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

This SDK tracks the MCP 2026-07-28 draft (the current development spec) while remaining fully compatible with the stable 2025-11-25 release and earlier versions. New 2026-07-28 features — server discovery & negotiation, transport-neutral subscriptions, long-running tasks, response caching, multi-round-trip requests, and standard HTTP routing headers — are documented below alongside the stable feature set. For the full MCP specification, see modelcontextprotocol.io.

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

Start 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(())
}

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::{ClientInfo, ClientLifecycleMode, ClientServiceExt, ProtocolVersion};

// Start directly with server/discover and include client metadata on every request.
let client = ClientInfo::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.
let client = ClientInfo::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

Build a transport

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

Build a service

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

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

Start the server

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

Interact with the server

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?;

Waiting for service shutdown

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(())
}

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.

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::new(
            ServerCapabilities::builder()
                .enable_resources()
                .build(),
        )
    }

    async fn list_resources(
        &self,
        _request: Option<PaginatedRequestParams>,
        _context: RequestContext<RoleServer>,
    ) -> Result<ListResourcesResult, McpError> {
        Ok(ListResourcesResult {
            resources: vec![
                Resource::new("file:///config.json", "config"),
                Resource::new("memo://insights", "insights"),
            ],
            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::new(vec![
                ResourceContents::text(r#"{"key": "value"}"#, &request.uri),
            ])),
            "memo://insights" => Ok(ReadResourceResult::new(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::new("file:///config.json"),
).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::new("file:///config.json"),
).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(
            Role::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::new(vec![
            PromptMessage::new_text(
                Role::User,
                format!("Review my {} code. Focus on: {}", args.language, focus.join(", ")),
            ),
        ])
        .with_description(format!("Code review for {}", args.language)))
    }
}

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

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

Deprecated (SEP-2577): Sampling is deprecated and will be removed in a future release. It remains fully functional for now. See SEP-2577.

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::new(
        vec![SamplingMessage::user_text("Explain this error: connection refused")],
        150,
    )
    .with_model_preferences(
        ModelPreferences::new()
            .with_hints(vec![ModelHint::new("claude")])
            .with_cost_priority(0.3)
            .with_speed_priority(0.8)
            .with_intelligence_priority(0.7),
    )
    .with_system_prompt("You are a helpful assistant.")
    .with_include_context(ContextInclusion::None)
    .with_temperature(0.7),
).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::new(
            SamplingMessage::assistant_text(response_text),
            "my-model".into(),
        )
        .with_stop_reason(CreateMessageResult::STOP_REASON_END_TURN))
    }
}

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

Roots

Deprecated (SEP-2577): Roots is deprecated and will be removed in a future release. It remains fully functional for now. See SEP-2577.

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::new(vec![
            Root::new("file:///home/user/project").with_name("My Project"),
        ]))
    }
}

Clients notify the server when roots change:

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

Logging

Deprecated (SEP-2577): Logging is deprecated and will be removed in a future release. It remains fully functional for now. See SEP-2577.

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::new(
            ServerCapabilities::builder()
                .enable_logging()
                .build(),
        )
    }

    // 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::new(
        LoggingLevel::Info,
        serde_json::json!({
            "message": "Processing completed",
            "items_processed": 42
        }),
    )
    .with_logger("my-server"),
).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::new(LoggingLevel::Warning)).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::new(
            ServerCapabilities::builder()
                .enable_completions()
                .enable_prompts()
                .build(),
        )
    }

    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();

        let completion = CompletionInfo::with_pagination(filtered, None, false)
            .map_err(|e| McpError::internal_error(e, None))?;
        Ok(CompleteResult::new(completion))
    }
}

Client-side

use rmcp::model::*;

let result = client.complete(CompleteRequestParams::new(
    Reference::for_prompt("sql_query"),
    ArgumentInfo::new("operation", "SEL"),
)).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::new(
            ProgressToken(NumberOrString::Number(i as i64)),
            i as f64,
        )
        .with_total(total_items as f64)
        .with_message(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::new(
    Some(the_request_id),
    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

Legacy clients send initialized after the initialize handshake completes. Clients using ClientLifecycleMode::Discover do not send this notification:

// Sent automatically by rmcp during the legacy 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

Protocol 2026-07-28 replaces resources/subscribe, resources/unsubscribe, and the standalone HTTP GET stream with the transport-neutral, long-lived subscriptions/listen request. Each requested notification category is opt-in.

