fix(mcp): the OAuth proxy advertises and returns token auth method none

The proxy issues no client secrets, but its authorization server metadata
named no token_endpoint_auth_methods_supported (RFC 8414 then implies
client_secret_basic) and /oauth/register echoed a requested secret-based
method. A client following that default registered with client_secret_basic
and then failed the token exchange ("client_secret_basic authentication
requires a client_secret"), reproduced with the SDK calls a self-hosted chat
client makes. The metadata now lists ["none"] and registration always answers
none (RFC 7591 lets the server replace a requested value).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/McpServer/autopilot-monitor-mcp/src/__tests__/oauth-register.test.ts

@@ -84,6 +84,12 @@ describe('/.well-known/oauth-authorization-server — RFC 8414 metadata', () =>
     expect(json.issuer).toBe(baseUrl);
     expect(json.authorization_response_iss_parameter_supported).toBe(true);
   });
+
+  it('advertises public clients only, so clients never pick a secret-based token auth method', async () => {
+    const res = await fetch(`${baseUrl}/.well-known/oauth-authorization-server`);
+    const json = (await res.json()) as Record<string, unknown>;
+    expect(json.token_endpoint_auth_methods_supported).toEqual(['none']);
+  });
 });
 
 describe('/oauth/register — dynamic client registration (RFC 7591)', () => {
@@ -99,6 +105,19 @@ describe('/oauth/register — dynamic client registration (RFC 7591)', () => {
     expect(json.token_endpoint_auth_method).toBe('none');
   });
 
+  it('answers a requested secret-based auth method with none and issues no secret', async () => {
+    // A self-hosted client following the RFC 8414 default registers with client_secret_basic;
+    // echoing it back made the client demand a secret at the token endpoint.
+    const { status, json } = await register({
+      client_name: 'Self-hosted client',
+      redirect_uris: ['http://localhost:3080/api/mcp/example/oauth/callback'],
+      token_endpoint_auth_method: 'client_secret_basic',
+    });
+    expect(status).toBe(201);
+    expect(json.token_endpoint_auth_method).toBe('none');
+    expect(json.client_secret).toBeUndefined();
+  });
+
   it('registers a VS Code client (loopback + vscode.dev redirect) — GitHub Copilot DCR', async () => {
     const { status, json } = await register({
       client_name: 'Visual Studio Code',

src/McpServer/autopilot-monitor-mcp/src/oauth.ts

@@ -581,6 +581,10 @@ export function createOAuthRouter(): Router {
       response_types_supported: ['code'],
       grant_types_supported: ['authorization_code', 'refresh_token'],
       code_challenge_methods_supported: ['S256'],
+      // Every client of this proxy is public (PKCE); it issues no client secrets. Without this
+      // field RFC 8414 implies client_secret_basic, and clients that follow it (LibreChat) register
+      // with a secret method and then fail the token exchange for lack of a secret.
+      token_endpoint_auth_methods_supported: ['none'],
       // MCP spec 2026-07-28 client registration: clients that support Client ID
       // Metadata Documents pick them over dynamic registration when this flag
       // is present (cimd.ts resolves the document at /oauth/authorize).
@@ -602,7 +606,7 @@ export function createOAuthRouter(): Router {
   // registration itself, an org-wide auth DoS. The .well-known discovery docs
   // stay unthrottled (static JSON, no outbound work, no logging).
   router.post('/oauth/register', oauthRateLimit, (req, res) => {
-    const { client_name, redirect_uris, grant_types, response_types, token_endpoint_auth_method } = req.body ?? {};
+    const { client_name, redirect_uris, grant_types, response_types } = req.body ?? {};
 
     // Field-level bounds — backstop the route-level body-size limit. The
     // parser caps total body bytes; these caps stop a single registration
@@ -671,7 +675,10 @@ export function createOAuthRouter(): Router {
       redirect_uris: uris,
       grant_types: grant_types ?? ['authorization_code', 'refresh_token'],
       response_types: response_types ?? ['code'],
-      token_endpoint_auth_method: token_endpoint_auth_method ?? 'none',
+      // Always 'none', whatever was requested: no secret is ever issued (RFC 7591 §3.2.1 lets the
+      // server replace a requested value). Echoing a secret-based method made SDK clients demand a
+      // client_secret at the token endpoint that does not exist.
+      token_endpoint_auth_method: 'none',
     });
   });