fix(mcp-worker): restore dynamic client registration endpoint - #581

Summary

• Re-adds clientRegistrationEndpoint: '/oauth/register' to the remote MCP worker's OAuthProvider config, reverting Remove unused registration endpoint. #577.
• Adds a strict Content-Security-Policy to the OAuth consent page so client-supplied metadata can never lead to script execution.

The Problem

#577 removed the registration endpoint as unused. It wasn't: @cloudflare/workers-oauth-provider implements RFC 7591 Dynamic Client Registration automatically whenever clientRegistrationEndpoint is set, and DCR is the only registration mechanism our server offers. Removing it dropped registration_endpoint from /.well-known/oauth-authorization-server and left /oauth/register returning 404.

MCP clients try pre-registered credentials, then CIMD, then DCR (spec). We support none of the first two, so clients fall through to DCR, find no registration_endpoint, and the MCP TypeScript SDK throws from registerClient():

SDK auth failed: Incompatible auth server: does not support dynamic client registration

That's the error users hit on claude mcp add --transport http devcycle https://mcp.devcycle.com/mcp (and npx mcp-remote) since #577 landed. The Claude Code MCP docs call it out by name.

Confirmed against production:

$ curl -s https://mcp.devcycle.com/.well-known/oauth-authorization-server
{... "client_id_metadata_document_supported":false}   # no registration_endpoint

$ curl -s -o /dev/null -w '%{http_code}' -X POST https://mcp.devcycle.com/oauth/register
404

Securing the consent page

DCR lets any client register a client_name / logo_uri, which the consent page reflects on devcycle.com. Rather than rely on output escaping alone, this makes script execution impossible there:

Content-Security-Policy: script-src 'none'; object-src 'none'; base-uri 'none'; frame-ancestors 'none'

The page has no first-party JavaScript (static HTML, inline <style>, a <form>), so script-src 'none' costs nothing and blocks inline <script>, injected onerror/onload handlers, and mutation-XSS variants, independently of escaping. Verified locally by registering a malicious client_name and loading the consent page: header present, page renders normally, name emitted as inert escaped text.

Related

• Reverts Remove unused registration endpoint. #577