diff --git a/packages/services/service-settings/src/manifests/sms.manifest.ts b/packages/services/service-settings/src/manifests/sms.manifest.ts
index 2bcbe2b56a..de29fedf34 100644
--- a/packages/services/service-settings/src/manifests/sms.manifest.ts
+++ b/packages/services/service-settings/src/manifests/sms.manifest.ts
@@ -21,6 +21,26 @@ const manifest = {
     { type: 'group', id: 'provider', label: 'Provider', required: false,
       description: 'Choose how this workspace sends outbound SMS.' },
 
+    // ⚠️ This options table is a CONTRACT, not a menu of aspirations: every
+    // value here must be one `@objectstack/service-sms` can actually build a
+    // transport for, and every transport it can build must appear here. The two
+    // sets are held equal by an executable assertion —
+    // `packages/services/service-sms/src/sms-manifest-providers.contract.test.ts`
+    // compares these values against `SMS_TRANSPORT_PROVIDERS` / `makeSmsTransport`
+    // and goes red the moment they diverge, in either direction, naming the
+    // value that drifted.
+    //
+    // Adding an option without a transport is not a harmless placeholder: the
+    // form validates, the save succeeds, and every send is silently downgraded
+    // to `LogSmsTransport` — a workspace that reports `status: 'sent'` and
+    // delivers nothing. That is exactly what mail shipped before #5094
+    // (`sendgrid` / `ses` offered with nothing behind them, while `resend` had a
+    // working transport nobody could pick), and what `os serve` did with an
+    // out-of-table `OS_SMS_PROVIDER` before #5713.
+    //
+    // `log` is listed and labelled for what it does. It is the one option that
+    // does not deliver, but it does not *pretend* to — which is what makes
+    // "offered" and "deliverable" the same set rather than merely overlapping.
     { type: 'select', key: 'provider', label: 'Provider', required: true, default: 'log',
       options: [
         { value: 'log', label: 'None (log only — no real delivery)' },
diff --git a/packages/services/service-sms/package.json b/packages/services/service-sms/package.json
index 3f68486298..9304c1ecc6 100644
--- a/packages/services/service-sms/package.json
+++ b/packages/services/service-sms/package.json
@@ -22,6 +22,7 @@
     "@objectstack/spec": "workspace:*"
   },
   "devDependencies": {
+    "@objectstack/service-settings": "workspace:*",
     "@types/node": "^26.1.2",
     "typescript": "^6.0.3",
     "vitest": "^4.1.10"
diff --git a/packages/services/service-sms/src/sms-manifest-providers.contract.test.ts b/packages/services/service-sms/src/sms-manifest-providers.contract.test.ts
new file mode 100644
index 0000000000..81fee5c241
--- /dev/null
+++ b/packages/services/service-sms/src/sms-manifest-providers.contract.test.ts
@@ -0,0 +1,203 @@
+// Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// The `sms` settings dropdown ↔ this package's transports (#5773).
+//
+// The invariant, made executable: **every provider an admin can select must be
+// one this package can actually deliver through, and every provider this
+// package can deliver through must be selectable.** Two halves of one fact, and
+// the mail side has already paid for both of them coming apart (#5094):
+// `sendgrid` / `ses` were offered by `mail.manifest.ts` with no transport
+// behind them — the form validated, the save succeeded, and no mail was ever
+// sent — while `resend`, which had a working transport all along, could not be
+// picked at all.
+//
+// The sms sides are, today, exactly equal. That is the whole reason this file
+// exists: nothing was holding them equal. `sms.manifest.ts` carried one literal
+// list and `transports/index.ts` carried another, maintained independently, and
+// a drift in either direction would have shipped in silence — an admin picking
+// a provider that logs instead of sending (the #5713 failure, from the settings
+// face rather than the `OS_SMS_PROVIDER` face), or a working transport nobody
+// can reach.
+//
+// This is deliberately a CROSS-PACKAGE assertion rather than a third mirrored
+// literal. A list pinned inside this package can be "fixed" by editing the
+// other list, which is precisely how two hand-maintained copies drift.
+// `@objectstack/service-settings` is therefore a **devDependency** here —
+// test-only, no runtime edge, and no cycle: service-settings' dependency
+// closure (core, metadata-core, platform-objects, spec, types) does not contain
+// this package. Same shape and same direction as the mail precedent, where
+// `plugin-email` (the transport owner) holds
+// `mail-manifest-providers.contract.test.ts` and devDepends on the settings
+// service. The registry of manifests must not have to know every provider
+// implementation; the provider proves it satisfies the published contract.
