diff --git a/tests-js/scripts/desktop-chat-smoke.ts b/tests-js/scripts/desktop-chat-smoke.ts
index 50dad62844..528d010379 100644
--- a/tests-js/scripts/desktop-chat-smoke.ts
+++ b/tests-js/scripts/desktop-chat-smoke.ts
@@ -16,6 +16,8 @@ export interface DesktopChatSmokeOptions {
   outDir: string
   expectCommit?: string
   provenanceCommit?: string
+  /** A commit the running app may report instead of expectCommit: the build a reused packaged app came from. */
+  reusedBuildCommit?: string
   observePrompts?: () => Promise<readonly string[]>
 }
 
@@ -75,9 +77,10 @@ export async function readChatIdentity(page: Page): Promise<ChatIdentity> {
   return { ...identity, commit: identity.commit ?? null }
 }
 
-/** Old desktops do not report a commit; their caller must verify the installed runtime. */
-export function assertChatCommit(identity: ChatIdentity, expected: string, provenanceCommit?: string): void {
-  if (identity.commit !== null && identity.commit !== expected) {
+/** Old desktops do not report a commit; their caller must verify the installed runtime.
+ * A reused packaged app reports the commit it was built from (`reusedBuildCommit`). */
+export function assertChatCommit(identity: ChatIdentity, expected: string, provenanceCommit?: string, reusedBuildCommit?: string): void {
+  if (identity.commit !== null && identity.commit !== expected && identity.commit !== reusedBuildCommit) {
     throw new Error(`Running commit ${identity.commit} does not equal expected ${expected}`)
   }
 
@@ -279,7 +282,7 @@ export async function runDesktopChatSmoke(page: Page, options: DesktopChatSmokeO
 
     const identity = await readChatIdentity(page)
 
-    if (expectCommit) { assertChatCommit(identity, expectCommit, options.provenanceCommit) }
+    if (expectCommit) { assertChatCommit(identity, expectCommit, options.provenanceCommit, options.reusedBuildCommit) }
     const beforeIds = (await readTranscript(page)).map((message: TranscriptMessage): string => message.id)
     const before = (await observe()).length
     await composer.click()
diff --git a/tests-js/scripts/desktop-reused-build-commit.test.ts b/tests-js/scripts/desktop-reused-build-commit.test.ts
new file mode 100644
index 0000000000..2f9424c1cd
--- /dev/null
+++ b/tests-js/scripts/desktop-reused-build-commit.test.ts
@@ -0,0 +1,48 @@
+import { spawnSync } from 'node:child_process'
+import fs from 'node:fs'
+import os from 'node:os'
+import path from 'node:path'
+
+import { expect, test } from 'vitest'
+
+import { assertChatCommit } from './desktop-chat-smoke.ts'
+import { reusedDesktopBuildCommit } from './desktop-smoke-process.ts'
+
+function git(repo: string, ...args: string[]): string {
+  const result = spawnSync('git', ['-C', repo, ...args], {
+    encoding: 'utf8',
+    env: { ...process.env, GIT_AUTHOR_NAME: 't', GIT_AUTHOR_EMAIL: 't@t.invalid', GIT_COMMITTER_NAME: 't', GIT_COMMITTER_EMAIL: 't@t.invalid' },
+  })
+  if (result.status !== 0) { throw new Error(result.stderr) }
+
+  return result.stdout.trim()
+}
+
+function commitFile(repo: string, name: string): string {
+  fs.writeFileSync(path.join(repo, name), `${name}\n`)
+  git(repo, 'add', name)
+  git(repo, 'commit', '-q', '-m', name)
+
+  return git(repo, 'rev-parse', 'HEAD')
+}
+
+test('a reused packaged app may report its build commit only across the synthetic NEXT commit', (): void => {
+  const repo = fs.mkdtempSync(path.join(os.tmpdir(), 'reused-build-'))
+
+  try {
+    git(repo, 'init', '-q')
+    const head = commitFile(repo, 'app.ts')
+    const next = commitFile(repo, '.hermes-e2e-next')
+    const app = { appVersion: '0.0.0', commit: head, hermesRoot: repo, platform: process.platform }
+
+    // HEAD -> NEXT: only the marker changed, so the app built at HEAD stands for NEXT.
+    const reused = reusedDesktopBuildCommit(repo, 'source', next)
+    expect(reused).toBe(head)
+    expect((): void => assertChatCommit(app, next, next, reused)).not.toThrow()
+
+    // A commit that changes anything else gives no allowance: a stale app stays a failure.
+    const real = commitFile(repo, 'feature.ts')
+    expect(reusedDesktopBuildCommit(repo, 'source', real)).toBeUndefined()
+    expect((): void => assertChatCommit({ ...app, commit: next }, real, real, reusedDesktopBuildCommit(repo, 'source', real))).toThrow('does not equal expected')
+  } finally { fs.rmSync(repo, { recursive: true, force: true }) }
+})
diff --git a/tests-js/scripts/desktop-smoke-process.ts b/tests-js/scripts/desktop-smoke-process.ts
index 34bdc39176..9191c451b0 100644
--- a/tests-js/scripts/desktop-smoke-process.ts
+++ b/tests-js/scripts/desktop-smoke-process.ts
@@ -63,6 +63,34 @@ export function readInstallationCommit(root: string, origin: 'source' | 'bundled
   return z.string().regex(/^[0-9a-f]{40}$/).parse(commit)
 }
 
