diff --git a/src/lib/jobs/__tests__/idempotency-cleanup.test.ts b/src/lib/jobs/__tests__/idempotency-cleanup.test.ts
new file mode 100644
index 0000000..265bf1e
--- /dev/null
+++ b/src/lib/jobs/__tests__/idempotency-cleanup.test.ts
@@ -0,0 +1,47 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import type { PrismaClient } from "@/generated/prisma/client";
+import { cleanupExpiredIdempotencyKeys } from "../idempotency-cleanup";
+
+function makePrismaMock(deletedCount: number) {
+  return {
+    idempotencyKey: {
+      deleteMany: vi.fn().mockResolvedValue({ count: deletedCount }),
+    },
+  } as unknown as PrismaClient;
+}
+
+describe("cleanupExpiredIdempotencyKeys", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-05-04T03:00:00Z"));
+  });
+
+  it("deletes all rows whose expires_at is in the past and returns the count", async () => {
+    const prisma = makePrismaMock(100);
+    const deleted = await cleanupExpiredIdempotencyKeys(prisma);
+
+    expect(deleted).toBe(100);
+    expect(prisma.idempotencyKey.deleteMany).toHaveBeenCalledTimes(1);
+    expect(prisma.idempotencyKey.deleteMany).toHaveBeenCalledWith({
+      where: { expiresAt: { lt: new Date("2026-05-04T03:00:00Z") } },
+    });
+  });
+
+  it("returns 0 when no rows are expired", async () => {
+    const prisma = makePrismaMock(0);
+    const deleted = await cleanupExpiredIdempotencyKeys(prisma);
+    expect(deleted).toBe(0);
+  });
+
+  it("propagates errors from prisma so the handler can warn", async () => {
+    const prisma = {
+      idempotencyKey: {
+        deleteMany: vi.fn().mockRejectedValue(new Error("connection lost")),
+      },
+    } as unknown as PrismaClient;
+
+    await expect(cleanupExpiredIdempotencyKeys(prisma)).rejects.toThrow(
+      "connection lost",
+    );
+  });
+});
diff --git a/src/lib/jobs/idempotency-cleanup.ts b/src/lib/jobs/idempotency-cleanup.ts
new file mode 100644
index 0000000..ec30f8f
--- /dev/null
+++ b/src/lib/jobs/idempotency-cleanup.ts
@@ -0,0 +1,18 @@
+/**
+ * Daily cleanup for the `idempotency_keys` table.
+ *
+ * `withIdempotency()` (src/lib/idempotency.ts) only purges a row lazily
+ * on the next lookup with the same `(userId, key, method, path)` tuple
+ * — most rows never see another retry, so without a sweeper the table
+ * grows unbounded with stale 24h-expired entries.
+ */
+import type { PrismaClient } from "@/generated/prisma/client";
+
+export async function cleanupExpiredIdempotencyKeys(
+  prisma: PrismaClient,
+): Promise<number> {
+  const { count } = await prisma.idempotencyKey.deleteMany({
+    where: { expiresAt: { lt: new Date() } },
+  });
+  return count;
+}
diff --git a/src/lib/jobs/reminder-worker.ts b/src/lib/jobs/reminder-worker.ts
index 9036bc0..b9b125b 100644
--- a/src/lib/jobs/reminder-worker.ts
+++ b/src/lib/jobs/reminder-worker.ts
@@ -27,6 +27,7 @@ import {
   recordError,
 } from "@/lib/jobs/worker-status";
 import { setGlobalBoss } from "@/lib/jobs/boss-instance";
+import { cleanupExpiredIdempotencyKeys } from "@/lib/jobs/idempotency-cleanup";
 import { deleteMessage } from "@/lib/telegram";
 import { decrypt, encrypt } from "@/lib/crypto";
 import { syncMoodLogEntries } from "@/lib/moodlog/sync";
@@ -90,6 +91,8 @@ const DATA_BACKUP_QUEUE = "data-backup";
 const DATA_BACKUP_CRON = "0 3 * * 0"; // weekly Sunday at 03:00
 const RATE_LIMIT_CLEANUP_QUEUE = "rate-limit-cleanup";
 const RATE_LIMIT_CLEANUP_CRON = "*/5 * * * *"; // every 5 minutes
+const IDEMPOTENCY_CLEANUP_QUEUE = "idempotency-cleanup";
+const IDEMPOTENCY_CLEANUP_CRON = "0 3 * * *"; // daily at 03:00 (Europe/Berlin)
 
 interface ReminderCheckPayload {
   triggeredAt: string;
@@ -141,6 +144,10 @@ interface RateLimitCleanupPayload {
   triggeredAt: string;
 }
 
+interface IdempotencyCleanupPayload {
+  triggeredAt: string;
+}
+
 // Re-export timezone utilities under local names for backward compatibility
 const getUserTodayBounds = getUserTodayBoundsUtil;
 const getDayOfWeekInTz = getDayOfWeekInTzUtil;
@@ -818,6 +825,21 @@ async function handleRateLimitCleanup(
   });
 }
 
+async function handleIdempotencyCleanup(
+  jobs: Job<IdempotencyCleanupPayload>[],
+) {
+  void jobs;
+  await withBackgroundEvent("job.idempotency_cleanup", async (evt) => {
+    const p = getWorkerPrisma();
+    try {
+      const deleted = await cleanupExpiredIdempotencyKeys(p);
+      evt.addMeta("idempotency_cleanup_deleted", deleted);
+    } catch (err) {
+      evt.addWarning(`idempotency-cleanup failed: ${err}`);
+    }
+  });
+}
+
 async function handleDataBackup(jobs: Job<DataBackupPayload>[]) {
   void jobs;
   await withBackgroundEvent("job.data_backup", async (evt) => {
@@ -998,6 +1020,7 @@ export async function startReminderWorker() {
     MOODLOG_SYNC_QUEUE,
     DATA_BACKUP_QUEUE,
     RATE_LIMIT_CLEANUP_QUEUE,
+    IDEMPOTENCY_CLEANUP_QUEUE,
   ];
 
   for (const q of allQueues) {
@@ -1017,6 +1040,7 @@ export async function startReminderWorker() {
     [MOODLOG_SYNC_QUEUE, MOODLOG_SYNC_CRON],
     [DATA_BACKUP_QUEUE, DATA_BACKUP_CRON],
     [RATE_LIMIT_CLEANUP_QUEUE, RATE_LIMIT_CLEANUP_CRON],
+    [IDEMPOTENCY_CLEANUP_QUEUE, IDEMPOTENCY_CLEANUP_CRON],
   ];
 
   for (const [name, cron] of schedules) {
@@ -1084,6 +1108,11 @@ export async function startReminderWorker() {
     { localConcurrency: 1 },
     handleRateLimitCleanup,
   );
+  await boss.work<IdempotencyCleanupPayload>(
+    IDEMPOTENCY_CLEANUP_QUEUE,
+    { localConcurrency: 1 },
+    handleIdempotencyCleanup,
+  );
 
   return boss;
 }