diff --git a/src/main/native-chat/ssh-transcript-host-routing.test.ts b/src/main/native-chat/ssh-transcript-host-routing.test.ts
index f48f2f2255c..30b10a2034d 100644
--- a/src/main/native-chat/ssh-transcript-host-routing.test.ts
+++ b/src/main/native-chat/ssh-transcript-host-routing.test.ts
@@ -285,7 +285,6 @@ describe('native chat SSH transcript host routing (#13663)', () => {
         return {
           size: bytes.length,
           identity: '1:7:11',
-          identityReliable: true,
           mtimeMs: 1,
           ctimeMs: 1
         }
@@ -770,7 +769,18 @@ describe('native chat SSH transcript host routing (#13663)', () => {
     const transcriptPath = '/tmp/large-append-session.jsonl'
     const initial = Buffer.from(claudeLine('initial', 'first'))
     const files = new Map([[transcriptPath, initial]])
-    const { provider, readFile, readFileRange } = memoryProvider(files)
+    const { provider, readFile, readFileRange, stat } = memoryProvider(files)
+    stat.mockImplementation(async (filePath: string): Promise<FileStat> => {
+      const bytes = files.get(filePath)!
+      return {
+        size: bytes.length,
+        type: 'file',
+        mtime: bytes.length,
+        mtimeMs: bytes.length,
+        dev: 7,
+        ino: 11
+      }
+    })
     mocks.getProvider.mockReturnValue(provider)
     const appended: string[] = []
     const replacements: string[][] = []
@@ -787,13 +797,24 @@ describe('native chat SSH transcript host routing (#13663)', () => {
     try {
       await vi.waitFor(() => expect(readFileRange).toHaveBeenCalled())
       readFileRange.mockClear()
+      const partial = Buffer.from('{"type":')
+      files.set(transcriptPath, Buffer.concat([initial, partial]))
+      await vi.waitFor(() =>
+        expect(readFileRange).toHaveBeenCalledWith(
+          transcriptPath,
+          initial.length,
+          partial.length,
+          expect.anything()
+        )
+      )
+      readFileRange.mockClear()
       const append = Buffer.from(
         Array.from({ length: 3_000 }, (_, index) =>
           claudeLine(`append-${index}`, `bounded remote append ${index}`)
         ).join('')
       )
       expect(append.length).toBeGreaterThan(MAX_FILE_RANGE_READ_BYTES)
-      files.set(transcriptPath, Buffer.concat([initial, append]))
+      files.set(transcriptPath, Buffer.concat([initial, partial, append]))
 
       await vi.waitFor(() => expect(replacements.at(-1)).toContain('append-2999'))
       expect(replacements.at(-1)).toHaveLength(40)
@@ -803,6 +824,9 @@ describe('native chat SSH transcript host routing (#13663)', () => {
       expect(requestedLengths.reduce((sum, length) => sum + length, 0)).toBeLessThanOrEqual(
         append.length + 256
       )
+      const afterReplacement = Buffer.from(claudeLine('after-replacement', 'still live'))
+      files.set(transcriptPath, Buffer.concat([initial, partial, append, afterReplacement]))
+      await vi.waitFor(() => expect(appended).toContain('after-replacement'), { timeout: 3_000 })
       expect(readFile).not.toHaveBeenCalled()
     } finally {
       subscription.unsubscribe()
diff --git a/src/main/native-chat/ssh-transcript-range-fs.test.ts b/src/main/native-chat/ssh-transcript-range-fs.test.ts
index d81cb78458e..d0113912d76 100644
--- a/src/main/native-chat/ssh-transcript-range-fs.test.ts
+++ b/src/main/native-chat/ssh-transcript-range-fs.test.ts
@@ -39,9 +39,11 @@ function mutableProvider(initial: string) {
   } as unknown as IFilesystemProvider
   return {
     provider,
-    replace(next: string) {
+    replace(next: string, touch = true) {
       bytes = Buffer.from(next)
-      mtimeMs++
+      if (touch) {
+        mtimeMs++
+      }
     }
   }
 }
@@ -53,13 +55,12 @@ beforeEach(() => {
 describe('SSH transcript range stability', () => {
   it.each([
     ['same-length rewrite', 'new generation'],
-    ['growing rewrite', 'new generation with more bytes'],
-    ['concurrent append', 'old generation append']
+    ['growing rewrite', 'new generation with more bytes']
   ])('invalidates a %s', async (_label, replacement) => {
     const remote = mutableProvider('old generation')
     mocks.provider = remote.provider
     const rangeFs = await createSshTranscriptRangeFs('ssh-owner')
-    const openingStamp = await rangeFs.stat(TRANSCRIPT_PATH)
+    const openingStamp = await rangeFs.stat(TRANSCRIPT_PATH, undefined, true)
 