+/** The file the HEAD -> NEXT legs' synthetic commit adds (installer-common.sh::resolve_update_ref,
+ * windows-e2e.ps1::New-NextCommit). */
+const SYNTHETIC_NEXT_MARKER = '.hermes-e2e-next'
+
+/** The commit a reused packaged app may still report for an installed source checkout.
+ *
+ * `hermes update` keeps the packaged app when no desktop build input changed, and that app keeps
+ * naming the commit it was built from (hermes_cli/desktop_build_reuse.py). The harness can prove
+ * that case only for the synthetic NEXT commit, whose sole change is one marker file: there the
+ * parent is the commit the app may report. Any other installed commit, and any bundled install,
+ * returns undefined. */
+export function reusedDesktopBuildCommit(root: string, origin: 'source' | 'bundled', installed: string): string | undefined {
+  if (origin !== 'source') {
+    return undefined
+  }
+
+  const git: string = process.env.HERMES_E2E_REAL_GIT || 'git'
+  const parents = nativeText(git, ['-C', root, 'rev-list', '--parents', '-n', '1', installed]).split(/\s+/).slice(1)
+
+  if (parents.length !== 1) {
+    return undefined
+  }
+
+  const changed = nativeText(git, ['-C', root, 'diff', '--name-only', parents[0], installed])
+
+  return changed === SYNTHETIC_NEXT_MARKER ? parents[0] : undefined
+}
+
 const bundleEnvSchema = z.record(z.string(), z.string().nullable())
 
 /** The baked runtime defaults/clears of a bundled artifact, recorded in the
diff --git a/tests/install/e2e-assets/desktop-smoke.ts b/tests/install/e2e-assets/desktop-smoke.ts
index 273ba4d65f..4261e40a44 100644
--- a/tests/install/e2e-assets/desktop-smoke.ts
+++ b/tests/install/e2e-assets/desktop-smoke.ts
@@ -12,7 +12,7 @@ import { z } from 'zod'
 import { resolveDesktopHermesHome } from '../../../apps/desktop/electron/data-paths.mjs'
 import { applyBundleEnvironment } from '../../../apps/desktop/scripts/bundle-env.mjs'
 import { type ChatIdentity, readChatIdentity, runDesktopChatSmoke, waitForChatReady } from '../../../tests-js/scripts/desktop-chat-smoke.ts'
-import { assertBackendOrigin, localBackendProcess, readBundledBundleEnv, readInstallationCommit } from '../../../tests-js/scripts/desktop-smoke-process.ts'
+import { assertBackendOrigin, localBackendProcess, readBundledBundleEnv, readInstallationCommit, reusedDesktopBuildCommit } from '../../../tests-js/scripts/desktop-smoke-process.ts'
 import { validateMockUrl, writeEnvFile, writeMockProviderConfig } from '../../../tests-js/scripts/mock-provider-config.ts'
 import { type MockServer, startMockServer } from '../../../tests-js/scripts/mock-server.ts'
 
@@ -353,7 +353,8 @@ export async function runInstalledDesktopSmoke(options: SmokeOptions, launchApp:
     if (provenanceCommit !== options['expect-commit']) { throw new Error('Installed commit differs from --expect-commit') }
     // Trace only chat actions. Connection probes can return credential-bearing logs.
     await app.context().tracing.start({ screenshots: true, snapshots: false, sources: false })
-    const chat = await runDesktopChatSmoke(page, { mockUrl, phase: options.phase, outDir: out, expectCommit: options['expect-commit'], provenanceCommit })
+    const reusedBuildCommit = reusedDesktopBuildCommit(options.root, options.origin, provenanceCommit)
+    const chat = await runDesktopChatSmoke(page, { mockUrl, phase: options.phase, outDir: out, expectCommit: options['expect-commit'], provenanceCommit, reusedBuildCommit })
     fs.writeFileSync(receiptPath, `${JSON.stringify({ ...chat, status: 'closing' }, null, 2)}\n`)
     const result = { ...chat, origin: options.origin, root: options.root, executable: options.exe, running, predictedHome,
       localModeConfigured: true, backend: { ...backend, command: redact(backend.command) }, launchKind: options.phase === 'new' ? 'post-update-launch' : 'installed-launch' }