diff --git a/src/lib/sw/diff-manifest.ts b/src/lib/sw/diff-manifest.ts
new file mode 100644
index 0000000..28a53b2
--- /dev/null
+++ b/src/lib/sw/diff-manifest.ts
@@ -0,0 +1,14 @@
+// Vergleicht die aktuelle Rezept-ID-Liste (vom Server) mit dem, was
+// der Cache schon hat. Der SW nutzt das Delta, um nur Neue zu laden
+// und Gelöschte abzuräumen.
+export type ManifestDiff = { toAdd: number[]; toRemove: number[] };
+
+export function diffManifest(currentIds: number[], cachedIds: number[]): ManifestDiff {
+  const current = new Set(currentIds);
+  const cached = new Set(cachedIds);
+  const toAdd: number[] = [];
+  const toRemove: number[] = [];
+  for (const id of current) if (!cached.has(id)) toAdd.push(id);
+  for (const id of cached) if (!current.has(id)) toRemove.push(id);
+  return { toAdd, toRemove };
+}
diff --git a/tests/unit/diff-manifest.test.ts b/tests/unit/diff-manifest.test.ts
new file mode 100644
index 0000000..55bf183
--- /dev/null
+++ b/tests/unit/diff-manifest.test.ts
@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { diffManifest } from '../../src/lib/sw/diff-manifest';
+
+describe('diffManifest', () => {
+  it('detects new IDs to add', () => {
+    const result = diffManifest([1, 2, 3, 4], [1, 2]);
+    expect(result.toAdd.sort()).toEqual([3, 4]);
+    expect(result.toRemove).toEqual([]);
+  });
+
+  it('detects removed IDs', () => {
+    const result = diffManifest([1, 2], [1, 2, 3, 4]);
+    expect(result.toAdd).toEqual([]);
+    expect(result.toRemove.sort()).toEqual([3, 4]);
+  });
+
+  it('detects both add and remove in one diff', () => {
+    const result = diffManifest([1, 3, 5], [1, 2, 3]);
+    expect(result.toAdd).toEqual([5]);
+    expect(result.toRemove).toEqual([2]);
+  });
+
+  it('returns empty arrays when identical', () => {
+    const result = diffManifest([1, 2, 3], [3, 2, 1]);
+    expect(result.toAdd).toEqual([]);
+    expect(result.toRemove).toEqual([]);
+  });
+
+  it('handles empty caches (first sync)', () => {
+    const result = diffManifest([1, 2], []);
+    expect(result.toAdd.sort()).toEqual([1, 2]);
+    expect(result.toRemove).toEqual([]);
+  });
+});