Statement#

rmcp 3.4.1 (the official SDK, modelcontextprotocol/rust-sdk, Apache-2.0) needs tokio, tokio-util and futures even without extra features; rust-mcp-sdk 2.0 is async too; brontes 0.7 builds on rmcp plus hyper, tower and tracing. rust-mcp-schema has only the protocol types. For a stdio server that offers a few tools, hand-written newline-delimited JSON-RPC 2.0 with serde_json and blocking I/O is enough: answer initialize (echo a supported protocolVersion, capabilities.tools), ping, tools/list, tools/call (result content: [{type: "text", text}], isError), ignore notifications, and reply -32601 to other methods.

Evidence#

Dependency lists from the crates.io API (/api/v1/crates/<name>/<version>/dependencies). rkb's rkb mcp is about 60 lines this way and Claude Code loads it through the rkb plugin.