diff --git a/.changeset/adr-0024-sso-domain-verification.md b/.changeset/adr-0024-sso-domain-verification.md
new file mode 100644
index 0000000000..ad3119afbd
--- /dev/null
+++ b/.changeset/adr-0024-sso-domain-verification.md
@@ -0,0 +1,26 @@
+---
+"@objectstack/plugin-auth": minor
+"@objectstack/platform-objects": minor
+---
+
+feat(auth): opt-in SSO domain verification (ADR-0024 ②)
+
+Add DNS-TXT domain-ownership verification for external SSO providers, gated
+behind a new `OS_SSO_DOMAIN_VERIFICATION` flag (off by default — today's
+register→login behavior is unchanged). When enabled, `@better-auth/sso` mounts
+`/sso/request-domain-verification` + `/sso/verify-domain` and enforces that a
+provider's email domain be DNS-verified before it may complete a login.
+
+- `auth-manager.ts`: new `ssoDomainVerification` enabled-flag (readBooleanEnv) →
+  passes `domainVerification: { enabled: true }` to `sso()`; public
+  `isSsoDomainVerificationEnabled()` helper.
+- `register-sso-provider.ts`: `runRequestDomainVerification` /
+  `runVerifyDomain` bridges — re-dispatch through the gated better-auth
+  endpoints and reshape the response into the `{ success, data }` envelope the
+  `sys_sso_provider` action `resultDialog` reads (request → ready-to-paste DNS
+  TXT record; verify → clear success/error). A bare 404 from the inner endpoint
+  is surfaced as "not enabled for this environment".
+- `auth-plugin.ts`: mount the two bridges as rawApp routes
+  (`/admin/sso/{request-domain-verification,verify-domain}`).
+- `sys_sso_provider`: `domain_verified` field + list column + the two actions;
+  `domainVerified` documented in `AUTH_SSO_PROVIDER_SCHEMA`.
diff --git a/packages/platform-objects/src/identity/sys-sso-provider.object.ts b/packages/platform-objects/src/identity/sys-sso-provider.object.ts
index f376709d36..b6c601699a 100644
--- a/packages/platform-objects/src/identity/sys-sso-provider.object.ts
+++ b/packages/platform-objects/src/identity/sys-sso-provider.object.ts
@@ -119,6 +119,56 @@ export const SysSsoProvider = ObjectSchema.create({
         { name: 'identifierFormat', label: 'NameID Format', type: 'text', required: false, placeholder: 'urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress', helpText: 'Optional. Requested SAML NameID format. Defaults to the IdP’s configured format.' },
       ],
     },
+    {
+      name: 'request_domain_verification',
+      label: 'Request Domain Verification',
+      icon: 'globe',
+      variant: 'secondary',
+      locations: ['list_item', 'record_header'],
+      type: 'api',
+      method: 'POST',
+      // ADR-0024 ② (opt-in OS_SSO_DOMAIN_VERIFICATION). Asks @better-auth/sso
+      // for a one-time DNS-TXT challenge and reveals the ready-to-paste record
+      // ONCE via `resultDialog`. Routed through the env bridge (plugin-auth /
+      // cloud AuthProxyPlugin) which reshapes the `{domainVerificationToken}`
+      // response into the `{ data: { dnsRecordName, dnsRecordValue } }` envelope
+      // the dialog reads. When the feature is OFF the bridge returns a clear
+      // "not enabled for this environment" error instead of a bare 404.
