diff --git a/psf-memo-indexer/test/property/backup-db.property.test.js b/psf-memo-indexer/test/property/backup-db.property.test.js
new file mode 100644
index 0000000..f12fba8
--- /dev/null
+++ b/psf-memo-indexer/test/property/backup-db.property.test.js
@@ -0,0 +1,66 @@
+/*
+  Property tests for the indexer's DB backup decision.
+
+  BackupDb.maybeBackupDb decides when the indexer should ask psf-memo-db to
+  back up its LevelDB. The decision is a pure function of the block height and
+  the configured epoch: a backup is requested exactly when the height is a
+  positive multiple of the epoch. These properties pin that invariant across a
+  broad range of heights and epochs, and confirm the decision is deterministic
+  (idempotent) for repeated calls.
+*/
+
+import test from 'node:test'
+
+import { seededRandom, forAll, intGen } from './harness.js'
+import BackupDb from '../../src/use-cases/backup-db.js'
+
+const rng = seededRandom(20260828)
+
+function makeBackupDb () {
+  const calls = []
+  const adapters = {
+    dbCtrl: {
+      backupDb: async (height, epoch) => {
+        calls.push({ height, epoch })
+        return true
+      }
+    }
+  }
+  const uut = new BackupDb({ adapters })
+  return { uut, calls }
+}
+
+test('maybeBackupDb requests a backup exactly at positive multiples of the epoch', async () => {
+  const heightGen = intGen(rng, 0, 1000000)
+  const epochGen = intGen(rng, 1, 10000)
+
+  await forAll(
+    (i) => ({ height: heightGen(), epoch: epochGen() }),
+    async ({ height, epoch }) => {
+      const { uut, calls } = makeBackupDb()
+      const result = await uut.maybeBackupDb(height, epoch)
+      const expected = height > 0 && height % epoch === 0
+      const requested = calls.length === 1 &&
+        calls[0].height === height &&
+        calls[0].epoch === epoch
+      return result === expected && requested === expected
+    },
+    { label: 'backup decision matches height % epoch invariant' }
+  )
+})
+
+test('maybeBackupDb is deterministic across repeated calls', async () => {
+  const heightGen = intGen(rng, 0, 1000000)
+  const epochGen = intGen(rng, 1, 10000)
+
+  await forAll(
+    (i) => ({ height: heightGen(), epoch: epochGen() }),
+    async ({ height, epoch }) => {
+      const { uut, calls } = makeBackupDb()
+      const first = await uut.maybeBackupDb(height, epoch)
+      const second = await uut.maybeBackupDb(height, epoch)
+      return first === second && calls.length === (first ? 2 : 0)
+    },
+    { label: 'backup decision idempotence' }
+  )
+})
diff --git a/psf-memo-indexer/test/unit/use-cases/backup-db.unit.js b/psf-memo-indexer/test/unit/use-cases/backup-db.unit.js
index ba8780a..034f833 100644
--- a/psf-memo-indexer/test/unit/use-cases/backup-db.unit.js
+++ b/psf-memo-indexer/test/unit/use-cases/backup-db.unit.js
@@ -19,6 +19,12 @@ describe('#BackupDb', () => {
 
   afterEach(() => sandbox.restore())
 
+  describe('constructor', () => {
+    it('should throw when adapters are missing', () => {
+      assert.throws(() => new BackupDb(), /Adapters required/)
+    })
+  })
+
   describe('#maybeBackupDb', () => {
     it('should request a backup at an epoch boundary', async () => {
       const result = await uut.maybeBackupDb(1000, 1000)