diff --git a/lib/queue-prune.js b/lib/queue-prune.js
new file mode 100644
index 0000000..81352f4
--- /dev/null
+++ b/lib/queue-prune.js
@@ -0,0 +1,59 @@
+// Queue auto-prune logic. Extracted from renderer/app.js handleBatchDone so
+// the algorithm can be unit-tested without needing a DOM or the renderer's
+// module-level state (queueJobs, _jobIndexById).
+//
+// Loaded both as a CommonJS module (Node tests) and as a browser global
+// (renderer/app.js via index.html script tag) so the same single
+// implementation backs both runtime and tests — no drift between them.
+//
+// Behaviour: when the number of terminal-status jobs (done / skipped /
+// error / aborted) in the queue exceeds `limit`, drop the oldest terminal
+// jobs (insertion order) until we're back at the limit. Non-terminal jobs
+// (queued / preview / uploading / retrying / getting-server) are always
+// kept — those are work the user can still act on. Without this cap a
+// long session accumulates thousands of done rows and every render becomes
+// O(N) on a perpetually-growing N.
+
+(function (root) {
+  'use strict';
+
+  const TERMINAL_STATUSES = new Set(['done', 'skipped', 'error', 'aborted']);
+
+  /**
+   * Compute which jobs to keep vs drop, given a queue and a terminal-jobs cap.
+   * @param {Array<{id: string, status: string}>} jobs the current queue
+   * @param {number} limit max terminal jobs to keep
+   * @returns {null | { kept: Array, dropped: Array }} null when nothing changed
+   */
+  function pruneOldestTerminalJobs(jobs, limit) {
+    if (!Array.isArray(jobs) || jobs.length === 0) return null;
+    if (!Number.isFinite(limit) || limit < 0) return null;
+
+    // Walk once, record indices of terminal jobs in insertion order.
+    const terminalIdxs = [];
+    for (let i = 0; i < jobs.length; i++) {
+      const j = jobs[i];
+      if (j && TERMINAL_STATUSES.has(j.status)) terminalIdxs.push(i);
+    }
+    if (terminalIdxs.length <= limit) return null;
+
+    const dropCount = terminalIdxs.length - limit;
+    const dropSet = new Set(terminalIdxs.slice(0, dropCount));
+
+    const kept = [];
+    const dropped = [];
+    for (let i = 0; i < jobs.length; i++) {
+      if (dropSet.has(i)) dropped.push(jobs[i]);
+      else kept.push(jobs[i]);
+    }
+    return { kept, dropped };
+  }
+
+  const api = { pruneOldestTerminalJobs, TERMINAL_STATUSES };
+
+  if (typeof module !== 'undefined' && module.exports) {
+    module.exports = api;
+  } else if (root) {
+    root.QueuePrune = api;
+  }
+})(typeof window !== 'undefined' ? window : this);
diff --git a/renderer/app.js b/renderer/app.js
index 634af35..7127eaa 100644
--- a/renderer/app.js
+++ b/renderer/app.js
@@ -2023,34 +2023,23 @@ function handleBatchDone(summary) {
     queueJobs = nextJobs;
     renderQueueTable();
   } else {
-    // Auto-prune for the default (removeOnDone=false) too: cap done/skipped
-    // jobs at the most recent N so the queue can't grow unbounded across
-    // long sessions. Without this, sortQueueJobs / _computeQueueStats /
-    // renderQueueTable all become O(N) on a forever-growing N and the UI
-    // starts visibly lagging once the queue passes a few thousand entries.
-    // 500 most-recent terminal jobs is enough for "see what just happened"
-    // while stopping the runaway growth.
+    // Auto-prune for the default (removeOnDone=false) too: cap terminal
+    // jobs (done/skipped/error/aborted) at the most recent N so the queue
+    // can't grow unbounded across long sessions. The algorithm lives in
+    // lib/queue-prune.js (same impl Node-tested, see tests/queue-prune.test.js)
+    // and the result tells us which jobs to drop so we can clean up the
+    // index + selection in one pass.
     const TERMINAL_KEEP_LIMIT = 500;
-    const terminalIdxs = [];
-    for (let i = 0; i < queueJobs.length; i++) {
-      const s = queueJobs[i].status;
-      if (s === 'done' || s === 'skipped' || s === 'error' || s === 'aborted') terminalIdxs.push(i);
-    }
-    if (terminalIdxs.length > TERMINAL_KEEP_LIMIT) {
-      const dropCount = terminalIdxs.length - TERMINAL_KEEP_LIMIT;
-      // terminalIdxs is in insertion order → first `dropCount` are the
-      // oldest. Remove those, keep everything else.
