SSE Transport

Server-Sent Events transport is supported for backward compatibility with older MCP clients.

Enable SSE on the Axum or Actix backend:

let server = create_axum_server(
    server_info,
    handler.to_mcp_server_handler(),
    AxumServerOptions {
        sse_support: true, // enables SSE alongside Streamable HTTP
        ..Default::default()
    },
);

SSE is enabled by default (true) for maximum compatibility.

Client-side SSE

use rust_mcp_sdk::{
    mcp_client::{client_runtime, McpClientOptions},
    schema::{ClientCapabilities, Implementation, InitializeRequestParams, ProtocolVersion},
    ClientSseTransport, ClientSseTransportOptions, ToMcpClientHandler,
};

// client name, version and capabilities
let client_details = InitializeRequestParams {
    capabilities: ClientCapabilities::default(),
    client_info: Implementation {
        name: "hello-client".into(),
        version: "0.1.0".into(),
        title: None,
        description: None,
        icons: vec![],
        website_url: None,
    },
    protocol_version: ProtocolVersion::V2025_11_25.into(),
    meta: None,
};

// Connect to the server's SSE endpoint
let transport = ClientSseTransport::new(
    "http://127.0.0.1:8080/sse",
    ClientSseTransportOptions {
        ..Default::default()
    },
)?;

// create the MCP client (`handler` implements `ClientHandler`)
let client = client_runtime::create_client(McpClientOptions {
    client_details,
    transport,
    handler: handler.to_mcp_client_handler(),
    task_store: None,
    server_task_store: None,
    message_observer: None,
});
client.clone().start().await?;

The ClientSseTransportOptions struct supports request timeouts, retry behavior, and custom HTTP headers - see ClientSseTransportOptions.