agnt-mcp

Synchronous Model Context Protocol stdio client for the agnt agent runtime. Spawns an MCP server as a child process, runs the 2024-11-05 initialize handshake, and bridges every remote tool into the native agnt_core::Tool trait.

use std::sync::{Arc, Mutex};
use agnt_core::Tool;
use agnt_mcp::{McpClient, McpTool};

# fn main() -> Result<(), Box<dyn std::error::Error>> {
let mut client = McpClient::start("mcp-server-filesystem", &["/tmp"])?;
let tools = client.list_tools()?;
let shared = Arc::new(Mutex::new(client));

let bridged: Vec<Box<dyn Tool>> = tools
    .into_iter()
    .map(|info| Box::new(McpTool::new(Arc::clone(&shared), info)) as Box<dyn Tool>)
    .collect();

// Register each `bridged` tool on your Agent's Registry as usual.
# Ok(()) }

Design notes

• No async runtime. The stdout reader runs in a dedicated std::thread, draining JSON-RPC frames into an mpsc::Receiver. Requests time out at 30 seconds (REQUEST_TIMEOUT).
• Bounded reader. v0.3.1 caps the inner reader at MAX_LINE_BYTES (4 MiB) so a hostile or buggy server cannot OOM the agent with a multi-gigabyte line. Overflow surfaces as McpError::Protocol and closes the stream.
• Typed errors. McpError splits into Io, Protocol, Timeout, and Closed so callers can tell DoS from protocol bugs from an exited child.
• Off by default in the flagship crate. cargo add agnt --features mcp pulls this in; without the feature the flagship stays lean.

See the flagship agnt crate for the agent runtime this plugs into.

License

Dual-licensed under MIT OR Apache-2.0.