agent-client-protocol-rmcp

rmcp integration for Agent Client Protocol MCP servers.

Overview

This crate bridges rmcp-based MCP server implementations with the runtime-agnostic MCP server framework from agent-client-protocol. It lets you define MCP tools in Rust, serve them directly, or attach them to an ACP proxy.

Attached servers are advertised with the opt-in native MCP-over-ACP transport: McpServer::Acp plus request-scoped mcp/message for MCP 2026-07-28, without an initialization prerequisite or connect/disconnect exchange. This crate does not enable the core SDK's unstable_mcp_over_acp feature merely to build or directly serve a server. Enable this crate's matching unstable_mcp_over_acp feature when using with_mcp_server. Use agent-client-protocol-polyfill when the final agent accepts HTTP but not ACP-transport MCP servers.

unstable_protocol_v2 and unstable_mcp_over_acp are independent passthrough features. Enable both when attaching a native MCP server to draft ACP v2.

Native attachments lazily initialize one shared rmcp application service. Standalone connections still invoke the factory separately. Each native request owns its metadata, notifications, cancellation, and supervised cleanup. Run the direct ACP example with:

cargo run -p agent-client-protocol-rmcp --example stateless_native_mcp \
  --features unstable_protocol_v2,unstable_mcp_over_acp

Usage

Use the McpServerExt trait to build an MCP server with tools:

use agent_client_protocol::{ConnectTo, mcp_server::McpServer, role::mcp};
use agent_client_protocol_rmcp::McpServerExt;

async fn serve(
    client_transport: impl ConnectTo<mcp::Server>,
) -> agent_client_protocol::Result<()> {
    let server = McpServer::<mcp::Client>::builder("my-tools").build();
    server.connect_to(client_transport).await
}

Choosing mcp::Client as the counterpart makes this a standalone MCP server that implements ConnectTo<mcp::Client>.

Or create an MCP server from an rmcp service:

use agent_client_protocol::mcp_server::McpServer;
use agent_client_protocol_rmcp::McpServerExt;

let server = McpServer::from_rmcp("my-server", MyRmcpService::new);

// Use as a handler in a proxy
Proxy.builder()
    .with_mcp_server(server)
    .connect_to(transport)
    .await?;

Why a Separate Crate?

This crate is separate from agent-client-protocol to avoid coupling the core protocol crate to the rmcp dependency. This allows:

• agent-client-protocol to remain focused on the ACP protocol
• agent-client-protocol-rmcp to track rmcp updates independently
• Integrations to choose compatible agent-client-protocol and rmcp major versions explicitly

Versioning

Both agent-client-protocol and rmcp are public dependencies of this crate: their types and traits appear in its public API. A source-incompatible major release of either dependency therefore requires a major release of this crate.

See the rmcp 4.x integration migration guide for migrating both public dependencies in the combined core 3.x / integration 4.x / rmcp 3.x release. The minimum supported Rust version remains 1.88. Native attachments also move to the request-scoped MCP binding for MCP 2026-07-28 only; the old connect/disconnect binding is removed. Standalone rmcp serving retains protocol negotiation. Custom transports and low-level callers should follow the connection-driver migration, and raw response adapters should follow the core 3.x migration guide.

Related Crates

• agent-client-protocol — Core ACP protocol types and traits
• agent-client-protocol-conductor — Proxy-chain orchestration

License

Apache-2.0