make client side client_id generation configurable in the oauth router (#734)

src/server/auth/clients.ts

@@ -16,5 +16,5 @@ export interface OAuthRegisteredClientsStore {
    * 
    * If unimplemented, dynamic client registration is unsupported.
    */
-  registerClient?(client: OAuthClientInformationFull): OAuthClientInformationFull | Promise<OAuthClientInformationFull>;
+  registerClient?(client: Omit<OAuthClientInformationFull, "client_id">): OAuthClientInformationFull | Promise<OAuthClientInformationFull>;
 }

src/server/auth/handlers/register.test.ts

@@ -218,6 +218,26 @@ describe('Client Registration Handler', () => {
       expect(response.body.client_secret_expires_at).toBe(0);
     });
 
+    it('sets no client_id when clientIdGeneration=false', async () => {
+      // Create handler with no expiry
+      const customApp = express();
+      const options: ClientRegistrationHandlerOptions = {
+        clientsStore: mockClientStoreWithRegistration,
+        clientIdGeneration: false
+      };
+
+      customApp.use('/register', clientRegistrationHandler(options));
+
+      const response = await supertest(customApp)
+        .post('/register')
+        .send({
+          redirect_uris: ['https://example.com/callback']
+        });
+
+      expect(response.status).toBe(201);
+      expect(response.body.client_id).toBeUndefined();
+    });
+
     it('handles client with all metadata fields', async () => {
       const fullClientMetadata: OAuthClientMetadata = {
         redirect_uris: ['https://example.com/callback'],

src/server/auth/handlers/register.ts

@@ -31,6 +31,13 @@ export type ClientRegistrationHandlerOptions = {
    * Registration endpoints are particularly sensitive to abuse and should be rate limited.
    */
   rateLimit?: Partial<RateLimitOptions> | false;
+
+  /**
+   * Whether to generate a client ID before calling the client registration endpoint.
+   *
+   * If not set, defaults to true.
+   */
+  clientIdGeneration?: boolean;
 };
 
 const DEFAULT_CLIENT_SECRET_EXPIRY_SECONDS = 30 * 24 * 60 * 60; // 30 days
@@ -38,7 +45,8 @@ const DEFAULT_CLIENT_SECRET_EXPIRY_SECONDS = 30 * 24 * 60 * 60; // 30 days
 export function clientRegistrationHandler({
   clientsStore,
   clientSecretExpirySeconds = DEFAULT_CLIENT_SECRET_EXPIRY_SECONDS,
-  rateLimit: rateLimitConfig
+  rateLimit: rateLimitConfig,
+  clientIdGeneration = true,
 }: ClientRegistrationHandlerOptions): RequestHandler {
   if (!clientsStore.registerClient) {
     throw new Error("Client registration store does not support registering clients");
@@ -78,7 +86,6 @@ export function clientRegistrationHandler({
       const isPublicClient = clientMetadata.token_endpoint_auth_method === 'none'
 
       // Generate client credentials
-      const clientId = crypto.randomUUID();
       const clientSecret = isPublicClient
         ? undefined
         : crypto.randomBytes(32).toString('hex');
@@ -89,14 +96,17 @@ export function clientRegistrationHandler({
       const secretExpiryTime = clientsDoExpire ? clientIdIssuedAt + clientSecretExpirySeconds : 0
       const clientSecretExpiresAt = isPublicClient ? undefined : secretExpiryTime
 
-      let clientInfo: OAuthClientInformationFull = {
+      let clientInfo: Omit<OAuthClientInformationFull, "client_id"> & { client_id?: string } = {
         ...clientMetadata,
-        client_id: clientId,
         client_secret: clientSecret,
         client_id_issued_at: clientIdIssuedAt,
         client_secret_expires_at: clientSecretExpiresAt,
       };
 
+      if (clientIdGeneration) {
+        clientInfo.client_id = crypto.randomUUID();
+      }
+
       clientInfo = await clientsStore.registerClient!(clientInfo);
       res.status(201).json(clientInfo);
     } catch (error) {

src/server/auth/router.ts

@@ -142,7 +142,7 @@ export function mcpAuthRouter(options: AuthRouterOptions): RequestHandler {
       new URL(oauthMetadata.registration_endpoint).pathname,
       clientRegistrationHandler({
         clientsStore: options.provider.clientsStore,
-        ...options,
+        ...options.clientRegistrationOptions,
       })
     );
   }