+      target: '/api/v1/auth/admin/sso/request-domain-verification',
+      params: [
+        { name: 'providerId', field: 'provider_id', defaultFromRow: true, required: true },
+        { name: 'domain', field: 'domain', defaultFromRow: true, required: false },
+      ],
+      resultDialog: {
+        title: 'Verify your domain',
+        description:
+          'Add the DNS TXT record below at your domain’s DNS provider, then run “Verify Domain”. The token is shown once.',
+        acknowledge: 'Done',
+        fields: [
+          { path: 'data.dnsRecordType', label: 'Record type', format: 'text' },
+          { path: 'data.dnsRecordName', label: 'Name / Host', format: 'secret' },
+          { path: 'data.dnsRecordValue', label: 'Value', format: 'secret' },
+        ],
+      },
+    },
+    {
+      name: 'verify_domain',
+      label: 'Verify Domain',
+      icon: 'shield-check',
+      variant: 'secondary',
+      locations: ['list_item', 'record_header'],
+      type: 'api',
+      method: 'POST',
+      // ADR-0024 ②. Re-checks the DNS-TXT record and flips `domain_verified`
+      // on success. Routed through the env bridge, which maps @better-auth/sso's
+      // empty 204 / 502 into a clear success/error toast.
+      target: '/api/v1/auth/admin/sso/verify-domain',
+      successMessage: 'Domain ownership verified',
+      refreshAfter: true,
+      params: [
+        { name: 'providerId', field: 'provider_id', defaultFromRow: true, required: true },
+      ],
+    },
     {
       name: 'delete_sso_provider',
       label: 'Delete SSO Provider',
@@ -144,7 +194,7 @@ export const SysSsoProvider = ObjectSchema.create({
       name: 'all',
       label: 'All',
       data: { provider: 'object', object: 'sys_sso_provider' },
-      columns: ['provider_id', 'issuer', 'domain', 'created_at'],
+      columns: ['provider_id', 'issuer', 'domain', 'domain_verified', 'created_at'],
       sort: [{ field: 'provider_id', order: 'asc' }],
       pagination: { pageSize: 50 },
       // Per-object empty state — the shared identity-object copy ("created
@@ -186,6 +236,15 @@ export const SysSsoProvider = ObjectSchema.create({
       group: 'Identity',
     }),
 
+    domain_verified: Field.boolean({
+      label: 'Domain Verified',
+      defaultValue: false,
+      readonly: true,
+      description:
+        'Whether DNS ownership of the email domain has been proven (ADR-0024 ②). Set by “Verify Domain” after the DNS TXT record resolves. Managed by better-auth — not directly editable. Only enforced when domain verification is enabled for the environment.',
+      group: 'Identity',
+    }),
+
     oidc_config: Field.textarea({
       label: 'OIDC Config',
       required: false,
diff --git a/packages/plugins/plugin-auth/src/auth-manager.ts b/packages/plugins/plugin-auth/src/auth-manager.ts
index 1086c99408..4c332edd9d 100644
--- a/packages/plugins/plugin-auth/src/auth-manager.ts
+++ b/packages/plugins/plugin-auth/src/auth-manager.ts
@@ -1101,6 +1101,15 @@ export class AuthManager {
     const oidcFromEnv = readBooleanEnv('OS_OIDC_PROVIDER_ENABLED');
     const ssoFromEnv = readBooleanEnv('OS_SSO_ENABLED');
     const scimFromEnv = readBooleanEnv('OS_SCIM_ENABLED');
+    // Opt-in DNS domain-verification for external SSO providers (ADR-0024 ②).
+    // OFF by default → today's behavior exactly (register → login immediately).
+    // ON → @better-auth/sso mounts /sso/{request-domain-verification,verify-domain}
+    // AND enforces a HARD login gate: a provider whose domain is not DNS-verified
+    // rejects logins ("Provider domain has not been verified"). The two are
+    // coupled in @better-auth/sso (the endpoints only register when
+    // `domainVerification.enabled`), so this single flag governs both. Requires
+    // `OS_SSO_ENABLED` (the sso plugin must be loaded to honor it).
+    const ssoDomainVerifyFromEnv = readBooleanEnv('OS_SSO_DOMAIN_VERIFICATION');
     // @better-auth/scim's `active:false` → ban runs through the admin plugin,
     // and org-scoped tokens need the organization plugin — so enabling SCIM
     // forces `admin` on (organization already defaults on). See ADR-0071.