+
+import { describe, it, expect } from 'vitest';
+import { smsSettingsManifest } from '@objectstack/service-settings';
+import {
+  makeSmsTransport,
+  SMS_TRANSPORT_PROVIDERS,
+  isSmsTransportProvider,
+  type MakeSmsTransportOptions,
+} from './transports/index.js';
+import { LogSmsTransport } from './sms-service.js';
+import { AliyunSmsTransport } from './transports/aliyun.js';
+import { TwilioSmsTransport } from './transports/twilio.js';
+
+/**
+ * The `provider` specifier, read off the shipped manifest.
+ *
+ * Every read in this file is scoped to `key === 'provider'` on purpose: the
+ * `sms` namespace is expected to grow keys that have nothing to do with the
+ * transport vocabulary (#2814 adds `daily_quota` / `daily_quota_per_tenant`
+ * number specifiers to this same manifest). Those carry no `options` table and
+ * a different key, so they never enter this assertion surface — adding them
+ * neither needs a change here nor can break these tests.
+ */
+function providerSpecifier(): Record<string, any> {
+  const spec = (smsSettingsManifest.specifiers as Array<Record<string, any>>).find(
+    (s) => s.key === 'provider',
+  );
+  expect(spec, 'sms manifest must declare a `provider` specifier').toBeDefined();
+  return spec!;
+}
+
+/** The `provider` select's option values. */
+function providerOptions(): string[] {
+  const options = providerSpecifier().options as Array<{ value: string }> | undefined;
+  expect(options, 'the `provider` specifier must declare an `options` table').toBeDefined();
+  return options!.map((o) => o.value);
+}
+
+/**
+ * Per-provider fixture: the minimal credentials that let the provider be built
+ * for real, and the transport class it must build INTO.
+ *
+ * Keeping the expected class beside the build args (rather than as a row of
+ * standalone `toBeInstanceOf` assertions) is what stops this file from becoming
+ * the third hand-maintained copy of the vocabulary it exists to police: the
+ * table's key set is asserted equal to `SMS_TRANSPORT_PROVIDERS` below, so a
+ * provider added to the vocabulary without a fixture fails here instead of
+ * quietly going unexercised.
+ */
+const PROVIDER_FIXTURES: Record<
+  string,
+  { args: MakeSmsTransportOptions; transport: abstract new (...args: any[]) => unknown }
+> = {
+  log: { args: { provider: 'log' }, transport: LogSmsTransport },
+  aliyun: {
+    args: {
+      provider: 'aliyun',
+      options: { accessKeyId: 'LTAI_test', accessKeySecret: 'test-secret', signName: 'ObjectStack' },
+    },
+    transport: AliyunSmsTransport,
+  },
+  twilio: {
+    args: {
+      provider: 'twilio',
+      options: { accountSid: 'AC_test', authToken: 'test-token', from: '+15005550006' },
+    },
+    transport: TwilioSmsTransport,
+  },
+};
+
+describe('sms settings dropdown ↔ sms transports', () => {
+  // The two directions are separate `it`s so each drift fails on its own and
+  // names its own values, rather than one set-equality that reds for both and
+  // leaves the reader to work out which way it went.
+
+  it('offers no provider the transports cannot build (⊆)', () => {
+    const buildable: readonly string[] = SMS_TRANSPORT_PROVIDERS;
+    const orphanOptions = providerOptions().filter((p) => !buildable.includes(p));
+    expect(
+      orphanOptions,
+      `sms.manifest.ts offers provider(s) with no transport behind them: ${JSON.stringify(orphanOptions)}. `
+        + `An admin can select one, the form validates, the save succeeds — and every send is silently `
+        + `downgraded to LogSmsTransport (the #5094 defect shape). Either add a transport to `
+        + `SMS_TRANSPORT_PROVIDERS / makeSmsTransport, or drop the option.`,
+    ).toEqual([]);
+  });
+
+  it('hides no provider the transports can build (⊇)', () => {
+    const offered = providerOptions();
+    const unreachable = SMS_TRANSPORT_PROVIDERS.filter((p) => !offered.includes(p));
+    expect(
+      unreachable,
+      `SMS_TRANSPORT_PROVIDERS can deliver through provider(s) the settings page never lists: `
+        + `${JSON.stringify(unreachable)}. That is the 'resend' half of #5094 — a working transport `
+        + `nobody can pick. Add the option to the \`provider\` select in sms.manifest.ts.`,
+    ).toEqual([]);
+  });
+
+  it('lists each provider exactly once', () => {
+    // Set equality above cannot see a duplicated option value; the rendered
+    // dropdown would show the same provider twice.
