diff --git a/lib/log-rotation.js b/lib/log-rotation.js
new file mode 100644
index 0000000..0242848
--- /dev/null
+++ b/lib/log-rotation.js
@@ -0,0 +1,52 @@
+// Generic numbered-backup log rotation. Used by the upload log + can be
+// reused by other long-lived log files (debug log, account-rotation log).
+//
+// Behaviour:
+// - File missing → no-op, returns false.
+// - File ≤ maxBytes → no-op, returns false.
+// - File > maxBytes → drop oldest .N backup, shift .K → .K+1, rename live
+//   file to .1, return true. Caller (or the next append) creates a fresh
+//   primary on demand.
+//
+// Errors are reported via `log` (e.g. debugLog) but never thrown — rotation
+// is best-effort; the caller's append happens anyway.
+
+const fs = require('fs');
+const path = require('path');
+
+function maybeRotateLogFile(filePath, maxBytes, maxBackups = 3, log = () => {}) {
+  if (!filePath || !Number.isFinite(maxBytes) || maxBytes <= 0) return false;
+  let size = 0;
+  try {
+    const st = fs.statSync(filePath);
+    size = st.size;
+  } catch (err) {
+    // ENOENT is normal — nothing to rotate yet.
+    if (err && err.code !== 'ENOENT') {
+      log(`logRotation: stat ${filePath} failed: ${err.message}`);
+    }
+    return false;
+  }
+  if (size <= maxBytes) return false;
+
+  const ext = path.extname(filePath);
+  const base = filePath.slice(0, filePath.length - ext.length);
+
+  // Drop the oldest backup if it exists, then shift each numbered backup up
+  // one slot. Errors are ignored: missing intermediate backups are normal,
+  // failed renames just mean we'll rotate again next time.
+  try { fs.unlinkSync(`${base}.${maxBackups}${ext}`); } catch {}
+  for (let i = maxBackups - 1; i >= 1; i--) {
+    try { fs.renameSync(`${base}.${i}${ext}`, `${base}.${i + 1}${ext}`); } catch {}
+  }
+  try {
+    fs.renameSync(filePath, `${base}.1${ext}`);
+    log(`logRotation: rotated ${filePath} (${(size / 1024 / 1024).toFixed(1)} MB) → ${base}.1${ext}`);
+    return true;
+  } catch (err) {
+    log(`logRotation: rename ${filePath} → ${base}.1${ext} failed: ${err.message}`);
+    return false;
+  }
+}
+
+module.exports = { maybeRotateLogFile };
diff --git a/main.js b/main.js
index 867cfc9..91b63cc 100644
--- a/main.js
+++ b/main.js
@@ -13,6 +13,7 @@ const { checkForUpdate, installUpdate, abortUpdate } = require('./lib/updater');
 const backupCrypto = require('./lib/backup-crypto');
 const FolderMonitor = require('./lib/folder-monitor');
 const RemoteServer = require('./lib/remote-server');
+const { maybeRotateLogFile } = require('./lib/log-rotation');
 
 let mainWindow;
 let _lastImportPath = null;
@@ -315,34 +316,6 @@ function _resolveUploadLogTarget() {
 const UPLOAD_LOG_MAX_BYTES = 50 * 1024 * 1024;
 const UPLOAD_LOG_MAX_BACKUPS = 3;
 
