diff --git a/src/app/cash-wallet-cutover/prepare.ts b/src/app/cash-wallet-cutover/prepare.ts
index 35793b208..f4834de54 100644
--- a/src/app/cash-wallet-cutover/prepare.ts
+++ b/src/app/cash-wallet-cutover/prepare.ts
@@ -16,6 +16,9 @@ type PreparePrimaryCashWalletCutoverResult = {
   report: CashWalletCutoverPreflightReport
   plannedMigrations: PrimaryCashWalletMigrationPlan[]
   migrations: CashWalletMigration[]
+  // Cohort accountIds that were requested but not found among unlocked
+  // accounts — surfaced so the operator notices typos/locked/missing ids.
+  cohortNotFound?: AccountId[]
 }
 
 export const preparePrimaryCashWalletCutover = async ({
@@ -24,18 +27,32 @@ export const preparePrimaryCashWalletCutover = async ({
   accountsRepo,
   walletsRepo,
   migrationsRepo,
+  accountIds,
 }: {
   cutoverVersion: number
   runId: string
   accountsRepo: Pick<IAccountsRepository, "listUnlockedAccounts">
   walletsRepo: Pick<IWalletsRepository, "listByAccountId">
   migrationsRepo: CashWalletMigrationRecordsRepository
+  // Phased cutover (runbook: internal accounts first, then beta cohort).
+  // When set, only these accounts are prepared; everything else is left
+  // untouched. When omitted, the whole unlocked population is prepared.
+  accountIds?: AccountId[]
 }): Promise<PreparePrimaryCashWalletCutoverResult | RepositoryError> => {
-  const discoveries = await discoverCashWalletCutoverAccounts({
+  const allDiscoveries = await discoverCashWalletCutoverAccounts({
     accountsRepo,
     walletsRepo,
   })
-  if (discoveries instanceof Error) return discoveries
+  if (allDiscoveries instanceof Error) return allDiscoveries
+
+  let discoveries = allDiscoveries
+  let cohortNotFound: AccountId[] | undefined
+  if (accountIds !== undefined) {
+    const requested = new Set<string>(accountIds)
+    discoveries = allDiscoveries.filter((d) => requested.has(d.accountId))
+    const found = new Set<string>(discoveries.map((d) => d.accountId))
+    cohortNotFound = accountIds.filter((id) => !found.has(id))
+  }
 
   const report = buildCashWalletCutoverPreflightReport({
     cutoverVersion,
@@ -44,7 +61,7 @@ export const preparePrimaryCashWalletCutover = async ({
   })
 
   if (!report.canStart) {
-    return { report, plannedMigrations: [], migrations: [] }
+    return { report, plannedMigrations: [], migrations: [], cohortNotFound }
   }
 
   const plannedMigrations = buildPrimaryCashWalletMigrationPlan({
@@ -59,5 +76,5 @@ export const preparePrimaryCashWalletCutover = async ({
   })
   if (migrations instanceof Error) return migrations
 
-  return { report, plannedMigrations, migrations }
+  return { report, plannedMigrations, migrations, cohortNotFound }
 }
diff --git a/src/scripts/cash-wallet-cutover.ts b/src/scripts/cash-wallet-cutover.ts
index 0a45500f7..cae95d1a7 100644
--- a/src/scripts/cash-wallet-cutover.ts
+++ b/src/scripts/cash-wallet-cutover.ts
@@ -57,6 +57,10 @@ const args = yargs(hideBin(process.argv))
   .option("max-provision-attempts", { type: "number", default: 5 })
   .option("dry-run", { type: "boolean", default: false })
   .option("account-id", { type: "string" })
+  .option("account-ids", {
+    type: "string",
+    describe: "comma-separated accountIds — prepare only this cohort (phased cutover)",
+  })
   .option("reason", { type: "string", default: "" })
   .option("lock-stale-seconds", { type: "number", default: 300 })
   .option("configPath", { type: "string", demandOption: true })
@@ -115,12 +119,19 @@ const run = async () => {
     }
 