@@ -1117,6 +1126,7 @@ export class AuthManager {
       deviceAuthorization: pluginConfig.deviceAuthorization ?? false,
       admin: pluginConfig.admin ?? scimEffective,
       sso: ssoFromEnv ?? (pluginConfig as any).sso ?? false,
+      ssoDomainVerification: ssoDomainVerifyFromEnv ?? (pluginConfig as any).ssoDomainVerification ?? false,
       scim: scimEffective,
     };
 
@@ -1482,8 +1492,15 @@ export class AuthManager {
       // an explicit default role (belt-and-suspenders over SecurityPlugin's
       // `member_default` fallback, which already grants baseline access to any
       // authenticated user). Requires the `organization` plugin — on by default.
+      // `domainVerification.enabled` (ADR-0024 ②, opt-in via OS_SSO_DOMAIN_VERIFICATION):
+      // when on, @better-auth/sso mounts /sso/request-domain-verification +
+      // /sso/verify-domain (DNS TXT proof-of-ownership) AND enforces that an
+      // external IdP's email domain be DNS-verified before it may complete a
+      // login — preventing an org admin from registering a provider for a domain
+      // they don't control. Off by default to preserve the register→login flow.
       plugins.push(sso({
         organizationProvisioning: { defaultRole: 'member' },
+        ...(enabled.ssoDomainVerification ? { domainVerification: { enabled: true } } : {}),
       }));
     }
 
@@ -2052,6 +2069,21 @@ export class AuthManager {
     return ssoFromEnv ?? (this.config.plugins as any)?.sso ?? false;
   }
 
+  /**
+   * Whether opt-in DNS domain-verification (ADR-0024 ②) is wired — i.e. the
+   * `/sso/request-domain-verification` + `/sso/verify-domain` endpoints are
+   * mounted (and the hard "domain must be verified to log in" gate is active).
+   * Resolved with the EXACT logic `buildPluginList` uses for the `sso()`
+   * `domainVerification.enabled` option, so the bridge can return a clear
+   * "not enabled for this environment" instead of a bare 404 when off.
+   * Implies `isSsoWired()` (the sso plugin must be loaded to honor it).
+   */
+  public isSsoDomainVerificationEnabled(): boolean {
+    if (!this.isSsoWired()) return false;
+    const fromEnv = readBooleanEnv('OS_SSO_DOMAIN_VERIFICATION');
+    return fromEnv ?? (this.config.plugins as any)?.ssoDomainVerification ?? false;
+  }
+
   /**
    * Whether enterprise SSO is actually *usable*, not merely wired: the plugin
    * is on AND at least one `sys_sso_provider` row exists. Per-email domain→IdP
diff --git a/packages/plugins/plugin-auth/src/auth-plugin.ts b/packages/plugins/plugin-auth/src/auth-plugin.ts
index 9089e3f525..f92b365ec8 100644
--- a/packages/plugins/plugin-auth/src/auth-plugin.ts
+++ b/packages/plugins/plugin-auth/src/auth-plugin.ts
@@ -12,7 +12,7 @@ import {
 import { SysOrganizationDetailPage, SysUserDetailPage } from '@objectstack/platform-objects/pages';
 import { AuthManager, type AuthManagerOptions } from './auth-manager.js';
 import { runSetInitialPassword } from './set-initial-password.js';
-import { runRegisterSsoProviderFromForm, runRegisterSamlProviderFromForm } from './register-sso-provider.js';
+import { runRegisterSsoProviderFromForm, runRegisterSamlProviderFromForm, runRequestDomainVerification, runVerifyDomain } from './register-sso-provider.js';
 import {
   authIdentityObjects,
   authPluginManifestHeader,
@@ -1084,6 +1084,41 @@ export class AuthPlugin implements Plugin {
       }
     });
 
+    // ────────────────────────────────────────────────────────────────────
+    // SSO domain verification (ADR-0024 ②, opt-in OS_SSO_DOMAIN_VERIFICATION).