-function _maybeRotateUploadLog(filePath) {
-  let size = 0;
-  try {
-    const st = fs.statSync(filePath);
-    size = st.size;
-  } catch (err) {
-    // ENOENT is the common case (file doesn't exist yet) — nothing to rotate.
-    if (err && err.code !== 'ENOENT') {
-      debugLog(`uploadLog stat for rotation failed: ${err.message}`);
-    }
-    return;
-  }
-  if (size <= UPLOAD_LOG_MAX_BYTES) return;
-  const ext = path.extname(filePath);
-  const base = filePath.slice(0, filePath.length - ext.length);
-  // Drop the oldest backup if it exists, then shift each numbered backup up.
-  try { fs.unlinkSync(`${base}.${UPLOAD_LOG_MAX_BACKUPS}${ext}`); } catch {}
-  for (let i = UPLOAD_LOG_MAX_BACKUPS - 1; i >= 1; i--) {
-    try { fs.renameSync(`${base}.${i}${ext}`, `${base}.${i + 1}${ext}`); } catch {}
-  }
-  try {
-    fs.renameSync(filePath, `${base}.1${ext}`);
-    debugLog(`uploadLog rotated (${(size / 1024 / 1024).toFixed(1)} MB) → ${base}.1${ext}`);
-  } catch (err) {
-    debugLog(`uploadLog rotation rename failed: ${err.message}`);
-  }
-}
-
 function _flushUploadLog() {
   if (_uploadLogWriting || _uploadLogBuffer.length === 0) return;
   const target = _resolveUploadLogTarget();
@@ -354,7 +327,7 @@ function _flushUploadLog() {
   try { fs.mkdirSync(path.dirname(target.path), { recursive: true }); } catch {}
   // Cheap size check + rotation right before the append, so we never grow
   // a single log file beyond the cap regardless of session length.
-  _maybeRotateUploadLog(target.path);
+  maybeRotateLogFile(target.path, UPLOAD_LOG_MAX_BYTES, UPLOAD_LOG_MAX_BACKUPS, debugLog);
   const chunk = _uploadLogBuffer.join('');
   _uploadLogBuffer.length = 0;
   _uploadLogWriting = true;
diff --git a/tasks/todo.md b/tasks/todo.md
index 8282bdb..c320d71 100644
--- a/tasks/todo.md
+++ b/tasks/todo.md
@@ -6,11 +6,13 @@
 - ✅ 3.3.2 — `fileuploader.log` Auto-Rotation bei 50 MB (max 3 Backups: .1 .2 .3)
 - ✅ 3.3.3 — `_jobLogCollector` Cap auf 1000 tracked jobs (FIFO-eviction beim Überschreiten)
 - ✅ 3.3.4 — `applyQueueSelectionClasses` + `applyRecentSelectionClasses` nutzen `getElementsByClassName` (live HTMLCollection statt querySelectorAll re-query bei jedem Klick)
+- ✅ 3.3.5 — Log-Rotation extrahiert nach `lib/log-rotation.js` + 10 neue Unit-Tests (cap, shift, eviction, idempotency, maxBackups=1, invalid input, no-extension)
 
 ## Open items (priorisiert)
 
 ### Code-Qualität
-- [ ] **Test-Coverage für 3.3.0** — keine Tests für die queue-cap-prune-Logik in handleBatchDone, sortQueueJobs dynamic-throttle, log-error-recovery.
+- [ ] queue-cap-prune-Logik (3.3.0 handleBatchDone) — DOM-abhängig, bräuchte jsdom; deferred bis sich anderes lohnt
+- [ ] sortQueueJobs dynamic-throttle (3.3.0) — modul-state-abhängig im renderer; deferred
 
