Claude Desktop (Electron, >= 0.13.x) and claude.ai web interface fail to connect to custom MCP servers using OAuth 2.1/SSE/Streamable HTTP, while Claude Code CLI connects successfully to the same server. The GitHub issue has 71+ reactions, 66 comments, was closed as completed by Anthropic staff. Multiple root causes confirmed: WAF blocking Anthropic outbound IPs, missing WWW-Authenticate header on 401 responses, CORS misconfiguration on .well-known endpoints, and DCR format issues (float client_secret_expires_at rejected by RFC 7591 integer parser, token_endpoint_auth_method: client_secret_post rejected by better-auth). Complete fix kit below with copyable config examples and verification curl commands.

A binding/secret is undefined inside a Cloudflare Worker.