diff --git a/packages/backend/convex/maintenanceDedup.ts b/packages/backend/convex/maintenanceDedup.ts
new file mode 100644
index 0000000..e8ba281
--- /dev/null
+++ b/packages/backend/convex/maintenanceDedup.ts
@@ -0,0 +1,21 @@
+/**
+ * Pure, deterministic dedup key for maintenance threads.
+ *
+ * Derives a stable key from the merge-base + head SHAs so that repeated
+ * scheduled sync runs against the same upstream state collapse onto a single
+ * maintenance thread instead of creating a new one each run. NEVER involves
+ * Date.now()/random — the key must be identical for identical inputs.
+ *
+ * Returns `null` when the head SHA is missing/empty (unresolvable head → the
+ * caller cannot dedup and should SKIP thread creation rather than mint a
+ * unique key).
+ */
+export const maintenanceDedupKey = (input: {
+  mergeBaseSha?: string;
+  headSha?: string;
+}): string | null => {
+  const head = input.headSha?.trim();
+  if (!head) return null;
+  const base = input.mergeBaseSha?.trim() || 'nobase';
+  return `${base}:${head}`;
+};
diff --git a/packages/backend/convex/schema.ts b/packages/backend/convex/schema.ts
index 824c8c0..241fd0b 100644
--- a/packages/backend/convex/schema.ts
+++ b/packages/backend/convex/schema.ts
@@ -797,6 +797,7 @@ const applicationTables = {
     ),
     ignoredCommitShas: v.optional(v.array(v.string())),
     ignoredReason: v.optional(v.string()),
+    dedupKey: v.optional(v.string()),
     createdAt: v.number(),
     updatedAt: v.number(),
     resolvedAt: v.optional(v.number()),
@@ -805,7 +806,8 @@ const applicationTables = {
     .index('by_owner_status', ['ownerId', 'status'])
     .index('by_spoon', ['spoonId'])
     .index('by_source', ['ownerId', 'source'])
-    .index('by_created', ['createdAt']),
+    .index('by_created', ['createdAt'])
+    .index('by_spoon_dedup', ['spoonId', 'dedupKey']),
   threadMessages: defineTable({
     threadId: v.id('threads'),
     ownerId: v.id('users'),
diff --git a/packages/backend/tests/unit/maintenance-dedup.test.ts b/packages/backend/tests/unit/maintenance-dedup.test.ts
new file mode 100644
index 0000000..af08a83
--- /dev/null
+++ b/packages/backend/tests/unit/maintenance-dedup.test.ts
@@ -0,0 +1,45 @@
+import { describe, expect, test } from 'vitest';
+
+import { maintenanceDedupKey } from '../../convex/maintenanceDedup';
+
+describe('maintenanceDedupKey', () => {
+  test('combines mergeBase and head into a stable key', () => {
+    expect(maintenanceDedupKey({ mergeBaseSha: 'a', headSha: 'b' })).toBe('a:b');
+  });
+
+  test('falls back to nobase when mergeBase is missing', () => {
+    expect(maintenanceDedupKey({ headSha: 'b' })).toBe('nobase:b');
+  });
+
+  test('returns null when head is missing', () => {
+    expect(maintenanceDedupKey({ mergeBaseSha: 'a' })).toBeNull();
+  });
+
+  test('returns null when head is empty', () => {
+    expect(maintenanceDedupKey({ mergeBaseSha: 'a', headSha: '' })).toBeNull();
+  });
+
+  test('returns null when head is only whitespace', () => {
+    expect(maintenanceDedupKey({ mergeBaseSha: 'a', headSha: '   ' })).toBeNull();
+  });
+
+  test('is deterministic across repeated calls (no Date.now/random)', () => {
+    const input = { mergeBaseSha: 'base123', headSha: 'head456' };
+    const first = maintenanceDedupKey(input);
+    const second = maintenanceDedupKey(input);
+    expect(first).toBe(second);
+    expect(first).toBe('base123:head456');
+  });
+
+  test('different heads produce different keys', () => {
+    const a = maintenanceDedupKey({ mergeBaseSha: 'base', headSha: 'h1' });
+    const b = maintenanceDedupKey({ mergeBaseSha: 'base', headSha: 'h2' });
+    expect(a).not.toBe(b);
+  });
+
+  test('trims surrounding whitespace on inputs', () => {
+    expect(
+      maintenanceDedupKey({ mergeBaseSha: '  a  ', headSha: '  b  ' }),
+    ).toBe('a:b');
+  });
+});