAPSHOT} must not be empty`).toBeGreaterThan(0)
        expect(promptSnapshot.endsWith('\n'), `${scenario.name}/${SYSTEM_PROMPT_SNAPSHOT} must end in a newline`).toBe(true)
-        expect(headerDeltaCount(fixture), `${scenario.name}: a pinning fixture must carry exactly its declared request/header-deltas`)
-          .toBe(scenario.expectedHeaderDeltas ?? 0)
+        expect(headerChangeCount(fixture), `${scenario.name}: a pinning fixture must carry exactly its declared changed headers`)
+          .toBe(scenario.expectedHeaderChanges ?? 0)
      }
    })

--- a/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/behavior.json
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/behavior.json
@@ -5,7 +5,7 @@
    "lines": [
      { "type": "session", "id": "{{SID}}", "createdAt": 100, "cwd": "{{CWD}}" },
      { "type": "request/header", "seq": 0, "time": 100, "data": { "header": { "config": { "model": "fake" }, "system": "SYS PROMPT", "tools": [{ "name": "t1", "description": "D1", "parameters": { "type": "object" } }] }, "reason": "initial" } },
-      { "type": "request/header-delta", "seq": 1, "time": 100, "data": { "system": { "keepStart": 1, "keepEnd": 0, "insert": ["NEW PROMPT LINE"] } } },
+      { "type": "request/header", "seq": 1, "time": 100, "data": { "header": { "config": { "model": "fake" }, "system": "SYS PROMPT\n\nNEW PROMPT LINE", "tools": [{ "name": "t1", "description": "D1", "parameters": { "type": "object" } }] }, "reason": "change" } },
      { "type": "turn/start", "seq": 2, "time": 100, "data": { "turn": 1 } }
    ]
  }]
--- a/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl
@@ -1,4 +1,4 @@
 {"type":"session","id":"12121212-3434-4545-8686-787878787878","createdAt":7,"cwd":"/rec/pin-cwd"}
 {"type":"request/header","seq":0,"time":7,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":[{"name":"t1","description":"D1","parameters":{"type":"object"}}]},"reason":"initial"}}
-{"type":"request/header-delta","seq":1,"time":7,"data":{"system":{"keepStart":1,"keepEnd":0,"insert":["{{system}}"]}}}
+{"type":"request/header","seq":1,"time":7,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":[{"name":"t1","description":"D1","parameters":{"type":"object"}}]},"reason":"change"}}
 {"type":"turn/start","seq":2,"time":7,"data":{"turn":1}}
--- a/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/system-prompt.golden.md
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/system-prompt.golden.md
@@ -1,5 +1,7 @@
 SYS PROMPT

-<!-- request/header-delta 1: keepStart=1, keepEnd=0 -->
+<!-- request/header change 1 -->
+
+SYS PROMPT

 NEW PROMPT LINE
--- a/packages/support/acp-snapshot/tests/normalize.spec.ts
+++ b/packages/support/acp-snapshot/tests/normalize.spec.ts
@@ -180,89 +180,19 @@ describe('scrubRequestHeaders', () => {
    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 } })
+  it('leaves malformed headers with no scrubbable payload byte-identical', () => {
    const headerless = JSON.stringify({ type: 'request/header', seq: 10, time: 9, data: { reason: 'initial' } })
    const nullData = JSON.stringify({ type: 'request/header', seq: 11, time: 9, data: null })
-    const raw = `${headerLine}\n${configOnly}\n${oddShapes}\n${headerless}\n${nullData}\n`
+    const raw = `${headerLine}\n${headerless}\n${nullData}\n`
    expect(scrubRequestHeaders(raw)).toBe(raw)
  })

