Stdio and HTTP for MCP Servers

I rewrote my MCP server to use Stdio input and output instead of HTTP. An MCP server that uses Stdio still receives and returns JSON-formatted messages, but no HTTP connection is made. Instead, a client (like llama-cpp) uses a sub-process to interact with the MCP server.

Stdio MCP Server

In this sense, a Stdio MCP server is not a real "server" but is just a regular console program. In any case, I used the Rust crate RMcp, which allowed me to replace 300 lines of code with about 60 lines of code.

With RMcp, it is possible to generate most of the necessary code with just Rust derive attributes.

In llama-cpp, Stdio MCP server support was added just a couple days ago. To add an Stdio MCP server, use the mcp-servers-json or mcp-servers-config arguments to llama-server, and specify a JSON object that defines mcpServers. This is the mcp.json file format. The MCP server tools will appear in the built-in tools in llama-cpp's web UI.