"Couldn't reach the MCP server" via Claude.ai web connector — requests never reach server (ofid references)

Description

When adding a remote MCP server via Claude.ai Settings → Integrations → Custom MCP, the connection always fails with "Couldn't reach the MCP server". Requests never reach the server (confirmed via server logs — zero inbound traffic during connection attempts).

Reference IDs

• ofid_c27fabdbf36c7d91
• ofid_24d939aad42a1bf8

Server details

• Transport: Streamable HTTP (MCP spec 2025-11-05)
• OAuth 2.1 + PKCE with Dynamic Client Registration
• /.well-known/oauth-protected-resource ✓
• /.well-known/oauth-authorization-server ✓
• WWW-Authenticate: Bearer realm=..., resource_metadata=... ✓
• Health endpoint returns HTTP 200 from external curl ✓
• CORS headers present on all responses ✓

Evidence that server is not at fault

• curl from external IPs → HTTP 200 /health ✓
• Claude Code CLI (local STDIO) → works perfectly, 165 tools loaded ✓
• Zero server-side logs during Claude.ai web connector attempts → requests never arrive

Expected behavior

Claude.ai web connector should forward requests to the MCP server and trigger OAuth discovery flow.

Actual behavior

Connection fails before reaching the server. The ofid reference IDs suggest failure in Anthropic's relay infrastructure.

Related issues

Possibly related to #227, #214, #246 in modelcontextprotocol/modelcontextprotocol