-  it('scrubs a one-sided tools delta and passes non-object schema entries through', () => {
-    const addedOnly = JSON.stringify({
-      type: 'request/header-delta', seq: 8, time: 9,
-      data: { tools: { added: [null, 'weird', { name: 'x', description: 'D' }] } },
-    })
-    const out = scrubRequestHeaders(`${headerLine}\n${addedOnly}\n`)
-    // Non-object entries survive untouched; the object entry keeps only name.
-    expect(out).toContain('"added":[null,"weird",{"name":"x","description":"{{tools}}"}]')
-    const changedOnly = JSON.stringify({
-      type: 'request/header-delta', seq: 8, time: 9,
-      data: { tools: { changed: [{ name: 'y', parameters: {} }] } },
-    })
-    expect(scrubRequestHeaders(`${headerLine}\n${changedOnly}\n`))
-      .toContain('"changed":[{"name":"y","parameters":"{{tools}}"}]')
-  })
-
-  it('scrubs a header-delta system payload but keeps its line positions and arity', () => {
-    const delta = JSON.stringify({
-      type: 'request/header-delta', seq: 8, time: 9,
-      data: { system: { keepStart: 1, keepEnd: 4, insert: ['leaked prompt line', 'second line'] }, config: { model: 'm2' } },
-    })
-    const out = scrubRequestHeaders(`${headerLine}\n${delta}\n`)
-    // One token PER inserted line: the edit's position AND extent survive.
-    expect(out).toContain('"insert":["{{system}}","{{system}}"]')
-    expect(out).toContain('"keepStart":1')
-    expect(out).toContain('"keepEnd":4')
-    expect(out).toContain('"config":{"model":"m2"}')
-    expect(out).not.toContain('leaked prompt line')
-    expect(out).not.toContain('{{tools}}') // no tools delta → none invented
-  })
-
-  it('scrubs a header-delta tools payload but keeps the added/removed/changed names', () => {
-    const delta = JSON.stringify({
-      type: 'request/header-delta', seq: 8, time: 9,
-      data: {
-        tools: {
-          added: [{ name: 'grep', description: 'Search files.', parameters: { type: 'object' } }],
-          removed: ['bash_kill'],
-          changed: [{ name: 'read', description: 'Read v2.', parameters: { type: 'object' } }],
-        },
-      },
-    })
-    const out = scrubRequestHeaders(`${headerLine}\n${delta}\n`)
-    // WHICH tools changed is behavior and survives; their bulk does not.
-    expect(out).toContain('"added":[{"name":"grep","description":"{{tools}}","parameters":"{{tools}}"}]')
-    expect(out).toContain('"removed":["bash_kill"]')
-    expect(out).toContain('"changed":[{"name":"read","description":"{{tools}}","parameters":"{{tools}}"}]')
-    expect(out).not.toContain('Search files')
-    expect(out).not.toContain('Read v2')
-  })
-
  it('passes every other line through byte-for-byte and is idempotent', () => {
    const other = JSON.stringify({ type: 'assistant/chunk', seq: 4, time: 9, data: { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text: 'hi' } } })
-    const delta = JSON.stringify({
-      type: 'request/header-delta', seq: 8, time: 9,
-      data: { system: { keepStart: 0, keepEnd: 0, insert: ['x'] }, tools: { added: [{ name: 't', description: 'd', parameters: {} }], removed: [], changed: [] } },
-    })
-    const raw = `${headerLine}\n${headerEvent({ config: { model: 'm' }, system: 's', tools: [] })}\n${delta}\n${other}\n`
+    const raw = `${headerLine}\n${headerEvent({ config: { model: 'm' }, system: 's', tools: [] })}\n${other}\n`
    const once = scrubRequestHeaders(raw)
    expect(once.split('\n')[0]).toBe(headerLine)
-    expect(once.split('\n')[3]).toBe(other)
+    expect(once.split('\n')[2]).toBe(other)
    expect(scrubRequestHeaders(once)).toBe(once)
  })
 })
@@ -280,12 +210,15 @@ describe('scrubSystemPrompts', () => {
        reason: 'initial',
      },
    })
-    const delta = JSON.stringify({
-      type: 'request/header-delta', seq: 2, time: 3,
+    const changed = JSON.stringify({
+      type: 'request/header', seq: 2, time: 3,
      data: {
-        system: { keepStart: 1, keepEnd: 2, insert: ['new prompt line'] },
-        tools: { changed: [{ name: 'read', description: 'changed schema' }] },
-        messagePrefix: [{ role: 'user', content: [{ type: 'text', text: 'changed prefix' }] }],
+        header: {
+          system: 'new prompt',
+          tools: [{ name: 'read', description: 'changed schema' }],
+          messagePrefix: [{ role: 'user', content: [{ type: 'text', text: 'changed prefix' }] }],
+        },
+        reason: 'change',
      },
    })
    const toolsOnly = JSON.stringify({
@@ -293,11 +226,10 @@ describe('scrubSystemPrompts', () => {
      data: { header: { tools: [{ name: 'read', description: 'schema only' }] }, reason: 'resume' },
    })