MCP Spec: Subscriptions

Server-side

Declare the notification capabilities you serve, return the accepted subset, and use the filter-enforcing subscription sink:

use rmcp::{
    ErrorData, ServerHandler,
    model::*,
    service::SubscriptionContext,
};

impl ServerHandler for MyServer {
    fn get_info(&self) -> ServerInfo {
        ServerInfo::new(
            ServerCapabilities::builder()
                .enable_tools()
                .enable_tool_list_changed()
                .build(),
        )
    }

    fn accepted_subscription_filter(
        &self,
        requested: &SubscriptionFilter,
    ) -> Option<SubscriptionFilter> {
        Some(requested.clone())
    }

    async fn listen(&self, context: SubscriptionContext) -> Result<(), ErrorData> {
        if context.accepted().tools_list_changed == Some(true) {
            context.sink().notify_tool_list_changed().await
                .map_err(|error| ErrorData::internal_error(error.to_string(), None))?;
        }
        context.cancelled().await;
        Ok(())
    }
}

The SDK intersects the handler's filter with the requested categories and the capabilities advertised by get_info(). It sends the acknowledgment before listen, tags every sink notification with the listen request ID, and rejects categories or resource URIs outside the accepted filter.

Client-side

use rmcp::model::*;

let mut subscription = client.listen(
    SubscriptionFilter::builder()
        .tools_list_changed()
        .resource_subscription("file:///config.json")
        .build(),
).await?;

println!("accepted: {:?}", subscription.acknowledged());
while let Some(notification) = subscription.next().await? {
    println!("notification: {notification:?}");
}

subscription.cancel().await?;

listen() buffers up to 64 notifications per subscription. Use listen_with_capacity() to choose a different non-zero capacity; if a consumer falls behind, Subscription::end() reports SubscriptionEnd::Lagged.

For older negotiated protocol versions, the deprecated subscribe() and unsubscribe() APIs retain their legacy wire behavior. Modern Streamable HTTP uses the listen POST response stream directly and does not use sessions, GET, DELETE, or Last-Event-ID. After an abrupt transport close, call listen again; subscription state is not resumed across HTTP or stdio reconnects.

See the modern subscription server and client examples.

Multi-Round-Trip Requests

Protocol 2026-07-28 adds Multi-Round-Trip Requests (MRTR, SEP-2322): a server can answer a tools/call, prompts/get, or resources/read with an InputRequiredResult instead of a final result, asking the client to fulfill one or more embedded server requests (elicitation, sampling, or roots) and then retry. The exchange is stateless — the server carries its progress in an opaque requestState that the client echoes back verbatim.

MCP Spec: Multiple Round-Trip Requests

Server-side

Return an InputRequiredResult via the outcome enum for the method (CallToolResponse, GetPromptResponse, or ReadResourceResponse). The SDK only forwards it to peers that negotiated 2026-07-28 or newer — older peers get a protocol error instead.

async fn call_tool(&self, request: CallToolRequestParams, _ctx: RequestContext<RoleServer>)
    -> Result<CallToolResponse, ErrorData>
{
    match request.request_state {
        // First round: ask the client for input, seal progress into requestState.
        None => {
            let mut input_requests = InputRequests::new();
            input_requests.insert("city".into(), InputRequest::Elicitation(elicit_city()));
            let sealed = self.codec.seal_json(&json!({ "awaiting": "city" }))?;
            Ok(InputRequiredResult::new(Some(input_requests), Some(sealed)).into())
        }
        // Retry round: verify the echoed state, read the responses, finish.
        Some(sealed) => {
            let _state = self.codec.open_json(&sealed)
                .map_err(|_| ErrorData::invalid_params("tampered request state", None))?;
            let city = request.input_responses.as_ref()
                .and_then(|r| r.get("city"));
            Ok(CallToolResult::success(vec![ContentBlock::text("It is sunny.")]).into())
        }
    }
}

requestState is untrusted. The client echoes it back verbatim, so a stateless server that stores meaningful data in it MUST verify integrity first. Enable the request-state feature and use RequestStateCodec to seal and open it (HMAC-tagged), or keep state server-side and use requestState only as an opaque handle.