-      const dropSet = new Set(terminalIdxs.slice(0, dropCount));
-      const nextJobs = [];
-      for (let i = 0; i < queueJobs.length; i++) {
-        if (dropSet.has(i)) {
-          removeJobFromIndex(queueJobs[i]);
-          selectedJobIds.delete(queueJobs[i].id);
-        } else {
-          nextJobs.push(queueJobs[i]);
-        }
+    // Optional-chain so the renderer still works if the prune script fails
+    // to load (e.g. file:// path issues during dev) — falls back to no-prune
+    // rather than crashing on every batch-done.
+    const result = window.QueuePrune?.pruneOldestTerminalJobs(queueJobs, TERMINAL_KEEP_LIMIT);
+    if (result) {
+      for (const j of result.dropped) {
+        removeJobFromIndex(j);
+        selectedJobIds.delete(j.id);
       }
-      queueJobs = nextJobs;
+      queueJobs = result.kept;
       renderQueueTable();
     }
   }
diff --git a/renderer/index.html b/renderer/index.html
index 0d1d425..a9a2ae9 100644
--- a/renderer/index.html
+++ b/renderer/index.html
@@ -330,6 +330,7 @@
     </div>
   </div>
 
+  <script src="../lib/queue-prune.js"></script>
   <script src="app.js"></script>
 </body>
 </html>
diff --git a/tasks/todo.md b/tasks/todo.md
index 43d8b0d..478b7ea 100644
--- a/tasks/todo.md
+++ b/tasks/todo.md
@@ -9,12 +9,12 @@
 - ✅ 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)
 - ✅ 3.3.6 — CSS `.queue-row` transition nur noch auf `:hover` (kein 150ms compositor-tween bei status-flips)
 - ✅ 3.3.7 — `_sessionTrackedJobs`/`_sessionDoneJobs` werden bei handleBatchDone gegen current queueJobs geprunt (no more unbounded session memory growth across batches)
+- ✅ 3.3.8 — queue-cap-prune-Logik nach `lib/queue-prune.js` extrahiert (dual-environment: Node + Browser-global) + 10 Unit-Tests (insertion-order, limit=0, malformed entries, large-queue 5000-job sweep)
 
 ## Open items (priorisiert)
 
 ### Code-Qualität (deferred — bräuchte jsdom für DOM/state)
-- [ ] queue-cap-prune-Logik (3.3.0 handleBatchDone)
-- [ ] sortQueueJobs dynamic-throttle (3.3.0)
+- [ ] sortQueueJobs dynamic-throttle (3.3.0) — modul-state-abhängig im renderer
 - [ ] removeFromQueueOnDone microtask-coalesce (3.3.1) — Microtask-Timing schwer zu testen ohne fake-timer setup
 
 ### Loop-Status
diff --git a/tests/queue-prune.test.js b/tests/queue-prune.test.js
new file mode 100644
index 0000000..0cf82e9
--- /dev/null
+++ b/tests/queue-prune.test.js
@@ -0,0 +1,115 @@
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+
+const { pruneOldestTerminalJobs, TERMINAL_STATUSES } = require('../lib/queue-prune');
+
+const j = (id, status) => ({ id, status });
+
+test('returns null on empty / non-array input', () => {
+  assert.equal(pruneOldestTerminalJobs([], 5), null);
+  assert.equal(pruneOldestTerminalJobs(null, 5), null);
+  assert.equal(pruneOldestTerminalJobs(undefined, 5), null);
+});
+
+test('returns null when all jobs are non-terminal regardless of limit', () => {
+  const jobs = [j('a', 'queued'), j('b', 'uploading'), j('c', 'preview')];
+  assert.equal(pruneOldestTerminalJobs(jobs, 0), null);
+  assert.equal(pruneOldestTerminalJobs(jobs, 100), null);
+});
+
+test('returns null when terminal count is at or under the limit', () => {
+  const jobs = [j('a', 'done'), j('b', 'done'), j('c', 'queued')];
+  assert.equal(pruneOldestTerminalJobs(jobs, 2), null, 'terminal=2, limit=2 → no-op');
+  assert.equal(pruneOldestTerminalJobs(jobs, 3), null, 'terminal=2, limit=3 → no-op');
+});
+
+test('drops oldest terminal jobs when over the limit, keeps non-terminal', () => {
