diff --git a/backend/migrations/0004_backfill_complete_relay_sync.sql b/backend/migrations/0004_backfill_complete_relay_sync.sql
new file mode 100644
index 0000000..dd4027a
--- /dev/null
+++ b/backend/migrations/0004_backfill_complete_relay_sync.sql
@@ -0,0 +1,29 @@
+-- A manual migration created relays without recording the `complete_relay_sync`
+-- activity that `complete_relay_sync` normally emits. Backfill one per synced relay
+-- that lacks it, mirroring `insert_activity_tx`: a fresh v4 UUID id, the relay's tenant,
+-- a NULL billed_at, and a snapshot of the relay's current plan and status. This
+-- activity type is not billable (see `list_billable_activity`), so the NULL
+-- billed_at bills no one.
+INSERT INTO activity (id, tenant_pubkey, created_at, activity_type, resource_type, resource_id, snapshot)
+SELECT
+  lower(
+    hex(randomblob(4)) || '-' ||
+    hex(randomblob(2)) || '-4' ||
+    substr(hex(randomblob(2)), 2) || '-' ||
+    substr('89ab', abs(random() % 4) + 1, 1) ||
+    substr(hex(randomblob(2)), 2) || '-' ||
+    hex(randomblob(6))
+  ),
+  relay.tenant_pubkey,
+  CAST(strftime('%s', 'now') AS INTEGER),
+  'complete_relay_sync',
+  'relay',
+  relay.id,
+  json_object('resource_type', 'relay', 'plan', relay.plan_id, 'status', relay.status)
+FROM relay
+WHERE relay.synced = 1
+  AND NOT EXISTS (
+    SELECT 1 FROM activity
+    WHERE activity.resource_id = relay.id
+      AND activity.activity_type = 'complete_relay_sync'
+  );