+    const offered = providerOptions();
+    const duplicated = offered.filter((p, i) => offered.indexOf(p) !== i);
+    expect(duplicated, `duplicate option value(s) in the \`provider\` select: ${JSON.stringify(duplicated)}`)
+      .toEqual([]);
+  });
+
+  it('exercises every provider in the vocabulary — no fixture, no coverage', () => {
+    // Deliberately a RUNTIME assertion rather than typing the table as
+    // `Record<SmsProviderTag, …>`: this package's tsconfig excludes its own
+    // test files (its TEST_DEBT entry in scripts/check-type-check-coverage.mjs),
+    // so a type-level exhaustiveness check written here would be evaluated by
+    // no tsc program at all — green forever, deletable without a trace. That is
+    // the phantom-check shape #5286 is about; an assertion vitest runs is not.
+    expect(new Set(Object.keys(PROVIDER_FIXTURES))).toEqual(new Set(SMS_TRANSPORT_PROVIDERS));
+  });
+
+  it('builds the right transport class for every option the dropdown offers', () => {
+    for (const provider of providerOptions()) {
+      const fixture = PROVIDER_FIXTURES[provider];
+      expect(fixture, `no build recipe for offered provider '${provider}'`).toBeDefined();
+      // Not just "did not throw" — the right transport class per tag. A
+      // `switch` arm that fell through to the wrong constructor would pass a
+      // bare truthiness check and deliver through the wrong vendor.
+      expect(
+        makeSmsTransport(fixture!.args),
+        `provider '${provider}' must build a ${fixture!.transport.name}`,
+      ).toBeInstanceOf(fixture!.transport);
+    }
+  });
+
+  it('defaults to a provider that is both offered and buildable', () => {
+    const fallback = providerSpecifier().default;
+    // `log` is the one option that does not deliver — and it does not pretend
+    // to: the label says "no real delivery". SMS has no credential-free real
+    // transport, so the default has to be the visible opt-out rather than a
+    // vendor that would fail on the first send.
+    expect(fallback).toBe('log');
+    expect(providerOptions()).toContain(fallback);
+    expect(isSmsTransportProvider(fallback)).toBe(true);
+  });
+});
+
+describe('isSmsTransportProvider ↔ the same option list', () => {
+  // The CLI's start-up refusal of an out-of-table `OS_SMS_PROVIDER` (#5713,
+  // PR #5771) judges operator input with this predicate. It and the settings
+  // dropdown must be the same vocabulary, or the two configuration faces of one
+  // provider choice disagree about what is configurable.
+
+  it('accepts every provider the dropdown offers', () => {
+    for (const provider of providerOptions()) {
+      expect(isSmsTransportProvider(provider), `dropdown offers '${provider}' but the predicate refuses it`)
+        .toBe(true);
+    }
+  });
+
+  it('refuses values the dropdown does not offer', () => {
+    // `twilo` is the real typo #5713 opens with: it reached makeSmsTransport,
+    // threw, was caught, and became a LogSmsTransport that answered every OTP
+    // send `status: 'sent'`.
+    for (const bogus of ['twilo', 'aliyun_sms', 'sendgrid', '', 'LOG']) {
+      expect(isSmsTransportProvider(bogus), `'${bogus}' must not narrow to a provider tag`).toBe(false);
+      expect(providerOptions()).not.toContain(bogus);
+    }
+  });
+
+  it('refuses to build a provider outside the vocabulary', () => {
+    expect(() => makeSmsTransport({ provider: 'twilo' as never }))
+      .toThrow(/unknown provider 'twilo'/);
+  });
+});
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 76e03abda7..51faca1b17 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -2349,6 +2349,9 @@ importers:
         specifier: workspace:*
         version: link:../../spec
     devDependencies:
+      '@objectstack/service-settings':
+        specifier: workspace:*
+        version: link:../service-settings
       '@types/node':
         specifier: ^26.1.2
         version: 26.1.2
diff --git a/scripts/check-type-check-coverage.mjs b/scripts/check-type-check-coverage.mjs
index 7efe8b5f93..0efd84aca2 100644
--- a/scripts/check-type-check-coverage.mjs
+++ b/scripts/check-type-check-coverage.mjs
@@ -262,7 +262,7 @@ const TEST_DEBT = {
   '@objectstack/platform-objects': { tests: 8, errors: 3, note: 'TS2339 x2, TS7006 x1.' },
   '@objectstack/plugin-sharing': { tests: 11, errors: 3, note: 'TS6133 x2, TS18048 x1.' },
   '@objectstack/http-conformance': { tests: 2, errors: 1, note: 'TS2740 x1.' },
-  '@objectstack/service-sms': { tests: 3, errors: 1, note: 'TS2493 x1.' },
+  '@objectstack/service-sms': { tests: 4, errors: 1, note: 'TS2493 x1, in transports.test.ts. Re-measured at 4 files in #5773, which added sms-manifest-providers.contract.test.ts: the count moved, the error did not — the new file is type-clean with the exclusion lifted.' },
   '@objectstack/connector-rest': { tests: 3, errors: 1, note: 'TS6133 x1.' },
 };