RMCP

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

Migrating to 1.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

For the full MCP specification, see modelcontextprotocol.io.

Table of Contents

• Usage
• Tools
• Resources
• Prompts
• Sampling
• Roots
• Logging
• Completions
• Notifications
• Subscriptions
• Examples
• OAuth Support
• Related Resources
• Related Projects
• Development

Usage

Import the crate

rmcp = { version = "0.16.0", features = ["server"] }
## or dev channel
rmcp = { 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

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(())
}

Build a Server

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

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

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

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

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

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

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(())
}

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.

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_templates() on the ServerHandler trait. Enable the resources capability in get_info().

use rmcp::{
    ErrorData as McpError, RoleServer, ServerHandler, ServiceExt,
    model::*,
    service::RequestContext,
    transport::stdio,
};
use serde_json::json;

#[derive(Clone)]
struct MyServer;

impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo {
            capabilities: ServerCapabilities::builder()
                .enable_resources()
                .build(),
            ..Default::default()
        }
    }

    async fn list_resources(
        &self,
        _request: Option<PaginatedRequestParams>,
        _context: RequestContext<RoleServer>,
    ) -> Result<ListResourcesResult, McpError> {
        Ok(ListResourcesResult {
            resources: vec![
                RawResource::new("file:///config.json", "config").no_annotation(),
                RawResource::new("memo://insights", "insights").no_annotation(),
            ],
            next_cursor: None,
            meta: None,
        })
    }

    async fn read_resource(
        &self,
        request: ReadResourceRequestParams,
        _context: RequestContext<RoleServer>,
    ) -> Result<ReadResourceResult, McpError> {
        match request.uri.as_str() {
            "file:///config.json" => Ok(ReadResourceResult {
                contents: vec![ResourceContents::text(r#"{"key": "value"}"#, &request.uri)],
            }),
            "memo://insights" => Ok(ReadResourceResult {
                contents: vec![ResourceContents::text("Analysis results...", &request.uri)],
            }),
            _ => Err(McpError::resource_not_found(
                "resource_not_found",
                Some(json!({ "uri": request.uri })),
            )),
        }
    }

    async fn list_resource_templates(
        &self,
        _request: Option<PaginatedRequestParams>,
        _context: RequestContext<RoleServer>,
    ) -> Result<ListResourceTemplatesResult, McpError> {
        Ok(ListResourceTemplatesResult {
            resource_templates: vec![],
            next_cursor: None,
            meta: None,
        })
    }
}

Client-side

use rmcp::model::{ReadResourceRequestParams};

// List all resources (handles pagination automatically)
let resources = client.list_all_resources().await?;

// Read a specific resource by URI
let result = client.read_resource(ReadResourceRequestParams {
    meta: None,
    uri: "file:///config.json".into(),
}).await?;

// List resource templates
let templates = client.list_all_resource_templates().await?;

Notifications

Servers can notify clients when the resource list changes or when a specific resource is updated:

// Notify that the resource list has changed (clients should re-fetch)
context.peer.notify_resource_list_changed().await?;

// Notify that a specific resource was updated
context.peer.notify_resource_updated(ResourceUpdatedNotificationParam {
    uri: "file:///config.json".into(),
}).await?;

Clients handle these via ClientHandler:

impl ClientHandler for MyClient {
    async fn on_resource_list_changed(
        &self,
        _context: NotificationContext<RoleClient>,
    ) {
        // Re-fetch the resource list
    }

    async fn on_resource_updated(
        &self,
        params: ResourceUpdatedNotificationParam,
        _context: NotificationContext<RoleClient>,
    ) {
        // Re-read the updated resource at params.uri
    }
}

Example: examples/servers/src/common/counter.rs (server), examples/clients/src/everything_stdio.rs (client)

Prompts

Prompts are reusable message templates that servers expose to clients. They accept typed arguments and return conversation messages. The #[prompt] macro handles argument validation and routing automatically.

MCP Spec: Prompts

Server-side

Use the #[prompt_router], #[prompt], and #[prompt_handler] macros to define prompts declaratively. Arguments are defined as structs deriving JsonSchema.

use rmcp::{
    ErrorData as McpError, RoleServer, ServerHandler, ServiceExt,
    handler::server::{router::prompt::PromptRouter, wrapper::Parameters},
    model::*,
    prompt, prompt_handler, prompt_router,
    schemars::JsonSchema,
    service::RequestContext,
    transport::stdio,
};
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize, JsonSchema)]
pub struct CodeReviewArgs {
    #[schemars(description = "Programming language of the code")]
    pub language: String,
    #[schemars(description = "Focus areas for the review")]
    pub focus_areas: Option<Vec<String>>,
}