-    const out = scrubSystemPrompts(`${header}\n${delta}\n${toolsOnly}\n`)
+    const out = scrubSystemPrompts(`${header}\n${changed}\n${toolsOnly}\n`)
    expect(out).toContain('"system":"{{system}}"')
-    expect(out).toContain('"insert":["{{system}}"]')
    expect(out).not.toContain('full prompt')
-    expect(out).not.toContain('new prompt line')
+    expect(out).not.toContain('new prompt')
    expect(out).toContain('full schema')
    expect(out).toContain('full prefix')
    expect(out).toContain('changed schema')
--- a/packages/support/acp-snapshot/tests/suite.spec.ts
+++ b/packages/support/acp-snapshot/tests/suite.spec.ts
@@ -9,9 +9,8 @@ import {
  childFixturePaths,
  fixtureContext,
  formatSystemPromptSnapshot,
-  headerDeltaCount,
+  headerChangeCount,
  normalizedHeaders,
-  normalizedSystemPromptDeltas,
  normalizedSystemPrompts,
  refreshFixtureReplacements,
  stabilizeRefreshLog,
@@ -51,7 +50,7 @@ const RECORD_SRC = fileURLToPath(new URL('./fixtures/record-suite', import.meta.
 // is what this suite can exercise; the real overlay boot is the acp-agent
 // example's code-mode scenarios).
 const REPLAY_SCENARIOS: Scenario[] = [
-  { name: 'pin-turn', hasModelTurn: true, recorded: true, pinsHeader: true, expectedHeaderDeltas: 1, headerClass: 'main' },
+  { name: 'pin-turn', hasModelTurn: true, recorded: true, pinsHeader: true, expectedHeaderChanges: 1, headerClass: 'main' },
  { name: 'plain-turn', hasModelTurn: true, recorded: true, childSessions: 1, headerClass: 'main', configPath: AGENT.configPath },
  { name: 'no-model', hasModelTurn: false, recorded: false, headerClass: 'main' },
  { name: 'blocked-log', hasModelTurn: false, comparesLog: true, recorded: false, headerClass: 'main' },
@@ -132,7 +131,9 @@ describe('defineAcpSnapshotSuite: refresh write-back', () => {
    expect(readFileSync(join(refreshDir, 'pin-turn', 'system-prompt.golden.md'), 'utf8')).toBe([
      'SYS PROMPT',
      '',
-      '<!-- request/header-delta 1: keepStart=1, keepEnd=0 -->',
+      '<!-- request/header change 1 -->',
+      '',
+      'SYS PROMPT',
      '',
      'NEW PROMPT LINE',
      '',
@@ -247,46 +248,30 @@ describe('normalizedSystemPrompts', () => {
  })
 })

-describe('normalizedSystemPromptDeltas', () => {
-  it('extracts and normalizes well-formed system edits', () => {
-    const log = [
-      '{"type":"request/header-delta","data":{"system":{"keepStart":1,"keepEnd":0,"insert":["work in /w"]}}}',
-      '{"type":"request/header-delta","data":{"tools":{"replace":[]}}}',
-      '{"type":"request/header-delta","data":{"system":{"keepStart":"1","keepEnd":0,"insert":[]}}}',
-      '{"type":"request/header-delta","data":{"system":{"keepStart":1,"keepEnd":0,"insert":[null]}}}',
-      '',
-    ].join('\n')
-    expect(normalizedSystemPromptDeltas(log, { sessionIds: [], cwd: '/w' })).toEqual([
-      { keepStart: 1, keepEnd: 0, insert: ['work in {{cwd}}'] },
-    ])
-  })
-})
-
 describe('formatSystemPromptSnapshot', () => {
  it('adds a missing terminal newline without changing an existing one', () => {
    expect(formatSystemPromptSnapshot('prompt')).toBe('prompt\n')
    expect(formatSystemPromptSnapshot('prompt\n')).toBe('prompt\n')
  })

