diff --git a/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/migration.sql b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/migration.sql
new file mode 100644
index 000000000..e569b3230
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/migration.sql
@@ -0,0 +1,16 @@
+-- Terminal subscription rows written before endedAt was derived on every sync
+-- (and terminal-on-first-sync rows, whose create path never set it) have
+-- endedAt = NULL. The status-agnostic isSubscriptionInEffect predicate reads
+-- NULL as "entitled forever", so these rows must be closed out.
+--
+-- endedAt = LEAST(currentPeriodEnd, NOW()) mirrors getEndedAtForSync's
+-- fallback when Stripe omits ended_at: the paid-through boundary if it's
+-- already past, otherwise now.
+--
+-- No batching/temp-index machinery: the affected population was measured in
+-- production at 7 rows (2026-07-17), and a plain UPDATE only takes row locks
+-- on matching rows, so the seq scan is safe regardless of table size.
+UPDATE "Subscription"
+SET "endedAt" = LEAST("currentPeriodEnd", NOW())
+WHERE "status" IN ('canceled', 'incomplete_expired', 'unpaid')
+  AND "endedAt" IS NULL;
diff --git a/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/backfills-terminal-rows.ts b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/backfills-terminal-rows.ts
new file mode 100644
index 000000000..07e30ff59
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/backfills-terminal-rows.ts
@@ -0,0 +1,72 @@
+import { randomUUID } from 'crypto';
+import type { Sql } from 'postgres';
+import { expect } from 'vitest';
+
+const insertSubscription = async (sql: Sql, options: {
+  tenancyId: string,
+  status: string,
+  currentPeriodEnd: Date,
+  endedAt?: Date | null,
+}) => {
+  const id = randomUUID();
+  await sql`
+    INSERT INTO "Subscription" (
+      "id", "tenancyId", "customerId", "customerType", "product", "quantity",
+      "stripeSubscriptionId", "status", "currentPeriodStart", "currentPeriodEnd",
+      "cancelAtPeriodEnd", "endedAt", "creationSource", "createdAt", "updatedAt"
+    ) VALUES (
+      ${id}::uuid, ${options.tenancyId}::uuid, ${`customer-${id}`}, 'TEAM',
+      ${sql.json({ displayName: 'Test Product', customerType: 'team' })}, 1,
+      ${`sub_${id}`}, ${options.status}::"SubscriptionStatus",
+      '2026-01-01', ${options.currentPeriodEnd},
+      false, ${options.endedAt ?? null}, 'PURCHASE_PAGE', NOW(), NOW()
+    )
+  `;
+  return id;
+};
+
+export const preMigration = async (sql: Sql) => {
+  const tenancyId = randomUUID();
+  const pastPeriodEnd = new Date('2026-02-01T00:00:00Z');
+
+  // One row per terminal status, all with null endedAt and a past period end.
+  const canceledId = await insertSubscription(sql, { tenancyId, status: 'canceled', currentPeriodEnd: pastPeriodEnd });
+  const incompleteExpiredId = await insertSubscription(sql, { tenancyId, status: 'incomplete_expired', currentPeriodEnd: pastPeriodEnd });
+  const unpaidId = await insertSubscription(sql, { tenancyId, status: 'unpaid', currentPeriodEnd: pastPeriodEnd });
+
+  // Terminal row whose period end is in the future: endedAt must be capped at
+  // the migration's NOW() (matching getEndedAtForSync's last-resort fallback),
+  // not scheduled for the future.
+  const futurePeriodEnd = new Date('2100-01-01T00:00:00Z');
+  const futureCanceledId = await insertSubscription(sql, { tenancyId, status: 'canceled', currentPeriodEnd: futurePeriodEnd });
+
+  // The DB's own clock, so the post-migration bound check compares
+  // apples-to-apples (the Subscription timestamp columns have no time zone,
+  // so JS-side Date comparisons would be off by the session's tz offset).
+  const [{ now: dbNowBeforeMigration }] = await sql`SELECT NOW() AS now`;
+
+  return { tenancyId, canceledId, incompleteExpiredId, unpaidId, futureCanceledId, dbNowBeforeMigration: dbNowBeforeMigration as Date };
+};
+
+export const postMigration = async (sql: Sql, ctx: Awaited<ReturnType<typeof preMigration>>) => {
+  // Compare in SQL so stored values never round-trip through JS Dates.
