RMCP

An official Rust Model Context Protocol SDK implementation with tokio async runtime.

Migrating to 3.x? See the migration guide for breaking changes and upgrade instructions.

This repository contains the following crates:

• rmcp: The core crate providing the RMCP protocol implementation - see rmcp
• rmcp-macros: A procedural macro crate for generating RMCP tool implementations - see rmcp-macros

This SDK implements the stable MCP 2026-07-28 specification while remaining fully compatible with the 2025-11-25 release and earlier versions. Features introduced in 2026-07-28 — server discovery & negotiation, transport-neutral subscriptions, long-running tasks, response caching, multi-round-trip requests, and standard HTTP routing headers — are documented below. For the full MCP specification, see modelcontextprotocol.io.

Table of Contents

• Usage
• Tools
• Resources
• Prompts
• Sampling
• Elicitation
• Roots
• Logging
• Completions
• Notifications
• Subscriptions
• Multi-Round-Trip Requests
• Tasks
• Caching
• Standard HTTP Headers
• Stateless Streamable HTTP
• Transports
• Pagination
• Capability & Protocol Version Negotiation
• JSON Schema 2020-12
• Examples
• OAuth Support
• Related Resources
• Related Projects
• Development

Usage

Import the crate

Add the latest published version with cargo:

cargo add rmcp --features server

Or use the dev channel:

cargo add rmcp --features server --git https://github.com/modelcontextprotocol/rust-sdk --branch main

Third Dependencies

Basic dependencies:

• tokio
• serde Json Schema generation (version 2020-12):
• schemars

Build a Client

<details> <summary>Start a client</summary>

use rmcp::{ServiceExt, transport::{TokioChildProcess, ConfigureCommandExt}};
use tokio::process::Command;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = ().serve(TokioChildProcess::new(Command::new("npx").configure(|cmd| {
        cmd.arg("-y").arg("@modelcontextprotocol/server-everything");
    }))?).await?;
    Ok(())
}

</details>

Client lifecycle modes

serve() uses the legacy MCP lifecycle: the client sends initialize, receives the negotiated server information, and then sends notifications/initialized. Use ClientServiceExt::serve_with_lifecycle to select another lifecycle explicitly:

use rmcp::{ClientLifecycleMode, ClientServiceExt, ProtocolVersion, model::ClientConfig};

// Start directly with server/discover and include client metadata on every request.
let client = ClientConfig::default()
    .serve_with_lifecycle(
        transport,
        ClientLifecycleMode::Discover {
            preferred_versions: vec![ProtocolVersion::V_2026_07_28],
        },
    )
    .await?;

// Or probe the discover lifecycle and fall back when a legacy server reports
// that server/discover is not implemented or does not respond within 10 seconds.
let client = ClientConfig::default()
    .serve_with_lifecycle(
        transport,
        ClientLifecycleMode::Auto {
            preferred_versions: vec![ProtocolVersion::V_2026_07_28],
            legacy_version: Some(ProtocolVersion::V_2025_11_25),
        },
    )
    .await?;

ClientLifecycleMode::Initialize is equivalent to the existing serve() behavior. Discover startup does not send notifications/initialized; discovery completes startup, and each subsequent request carries its protocol version, client information, and capabilities in _meta.

Build a Server

<details> <summary>Build a transport</summary>

use tokio::io::{stdin, stdout};
let transport = (stdin(), stdout());

</details> <details> <summary>Build a service</summary>

You can easily build a service by using ServerHandler or ClientHandler.

let service = common::counter::Counter::new();

</details> <details> <summary>Start the server</summary>

// this call will finish the initialization process
let server = service.serve(transport).await?;

</details> <details> <summary>Interact with the server</summary>

Once the server is initialized, you can send requests or notifications:

// request
let roots = server.list_roots().await?;

// or send notification
server.notify_cancelled(...).await?;

</details> <details> <summary>Waiting for service shutdown</summary>

let quit_reason = server.waiting().await?;
// or cancel it
let quit_reason = server.cancel().await?;

</details>

---

Tools

Tools let servers expose callable functions to clients. Each tool has a name, description, and a JSON Schema for its parameters. Clients discover tools via list_tools and invoke them via call_tool.