     await rangeFs.read(TRANSCRIPT_PATH, 0, 3)
     remote.replace(replacement)
@@ -68,4 +69,28 @@ describe('SSH transcript range stability', () => {
       TranscriptRangeReadInvalidatedError
     )
   })
+
+  it('accepts a concurrent append that preserves the opening boundary', async () => {
+    const remote = mutableProvider('old generation')
+    mocks.provider = remote.provider
+    const rangeFs = await createSshTranscriptRangeFs('ssh-owner')
+    const openingStamp = await rangeFs.stat(TRANSCRIPT_PATH, undefined, true)
+
+    remote.replace('old generation append')
+
+    await expect(rangeFs.assertStable(TRANSCRIPT_PATH, openingStamp)).resolves.toBeUndefined()
+  })
+
+  it('rejects a growing rewrite even when remote mtime is coarse', async () => {
+    const remote = mutableProvider('old generation')
+    mocks.provider = remote.provider
+    const rangeFs = await createSshTranscriptRangeFs('ssh-owner')
+    const openingStamp = await rangeFs.stat(TRANSCRIPT_PATH, undefined, true)
+
+    remote.replace('new generation with more bytes', false)
+
+    await expect(rangeFs.assertStable(TRANSCRIPT_PATH, openingStamp)).rejects.toBeInstanceOf(
+      TranscriptRangeReadInvalidatedError
+    )
+  })
 })
diff --git a/src/main/native-chat/ssh-transcript-range-fs.ts b/src/main/native-chat/ssh-transcript-range-fs.ts
index 679dcdc438d..92bb5e33cc2 100644
--- a/src/main/native-chat/ssh-transcript-range-fs.ts
+++ b/src/main/native-chat/ssh-transcript-range-fs.ts
@@ -13,6 +13,8 @@ type ProviderGeneration = {
   generation: number
 }
 