+    // Re-dispatch through @better-auth/sso's /sso/{request-domain-verification,
+    // verify-domain} (so the per-provider admin gate runs) and reshape into the
+    // `{ success, data }` envelope the action `resultDialog` / toast reads:
+    // request returns the ready-to-paste DNS TXT record, verify returns a clear
+    // success/error. A 404 from the inner endpoint = feature OFF for this env.
+    rawApp.post(`${basePath}/admin/sso/request-domain-verification`, async (c: any) => {
+      try {
+        const { status, body } = await runRequestDomainVerification(
+          (req) => this.authManager!.handleRequest(req),
+          c.req.raw,
+        );
+        return c.json(body, status as any);
+      } catch (error) {
+        const err = error instanceof Error ? error : new Error(String(error));
+        ctx.logger.error('[AuthPlugin] sso/request-domain-verification bridge failed', err);
+        return c.json({ success: false, error: { code: 'internal', message: err.message } }, 500);
+      }
+    });
+
+    rawApp.post(`${basePath}/admin/sso/verify-domain`, async (c: any) => {
+      try {
+        const { status, body } = await runVerifyDomain(
+          (req) => this.authManager!.handleRequest(req),
+          c.req.raw,
+        );
+        return c.json(body, status as any);
+      } catch (error) {
+        const err = error instanceof Error ? error : new Error(String(error));
+        ctx.logger.error('[AuthPlugin] sso/verify-domain bridge failed', err);
+        return c.json({ success: false, error: { code: 'internal', message: err.message } }, 500);
+      }
+    });
+
     // ────────────────────────────────────────────────────────────────────
     // OAuth self-service: register an OAuth application for the signed-in
     // user. Thin wrapper over better-auth's `/oauth2/create-client`
diff --git a/packages/plugins/plugin-auth/src/auth-schema-config.ts b/packages/plugins/plugin-auth/src/auth-schema-config.ts
index b6a4fe51b9..db982ba0db 100644
--- a/packages/plugins/plugin-auth/src/auth-schema-config.ts
+++ b/packages/plugins/plugin-auth/src/auth-schema-config.ts
@@ -679,6 +679,12 @@ export const AUTH_SSO_PROVIDER_SCHEMA = {
     samlConfig: 'saml_config',
     userId: 'user_id',
     organizationId: 'organization_id',
+    // DNS domain-ownership proof (ADR-0024 ②). @better-auth/sso writes
+    // `domainVerified` on its `ssoProvider` model when domain verification is
+    // enabled; map it so the env can surface a verified/unverified badge. The
+    // one-time `domainVerificationToken` is NOT a provider column — it lives in
+    // the verification table and is returned only from request-domain-verification.
+    domainVerified: 'domain_verified',
   },
 } as const;
 
diff --git a/packages/plugins/plugin-auth/src/register-sso-provider.ts b/packages/plugins/plugin-auth/src/register-sso-provider.ts
index 3fddbf2c6b..7fff6655f7 100644
--- a/packages/plugins/plugin-auth/src/register-sso-provider.ts
+++ b/packages/plugins/plugin-auth/src/register-sso-provider.ts
@@ -263,3 +263,156 @@ export async function runRegisterSamlProviderFromForm(
   }
   return { status: 200, body: { success: true, data: { providerId: parsed?.providerId ?? providerId }, acsUrl, spMetadataUrl } };
 }
+
+
+// ── Domain verification (ADR-0024 ②, opt-in OS_SSO_DOMAIN_VERIFICATION) ──────
+//
+// `@better-auth/sso` proves an external IdP's email DOMAIN is controlled by the
+// registrant via a DNS-TXT challenge, mounted ONLY when `domainVerification` is
+// enabled on `sso()`. The two endpoints are:
+//   • POST /sso/request-domain-verification {providerId} → 201 {domainVerificationToken}
+//   • POST /sso/verify-domain {providerId}               → 204 (or 502 if the TXT
+//     record is absent / not yet propagated)
+// The token alone is not actionable — the admin needs the full DNS record
+// (name `_better-auth-token-<providerId>.<domain>`, value
+// `_better-auth-token-<providerId>=<token>`; the prefix is @better-auth/sso's
+// default `tokenPrefix`, which we do not override). These bridges re-dispatch
+// through the real endpoints (so the per-provider admin gate runs) and reshape
+// the response into the `{ success, data }` envelope the action `resultDialog`
+// reads — request returns the ready-to-paste DNS record; verify returns a
+// friendly success/error message. A `404` from the inner endpoint means the
+// feature is OFF for this env (endpoints unmounted) → surfaced as such, not a
+// bare "not found".
