diff --git a/supabase/migrations/20260602145425_lock_down_oauth_used_codes.sql b/supabase/migrations/20260602145425_lock_down_oauth_used_codes.sql
new file mode 100644
index 00000000..9fd0ed41
--- /dev/null
+++ b/supabase/migrations/20260602145425_lock_down_oauth_used_codes.sql
@@ -0,0 +1,9 @@
+-- OAuth authorization-code replay tracking is internal-only. The token
+-- endpoint uses a service-role client, so browser-facing API roles must not
+-- access this table.
+ALTER TABLE public.oauth_used_codes ENABLE ROW LEVEL SECURITY;
+
+REVOKE ALL ON TABLE public.oauth_used_codes FROM anon, authenticated;
+
+-- Privilege change: tell PostgREST to reload its schema/role cache.
+NOTIFY pgrst, 'reload schema';
diff --git a/tests/pg/oauth-used-codes-lockdown.pg.test.ts b/tests/pg/oauth-used-codes-lockdown.pg.test.ts
new file mode 100644
index 00000000..135271b7
--- /dev/null
+++ b/tests/pg/oauth-used-codes-lockdown.pg.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, it } from 'vitest'
+import { getClient } from '@/tests/pg/setup'
+
+/**
+ * Migration 20260602145425_lock_down_oauth_used_codes.sql enables RLS on
+ * public.oauth_used_codes and REVOKEs all privileges from the browser-facing
+ * `anon` and `authenticated` roles. The OAuth token endpoint reaches this
+ * replay-tracking table only through a service-role client (which bypasses
+ * both RLS and the revoke), so the API roles must be denied. These tests lock
+ * that contract in so a future migration cannot silently re-expose the table.
+ */
+describe('oauth_used_codes lockdown (pg)', () => {
+  async function expectDenied(role: 'anon' | 'authenticated', sql: string) {
+    const client = await getClient()
+    try {
+      await client.query('BEGIN')
+      await client.query(`SET LOCAL ROLE ${role}`)
+      await expect(client.query(sql)).rejects.toThrow(/permission denied/i)
+    } finally {
+      await client.query('ROLLBACK').catch(() => {})
+      client.release()
+    }
+  }
+
+  it('denies SELECT to the anon role', async () => {
+    await expectDenied('anon', 'SELECT * FROM public.oauth_used_codes LIMIT 1')
+  })
+
+  it('denies SELECT to the authenticated role', async () => {
+    await expectDenied('authenticated', 'SELECT * FROM public.oauth_used_codes LIMIT 1')
+  })
+
+  it('denies INSERT to the authenticated role', async () => {
+    await expectDenied(
+      'authenticated',
+      `INSERT INTO public.oauth_used_codes (code_hash) VALUES ('lockdown-test')`,
+    )
+  })
+
+  it('still allows the privileged (service-role / owner) connection to read', async () => {
+    // The app reaches this table through a service-role client, which bypasses
+    // RLS and the anon/authenticated revoke. Model that with the default
+    // privileged pg connection (table owner) the pg-real harness uses.
+    const client = await getClient()
+    try {
+      const res = await client.query<{ n: number }>(
+        'SELECT count(*)::int AS n FROM public.oauth_used_codes',
+      )
+      expect(res.rows[0]!.n).toBeGreaterThanOrEqual(0)
+    } finally {
+      client.release()
+    }
+  })
+})