#[derive(Clone)]
pub struct MyServer {
    prompt_router: PromptRouter<Self>,
}

#[prompt_router]
impl MyServer {
    fn new() -> Self {
        Self { prompt_router: Self::prompt_router() }
    }

    /// Simple prompt without parameters
    #[prompt(name = "greeting", description = "A simple greeting")]
    async fn greeting(&self) -> Vec<PromptMessage> {
        vec![PromptMessage::new_text(
            PromptMessageRole::User,
            "Hello! How can you help me today?",
        )]
    }

    /// Prompt with typed arguments
    #[prompt(name = "code_review", description = "Review code in a given language")]
    async fn code_review(
        &self,
        Parameters(args): Parameters<CodeReviewArgs>,
    ) -> Result<GetPromptResult, McpError> {
        let focus = args.focus_areas
            .unwrap_or_else(|| vec!["correctness".into()]);

        Ok(GetPromptResult {
            description: Some(format!("Code review for {}", args.language)),
            messages: vec![
                PromptMessage::new_text(
                    PromptMessageRole::User,
                    format!("Review my {} code. Focus on: {}", args.language, focus.join(", ")),
                ),
            ],
        })
    }
}

#[prompt_handler]
impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo {
            capabilities: ServerCapabilities::builder().enable_prompts().build(),
            ..Default::default()
        }
    }
}

Prompt functions support several return types:

• Vec<PromptMessage> -- simple message list
• GetPromptResult -- messages with an optional description
• Result<T, McpError> -- either of the above, with error handling

Client-side

use rmcp::model::GetPromptRequestParams;

// List all prompts
let prompts = client.list_all_prompts().await?;

// Get a prompt with arguments
let result = client.get_prompt(GetPromptRequestParams {
    meta: None,
    name: "code_review".into(),
    arguments: Some(rmcp::object!({
        "language": "Rust",
        "focus_areas": ["performance", "safety"]
    })),
}).await?;

Notifications

// Server: notify that available prompts have changed
context.peer.notify_prompt_list_changed().await?;

Example: examples/servers/src/prompt_stdio.rs (server), examples/clients/src/everything_stdio.rs (client)

Sampling

Sampling flips the usual direction: the server asks the client to run an LLM completion. The server sends a create_message request, the client processes it through its LLM, and returns the result.

MCP Spec: Sampling

Server-side (requesting sampling)

Access the client's sampling capability through context.peer.create_message():

use rmcp::model::*;

// Inside a ServerHandler method (e.g., call_tool):
let response = context.peer.create_message(CreateMessageRequestParams {
    meta: None,
    task: None,
    messages: vec![SamplingMessage::user_text("Explain this error: connection refused")],
    model_preferences: Some(ModelPreferences {
        hints: Some(vec![ModelHint { name: Some("claude".into()) }]),
        cost_priority: Some(0.3),
        speed_priority: Some(0.8),
        intelligence_priority: Some(0.7),
    }),
    system_prompt: Some("You are a helpful assistant.".into()),
    include_context: Some(ContextInclusion::None),
    temperature: Some(0.7),
    max_tokens: 150,
    stop_sequences: None,
    metadata: None,
    tools: None,
    tool_choice: None,
}).await?;

// Extract the response text
let text = response.message.content
    .first()
    .and_then(|c| c.as_text())
    .map(|t| &t.text);

Client-side (handling sampling)

On the client side, implement ClientHandler::create_message(). This is where you'd call your actual LLM:

use rmcp::{ClientHandler, model::*, service::{RequestContext, RoleClient}};

#[derive(Clone, Default)]
struct MyClient;

impl ClientHandler for MyClient {
    async fn create_message(
        &self,
        params: CreateMessageRequestParams,
        _context: RequestContext<RoleClient>,
    ) -> Result<CreateMessageResult, ErrorData> {
        // Forward to your LLM, or return a mock response:
        let response_text = call_your_llm(&params.messages).await;

        Ok(CreateMessageResult {
            message: SamplingMessage::assistant_text(response_text),
            model: "my-model".into(),
            stop_reason: Some(CreateMessageResult::STOP_REASON_END_TURN.into()),
        })
    }
}

Example: examples/servers/src/sampling_stdio.rs (server), examples/clients/src/sampling_stdio.rs (client)

Roots

Roots tell servers which directories or projects the client is working in. A root is a URI (typically file://) pointing to a workspace or repository. Servers can query roots to know where to look for files and how to scope their work.

