diff --git a/src/cache-manager.ts b/src/cache-manager.ts
index 90b5c63..5a3ea6e 100644
--- a/src/cache-manager.ts
+++ b/src/cache-manager.ts
@@ -131,14 +131,17 @@ export class CacheManager {
     const entry = this.cache[filePath];
     if (!entry) return false;
     const now = new Date().toISOString();
-    entry.cards = (cards || []).map((card: ResolvedCard) => ({
-      title: card.title,
-      anchor: card.anchor,
-      gist: card.gist,
-      bullets: card.bullets || [],
-    }));
-    entry.updatedAt = now;
-    entry.lastAccessedAt = now;
+    this.cache[filePath] = {
+      ...entry,
+      cards: (cards || []).map((card: ResolvedCard) => ({
+        title: card.title,
+        anchor: card.anchor,
+        gist: card.gist,
+        bullets: card.bullets || [],
+      })),
+      updatedAt: now,
+      lastAccessedAt: now,
+    };
     await this.save();
     return true;
   }
diff --git a/src/cache.ts b/src/cache.ts
index 3718f9a..6b8b846 100644
--- a/src/cache.ts
+++ b/src/cache.ts
@@ -4,8 +4,7 @@ import type { CacheEntry } from './types';
 
 export function touchCacheEntry(entry: CacheEntry | null, now?: string): CacheEntry | null {
   if (!entry) return null;
-  entry.lastAccessedAt = now || new Date().toISOString();
-  return entry;
+  return { ...entry, lastAccessedAt: now || new Date().toISOString() };
 }
 
 export function serializeCacheFile(entries: Record<string, CacheEntry>): string {
diff --git a/tests/main.test.js b/tests/main.test.js
index 84fd7cd..387b5bd 100644
--- a/tests/main.test.js
+++ b/tests/main.test.js
@@ -281,8 +281,10 @@ assert.deepStrictEqual(t.folderPathsForTarget('Reading'), ['Reading']);
 assert.deepStrictEqual(t.folderPathsForTarget(''), []);
 const untouchedCacheEntry = { generatedAt: '2024-01-01T00:00:00.000Z' };
 assert.strictEqual(t.touchCacheEntry(null), null);
-assert.strictEqual(t.touchCacheEntry(untouchedCacheEntry, '2024-01-05T00:00:00.000Z'), untouchedCacheEntry);
-assert.strictEqual(untouchedCacheEntry.lastAccessedAt, '2024-01-05T00:00:00.000Z');
+const touchedEntry = t.touchCacheEntry(untouchedCacheEntry, '2024-01-05T00:00:00.000Z');
+assert.notStrictEqual(touchedEntry, untouchedCacheEntry, 'touchCacheEntry returns new object');
+assert.strictEqual(touchedEntry.lastAccessedAt, '2024-01-05T00:00:00.000Z', 'touched entry has updated timestamp');
+assert.strictEqual(untouchedCacheEntry.lastAccessedAt, undefined, 'original entry not mutated');
 assert.strictEqual(t.shouldConfirmRegenerate({ updatedAt: '2024-01-05T00:00:00.000Z' }, true), true);
 assert.strictEqual(t.shouldConfirmRegenerate({ updatedAt: '2024-01-05T00:00:00.000Z' }, false), false);
 assert.strictEqual(t.shouldConfirmRegenerate({ generatedAt: '2024-01-01T00:00:00.000Z' }, true), false);
diff --git a/tests/modules.test.js b/tests/modules.test.js
index 77e251e..42a2c72 100644
--- a/tests/modules.test.js
+++ b/tests/modules.test.js
@@ -90,8 +90,9 @@ assert.deepStrictEqual(
 
 assert.strictEqual(t.touchCacheEntry(null), null, 'touchCacheEntry on null returns null');
 const entry = { generatedAt: '2024-01-01T00:00:00.000Z' };
-t.touchCacheEntry(entry, '2024-06-01T00:00:00.000Z');
-assert.strictEqual(entry.lastAccessedAt, '2024-06-01T00:00:00.000Z', 'touchCacheEntry sets lastAccessedAt');
+const touched = t.touchCacheEntry(entry, '2024-06-01T00:00:00.000Z');
+assert.strictEqual(touched.lastAccessedAt, '2024-06-01T00:00:00.000Z', 'touchCacheEntry sets lastAccessedAt on returned entry');
+assert.strictEqual(entry.lastAccessedAt, undefined, 'touchCacheEntry does not mutate original entry');
 
 const serialized = t.serializeCacheFile({ 'a.md': { cards: [] } });
 const parsed = JSON.parse(serialized);