agent-client-protocol-rmcp

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.

Each supported rmcp major version has its own module behind a matching Cargo feature. No rmcp feature is enabled by default; without one, this crate is empty and has no dependencies. Enable the feature for the rmcp major version your application uses:

Feature	Module	rmcp
rmcp_3	agent_client_protocol_rmcp::rmcp_3	3.x

[dependencies]
agent-client-protocol-rmcp = { version = "5", features = ["rmcp_3"] }
rmcp = "3"

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_mcp_over_acp is a passthrough for the core SDK feature and applies to every rmcp module. The native path follows the core feature however it is enabled. To attach a native MCP server to draft ACP v2, also enable unstable_protocol_v2 on agent-client-protocol itself.

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 rmcp_3,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::rmcp_3::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::rmcp_3::McpServerExt;

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

// Use as a handler in a proxy
Proxy.v1()
    .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 or its features. This allows:

• agent-client-protocol to remain focused on the ACP protocol, with no rmcp dependency and no rmcp features
• agent-client-protocol-rmcp to support several rmcp major versions side by side, one feature and module per major version
• Integrations to adopt a new rmcp major version without waiting for an ACP major release

Versioning

Both agent-client-protocol and rmcp are public dependencies of this crate: their types and traits appear in its public API. Each supported rmcp major version therefore gets its own feature and module, such as rmcp_3 for rmcp 3.x, so that the rmcp types in a module's API never change major version:

• No rmcp feature is enabled by default. Enable the one matching your rmcp dependency.
• The features are additive. Enabling one adds its module and changes nothing else, so an application can enable two while it migrates between rmcp major versions.
• Support for a new rmcp major version arrives as a new feature and module in a minor release of this crate.
• Dropping support for an rmcp major version removes its feature and module, which is a breaking change and only happens in an ACP major release.

A source-incompatible major release of agent-client-protocol still requires a major release of this crate; it shares its version with the other ACP crates.

agent-client-protocol-rmcp	agent-client-protocol	rmcp
5.x	5.x	3.x (rmcp_3 feature)
4.x	3.x	3.x
3.x	2.x	2.x
2.x	1.x	2.x
1.x	1.x	1.x

See the v5 migration guide for moving to the rmcp_3 feature and module, and 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