-  it('renders readable system-prompt delta sections', () => {
-    expect(formatSystemPromptSnapshot('prompt', [
-      { keepStart: 1, keepEnd: 0, insert: ['new', 'lines'] },
-    ])).toBe('prompt\n\n<!-- request/header-delta 1: keepStart=1, keepEnd=0 -->\n\nnew\nlines\n')
+  it('renders readable changed-prompt sections', () => {
+    expect(formatSystemPromptSnapshot('prompt', ['new\nlines']))
+      .toBe('prompt\n\n<!-- request/header change 1 -->\n\nnew\nlines\n')
  })

-  it('does not double the newline of a delta insert with a trailing blank line', () => {
-    expect(formatSystemPromptSnapshot('prompt\n', [
-      { keepStart: 2, keepEnd: 1, insert: ['tail', ''] },
-    ])).toBe('prompt\n\n<!-- request/header-delta 1: keepStart=2, keepEnd=1 -->\n\ntail\n')
+  it('does not double the newline of a changed prompt', () => {
+    expect(formatSystemPromptSnapshot('prompt\n', ['changed\n']))
+      .toBe('prompt\n\n<!-- request/header change 1 -->\n\nchanged\n')
  })
 })

-describe('headerDeltaCount', () => {
-  it('counts request/header-delta events, ignoring blanks and other lines', () => {
-    const delta = JSON.stringify({ type: 'request/header-delta', seq: 2, time: 9, data: {} })
-    const other = JSON.stringify({ type: 'request/header', seq: 0, time: 9, data: {} })
-    expect(headerDeltaCount(`${other}\n\n${delta}\n${delta}\n`)).toBe(2)
-    expect(headerDeltaCount(`${other}\n`)).toBe(0)
+describe('headerChangeCount', () => {
+  it('counts changed request headers, ignoring anchors, blanks, and other lines', () => {
+    const change = JSON.stringify({ type: 'request/header', seq: 2, time: 9, data: { reason: 'change' } })
+    const anchor = JSON.stringify({ type: 'request/header', seq: 0, time: 9, data: { reason: 'initial' } })
+    const other = JSON.stringify({ type: 'turn/start', seq: 1, time: 9, data: {} })
+    expect(headerChangeCount(`${anchor}\n${other}\n\n${change}\n${change}\n`)).toBe(2)
+    expect(headerChangeCount(`${anchor}\n`)).toBe(0)
  })
 })

--- a/packages/support/invariants/README.md
+++ b/packages/support/invariants/README.md
@@ -39,7 +39,7 @@ Agent status (per agent):

 Model requests (on `llm/stream`):

- **a loop-built request is exactly what the log reconstructs** — a frozen request with a live `sessionId` (the loop-built marker; hand-built one-shots like compaction's summarize are unfrozen and skipped) must carry frozen `messages` deep-equal to the derivation over the log prefix strictly before the in-flight step's `step/start` (rebuilt through a FRESH `Session`, so the live cache cannot vouch for itself — and boundary-correct: content logged after `step/start` legitimately belongs to the next request), and every non-content field must equal the fold of the log's `request/header*` events (see [the reconstructability RFC](../../../docs/rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)). Registered with `prepend: true` so a short-circuiting `llm/stream` listener (the replay adapter) cannot silence it; prepend orders it against append-registered listeners only — correctness rests on the seq-bounded rebuild, never listener timing.
+- **a loop-built request is exactly what the log reconstructs** — a frozen request with a live `sessionId` (the loop-built marker; hand-built one-shots like compaction's summarize are unfrozen and skipped) must carry frozen `messages` deep-equal to the derivation over the log prefix strictly before the in-flight step's `step/start` (rebuilt through a FRESH `Session`, so the live cache cannot vouch for itself — and boundary-correct: content logged after `step/start` legitimately belongs to the next request), and every non-content field must equal the latest logged `request/header` (see [the reconstructability RFC](../../../docs/rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)). Registered with `prepend: true` so a short-circuiting `llm/stream` listener (the replay adapter) cannot silence it; prepend orders it against append-registered listeners only — correctness rests on the seq-bounded rebuild, never listener timing.