+  const jobs = [
+    j('t1', 'done'),     // oldest terminal — should be dropped
+    j('t2', 'done'),     // should be dropped
+    j('queued1', 'queued'),
+    j('t3', 'error'),    // newest of the dropped block
+    j('uploading1', 'uploading'),
+    j('t4', 'done'),     // kept (within limit window)
+    j('t5', 'skipped'),  // kept
+    j('t6', 'aborted'),  // kept
+  ];
+  // 6 terminal, limit 3 → drop 3 oldest (t1, t2, t3)
+  const result = pruneOldestTerminalJobs(jobs, 3);
+  assert.notEqual(result, null);
+  const droppedIds = result.dropped.map(x => x.id).sort();
+  assert.deepEqual(droppedIds, ['t1', 't2', 't3']);
+  // Non-terminal jobs always kept; surviving terminals are the newest 3
+  const keptIds = result.kept.map(x => x.id);
+  assert.deepEqual(keptIds, ['queued1', 'uploading1', 't4', 't5', 't6']);
+});
+
+test('respects insertion order (oldest by index, not by status)', () => {
+  const jobs = [
+    j('older-error', 'error'),
+    j('newer-done', 'done'),
+    j('newest-aborted', 'aborted'),
+  ];
+  const result = pruneOldestTerminalJobs(jobs, 1);
+  assert.deepEqual(result.dropped.map(x => x.id), ['older-error', 'newer-done']);
+  assert.deepEqual(result.kept.map(x => x.id), ['newest-aborted']);
+});
+
+test('drops everything terminal when limit is 0', () => {
+  const jobs = [
+    j('q', 'queued'),
+    j('d1', 'done'),
+    j('d2', 'done'),
+    j('e1', 'error'),
+  ];
+  const result = pruneOldestTerminalJobs(jobs, 0);
+  assert.deepEqual(result.dropped.map(x => x.id), ['d1', 'd2', 'e1']);
+  assert.deepEqual(result.kept.map(x => x.id), ['q']);
+});
+
+test('rejects negative or non-finite limits', () => {
+  const jobs = [j('a', 'done'), j('b', 'done')];
+  assert.equal(pruneOldestTerminalJobs(jobs, -1), null);
+  assert.equal(pruneOldestTerminalJobs(jobs, NaN), null);
+  assert.equal(pruneOldestTerminalJobs(jobs, Infinity), null,
+    'Infinity is technically not finite; safer to treat as no-op');
+});
+
+test('TERMINAL_STATUSES set covers all 4 terminal kinds', () => {
+  assert.ok(TERMINAL_STATUSES.has('done'));
+  assert.ok(TERMINAL_STATUSES.has('skipped'));
+  assert.ok(TERMINAL_STATUSES.has('error'));
+  assert.ok(TERMINAL_STATUSES.has('aborted'));
+  assert.equal(TERMINAL_STATUSES.size, 4);
+  // Non-terminal must not be in the set
+  for (const s of ['queued', 'preview', 'uploading', 'retrying', 'getting-server']) {
+    assert.equal(TERMINAL_STATUSES.has(s), false, `${s} must not be terminal`);
+  }
+});
+
+test('handles malformed entries (null / missing status) without throwing', () => {
+  const jobs = [
+    null,
+    j('a', 'done'),
+    { id: 'no-status' }, // no status
+    j('b', 'done'),
+  ];
+  // 2 terminal, limit 1 → drop oldest (a). null and no-status entries stay
+  // because they aren't terminal. The function must not throw on them.
+  const result = pruneOldestTerminalJobs(jobs, 1);
+  assert.notEqual(result, null);
+  assert.deepEqual(result.dropped.map(x => x && x.id), ['a']);
+  assert.equal(result.kept.length, 3);
+});
+
+test('large queue: keeps the newest `limit` terminals', () => {
+  const jobs = [];
+  for (let i = 0; i < 5000; i++) jobs.push(j(`done-${i}`, 'done'));
+  const result = pruneOldestTerminalJobs(jobs, 500);
+  assert.notEqual(result, null);
+  assert.equal(result.dropped.length, 4500);
+  assert.equal(result.kept.length, 500);
+  // First kept = done-4500 (the 4501st original entry)
+  assert.equal(result.kept[0].id, 'done-4500');
+  assert.equal(result.kept[result.kept.length - 1].id, 'done-4999');
+});