+const STABILITY_BOUNDARY_BYTES = 64
+
 function requireSameProvider(connectionId: string, expected: ProviderGeneration): void {
   const current = getSshFilesystemProviderSnapshot(connectionId)
   if (
@@ -41,49 +43,85 @@ export async function createSshTranscriptRangeFs(
   const providerSnapshot: ProviderGeneration = snapshot
   requireSameProvider(connectionId, providerSnapshot)
   const provider = providerSnapshot.provider
-  async function stat(filePath: string, statSignal?: AbortSignal): Promise<TranscriptFileStamp> {
+  async function read(
+    filePath: string,
+    position: number,
+    length: number,
+    readSignal?: AbortSignal
+  ) {
+    readSignal?.throwIfAborted()
+    const parts: Buffer[] = []
+    let remaining = length
+    let cursor = position
+    while (remaining > 0) {
+      requireSameProvider(connectionId, providerSnapshot)
+      const window = Math.min(remaining, MAX_FILE_RANGE_READ_BYTES)
+      const result = await provider.readFileRange!(filePath, cursor, window, {
+        signal: readSignal
+      })
+      requireSameProvider(connectionId, providerSnapshot)
+      const bytes = result.bytes.subarray(0, result.bytesRead)
+      parts.push(bytes)
+      cursor += bytes.length
+      remaining -= bytes.length
+      if (bytes.length < window) {
+        break
+      }
+    }
+    return parts.length === 1 ? parts[0]! : Buffer.concat(parts)
+  }
+  async function stat(
+    filePath: string,
+    statSignal?: AbortSignal,
+    captureBoundary = false
+  ): Promise<TranscriptFileStamp> {
     statSignal?.throwIfAborted()
     requireSameProvider(connectionId, providerSnapshot)
     const stamp = await provider.stat(filePath, { signal: statSignal })
     requireSameProvider(connectionId, providerSnapshot)
     const mtimeMs = stamp.mtimeMs ?? stamp.mtime
+    const boundaryLength = Math.min(stamp.size, STABILITY_BOUNDARY_BYTES)
+    const boundaryFingerprint =
+      captureBoundary && boundaryLength > 0
+        ? (await read(filePath, stamp.size - boundaryLength, boundaryLength, statSignal)).toString(
+            'base64'
+          )
+        : undefined
     return {
       size: stamp.size,
       identity: `${providerSnapshot.generation}:${stamp.dev ?? 0}:${stamp.ino ?? 0}`,
-      identityReliable: stamp.dev !== undefined && stamp.ino !== undefined,
       mtimeMs,
-      ctimeMs: mtimeMs
+      ctimeMs: mtimeMs,
+      ...(boundaryFingerprint ? { boundaryFingerprint } : {})
     }
   }
   const rangeFs: TranscriptRangeFs = {
     stat,
-    async read(filePath, position, length, readSignal) {
-      readSignal?.throwIfAborted()
-      const parts: Buffer[] = []
-      let remaining = length
-      let cursor = position
-      while (remaining > 0) {
-        requireSameProvider(connectionId, providerSnapshot)
-        const window = Math.min(remaining, MAX_FILE_RANGE_READ_BYTES)
-        const result = await provider.readFileRange!(filePath, cursor, window, {
-          signal: readSignal
-        })
-        requireSameProvider(connectionId, providerSnapshot)
-        const bytes = result.bytes.subarray(0, result.bytesRead)
-        parts.push(bytes)
-        cursor += bytes.length
-        remaining -= bytes.length
-        if (bytes.length < window) {
-          break
-        }
-      }
-      return parts.length === 1 ? parts[0]! : Buffer.concat(parts)
-    },
+    read,
     async assertStable(filePath, openingStamp, stableSignal) {
       const closingStamp = await stat(filePath, stableSignal)
       const identityChanged = closingStamp.identity !== openingStamp.identity
       const versionChanged = closingStamp.mtimeMs !== openingStamp.mtimeMs
-      if (identityChanged || versionChanged) {
+      if (identityChanged || closingStamp.size < openingStamp.size) {
+        throw new TranscriptRangeReadInvalidatedError()
+      }
+      if (closingStamp.size === openingStamp.size) {
+        if (versionChanged) {
+          throw new TranscriptRangeReadInvalidatedError()
+        }
+        return
+      }
+      if (!openingStamp.boundaryFingerprint) {
+        throw new TranscriptRangeReadInvalidatedError()
+      }
+      const boundaryLength = Math.min(openingStamp.size, STABILITY_BOUNDARY_BYTES)
+      const boundary = await read(
+        filePath,
+        openingStamp.size - boundaryLength,
+        boundaryLength,
+        stableSignal
+      )
+      if (boundary.toString('base64') !== openingStamp.boundaryFingerprint) {
         throw new TranscriptRangeReadInvalidatedError()
       }
     }
diff --git a/src/main/native-chat/transcript-incremental-reader.ts b/src/main/native-chat/transcript-incremental-reader.ts
index c6b625abf76..81f76a57b89 100644
--- a/src/main/native-chat/transcript-incremental-reader.ts
+++ b/src/main/native-chat/transcript-incremental-reader.ts
@@ -40,7 +40,7 @@ export async function readIncrementalTranscriptMessages(
   signal?: AbortSignal,
   rangeFs?: TranscriptRangeFs
 ): Promise<NativeChatMessage[]> {
-  const openingStamp = rangeFs ? await rangeFs.stat(filePath, signal) : null
+  const openingStamp = rangeFs ? await rangeFs.stat(filePath, signal, true) : null
   const end = openingStamp?.size ?? (await wslGatedStat(filePath, 'exact', signal)).size
   if (end <= state.offset) {
     return []
diff --git a/src/main/native-chat/transcript-range-fs.ts b/src/main/native-chat/transcript-range-fs.ts
index 2cdd668874a..156a0b2fba5 100644
--- a/src/main/native-chat/transcript-range-fs.ts
+++ b/src/main/native-chat/transcript-range-fs.ts
@@ -1,9 +1,9 @@
 export type TranscriptFileStamp = {
   size: number
   identity: string
-  identityReliable: boolean
   mtimeMs: number
   ctimeMs: number
+  boundaryFingerprint?: string
 }
 
