Module mcp 

Model Context Protocol (MCP) server for grr.

Hand-rolled JSON-RPC 2.0 over stdio — no SDK crate: a tool-only server needs four RPCs. Newline-delimited requests in, one response line per request out, logs to stderr ONLY (stdout is the MCP stream, the same discipline as the rest of grr).

• server/discover — the stateless revision’s discovery RPC: revisions served, capabilities, identity, and it needs no prior call
• tools/list — every method in the Discovery index as a tool
• tools/call — one method invocation through the shared call path

2026-07-28 is the newest revision served, so the handshake is gone from the happy path — but initialize is answered with a minimal legacy handshake (server/discover remains the discovery RPC), because the deployed client ecosystem still opens with it. ping and logging/setLevel answer with a JSON-RPC error naming the revision to speak instead. Notifications (notifications/initialized, whatever the revision) carry no id, so they are never answered at all.

Tool names are the dotted method ids grr api call takes (gmail.users.messages.list), so an agent reaches every method Google publishes through the same resolution the CLI uses. Errors never kill the server: a tool failure is an isError result (MCP’s tool-error contract), a protocol failure a JSON-RPC error object.

{"jsonrpc":"2.0","id":1,"method":"server/discover"}
{"jsonrpc":"2.0","id":2,"method":"tools/list"}
{"jsonrpc":"2.0","id":3,"method":"tools/call",
 "params":{"name":"gmail.users.messages.list","arguments":{"userId":"me","dry_run":true}}}

Structs

McpServer

One server instance: one credential, one safety profile, every tool. The stdio loop is single-tasked, so no Arc is needed.

Functions

serve

The stdio loop: newline-delimited JSON-RPC in, one response line per request out. Generic over the reader/writer so tests can run the whole handshake in-process. EOF on stdin (the client closed the session) is a clean shutdown.