Open-sourcing cimd-proxy: connect MCP clients to any OIDC provider by URL alone

DerMozart · reddit · 2026-09-09

After wiring MCP auth against Claude, ChatGPT, Mistral and Grok, the author open-sourced cimd-proxy (Apache-2.0). Key findings: Claude needs both CIMD flags advertised or it silently falls back; ChatGPT ships non-draft fields that break strict parsers; Mistral re-registers per connection; Grok uses path-inserted metadata URLs; Keycloak's CIMD support is experimental and lacks RFC 8707 resource indicators required by MCP. The proxy presents an OAuth 2.1 facade, federates to your existing provider, never reads tokens (relay verbatim, refresh via sealed envelope, no server-side session), and allowlists metadata hosts against SSRF.

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