 export class TranscriptRangeReadInvalidatedError extends Error {
@@ -14,7 +14,11 @@ export class TranscriptRangeReadInvalidatedError extends Error {
 }
 
 export type TranscriptRangeFs = {
-  stat(filePath: string, signal?: AbortSignal): Promise<TranscriptFileStamp>
+  stat(
+    filePath: string,
+    signal?: AbortSignal,
+    captureBoundary?: boolean
+  ): Promise<TranscriptFileStamp>
   read(filePath: string, position: number, length: number, signal?: AbortSignal): Promise<Buffer>
   assertStable(
     filePath: string,
diff --git a/src/main/native-chat/transcript-tail-reader.ts b/src/main/native-chat/transcript-tail-reader.ts
index f46a6e3bf5c..9cb0e3ac8e7 100644
--- a/src/main/native-chat/transcript-tail-reader.ts
+++ b/src/main/native-chat/transcript-tail-reader.ts
@@ -73,7 +73,7 @@ export async function readNativeChatTranscriptTailFile(
   oversizedRecordCount?: number
 }> {
   signal?.throwIfAborted()
-  const openingStamp = rangeFs ? await rangeFs.stat(filePath, signal) : null
+  const openingStamp = rangeFs ? await rangeFs.stat(filePath, signal, true) : null
   const end = Math.min(
     openingStamp?.size ?? (await wslGatedStat(filePath, 'exact', signal)).size,
     endOffset ?? Number.MAX_SAFE_INTEGER
diff --git a/src/main/native-chat/transcript-watch-engine.ts b/src/main/native-chat/transcript-watch-engine.ts
index 58f4087c706..df0b9af026b 100644
--- a/src/main/native-chat/transcript-watch-engine.ts
+++ b/src/main/native-chat/transcript-watch-engine.ts
@@ -33,9 +33,7 @@ const ROTATION_RETRY_MS = 25
 const MAX_ROTATION_RETRY_MS = 2_000
 let activeWatcherCount = 0
 
-export function getActiveNativeChatWatcherCount(): number {
-  return activeWatcherCount
-}
+export const getActiveNativeChatWatcherCount = (): number => activeWatcherCount
 
 /** Installs a live tail on an already-resolved path; missing files return null. */
 export async function installTranscriptWatcher(
@@ -191,6 +189,7 @@ export async function installTranscriptWatcher(
       return
     }
     if (replacementSnapshot && onReplace) {
+      resetIncrementalTranscriptState(state)
       state.offset = replacementSnapshot.consumedTo
       state.pendingStart = state.offset
       onReplace(