+
+/** @better-auth/sso default verification token prefix (we don't override `tokenPrefix`). */
+const SSO_DOMAIN_TOKEN_PREFIX = 'better-auth-token';
+
+/**
+ * Strip protocol / path / port so `https://acme.com/` → `acme.com` for the DNS
+ * record. Regex-free on purpose — `domain` is request-controlled input, so a
+ * backtracking pattern here would be a ReDoS vector (CodeQL js/polynomial-redos).
+ */
+function bareHostname(domain: string): string {
+  let d = domain.trim();
+  if (!d) return d;
+  const schemeIdx = d.indexOf('://');
+  if (schemeIdx !== -1) {
+    try {
+      return new URL(d).hostname;
+    } catch {
+      d = d.slice(schemeIdx + 3); // malformed URL — drop the scheme and strip manually
+    }
+  }
+  // Truncate at the first path / port / query / fragment separator.
+  for (const sep of ['/', ':', '?', '#']) {
+    const i = d.indexOf(sep);
+    if (i !== -1) d = d.slice(0, i);
+  }
+  return d;
+}
+
+function rewriteSsoAdminUrl(request: Request, fromSuffix: RegExp, toPath: string): { innerUrl: string; origin: string } | null {
+  try {
+    const url = new URL(request.url);
+    return { origin: url.origin, innerUrl: `${url.origin}${url.pathname.replace(fromSuffix, toPath)}` };
+  } catch {
+    return null;
+  }
+}
+
+function forwardAuthHeaders(request: Request, origin: string): Headers {
+  const headers = new Headers({ 'content-type': 'application/json' });
+  const cookie = request.headers.get('cookie');
+  if (cookie) headers.set('cookie', cookie);
+  const authz = request.headers.get('authorization');
+  if (authz) headers.set('authorization', authz);
+  headers.set('origin', request.headers.get('origin') || origin);
+  return headers;
+}
+
+/**
+ * Request a DNS-TXT domain-verification challenge for a registered provider and
+ * return the ready-to-paste DNS record (for a one-shot `resultDialog`).
+ *
+ * Body: `{ providerId, domain? }` (domain only shapes the displayed record name).