 ### UX-Politur
 - [ ] **CSS `.queue-row` transition** auf `:hover` scopen (aktuell auf jedem row → unnötiger Repaint bei Status-Flip).
diff --git a/tests/log-rotation.test.js b/tests/log-rotation.test.js
new file mode 100644
index 0000000..d17d3b8
--- /dev/null
+++ b/tests/log-rotation.test.js
@@ -0,0 +1,134 @@
+const { test, beforeEach, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+
+const { maybeRotateLogFile } = require('../lib/log-rotation');
+
+let tmpDir;
+let logFile;
+
+beforeEach(() => {
+  tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'mhu-log-rotation-'));
+  logFile = path.join(tmpDir, 'fileuploader.log');
+});
+
+afterEach(() => {
+  try { fs.rmSync(tmpDir, { recursive: true, force: true }); } catch {}
+});
+
+function writeBytes(p, n, fill = 'a') {
+  fs.writeFileSync(p, fill.repeat(n), 'utf-8');
+}
+
+test('returns false and skips rotation when file does not exist', () => {
+  const result = maybeRotateLogFile(logFile, 100);
+  assert.equal(result, false);
+  assert.equal(fs.existsSync(logFile), false);
+});
+
+test('returns false when file is below the size cap', () => {
+  writeBytes(logFile, 50);
+  const result = maybeRotateLogFile(logFile, 100);
+  assert.equal(result, false);
+  assert.equal(fs.statSync(logFile).size, 50, 'live file untouched');
+  assert.equal(fs.existsSync(logFile + '.1'), false, 'no .1 created');
+});
+
+test('rotates live file to .1 when over cap', () => {
+  writeBytes(logFile, 200, 'X');
+  const result = maybeRotateLogFile(logFile, 100, 3);
+  assert.equal(result, true);
+  assert.equal(fs.existsSync(logFile), false, 'live file moved away');
+  const expectedBackup = path.join(tmpDir, 'fileuploader.1.log');
+  assert.equal(fs.existsSync(expectedBackup), true, '.1 backup exists');
+  assert.equal(fs.statSync(expectedBackup).size, 200);
+});
+
+test('shifts existing backups up: .1 → .2, .2 → .3 on rotation', () => {
+  writeBytes(path.join(tmpDir, 'fileuploader.2.log'), 10, 'B');
+  writeBytes(path.join(tmpDir, 'fileuploader.1.log'), 20, 'A');
+  writeBytes(logFile, 200, 'L');
+
+  const result = maybeRotateLogFile(logFile, 100, 3);
+  assert.equal(result, true);
+
+  // Live file → .1 (latest live data)
+  assert.equal(fs.statSync(path.join(tmpDir, 'fileuploader.1.log')).size, 200);
+  // Old .1 → .2
+  assert.equal(fs.statSync(path.join(tmpDir, 'fileuploader.2.log')).size, 20);
+  // Old .2 → .3
+  assert.equal(fs.statSync(path.join(tmpDir, 'fileuploader.3.log')).size, 10);
+});
+
+test('drops oldest backup when at maxBackups limit', () => {
+  // Pre-populate all three backup slots.
+  writeBytes(path.join(tmpDir, 'fileuploader.3.log'), 5, 'C');  // oldest, will be dropped
+  writeBytes(path.join(tmpDir, 'fileuploader.2.log'), 10, 'B');
+  writeBytes(path.join(tmpDir, 'fileuploader.1.log'), 20, 'A');
+  writeBytes(logFile, 200, 'L');
+
+  const result = maybeRotateLogFile(logFile, 100, 3);
+  assert.equal(result, true);
+
+  // Old .3 (5 bytes 'C') gone, replaced by old .2.
+  const f3 = fs.statSync(path.join(tmpDir, 'fileuploader.3.log'));
+  assert.equal(f3.size, 10, 'old .2 became new .3 (the C-file was dropped)');
+  // .2 = old .1
+  assert.equal(fs.statSync(path.join(tmpDir, 'fileuploader.2.log')).size, 20);
+  // .1 = the live file we just rotated
+  assert.equal(fs.statSync(path.join(tmpDir, 'fileuploader.1.log')).size, 200);
+});
+
+test('is idempotent — second call on still-large file rotates again', () => {
+  writeBytes(logFile, 200, 'X');
+  maybeRotateLogFile(logFile, 100, 3);
+  // Simulate fresh writes after the first rotation
+  writeBytes(logFile, 200, 'Y');
+  const result = maybeRotateLogFile(logFile, 100, 3);
+  assert.equal(result, true);
+  // The .Y file is now .1, the .X file moved to .2
+  assert.equal(fs.readFileSync(path.join(tmpDir, 'fileuploader.1.log'), 'utf-8')[0], 'Y');
+  assert.equal(fs.readFileSync(path.join(tmpDir, 'fileuploader.2.log'), 'utf-8')[0], 'X');
+});
+
+test('maxBackups=1: only keeps a single .1 backup, never .2', () => {
+  writeBytes(logFile, 200, 'L');
+  maybeRotateLogFile(logFile, 100, 1);
+  writeBytes(logFile, 200, 'M');
+  maybeRotateLogFile(logFile, 100, 1);
+
+  // .1 holds the latest rotated content (M)
+  assert.equal(fs.readFileSync(path.join(tmpDir, 'fileuploader.1.log'), 'utf-8')[0], 'M');
+  // .2 must NOT exist
+  assert.equal(fs.existsSync(path.join(tmpDir, 'fileuploader.2.log')), false);
+});
+
+test('invalid maxBytes (0, negative, NaN) is a no-op', () => {
+  writeBytes(logFile, 1000, 'X');
+  for (const max of [0, -1, NaN]) {
+    const r = maybeRotateLogFile(logFile, max);
+    assert.equal(r, false, `maxBytes=${max} should be no-op`);
+  }
+  assert.equal(fs.existsSync(logFile), true);
+  assert.equal(fs.existsSync(logFile + '.1'), false);
+});
+
+test('logs through provided debug callback on rotation', () => {
+  writeBytes(logFile, 200, 'X');
+  const messages = [];
+  maybeRotateLogFile(logFile, 100, 3, (m) => messages.push(m));
+  assert.ok(messages.length >= 1, 'at least one log message');
+  assert.ok(messages.some(m => m.includes('rotated')), `expected "rotated" in: ${messages.join(' | ')}`);
+});
+
+test('handles file without extension correctly', () => {
+  const noExtFile = path.join(tmpDir, 'plainlog');
+  writeBytes(noExtFile, 200, 'P');
+  const result = maybeRotateLogFile(noExtFile, 100, 3);
+  assert.equal(result, true);
+  // base = the full path, ext = '', so backup name is "plainlog.1"
+  assert.equal(fs.existsSync(path.join(tmpDir, 'plainlog.1')), true);
+  assert.equal(fs.existsSync(noExtFile), false);
+});