Message Observer

The McpObserver trait provides a non-blocking hook for intercepting all MCP messages flowing through a server or client.

use std::sync::Arc;
use rust_mcp_sdk::{
    McpObserver,
    mcp_server::{McpServerOptions, server_runtime},
    schema::schema_utils::{ClientMessage, ServerMessage},
};

struct SimpleObserver;

impl McpObserver<ClientMessage, ServerMessage> for SimpleObserver {
    fn on_receive(&self, message: &ClientMessage) {
        println!("→ received: {message:?}");
    }

    fn on_send(&self, message: &ServerMessage) {
        println!("← sent: {message:?}");
    }
}

Both callbacks are synchronous and run in the critical message path - keep them fast (a few milliseconds at most) and non-blocking. For slow work such as network calls, spawn a task instead.

Attach the observer during server creation:

let server = server_runtime::create_server(McpServerOptions {
    server_details,
    transport,
    handler: handler.to_mcp_server_handler(),
    task_store: None,
    client_task_store: None,
    // stored as Arc<dyn McpObserver<ClientMessage, ServerMessage>>
    message_observer: Some(Arc::new(SimpleObserver)),
});

See example implementations in examples/common/server_observer.rs and client_observer.rs.