From a1d135e233b12f831da4137544de6654679acc8e Mon Sep 17 00:00:00 2001
From: Jinwoo Hong <73622457+Jinwoo-H@users.noreply.github.com>
Date: Sun, 13 Sep 2026 14:08:12 -0400
Subject: [PATCH] fix(ai-vault): re-read a transcript rewritten to its previous
 size (#20261)

---
 .../ai-vault/session-parse-cache-store.ts     |  2 ++
 .../session-transcript-consumers.test.ts      | 36 ++++++++++++++++++-
 .../ai-vault/session-transcript-reader.ts     | 11 +++++-
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/src/main/ai-vault/session-parse-cache-store.ts b/src/main/ai-vault/session-parse-cache-store.ts
index 1f1d624c85d..cbf950cfe7e 100644
--- a/src/main/ai-vault/session-parse-cache-store.ts
+++ b/src/main/ai-vault/session-parse-cache-store.ts
@@ -9,6 +9,8 @@ const MAX_CACHE_ENTRIES = 4096
 
 export type SessionParseResumePoint = {
   state: ResumableSessionParseState
+  mtimeMs: number
+  sizeBytes: number | undefined
   // Byte offset just past the last complete ('\n'-terminated) line consumed;
   // a trailing unterminated line is deliberately left before this point.
   byteOffset: number
diff --git a/src/main/ai-vault/session-transcript-consumers.test.ts b/src/main/ai-vault/session-transcript-consumers.test.ts
index cf9a6068673..856e426b658 100644
--- a/src/main/ai-vault/session-transcript-consumers.test.ts
+++ b/src/main/ai-vault/session-transcript-consumers.test.ts
@@ -1,4 +1,4 @@
-import { appendFile, mkdir, mkdtemp, rm, stat, writeFile } from 'node:fs/promises'
+import { appendFile, mkdir, mkdtemp, rm, stat, truncate, utimes, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { afterEach, expect, it, vi } from 'vitest'
@@ -170,6 +170,8 @@ it('replays only the appended lines on a resumed read', async () => {
   expect(firstRead?.outcome?.incomplete).toBe(false)
 
   await appendFile(transcript, `${jsonLines(claudeTurns(5, 5))}\n`)
+  const changedAt = new Date(firstRead!.start.candidate.file.mtimeMs + 2000)
+  await utimes(transcript, changedAt, changedAt)
   consumer.reads.length = 0
   await scanAiVaultSessions({ ...roots, platform: 'darwin', limit: 20 })
 
@@ -183,6 +185,38 @@ it('replays only the appended lines on a resumed read', async () => {
   ])
 })
 
+it.each(['rewrite', 'truncate then regrow'])(
+  're-reads a same-size %s from zero',
+  async (operation) => {
+    const { transcript } = await writeClaudeFixture()
+    const before = await claudeCandidate(transcript)
+    await parseAgentSessionFileCached(before, 'darwin')
+    const consumer = recordingConsumer()
+    const rewritten = `${jsonLines(claudeTurns(1, 4))}\n`.replace('reply 4', 'fresh 4')
+    expect(Buffer.byteLength(rewritten)).toBe(before.file.sizeBytes)
+
+    if (operation === 'truncate then regrow') {
+      await truncate(transcript, 0)
+      await appendFile(transcript, rewritten)
+    } else {
+      await writeFile(transcript, rewritten)
+    }
+    const changedAt = new Date(before.file.mtimeMs + 2000)
+    await utimes(transcript, changedAt, changedAt)
+    const session = await parseAgentSessionFileCached(await claudeCandidate(transcript), 'darwin')
+
+    expect(consumer.reads).toHaveLength(1)
+    expect(consumer.reads[0].start.mode).toBe('replace')
+    expect(consumer.reads[0].start.previousByteOffset).toBe(0)
+    expect(textsFor(consumer.reads, 'claude')).toContain('assistant:fresh 4')
+    expect(textsFor(consumer.reads, 'claude')).not.toContain('assistant:reply 4')
+    resetSessionParseCacheForTests()
+    expect(session).toEqual(
+      await parseAgentSessionFileCached(await claudeCandidate(transcript), 'darwin')
+    )
+  }
+)
+
 it('publishes a trailing unterminated line once, when it is complete', async () => {
   const { roots, transcript } = await writeClaudeFixture()
   const consumer = recordingConsumer()
diff --git a/src/main/ai-vault/session-transcript-reader.ts b/src/main/ai-vault/session-transcript-reader.ts
index 3fc0ddcc146..e76bde231f2 100644
--- a/src/main/ai-vault/session-transcript-reader.ts
+++ b/src/main/ai-vault/session-transcript-reader.ts
@@ -68,6 +68,9 @@ export async function readResumableTranscript(args: {
     resume !== null &&
     typeof file.sizeBytes === 'number' &&
     file.sizeBytes >= resume.byteOffset &&
+    file.mtimeMs >= resume.mtimeMs &&
+    // A changed timestamp without growth signals a rewrite, even at a valid line boundary.
+    (file.mtimeMs === resume.mtimeMs || file.sizeBytes > (resume.sizeBytes ?? resume.byteOffset)) &&
     (resume.byteOffset === 0 || (await endsWithNewlineAt(file.path, resume.byteOffset)))
 
   // Clone before consuming: a failed read must not corrupt the cached state,
@@ -127,7 +130,13 @@ export async function readResumableTranscript(args: {
     channel.finishRead({ session, byteOffset: readResult.consumedThrough, incomplete: false })
     return {
       session,
-      resume: { state, byteOffset: readResult.consumedThrough, channel }
+      resume: {
+        state,
+        byteOffset: readResult.consumedThrough,
+        mtimeMs: file.mtimeMs,
+        sizeBytes: file.sizeBytes,
+        channel
+      }
     }
   } catch (error) {
     channel.finishRead({ session: null, byteOffset: startOffset, incomplete: true })