diff --git a/CHANGELOG.md b/CHANGELOG.md
index 87120cca..2b683db5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,8 @@
   reflow moves an already-bottom scroll position.
 - Add native PTY coverage for queued notifications, transcript paging and
   double-`Esc` cancellation.
+- Retry bounded Windows executable activation when a transient security scan
+  briefly holds the freshly verified candidate, while preserving atomic rollback.
 
 ## 0.1.44-beta.5 - 2026-09-09
 
diff --git a/src/mendcode/install.ps1 b/src/mendcode/install.ps1
index 0f9f9601..24d6e8e4 100644
--- a/src/mendcode/install.ps1
+++ b/src/mendcode/install.ps1
@@ -450,9 +450,28 @@ function Activate-Candidate {
   }
   $script:Phase = "activating"
   Write-UpdateStatus
-  # File.Replace is one same-volume operation and preserves the original as its backup.
-  if ([IO.File]::Exists($destination)) { [IO.File]::Replace($candidate, $destination, $previous) }
-  else { [IO.File]::Move($candidate, $destination) }
+  if ([IO.File]::Exists($destination)) {
+    # Windows security scanners can briefly hold a freshly executed binary.
+    # Keep the replace atomic while allowing that bounded transient lock to clear.
+    $maxAttempts = 8
+    for ($attempt = 1; $attempt -le $maxAttempts; $attempt++) {
+      try {
+        # File.Replace is one same-volume operation and preserves the original as its backup.
+        [IO.File]::Replace($candidate, $destination, $previous)
+        break
+      } catch {
+        $exception = $_.Exception
+        $inner = $exception.InnerException
+        $isIoFailure = $exception -is [IO.IOException] -or $inner -is [IO.IOException]
+        $isAccessFailure = $exception -is [UnauthorizedAccessException] -or $inner -is [UnauthorizedAccessException]
+        $isSharingViolation = $exception.HResult -in @(-2147024864, -2147024863) -or $inner.HResult -in @(-2147024864, -2147024863)
+        if (-not ($isIoFailure -or $isAccessFailure -or $isSharingViolation) -or $attempt -eq $maxAttempts) { throw }
+        $delay = 250 * $attempt
+        Write-Warning "Executable activation is temporarily locked; retrying in ${delay}ms (attempt $attempt/$maxAttempts)."
+        Start-Sleep -Milliseconds $delay
+      }
+    }
+  } else { [IO.File]::Move($candidate, $destination) }
   $script:Phase = "activated"
   Write-UpdateStatus
   Write-Ok "Installed $destination"
diff --git a/src/mendcode/script/windows-installer-smoke.ts b/src/mendcode/script/windows-installer-smoke.ts
index 5c000f6c..cc0a50f4 100644
--- a/src/mendcode/script/windows-installer-smoke.ts
+++ b/src/mendcode/script/windows-installer-smoke.ts
@@ -11,16 +11,42 @@ if (!binary || !version) throw new Error("Provide MENDCODE_INSTALLER_TEST_BINARY
 const root = await fs.mkdtemp(path.join(os.tmpdir(), "mendcode-windows-installer-"))
 const digest = (bytes: Uint8Array) => createHash("sha256").update(bytes).digest("hex")
 const quote = (value: string) => `'${value.replaceAll("'", "''")}'`
-async function powershell(source: string, env: Record<string, string | undefined> = {}) {
-  const child = Bun.spawn(["powershell.exe", "-NoProfile", "-NonInteractive", "-EncodedCommand", Buffer.from(source, "utf16le").toString("base64")], {
+const encodePowerShell = (source: string) => Buffer.from(source, "utf16le").toString("base64")
+const spawnPowerShell = (source: string, env: Record<string, string | undefined> = {}) =>
+  Bun.spawn(["powershell.exe", "-NoProfile", "-NonInteractive", "-EncodedCommand", encodePowerShell(source)], {
     env: { ...process.env, ...env }, stdout: "pipe", stderr: "pipe",
   })
+async function powershell(source: string, env: Record<string, string | undefined> = {}) {
+  const child = spawnPowerShell(source, env)
   const timer = setTimeout(() => child.kill(), 90_000)
   try {
     const [code, stdout, stderr] = await Promise.all([child.exited, new Response(child.stdout).text(), new Response(child.stderr).text()])
     return { code, output: stdout + stderr }
   } finally { clearTimeout(timer) }
 }
+async function waitForFile(file: string, diagnostic?: string, timeout = 10_000) {
+  const deadline = Date.now() + timeout
+  while (!(await Bun.file(file).exists()) && Date.now() < deadline) await Bun.sleep(25)
+  if (diagnostic && (await Bun.file(diagnostic).exists())) throw new Error(await fs.readFile(diagnostic, "utf8"))
+  assert.equal(await Bun.file(file).exists(), true, `Timed out waiting for ${file}`)
+}
+async function waitForActivation(directory: string, release: string) {
+  const deadline = Date.now() + 90_000
+  while (!(await Bun.file(release).exists()) && Date.now() < deadline) {
+    const entries = await fs.readdir(directory, { withFileTypes: true }).catch(() => [])
+    for (const entry of entries) {
+      if (!entry.isDirectory() || !entry.name.startsWith(".update.")) continue
+      const status = await fs.readFile(path.join(directory, entry.name, "status"), "utf8").catch(() => "")
+      if (!status.includes("phase=activating")) continue
+      await Bun.sleep(500)
+      await fs.writeFile(release, "release")
+      return
+    }
+    await Bun.sleep(25)
+  }
+  if (await Bun.file(release).exists()) return
+  throw new Error(`Timed out waiting for installer activation in ${directory}`)
+}
 const archive = path.join(root, "candidate.zip")
 const installer = path.resolve(import.meta.dir, "../install.ps1")
 const parsed = await powershell(`$tokens=$null; $errors=$null; [void][System.Management.Automation.Language.Parser]::ParseFile(${quote(installer)}, [ref]$tokens, [ref]$errors); if ($errors.Count) { $errors | Out-String | Write-Error; exit 1 }`)
@@ -41,25 +67,74 @@ const server = Bun.serve({ hostname: "127.0.0.1", port: 0, fetch(request) {
   return new Response("Not found", { status: 404 })
 } })
 try {
-  for (scenario of ["success", "checksum-failure", "http-failure", "truncated"]) {
+  for (scenario of ["success", "transient-lock", "checksum-failure", "http-failure", "truncated"]) {
+    const expectSuccess = scenario === "success" || scenario === "transient-lock"
     const home = path.join(root, scenario)
     const installed = path.join(home, ".mendcode", "bin", "mendcode.exe")
     await fs.mkdir(path.dirname(installed), { recursive: true })
     // An unusable existing executable must not be run to perform recovery.
