test(acp): snapshot system prompts as Markdown

packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/session.jsonl

@@ -1,2 +1,2 @@
 {"type":"session","id":"ccdc749f-56f3-4267-9750-598b5c60b7b2","createdAt":600,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-nOQ4Gy"}
-{"type":"request/header","seq":0,"time":4,"data":{"header":{"config":{"model":"fake"},"system":"SYS PROMPT","tools":[{"name":"t1","description":"D1","parameters":{"type":"object"}}]},"reason":"initial"}}
+{"type":"request/header","seq":0,"time":4,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":[{"name":"t1","description":"D1","parameters":{"type":"object"}}]},"reason":"initial"}}

packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/system-prompt.golden.md

@@ -0,0 +1 @@
+SYS PROMPT

packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl

@@ -1,3 +1,3 @@
 {"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":"SYS PROMPT","tools":[{"name":"t1","description":"D1","parameters":{"type":"object"}}]},"reason":"initial"}}
+{"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":"turn/start","seq":1,"time":7,"data":{"turn":1}}

packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/system-prompt.golden.md

@@ -0,0 +1 @@
+SYS PROMPT

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

@@ -1,5 +1,11 @@
 import { describe, expect, it } from 'vitest'
-import { type NormalizeContext, normalizeSessionLog, normalizeStdout, scrubRequestHeaders } from '../src/normalize.ts'
+import {
+  type NormalizeContext,
+  normalizeSessionLog,
+  normalizeStdout,
+  scrubRequestHeaders,
+  scrubSystemPrompts,
+} from '../src/normalize.ts'
 
 /**
  * Unit tests for the pure snapshot normalizers. Live as a *.spec.ts (runs in
@@ -260,3 +266,43 @@ describe('scrubRequestHeaders', () => {
     expect(scrubRequestHeaders(once)).toBe(once)
   })
 })
+
+describe('scrubSystemPrompts', () => {
+  it('scrubs only system prompt payloads while keeping tools and prefixes verbatim', () => {
+    const header = JSON.stringify({
+      type: 'request/header', seq: 1, time: 2,
+      data: {
+        header: {
+          system: 'full prompt',
+          tools: [{ name: 'read', description: 'full schema' }],
+          messagePrefix: [{ role: 'user', content: [{ type: 'text', text: 'full prefix' }] }],
+        },
+        reason: 'initial',
+      },
+    })
+    const delta = JSON.stringify({
+      type: 'request/header-delta', 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' }] }],
+      },
+    })
+    const toolsOnly = JSON.stringify({
+      type: 'request/header', seq: 3, time: 4,
+      data: { header: { tools: [{ name: 'read', description: 'schema only' }] }, reason: 'resume' },
+    })
+
+    const out = scrubSystemPrompts(`${header}\n${delta}\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).toContain('full schema')
+    expect(out).toContain('full prefix')
+    expect(out).toContain('changed schema')
+    expect(out).toContain('changed prefix')
+    expect(out.split('\n')[2]).toBe(toolsOnly)
+    expect(scrubSystemPrompts(out)).toBe(out)
+  })
+})

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

@@ -8,8 +8,10 @@ import { defineAcpSnapshotSuite, type HarvestedLog, type Scenario } from '../src
 import {
   childFixturePaths,
   fixtureContext,
+  formatSystemPromptSnapshot,
   headerDeltaCount,
   normalizedHeaders,
+  normalizedSystemPrompts,
   refreshFixtureReplacements,
   stabilizeRefreshLog,
 } from '../src/suite.ts'
@@ -78,6 +80,7 @@ afterAll(async () => {
 
 function staleRefreshFixtures(dir: string): void {
   writeFileSync(join(dir, 'plain-turn', 'stdout.golden.jsonl'), 'stale stdout\n')
+  writeFileSync(join(dir, 'pin-turn', 'system-prompt.golden.md'), 'STALE PROMPT\n')
 
   const plainBehaviorFile = join(dir, 'plain-turn', 'behavior.json')
   const plainBehavior = JSON.parse(readFileSync(plainBehaviorFile, 'utf8')) as Record<string, unknown>
@@ -124,6 +127,8 @@ describe('defineAcpSnapshotSuite: refresh write-back', () => {
     const authored = readFileSync(join(refreshDir, 'authored-error', 'session.jsonl'), 'utf8')
     expect(authored).toContain('"error":"model exploded"')
     expect(authored).not.toContain('"error":"stale"')
+
+    expect(readFileSync(join(refreshDir, 'pin-turn', 'system-prompt.golden.md'), 'utf8')).toBe('SYS PROMPT\n')
   })
 })
 
@@ -219,6 +224,28 @@ describe('normalizedHeaders', () => {
   })
 })
 
+describe('normalizedSystemPrompts', () => {
+  it('extracts normalized string prompts and omits absent or non-string fields', () => {
+    const log = [
+      '{"type":"session","id":"a","createdAt":5,"cwd":"/w"}',
+      '{"type":"request/header","seq":0,"time":9,"data":{"header":{"system":"work in /w"}}}',
+      '{"type":"request/header","seq":1,"time":9,"data":{"header":{}}}',
+      '{"type":"request/header","seq":2,"time":9,"data":{"header":{"system":null}}}',
+      '{"type":"request/header","seq":3,"time":9,"data":{"header":null}}',
+      '{"type":"request/header","seq":4,"time":9,"data":{"header":"invalid"}}',
+      '',
+    ].join('\n')
+    expect(normalizedSystemPrompts(log, { sessionIds: [], cwd: '/w' })).toEqual(['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')
+  })
+})
+
 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: {} })