MCP Spec: Roots

Server-side

Ask the client for its root list, and handle change notifications:

use rmcp::{ServerHandler, model::*, service::{NotificationContext, RoleServer}};

impl ServerHandler for MyServer {
    // Query the client for its roots
    async fn call_tool(
        &self,
        request: CallToolRequestParams,
        context: RequestContext<RoleServer>,
    ) -> Result<CallToolResult, ErrorData> {
        let roots = context.peer.list_roots().await?;
        // Use roots.roots to understand workspace boundaries
        // ...
    }

    // Called when the client's root list changes
    async fn on_roots_list_changed(
        &self,
        _context: NotificationContext<RoleServer>,
    ) {
        // Re-fetch roots to stay current
    }
}

Client-side

Clients declare roots capability and implement list_roots():

use rmcp::{ClientHandler, model::*};

impl ClientHandler for MyClient {
    async fn list_roots(
        &self,
        _context: RequestContext<RoleClient>,
    ) -> Result<ListRootsResult, ErrorData> {
        Ok(ListRootsResult {
            roots: vec![
                Root {
                    uri: "file:///home/user/project".into(),
                    name: Some("My Project".into()),
                },
            ],
        })
    }
}

Clients notify the server when roots change:

// After adding or removing a workspace root:
client.notify_roots_list_changed().await?;

Logging

Servers can send structured log messages to clients. The client sets a minimum severity level, and the server sends messages through the peer notification interface.

MCP Spec: Logging

Server-side

Enable the logging capability, handle level changes from the client, and send log messages via the peer:

use rmcp::{ServerHandler, model::*, service::RequestContext};

impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo {
            capabilities: ServerCapabilities::builder()
                .enable_logging()
                .build(),
            ..Default::default()
        }
    }

    // Client sets the minimum log level
    async fn set_level(
        &self,
        request: SetLevelRequestParams,
        _context: RequestContext<RoleServer>,
    ) -> Result<(), ErrorData> {
        // Store request.level and filter future log messages accordingly
        Ok(())
    }
}

// Send a log message from any handler with access to the peer:
context.peer.notify_logging_message(LoggingMessageNotificationParam {
    level: LoggingLevel::Info,
    logger: Some("my-server".into()),
    data: serde_json::json!({
        "message": "Processing completed",
        "items_processed": 42
    }),
}).await?;

Available log levels (from least to most severe): Debug, Info, Notice, Warning, Error, Critical, Alert, Emergency.

Client-side

Clients handle incoming log messages via ClientHandler:

impl ClientHandler for MyClient {
    async fn on_logging_message(
        &self,
        params: LoggingMessageNotificationParam,
        _context: NotificationContext<RoleClient>,
    ) {
        println!("[{}] {}: {}", params.level,
            params.logger.unwrap_or_default(), params.data);
    }
}

Clients can also set the server's log level:

client.set_level(SetLevelRequestParams {
    level: LoggingLevel::Warning,
    meta: None,
}).await?;

Completions

Completions give auto-completion suggestions for prompt or resource template arguments. As a user fills in arguments, the client can ask the server for suggestions based on what's already been entered.

MCP Spec: Completions

Server-side

Enable the completions capability and implement the complete() handler. Use request.context to inspect previously filled arguments:

use rmcp::{ErrorData as McpError, ServerHandler, model::*, service::RequestContext, RoleServer};

impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo {
            capabilities: ServerCapabilities::builder()
                .enable_completions()
                .enable_prompts()
                .build(),
            ..Default::default()
        }
    }

    async fn complete(
        &self,
        request: CompleteRequestParams,
        _context: RequestContext<RoleServer>,
    ) -> Result<CompleteResult, McpError> {
        let values = match &request.r#ref {
            Reference::Prompt(prompt_ref) if prompt_ref.name == "sql_query" => {
                match request.argument.name.as_str() {
                    "operation" => vec!["SELECT", "INSERT", "UPDATE", "DELETE"],
                    "table" => vec!["users", "orders", "products"],
                    "columns" => {
                        // Adapt suggestions based on previously filled arguments
                        if let Some(ctx) = &request.context {
                            if let Some(op) = ctx.get_argument("operation") {
                                match op.to_uppercase().as_str() {
                                    "SELECT" | "UPDATE" => {
                                        vec!["id", "name", "email", "created_at"]
                                    }
                                    _ => vec![],
                                }
                            } else { vec![] }
                        } else { vec![] }
                    }
                    _ => vec![],
                }
            }
            _ => vec![],
        };

        // Filter by the user's partial input
        let filtered: Vec<String> = values.into_iter()
            .map(String::from)
            .filter(|v| v.to_lowercase().contains(&request.argument.value.to_lowercase()))
            .collect();

        Ok(CompleteResult {
            completion: CompletionInfo {
                values: filtered,
                total: None,
                has_more: Some(false),
            },
        })
    }
}

Client-side

use rmcp::model::*;

let result = client.complete(CompleteRequestParams {
    meta: None,
    r#ref: Reference::Prompt(PromptReference {
        name: "sql_query".into(),
    }),
    argument: ArgumentInfo {
        name: "operation".into(),
        value: "SEL".into(),
    },
    context: None,
}).await?;

// result.completion.values contains suggestions like ["SELECT"]

Example: examples/servers/src/completion_stdio.rs

Notifications

Notifications are fire-and-forget messages -- no response is expected. They cover progress updates, cancellation, and lifecycle events. Both sides can send and receive them.

