[MCPB]: Add a Claude Desktop bundle for the MCP server

[lloydwatkin]

Claude Code can talk to the server over HTTP directly, but Claude Desktop cannot — its remote connector UI has no way to send an API token header, so Desktop users had no route in at all.

This adds an MCPB bundle under mcpb/: a .mcpb file that installs into Claude Desktop with a double-click. Inside is a dependency-free Node script that speaks stdio to Desktop and forwards every message verbatim to the server over HTTPS with the token attached — so it understands nothing about MCP and gains no capabilities of its own. The tools a user sees are exactly the ones their account can already reach in admin.

Claude Desktop  ──stdio──▶  mcpb proxy  ──HTTPS + token──▶  Rails app

Three user_config fields (server URL, API token, auth header name) keep the bundle generic, so anyone using the gem can pack their own. The token is stored in the OS keychain and revocable from the existing MCP Tokens page.

Packs to a 2.9 kB, three-file bundle with no node_modules.

Testing Notes

Automated: cd mcpb && npm test — 13 tests spawning the proxy as a real child process against a stub HTTP server: header injection (default and custom), chunk-split and multi-message stdin, the 204 notification case, unreachable server, non-JSON error bodies, parse errors, and missing configuration. CI runs these and attaches a packed bundle as an artifact.

Manual — happy path:

1. cd mcpb && npx @anthropic-ai/mcpb pack . ../activeadmin-mcp.mcpb
2. Generate a token from admin → MCP Tokens.
3. Double-click the .mcpb, fill in the server URL, token, and (for Olio) X-MCP-Authorization as the auth header.
4. New chat → ask Claude to list the admin resources. Expect the resources your account can read.

Verified end to end against production, which uses the custom header:

$ printf '%s\n%s\n%s\n' '{"jsonrpc":"2.0","id":1,"method":"initialize",...}' \
    '{"jsonrpc":"2.0","method":"notifications/initialized"}' \
    '{"jsonrpc":"2.0","id":2,"method":"tools/list"}' | node server/index.js

id 1 {"protocolVersion": "2025-06-18", "serverInfo": {"name": "activeadmin-mcp", "version": "0.0.3"}, ...}
id 2 tools: ['list_resources', 'query', 'update']

The notification correctly produced no reply.

Error cases:

• Wrong token → {"jsonrpc":"2.0","id":1,"error":{"code":-32603,"message":"The server rejected the API token (HTTP 401). Generate a new token in the admin panel and update the extension's settings."}} — confirmed against production.
• Unreachable URL → a -32603 error naming the URL, rather than Desktop hanging forever on a reply that never comes.
• A notification that fails logs to stderr instead of replying, since JSON-RPC has no id to reply to.

README: new "Claude Desktop (MCPB bundle)" section covering how it works, building, installing, revoking access, and troubleshooting.

🤖 Generated with Claude Code