feat(oauth): PKCE exemption for allowlisted confidential clients (Copilot Studio) - #884
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Power Platform connectors (Microsoft Copilot Studio) run a plain OAuth 2.0 authorization code flow with a client secret and cannot send PKCE, so the S256 requirement on /authorize locked them out. OAUTH_PKCE_EXEMPT_CLIENT_IDS names the clients that may omit the challenge; the exemption only applies while the registration is confidential (client_secret_basic/post with a stored secret), so the token endpoint still refuses the code without the secret. Unlisted clients, including every DCR client, are unchanged and never cause a database lookup.
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Power Platform connectors, which is what a Microsoft Copilot Studio certified MCP connector is, use a plain OAuth 2.0 authorization code flow with a client secret and cannot send PKCE. Our /authorize requires S256, so such a connector could never sign in.
OAUTH_PKCE_EXEMPT_CLIENT_IDS(comma-separated) lists clients that may omit the challenge.client_secret_basic/client_secret_postwith a stored secret. The token endpoint then refuses the code without the secret, and the code stays bound to the registered redirect URI.docker-compose.cloud.yml(default empty) and documented in.env.example.Tests: 9 new cases in
authorize-pkce.middleware.spec.ts;src/auth+src/mcp-server563/563 green.