fix: Prefer the token_endpoint_auth_method response from DCR registration

src/client/auth.test.ts

@@ -10,7 +10,8 @@ import {
     discoverOAuthProtectedResourceMetadata,
     extractResourceMetadataUrl,
     auth,
-    type OAuthClientProvider
+    type OAuthClientProvider,
+    selectClientAuthMethod
 } from './auth.js';
 import { ServerError } from '../server/auth/errors.js';
 import { AuthorizationServerMetadata } from '../shared/auth.js';
@@ -881,6 +882,25 @@ describe('OAuth Authorization', () => {
         });
     });
 
+    describe('selectClientAuthMethod', () => {
+        it('selects the correct client authentication method from client information', () => {
+            const clientInfo = {
+                client_id: 'test-client-id',
+                client_secret: 'test-client-secret',
+                token_endpoint_auth_method: 'client_secret_basic'
+            };
+            const supportedMethods = ['client_secret_post', 'client_secret_basic', 'none'];
+            const authMethod = selectClientAuthMethod(clientInfo, supportedMethods);
+            expect(authMethod).toBe('client_secret_basic');
+        });
+        it('selects the correct client authentication method from supported methods', () => {
+            const clientInfo = { client_id: 'test-client-id' };
+            const supportedMethods = ['client_secret_post', 'client_secret_basic', 'none'];
+            const authMethod = selectClientAuthMethod(clientInfo, supportedMethods);
+            expect(authMethod).toBe('none');
+        });
+    });
+
     describe('startAuthorization', () => {
         const validMetadata = {
             issuer: 'https://auth.example.com',

src/client/auth.ts

@@ -3,6 +3,7 @@ import { LATEST_PROTOCOL_VERSION } from '../types.js';
 import {
     OAuthClientMetadata,
     OAuthClientInformation,
+    OAuthClientInformationMixed,
     OAuthTokens,
     OAuthMetadata,
     OAuthClientInformationFull,
@@ -56,7 +57,7 @@ export interface OAuthClientProvider {
      * server, or returns `undefined` if the client is not registered with the
      * server.
      */
-    clientInformation(): OAuthClientInformation | undefined | Promise<OAuthClientInformation | undefined>;
+    clientInformation(): OAuthClientInformationMixed | undefined | Promise<OAuthClientInformationMixed | undefined>;
 
     /**
      * If implemented, this permits the OAuth client to dynamically register with
@@ -66,7 +67,7 @@ export interface OAuthClientProvider {
      * This method is not required to be implemented if client information is
      * statically known (e.g., pre-registered).
      */
-    saveClientInformation?(clientInformation: OAuthClientInformationFull): void | Promise<void>;
+    saveClientInformation?(clientInformation: OAuthClientInformationMixed): void | Promise<void>;
 
     /**
      * Loads any existing OAuth tokens for the current session, or returns
@@ -149,6 +150,10 @@ export class UnauthorizedError extends Error {
 
 type ClientAuthMethod = 'client_secret_basic' | 'client_secret_post' | 'none';
 
+function isClientAuthMethod(method: string): method is ClientAuthMethod {
+    return ['client_secret_basic', 'client_secret_post', 'none'].includes(method);
+}
+
 const AUTHORIZATION_CODE_RESPONSE_TYPE = 'code';
 const AUTHORIZATION_CODE_CHALLENGE_METHOD = 'S256';
 
@@ -164,14 +169,24 @@ const AUTHORIZATION_CODE_CHALLENGE_METHOD = 'S256';
  * @param supportedMethods - Authentication methods supported by the authorization server
  * @returns The selected authentication method
  */
-function selectClientAuthMethod(clientInformation: OAuthClientInformation, supportedMethods: string[]): ClientAuthMethod {
+export function selectClientAuthMethod(clientInformation: OAuthClientInformationMixed, supportedMethods: string[]): ClientAuthMethod {
     const hasClientSecret = clientInformation.client_secret !== undefined;
 
     // If server doesn't specify supported methods, use RFC 6749 defaults
     if (supportedMethods.length === 0) {
         return hasClientSecret ? 'client_secret_post' : 'none';
     }
 