     await fs.writeFile(installed, "previous damaged executable")
     const previousDigest = digest(await Bun.file(installed).bytes())
-    const result = await powershell(`& ${quote(installer)} -Version ${quote(version)} -SkipSetup -NoModifyPath; exit $LASTEXITCODE`, {
-      OPENCODE_TEST_HOME: home, MENDCODE_GITHUB_BASE_URL: server.url.toString().replace(/\/$/, ""),
-      MENDCODE_UPDATE_PARENT_PID: undefined, MENDCODE_VERIFIED_SUMS_FILE: undefined,
-      MENDCODE_DB: path.join(home, "data", "test.db"),
-    })
+    const lockReady = path.join(home, "lock-ready")
+    const lockDiagnostic = path.join(home, "lock-diagnostic")
+    const lockProcess =
+      scenario === "transient-lock"
+        ? spawnPowerShell(`
+$installed = ${quote(installed)}
+$ready = ${quote(lockReady)}
+$release = ${quote(path.join(home, "lock-release"))}
+$diagnostic = ${quote(lockDiagnostic)}
+$stream = $null
+try {
+  $stream = [IO.File]::Open($installed, [IO.FileMode]::Open, [IO.FileAccess]::Read, [IO.FileShare]::Read)
+  [IO.File]::WriteAllText($ready, "ready")
+  $deadline = [DateTime]::UtcNow.AddSeconds(90)
+  while (-not [IO.File]::Exists($release) -and [DateTime]::UtcNow -lt $deadline) { Start-Sleep -Milliseconds 25 }
+  if (-not [IO.File]::Exists($release)) { throw "Did not receive the transient installer lock release." }
+} catch {
+  [IO.File]::WriteAllText($diagnostic, ($_ | Out-String))
+  exit 1
+} finally {
+  if ($null -ne $stream) { $stream.Dispose() }
+}
+`)
+        : undefined
+    const result = await (async () => {
+      try {
+        if (lockProcess) {
+          await waitForFile(lockReady, lockDiagnostic, 90_000)
+          const activation = waitForActivation(path.dirname(installed), path.join(home, "lock-release"))
+          try {
+            return await powershell(`& ${quote(installer)} -Version ${quote(version)} -SkipSetup -NoModifyPath; exit $LASTEXITCODE`, {
+              OPENCODE_TEST_HOME: home, MENDCODE_GITHUB_BASE_URL: server.url.toString().replace(/\/$/, ""),
+              MENDCODE_UPDATE_PARENT_PID: undefined, MENDCODE_VERIFIED_SUMS_FILE: undefined,
+              MENDCODE_DB: path.join(home, "data", "test.db"),
+            })
+          } finally {
+            await fs.writeFile(path.join(home, "lock-release"), "release")
+            await activation
+          }
+        }
+        return await powershell(`& ${quote(installer)} -Version ${quote(version)} -SkipSetup -NoModifyPath; exit $LASTEXITCODE`, {
+          OPENCODE_TEST_HOME: home, MENDCODE_GITHUB_BASE_URL: server.url.toString().replace(/\/$/, ""),
+          MENDCODE_UPDATE_PARENT_PID: undefined, MENDCODE_VERIFIED_SUMS_FILE: undefined,
+          MENDCODE_DB: path.join(home, "data", "test.db"),
+        })
+      } finally {
+        if (lockProcess) {
+          lockProcess.kill()
+          await lockProcess.exited
+        }
+      }
+    })()
     await fs.writeFile(path.join(home, "installer.log"), result.output)
-    if (scenario === "success") assert.equal(result.code, 0, result.output)
+    if (expectSuccess) assert.equal(result.code, 0, result.output)
+    if (scenario === "transient-lock") assert.match(result.output, /temporarily locked; retrying/i)
     const operations = (await fs.readdir(path.dirname(installed))).filter((name) => name.startsWith(".update."))
     assert.equal(operations.length, 1, `${scenario}: ${result.output}`)
     const operation = path.join(path.dirname(installed), operations[0])
     const status = await fs.readFile(path.join(operation, "status"), "utf8")
-    if (scenario === "success") {
+    if (expectSuccess) {
       assert.equal(result.code, 0, result.output)
       assert.equal(digest(await Bun.file(installed).bytes()), binaryDigest)
       assert.equal(digest(await Bun.file(path.join(operation, "previous")).bytes()), previousDigest)