MCP Spec: Tools

Server-side

The #[tool], #[tool_router], and #[tool_handler] macros handle all the wiring. For a tools-only server you can use #[tool_router(server_handler)] to skip the separate ServerHandler impl:

use rmcp::{handler::server::wrapper::Parameters, schemars, tool, tool_router, ServiceExt, transport::stdio};

#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
struct AddParams {
    a: i32,
    b: i32,
}

#[derive(Clone)]
struct Calculator;

#[tool_router(server_handler)]
impl Calculator {
    #[tool(description = "Add two numbers")]
    fn add(&self, Parameters(AddParams { a, b }): Parameters<AddParams>) -> String {
        (a + b).to_string()
    }
}

#[tokio::main]
async fn main() -> anyhow::Result<()> {
    let service = Calculator.serve(stdio()).await?;
    service.waiting().await?;
    Ok(())
}

The generated tool inputSchema and outputSchema are derived from the fields of T. The type name and documentation on T are ignored; only field names, field types, and field documentation are used.

2026-07-28 (SEP-2106): outputSchema may now be any JSON Schema type (not just object), and a tool result's structuredContent may be any JSON value (string, array, number, …) rather than only an object. Existing object-typed tools are unaffected.

When you need custom server metadata or multiple capabilities (tools + prompts), use explicit #[tool_handler]:

use rmcp::{handler::server::wrapper::Parameters, schemars, tool, tool_router, tool_handler, ServerHandler, ServiceExt};

#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
struct AddParams {
    a: i32,
    b: i32,
}

#[derive(Clone)]
struct Calculator;

#[tool_router]
impl Calculator {
    #[tool(description = "Add two numbers")]
    fn add(&self, Parameters(AddParams { a, b }): Parameters<AddParams>) -> String {
        (a + b).to_string()
    }
}

#[tool_handler(name = "calculator", version = "1.0.0", instructions = "A simple calculator")]
impl ServerHandler for Calculator {}

See crates/rmcp-macros for full macro documentation.

Tool result content types

Beyond a plain String, tools can return images, audio, embedded resources, and mixed content. Build a CallToolResult from a Vec<ContentBlock>:

use rmcp::model::{CallToolResult, ContentBlock, ResourceContents};

#[tool(description = "Render a chart")]
async fn chart(&self) -> Result<CallToolResult, McpError> {
    let png_base64 = render_png(); // base64-encoded image bytes
    let wav_base64 = render_wav(); // base64-encoded audio bytes

    Ok(CallToolResult::success(vec![
        // Text
        ContentBlock::text("Here is your chart:"),
        // Image — base64 data + MIME type
        ContentBlock::image(png_base64, "image/png"),
        // Audio — base64 data + MIME type
        ContentBlock::audio(wav_base64, "audio/wav"),
        // Embedded resource — inline text (or ResourceContents::blob for binary)
        ContentBlock::resource(ResourceContents::text(
            "chart source data",
            "chart://last/data.csv",
        )),
    ]))
}
# fn render_png() -> String { String::new() }
# fn render_wav() -> String { String::new() }

Image and audio data are base64 strings with a MIME type. For embedded resources, ResourceContents::text(..) inlines text and ResourceContents::blob(base64, uri) inlines binary.

Error handling

Two failure modes, chosen by whose problem it is:

• Tool-level error — Ok(CallToolResult::error(vec![...])). The tool ran but failed in a way the caller should see (no rows matched, upstream 500). The client renders your content, so the message reaches the user. Use this for almost every "the tool ran and didn't work" case.
• Protocol error — Err(McpError) with a JSON-RPC code (e.g. McpError::invalid_params(..)). Use this when the server can't route or process the request at all; clients render these opaquely, so the caller does not see your message.

use rmcp::model::{CallToolResult, ContentBlock};
use rmcp::ErrorData as McpError;

#[tool(description = "Look up a record")]
async fn lookup(&self, Parameters(args): Parameters<LookupArgs>) -> Result<CallToolResult, McpError> {
    // Malformed request — the server can't run anything → protocol error.
    if args.query.is_empty() {
        return Err(McpError::invalid_params("query must be non-empty", None));
    }

    // Tool ran, no result → tool-level error the user should see.
    let rows = self.run_query(&args.query).await;
    if rows.is_empty() {
        return Ok(CallToolResult::error(vec![ContentBlock::text(
            format!("no rows matched '{}'", args.query),
        )]));
    }

    Ok(CallToolResult::success(vec![ContentBlock::text(format_rows(&rows))]))
}

Client-side

use rmcp::model::CallToolRequestParams;

// List all tools
let tools = client.list_all_tools().await?;

// Call a tool by name
let result = client.call_tool(CallToolRequestParams::new("add")).await?;

Example: examples/servers/src/common/calculator.rs (server), examples/servers/src/calculator_stdio.rs (stdio runner)

---

Resources

Resources let servers expose data (files, database records, API responses) that clients can read. Each resource is identified by a URI and returns content as text or binary (base64-encoded) data. Resource templates allow servers to declare URI patterns with dynamic parameters.

MCP Spec: Resources

Server-side

Implement list_resources(), read_resource(), and optionally `list_resource_t