When connecting via Claude Code's HTTP MCP transport, the connection fails with:

HTTP 404: Invalid OAuth error response: SyntaxError: JSON Parse error: Unexpected identifier "Not". Raw body: Not Found

Root cause

Claude Code's HTTP MCP client performs OAuth 2.0 discovery before connecting, by hitting:

GET https://mcp.data.gouv.fr/.well-known/oauth-authorization-server

This endpoint returns a plain-text Not Found (HTTP 404). Claude Code then tries to parse that as a JSON OAuth error response, fails, and aborts the connection.

Expected behavior

The server should either:

• Return a valid JSON response at /.well-known/oauth-authorization-server indicating no authentication is required, or
• Return a properly-formatted JSON error (e.g. {"error": "..."}) so the client can handle it gracefully

Workaround

None currently

Environment

• Client: Claude Code
• Configured via: claude mcp add --transport http datagouv https://mcp.data.gouv.fr/mcp
• The MCP endpoint itself responds correctly (verified via curl with appropriate headers)