diff --git a/server/src/app.ts b/server/src/app.ts
index 85e06990..38e70c36 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -108,6 +108,7 @@ export function createApp(): express.Application {
         req.path.startsWith('/.well-known/') ||
         req.path === '/oauth/register' ||
         req.path === '/oauth/authorize' ||
+        req.path === '/oauth/userinfo' ||
         req.path === '/mcp'
       ) {
         cors({ origin: '*', credentials: false })(req, _res, next);
@@ -424,10 +425,15 @@ export function createApp(): express.Application {
   });
 
   // ChatGPT (and other OIDC-first clients) bootstrap OAuth discovery via
-  // /.well-known/openid-configuration. Serve the same typed AS metadata so
-  // they can find registration_endpoint, authorization_endpoint, token_endpoint.
+  // /.well-known/openid-configuration. Serve the AS metadata plus the OIDC
+  // userinfo_endpoint so ChatGPT can fetch the authenticated user's email
+  // for authorization domain claiming.
   app.get('/.well-known/openid-configuration', (_req: Request, res: Response) => {
-    res.json(getOAuthMetadata());
+    const meta = getOAuthMetadata();
+    res.json({
+      ...meta,
+      userinfo_endpoint: `${meta.issuer}/oauth/userinfo`,
+    });
   });
 
   // SDK authorize handler: validates OAuth params, calls provider.authorize() which redirects
diff --git a/server/src/routes/oauth.ts b/server/src/routes/oauth.ts
index 5f70d1b3..91558ff0 100644
--- a/server/src/routes/oauth.ts
+++ b/server/src/routes/oauth.ts
@@ -20,6 +20,7 @@ import {
   rotateOAuthClientSecret,
   listOAuthSessions,
   revokeSession,
+  getUserByAccessToken,
   AuthorizeParams,
 } from '../services/oauthService';
 import { writeAudit, getClientIp, logWarn } from '../services/auditLog';
@@ -153,6 +154,29 @@ oauthPublicRouter.post('/oauth/token', tokenLimiter, (req: Request, res: Respons
   return res.status(400).json({ error: 'unsupported_grant_type', error_description: `Unsupported grant_type: ${grant_type}` });
 });
 
+// OIDC UserInfo endpoint (RFC 9068 / OpenID Connect Core §5.3)
+// ChatGPT hits this after OAuth to fetch the authenticated user's email for domain claiming.
+oauthPublicRouter.get('/oauth/userinfo', (req: Request, res: Response) => {
+  if (!isAddonEnabled(ADDON_IDS.MCP)) return res.status(404).end();
+  const auth = req.headers['authorization'];
+  if (!auth || !auth.toLowerCase().startsWith('bearer ')) {
+    res.set('WWW-Authenticate', 'Bearer realm="TREK MCP"');
+    return res.status(401).json({ error: 'invalid_token' });
+  }
+  const token = auth.slice(7);
+  const info = getUserByAccessToken(token);
+  if (!info) {
+    res.set('WWW-Authenticate', 'Bearer realm="TREK MCP", error="invalid_token"');
+    return res.status(401).json({ error: 'invalid_token' });
+  }
+  return res.json({
+    sub:            String(info.user.id),
+    email:          info.user.email,
+    email_verified: true,
+    preferred_username: info.user.username,
+  });
+});
+
 // Token revocation endpoint (RFC 7009)
 oauthPublicRouter.post('/oauth/revoke', revokeLimiter, (req: Request, res: Response) => {
   if (!isAddonEnabled(ADDON_IDS.MCP)) return res.status(404).end();