Claude web rejects /mcp after successful OAuth and authenticated tools/list (ofid_4779c952258aa88e)

### MCP server URL
`https://mis-mcp-server.onrender.com/mcp`

### Reference ID
`ofid_4779c952258aa88e`

### Client and symptom
Claude web completed account authorization, then displayed: “no MCP server was found at the provided URL, or your account doesn’t have access to it.”

### Server-side isolation
The exact production revision was healthy and ready. A fresh DCR/PKCE flow using Claude’s documented hosted callback (`https://claude.ai/api/mcp/auth_callback`) completed successfully against production: public client registration, authorization code exchange, 250 ms token response, audience-bound bearer and refresh tokens, authenticated MCP initialize (`2025-03-26`), and authenticated `tools/list` returning 17 tools. The MCP URL and OAuth resource are exactly lowercase with no trailing slash. There are no cross-host redirects; protected-resource and authorization-server metadata return 200.

The server intentionally returns 404 for `POST /`; MCP transport is only at the published `/mcp` path. This resembles #806, although this server hosts its authorization server on the same origin. Please use the reference ID to confirm which transport URL Claude requested after token exchange and whether stale connector URL state was involved.

### Reconnect procedure attempted/requested
Remove the failed custom connector record, add a new Web custom connector with exactly `https://mis-mcp-server.onrender.com/mcp` (no advanced client credentials), authorize, then connect/enable it under Customize → Connectors. If it still fails, capture the new `ofid_` because reference IDs are time-limited.