+    // Prefer the method returned by the server during client registration if valid and supported
+    if (
+        'token_endpoint_auth_method' in clientInformation &&
+        clientInformation.token_endpoint_auth_method &&
+        isClientAuthMethod(clientInformation.token_endpoint_auth_method) &&
+        supportedMethods.includes(clientInformation.token_endpoint_auth_method)
+    ) {
+        return clientInformation.token_endpoint_auth_method;
+    }
+
     // Try methods in priority order (most secure first)
     if (hasClientSecret && supportedMethods.includes('client_secret_basic')) {
         return 'client_secret_basic';
@@ -793,7 +808,7 @@ export async function startAuthorization(
         resource
     }: {
         metadata?: AuthorizationServerMetadata;
-        clientInformation: OAuthClientInformation;
+        clientInformation: OAuthClientInformationMixed;
         redirectUrl: string | URL;
         scope?: string;
         state?: string;
@@ -876,7 +891,7 @@ export async function exchangeAuthorization(
         fetchFn
     }: {
         metadata?: AuthorizationServerMetadata;
-        clientInformation: OAuthClientInformation;
+        clientInformation: OAuthClientInformationMixed;
         authorizationCode: string;
         codeVerifier: string;
         redirectUri: string | URL;
@@ -955,7 +970,7 @@ export async function refreshAuthorization(
         fetchFn
     }: {
         metadata?: AuthorizationServerMetadata;
-        clientInformation: OAuthClientInformation;
+        clientInformation: OAuthClientInformationMixed;
         refreshToken: string;
         resource?: URL;
         addClientAuthentication?: OAuthClientProvider['addClientAuthentication'];

src/examples/client/simpleOAuthClient.ts

@@ -6,7 +6,7 @@ import { URL } from 'node:url';
 import { exec } from 'node:child_process';
 import { Client } from '../../client/index.js';
 import { StreamableHTTPClientTransport } from '../../client/streamableHttp.js';
-import { OAuthClientInformation, OAuthClientInformationFull, OAuthClientMetadata, OAuthTokens } from '../../shared/auth.js';
+import { OAuthClientInformationMixed, OAuthClientMetadata, OAuthTokens } from '../../shared/auth.js';
 import { CallToolRequest, ListToolsRequest, CallToolResultSchema, ListToolsResultSchema } from '../../types.js';
 import { OAuthClientProvider, UnauthorizedError } from '../../client/auth.js';
 
@@ -20,7 +20,7 @@ const CALLBACK_URL = `http://localhost:${CALLBACK_PORT}/callback`;
  * In production, you should persist tokens securely
  */
 class InMemoryOAuthClientProvider implements OAuthClientProvider {
-    private _clientInformation?: OAuthClientInformationFull;
+    private _clientInformation?: OAuthClientInformationMixed;
     private _tokens?: OAuthTokens;
     private _codeVerifier?: string;
 
@@ -46,11 +46,11 @@ class InMemoryOAuthClientProvider implements OAuthClientProvider {
         return this._clientMetadata;
     }
 
-    clientInformation(): OAuthClientInformation | undefined {
+    clientInformation(): OAuthClientInformationMixed | undefined {
         return this._clientInformation;
     }
 
-    saveClientInformation(clientInformation: OAuthClientInformationFull): void {
+    saveClientInformation(clientInformation: OAuthClientInformationMixed): void {
         this._clientInformation = clientInformation;
     }
 

src/shared/auth.ts

@@ -226,6 +226,7 @@ export type OAuthErrorResponse = z.infer<typeof OAuthErrorResponseSchema>;
 export type OAuthClientMetadata = z.infer<typeof OAuthClientMetadataSchema>;
 export type OAuthClientInformation = z.infer<typeof OAuthClientInformationSchema>;
 export type OAuthClientInformationFull = z.infer<typeof OAuthClientInformationFullSchema>;
+export type OAuthClientInformationMixed = OAuthClientInformation | OAuthClientInformationFull;
 export type OAuthClientRegistrationError = z.infer<typeof OAuthClientRegistrationErrorSchema>;
 export type OAuthTokenRevocationRequest = z.infer<typeof OAuthTokenRevocationRequestSchema>;
 export type OAuthProtectedResourceMetadata = z.infer<typeof OAuthProtectedResourceMetadataSchema>;