+ */
+export async function runRequestDomainVerification(
+  handle: AuthRequestHandler,
+  request: Request,
+): Promise<RegisterSsoFormResult & { body: RegisterSsoFormResult['body'] & { data?: any } }> {
+  let body: any;
+  try { body = await request.json(); } catch { body = {}; }
+  const str = (v: unknown): string => (typeof v === 'string' ? v.trim() : '');
+  const providerId = str(body?.providerId);
+  const domain = bareHostname(str(body?.domain));
+  if (!providerId) {
+    return { status: 400, body: { success: false, error: { code: 'invalid_request', message: 'Missing required field: providerId' } } };
+  }
+
+  const rw = rewriteSsoAdminUrl(request, /\/admin\/sso\/request-domain-verification$/, '/sso/request-domain-verification');
+  if (!rw) return { status: 400, body: { success: false, error: { code: 'invalid_request', message: 'Bad request URL' } } };
+  const headers = forwardAuthHeaders(request, rw.origin);
+
+  const resp = await handle(new Request(rw.innerUrl, { method: 'POST', headers, body: JSON.stringify({ providerId }) }));
+  let parsed: any = {};
+  try { const t = await resp.text(); parsed = t ? JSON.parse(t) : {}; } catch { parsed = {}; }
+  if (!resp.ok) {
+    if (resp.status === 404 && !parsed?.code) {
+      return { status: 400, body: { success: false, error: { code: 'domain_verification_disabled', message: 'Domain verification is not enabled for this environment (set OS_SSO_DOMAIN_VERIFICATION).' } } };
+    }
+    return { status: resp.status, body: { success: false, error: { code: parsed?.code || 'request_domain_verification_failed', message: parsed?.message || 'Failed to request domain verification' } } };
+  }
+
+  const token = str(parsed?.domainVerificationToken);
+  const label = `_${SSO_DOMAIN_TOKEN_PREFIX}-${providerId}`;
+  const dnsRecordName = domain ? `${label}.${domain}` : label;
+  const dnsRecordValue = `${label}=${token}`;
+  return {
+    status: 200,
+    body: {
+      success: true,
+      data: { providerId, domain, token, dnsRecordType: 'TXT', dnsRecordName, dnsRecordValue },
+    },
+  };
+}
+
+/**
+ * Verify a provider's domain ownership (re-checks the DNS-TXT record). Reshapes
+ * @better-auth/sso's empty `204` / `502` into a `{ success, data:{ message } }`
+ * envelope so the action surfaces a clear toast.
+ *
+ * Body: `{ providerId }`.
+ */
+export async function runVerifyDomain(
+  handle: AuthRequestHandler,
+  request: Request,
+): Promise<RegisterSsoFormResult & { body: RegisterSsoFormResult['body'] & { data?: any } }> {
+  let body: any;
+  try { body = await request.json(); } catch { body = {}; }
+  const str = (v: unknown): string => (typeof v === 'string' ? v.trim() : '');
+  const providerId = str(body?.providerId);
+  if (!providerId) {
+    return { status: 400, body: { success: false, error: { code: 'invalid_request', message: 'Missing required field: providerId' } } };
+  }
+
+  const rw = rewriteSsoAdminUrl(request, /\/admin\/sso\/verify-domain$/, '/sso/verify-domain');
+  if (!rw) return { status: 400, body: { success: false, error: { code: 'invalid_request', message: 'Bad request URL' } } };
+  const headers = forwardAuthHeaders(request, rw.origin);
+
+  const resp = await handle(new Request(rw.innerUrl, { method: 'POST', headers, body: JSON.stringify({ providerId }) }));
+  let parsed: any = {};
+  try { const t = await resp.text(); parsed = t ? JSON.parse(t) : {}; } catch { parsed = {}; }
+  if (resp.ok) {
+    return { status: 200, body: { success: true, data: { providerId, verified: true, message: 'Domain ownership verified — this provider can now sign users in.' } } };
+  }
+  // Friendlier copy for the expected failure modes.
+  let message = parsed?.message || 'Domain verification failed';
+  if (resp.status === 404 && !parsed?.code) {
+    message = 'Domain verification is not enabled for this environment (set OS_SSO_DOMAIN_VERIFICATION).';
+  } else if (parsed?.code === 'NO_PENDING_VERIFICATION') {
+    message = 'No pending verification — click “Request Domain Verification” first to get the DNS record.';
+  } else if (parsed?.code === 'DOMAIN_VERIFICATION_FAILED') {
+    message = 'DNS TXT record not found yet. Add the record shown when you requested verification, allow time for DNS to propagate, then retry.';
+  }
+  return { status: resp.status, body: { success: false, error: { code: parsed?.code || 'verify_domain_failed', message } } };
+}