MCP Spec: Notifications

Progress notifications

Servers can report progress during long-running operations:

use rmcp::model::*;

// Inside a tool handler:
for i in 0..total_items {
    process_item(i).await;

    context.peer.notify_progress(ProgressNotificationParam {
        progress_token: ProgressToken(NumberOrString::Number(i as i64)),
        progress: i as f64,
        total: Some(total_items as f64),
        message: Some(format!("Processing item {}/{}", i + 1, total_items)),
    }).await?;
}

Cancellation

Either side can cancel an in-progress request:

// Send a cancellation
context.peer.notify_cancelled(CancelledNotificationParam {
    request_id: the_request_id,
    reason: Some("User requested cancellation".into()),
}).await?;

Handle cancellation in ServerHandler or ClientHandler:

impl ServerHandler for MyServer {
    async fn on_cancelled(
        &self,
        params: CancelledNotificationParam,
        _context: NotificationContext<RoleServer>,
    ) {
        // Abort work for params.request_id
    }
}

Initialized notification

Clients send initialized after the handshake completes:

// Sent automatically by rmcp during the serve() handshake.
// Servers handle it via:
impl ServerHandler for MyServer {
    async fn on_initialized(
        &self,
        _context: NotificationContext<RoleServer>,
    ) {
        // Server is ready to receive requests
    }
}

List-changed notifications

When available tools, prompts, or resources change, tell the client:

context.peer.notify_tool_list_changed().await?;
context.peer.notify_prompt_list_changed().await?;
context.peer.notify_resource_list_changed().await?;

Example: examples/servers/src/common/progress_demo.rs

Subscriptions

Clients can subscribe to specific resources. When a subscribed resource changes, the server sends a notification and the client can re-read it.

MCP Spec: Resources - Subscriptions

Server-side

Enable subscriptions in the resources capability and implement the subscribe() / unsubscribe() handlers:

use rmcp::{ErrorData as McpError, ServerHandler, model::*, service::RequestContext, RoleServer};
use std::sync::Arc;
use tokio::sync::Mutex;
use std::collections::HashSet;

#[derive(Clone)]
struct MyServer {
    subscriptions: Arc<Mutex<HashSet<String>>>,
}

impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo {
            capabilities: ServerCapabilities::builder()
                .enable_resources()
                .enable_resources_subscribe()
                .build(),
            ..Default::default()
        }
    }

    async fn subscribe(
        &self,
        request: SubscribeRequestParams,
        _context: RequestContext<RoleServer>,
    ) -> Result<(), McpError> {
        self.subscriptions.lock().await.insert(request.uri);
        Ok(())
    }

    async fn unsubscribe(
        &self,
        request: UnsubscribeRequestParams,
        _context: RequestContext<RoleServer>,
    ) -> Result<(), McpError> {
        self.subscriptions.lock().await.remove(&request.uri);
        Ok(())
    }
}

When a subscribed resource changes, notify the client:

// Check if the resource has subscribers, then notify
context.peer.notify_resource_updated(ResourceUpdatedNotificationParam {
    uri: "file:///config.json".into(),
}).await?;

Client-side

use rmcp::model::*;

// Subscribe to updates for a resource
client.subscribe(SubscribeRequestParams {
    meta: None,
    uri: "file:///config.json".into(),
}).await?;

// Unsubscribe when no longer needed
client.unsubscribe(UnsubscribeRequestParams {
    meta: None,
    uri: "file:///config.json".into(),
}).await?;

Handle update notifications in ClientHandler:

impl ClientHandler for MyClient {
    async fn on_resource_updated(
        &self,
        params: ResourceUpdatedNotificationParam,
        _context: NotificationContext<RoleClient>,
    ) {
        // Re-read the resource at params.uri
    }
}

Examples

See examples.

OAuth Support

See Oauth_support for details.

Related Resources

• MCP Specification
• Schema

Related Projects

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Development

Tips for Contributors

See docs/CONTRIBUTE.MD to get some tips for contributing.

Using Dev Container

If you want to use dev container, see docs/DEVCONTAINER.md for instructions on using Dev Container for development.