+  const rows = await sql`
+    SELECT
+      "id",
+      ("endedAt" IS NOT NULL) AS "hasEndedAt",
+      ("endedAt" = "currentPeriodEnd") AS "matchesPeriodEnd",
+      ("endedAt" >= ${ctx.dbNowBeforeMigration} AND "endedAt" <= NOW() AND "endedAt" < "currentPeriodEnd") AS "cappedAtNow"
+    FROM "Subscription" WHERE "tenancyId" = ${ctx.tenancyId}::uuid
+  `;
+  const byId = new Map(rows.map((row) => [row.id, row]));
+
+  // Past period end: backfilled to exactly currentPeriodEnd.
+  for (const id of [ctx.canceledId, ctx.incompleteExpiredId, ctx.unpaidId]) {
+    expect(byId.get(id)?.hasEndedAt).toBe(true);
+    expect(byId.get(id)?.matchesPeriodEnd).toBe(true);
+  }
+
+  // Future period end: capped at the migration's NOW(), not scheduled for 2100.
+  expect(byId.get(ctx.futureCanceledId)?.hasEndedAt).toBe(true);
+  expect(byId.get(ctx.futureCanceledId)?.cappedAtNow).toBe(true);
+};
diff --git a/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/leaves-other-rows-untouched.ts b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/leaves-other-rows-untouched.ts
new file mode 100644
index 000000000..f955c5ea9
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260717000000_backfill_terminal_subscription_ended_at/tests/leaves-other-rows-untouched.ts
@@ -0,0 +1,60 @@
+import { randomUUID } from 'crypto';
+import type { Sql } from 'postgres';
+import { expect } from 'vitest';
+
+const insertSubscription = async (sql: Sql, options: {
+  tenancyId: string,
+  status: string,
+  endedAt: Date | null,
+}) => {
+  const id = randomUUID();
+  await sql`
+    INSERT INTO "Subscription" (
+      "id", "tenancyId", "customerId", "customerType", "product", "quantity",
+      "stripeSubscriptionId", "status", "currentPeriodStart", "currentPeriodEnd",
+      "cancelAtPeriodEnd", "endedAt", "creationSource", "createdAt", "updatedAt"
+    ) VALUES (
+      ${id}::uuid, ${options.tenancyId}::uuid, ${`customer-${id}`}, 'TEAM',
+      ${sql.json({ displayName: 'Test Product', customerType: 'team' })}, 1,
+      ${`sub_${id}`}, ${options.status}::"SubscriptionStatus",
+      '2026-01-01', '2026-02-01',
+      false, ${options.endedAt}, 'PURCHASE_PAGE', NOW(), NOW()
+    )
+  `;
+  return id;
+};
+
+export const preMigration = async (sql: Sql) => {
+  const tenancyId = randomUUID();
+  const existingEndedAt = new Date('2026-01-15T12:00:00Z');
+
+  // Non-terminal rows with null endedAt: live subscriptions, must stay null.
+  const activeId = await insertSubscription(sql, { tenancyId, status: 'active', endedAt: null });
+  const trialingId = await insertSubscription(sql, { tenancyId, status: 'trialing', endedAt: null });
+  // `incomplete` is non-terminal (Stripe expires it to incomplete_expired
+  // within ~24h, which arrives via webhook) — deliberately not backfilled.
+  const incompleteId = await insertSubscription(sql, { tenancyId, status: 'incomplete', endedAt: null });
+  const pastDueId = await insertSubscription(sql, { tenancyId, status: 'past_due', endedAt: null });
+
+  // Terminal row whose endedAt is already set: must keep its original value.
+  const alreadyEndedId = await insertSubscription(sql, { tenancyId, status: 'canceled', endedAt: existingEndedAt });
+
+  return { tenancyId, activeId, trialingId, incompleteId, pastDueId, alreadyEndedId, existingEndedAt };
+};
+
+export const postMigration = async (sql: Sql, ctx: Awaited<ReturnType<typeof preMigration>>) => {
+  // Compare the already-set endedAt in SQL against the same JS Date param used
+  // at insert time — both cross the driver's tz-naive conversion identically,
+  // so equality holds regardless of the session time zone.
+  const rows = await sql`
+    SELECT "id", "endedAt", ("endedAt" = ${ctx.existingEndedAt}) AS "matchesOriginal"
+    FROM "Subscription" WHERE "tenancyId" = ${ctx.tenancyId}::uuid
+  `;
+  const byId = new Map(rows.map((row) => [row.id, row]));
+
+  for (const id of [ctx.activeId, ctx.trialingId, ctx.incompleteId, ctx.pastDueId]) {
+    expect(byId.get(id)?.endedAt).toBeNull();
+  }
+
+  expect(byId.get(ctx.alreadyEndedId)?.matchesOriginal).toBe(true);
+};