Set a preferred auth method on the client registration

Author: kyle-leonhard

client/src/lib/oauth-state-machine.ts

@@ -1,17 +1,17 @@
-import { OAuthStep, AuthDebuggerState } from "./auth-types";
-import { DebugInspectorOAuthClientProvider } from "./auth";
 import {
   discoverOAuthMetadata,
-  registerClient,
-  startAuthorization,
-  exchangeAuthorization,
   discoverOAuthProtectedResourceMetadata,
+  exchangeAuthorization,
+  registerClient,
   selectResourceURL,
+  startAuthorization,
 } from "@modelcontextprotocol/sdk/client/auth.js";
 import {
   OAuthMetadataSchema,
   OAuthProtectedResourceMetadata,
 } from "@modelcontextprotocol/sdk/shared/auth.js";
+import { DebugInspectorOAuthClientProvider } from "./auth";
+import { AuthDebuggerState, OAuthStep } from "./auth-types";
 
 export interface StateMachineContext {
   state: AuthDebuggerState;
@@ -88,6 +88,16 @@ export const oauthTransitions: Record<OAuthStep, StateTransition> = {
         clientMetadata.scope = scopesSupported.join(" ");
       }
 
+      // Select a preferred auth method
+      const authMethodsSupported =
+        metadata.token_endpoint_auth_methods_supported;
+      for (const authMethod of ["client_secret_basic", "client_secret_post"]) {
+        if (authMethodsSupported && authMethodsSupported.includes(authMethod)) {
+          clientMetadata.token_endpoint_auth_method = authMethod;
+          break;
+        }
+      }
+
       const fullInformation = await registerClient(context.serverUrl, {
         metadata,
         clientMetadata,