     case "prepare": {
+      const cohort = args["account-ids"]
+        ? (args["account-ids"]
+            .split(",")
+            .map((s) => s.trim())
+            .filter(Boolean) as AccountId[])
+        : undefined
       const result = await CashWalletCutover.preparePrimaryCashWalletCutover({
         cutoverVersion,
         runId,
         accountsRepo: AccountsRepository(),
         walletsRepo: WalletsRepository(),
         migrationsRepo: repository,
+        accountIds: cohort,
       })
       if (result instanceof Error) throw result
       toJson(result)
diff --git a/test/flash/unit/app/cash-wallet-cutover/prepare-cohort.spec.ts b/test/flash/unit/app/cash-wallet-cutover/prepare-cohort.spec.ts
new file mode 100644
index 000000000..b8fe40e1e
--- /dev/null
+++ b/test/flash/unit/app/cash-wallet-cutover/prepare-cohort.spec.ts
@@ -0,0 +1,84 @@
+const discoverMock = jest.fn()
+jest.mock("@app/cash-wallet-cutover/discovery", () => ({
+  discoverCashWalletCutoverAccounts: (...args: unknown[]) => discoverMock(...args),
+}))
+
+const upsertMock = jest.fn()
+jest.mock("@app/cash-wallet-cutover/migration-records", () => ({
+  upsertPrimaryCashWalletMigrationRecords: (args: { plans: unknown[] }) => {
+    upsertMock(args)
+    // echo plans back as "migrations" so prepare returns cleanly
+    return Promise.resolve(args.plans)
+  },
+}))
+
+import { preparePrimaryCashWalletCutover } from "@app/cash-wallet-cutover/prepare"
+
+const legacyDefault = (id: string) => ({
+  status: "legacy_default" as const,
+  accountId: id as AccountId,
+  legacyUsdWalletId: `${id}-usd` as WalletId,
+  destinationUsdtWalletId: `${id}-usdt` as WalletId,
+  previousDefaultWalletId: `${id}-usd` as WalletId,
+})
+
+const repos = {
+  accountsRepo: {} as never,
+  walletsRepo: {} as never,
+  migrationsRepo: {} as never,
+}
+
+describe("preparePrimaryCashWalletCutover — cohort filter (phased cutover)", () => {
+  beforeEach(() => {
+    jest.clearAllMocks()
+    discoverMock.mockResolvedValue([
+      legacyDefault("acc-1"),
+      legacyDefault("acc-2"),
+      legacyDefault("acc-3"),
+    ])
+  })
+
+  it("prepares the whole population when no cohort is given", async () => {
+    const result = await preparePrimaryCashWalletCutover({
+      cutoverVersion: 1,
+      runId: "run-1",
+      ...repos,
+    })
+    if (result instanceof Error) throw result
+    expect(result.plannedMigrations.map((m) => m.accountId).sort()).toEqual([
+      "acc-1",
+      "acc-2",
+      "acc-3",
+    ])
+    expect(result.cohortNotFound).toBeUndefined()
+  })
+
+  it("prepares only the requested cohort, leaving the rest untouched", async () => {
+    const result = await preparePrimaryCashWalletCutover({
+      cutoverVersion: 1,
+      runId: "run-1",
+      accountIds: ["acc-1", "acc-3"] as AccountId[],
+      ...repos,
+    })
+    if (result instanceof Error) throw result
+    expect(result.plannedMigrations.map((m) => m.accountId).sort()).toEqual([
+      "acc-1",
+      "acc-3",
+    ])
+    expect(result.cohortNotFound).toEqual([])
+  })
+
+  it("reports requested ids that were not found among unlocked accounts", async () => {
+    const result = await preparePrimaryCashWalletCutover({
+      cutoverVersion: 1,
+      runId: "run-1",
+      accountIds: ["acc-2", "acc-missing"] as AccountId[],
+      ...repos,
+    })
+    if (result instanceof Error) throw result
+    expect(result.plannedMigrations.map((m) => m.accountId)).toEqual(["acc-2"])
+    expect(result.cohortNotFound).toEqual(["acc-missing"])
+    // the preflight report is scoped to the cohort, not the full population
+    expect(result.report.totalAccounts).toBe(1)
+  })
+})