 On any violation it throws `InvariantError` (`code: 'INVARIANT'`).

--- a/packages/support/invariants/src/index.ts
+++ b/packages/support/invariants/src/index.ts
@@ -60,8 +60,8 @@ interface SessionTrace {
  /** Every seq seen so far — validates `sourceEventSeqs` references. */
  knownSeqs: Set<number>
  /**
-   * The seqs currently on the surface linked list, in linked-list order
-   * (head to tail). A replace reorders this relative to seq order (the new
+   * The seqs currently on the surface, in derived-message order. A replace
+   * reorders this relative to seq order (the new
   * node takes the replaced range's position), so range validation is
   * positional, not by seq comparison.
   */
@@ -154,7 +154,7 @@ function validateEvent(trace: SessionTrace, event: SessionEvent): SessionTraceTr
      }
    }
  }
-  // Fold this event into the tracked surface linked list, validating the
+  // Fold this event into the tracked surface order, validating the
  // replace contract as we go. `append` adds a tail node; `replace` shadows a
  // positional range — every shadowed node must appear in sourceEventSeqs.
  if (se.surfaceOp !== undefined) {
@@ -497,7 +497,7 @@ export function apply(ctx: Context): void {
  //   the boundary (an `agent/request`-window inject) is legitimately absent
  //   from this request, and a current-surface comparison would false-fire.
  // - header: every non-content field must equal the fold of the log's
-  //   `request/header*` events — the loop logs the header event BEFORE
+  //   `request/header` events — the loop logs the header event BEFORE
  //   dispatch, so the fold already covers this request.
  //
  // Registered with `prepend: true` so a short-circuiting llm/stream listener
--- a/packages/support/invariants/tests/invariants.spec.ts
+++ b/packages/support/invariants/tests/invariants.spec.ts
@@ -635,7 +635,7 @@ describe('surface invariants', () => {
    session.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 2
    session.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 3
    // Replace node 2 (position 0) with seq 4 — surface is now [4, 3], so seq 4
-    // precedes seq 3 in linked-list order even though 4 > 3 numerically.
+    // precedes seq 3 in surface order even though 4 > 3 numerically.
    session.append('assistant/message', { turn: 1, step: 1, content: [{ type: 'text', text: 's' }] }, { surfaceOp: { op: 'replace', start: 2, end: 2 }, sourceEventSeqs: [2] }) // seq 4
    // A replace with start=3, end=4 passes the seq check (3 <= 4) but is
    // reversed positionally (3 is at pos 1, 4 is at pos 0).
@@ -745,7 +745,7 @@ describe('request-reconstruction cross-check (llm/stream)', () => {
  it('expects the folded header\'s session prefix ahead of the derivation (prefix + derived)', async () => {
    const { ctx, session, boundary } = await requestSetup()
    const prefix = { role: 'user' as const, content: [{ type: 'text' as const, text: '<system-reminder>catalog</system-reminder>' }] }
-    session.append('request/header-delta', { messagePrefix: [prefix] })
+    session.append('request/header', { header: { config: { model: 'm' }, messagePrefix: [prefix] }, reason: 'change' })
    // The prefixed request matches the fold…
    const prefixed = Object.freeze({ model: 'm', messages: Object.freeze([prefix, ...boundary]), sessionId: session.id })
    expect(() => { dispatch(ctx, prefixed) }).not.toThrow()