Client-side

The high-level call_tool, get_prompt, and read_resource helpers drive MRTR automatically: they fulfill each embedded request through the local ClientHandler and retry, up to DEFAULT_MRTR_MAX_ROUNDS (10).

// Auto mode: the SDK fulfills embedded requests and retries for you.
let result = client.call_tool(CallToolRequestParams::new("weather")).await?;

// Choose a custom round cap.
let result = client
    .call_tool_with_mrtr_max_rounds(CallToolRequestParams::new("weather"), 3)
    .await?;

// Manual mode: get the intermediate InputRequiredResult and drive rounds yourself.
match client.call_tool_once(CallToolRequestParams::new("weather")).await? {
    CallToolResponse::InputRequired(input_required) => { /* fulfill + retry */ }
    CallToolResponse::Complete(result) => { /* done */ }
    _ => {}
}

Example: examples/servers/src/mrtr.rs (end-to-end server + client)

Tasks (long-running tool invocations)

rmcp implements the MCP Tasks extension (SEP-2663, io.modelcontextprotocol/tasks). A client declares the extension in its capabilities; the server then decides per request whether to materialize a tools/call as a task, returning a CreateTaskResult (resultType: "task"). The client polls tasks/get, answers in-task input requests via tasks/update, and may request cooperative cancellation via tasks/cancel. Use rmcp::task_manager::TaskManager to manage task lifecycles server-side.

// Client: declare the tasks extension capability.
let caps = ClientCapabilities::builder().enable_tasks().build();

// Server: decide per request whether to materialize a task.
async fn call_tool(&self, request: CallToolRequestParams, context: RequestContext<RoleServer>)
    -> Result<CallToolResponse, McpError>
{
    let client_supports_tasks = context
        .client_capabilities()
        .is_some_and(|caps| caps.supports_tasks());
    if client_supports_tasks {
        let task = self.tasks.spawn(TaskOptions::default(), move |_ctx| {
            Box::pin(async move { /* long-running work -> Ok(CallToolResult) */ })
        });
        return Ok(CallToolResponse::Task(CreateTaskResult::new(task)));
    }
    // ... fall back to synchronous execution
}

See servers_task_stdio and the matching clients_task_stdio for a runnable end-to-end example.

Caching

rmcp clients transparently cache responses that carry the SEP-2549 caching hints (ttlMs / cacheScope) for server/discover, tools/list, prompts/list, resources/list, resources/templates/list, and resources/read.

Caching is on by default but only stores a response when the server sends a positive ttlMs, so servers that omit the hint behave exactly as before. Entries expire after their TTL, are partitioned by cache scope, and are invalidated automatically by the matching list_changed / resource updated notifications.

No call-site changes are needed — existing calls benefit automatically:

let tools = peer.list_tools(None).await?;     // served from cache while fresh
let res   = peer.read_resource(params).await?; // cached per-URI

Tune or disable it per connection via the Peer:

use std::time::Duration;
use rmcp::ClientCacheConfig;

// Customize behavior.
peer.set_response_cache_config(
    ClientCacheConfig::default()
        .with_default_ttl(Duration::from_secs(30)) // TTL for servers that omit ttlMs
        .with_max_ttl(Duration::from_secs(3600))   // upper bound on any TTL
        .with_max_entries(1024)
        .with_private_partition(user_id)            // separate private caches per principal
        .with_serve_stale_on_error(false),          // surface errors instead of stale data
).await;

// Or turn it off entirely.
peer.set_response_cache_config(ClientCacheConfig::disabled()).await;

// Manually flush.
peer.clear_response_cache().await;

Note: with the default serve_stale_on_error, a failed re-fetch returns the last cached response (even if expired) as Ok(..) instead of an error. Set with_serve_stale_on_error(false) if callers must observe fetch failures.

Standard HTTP Headers

Protocol 2026-07-28 standardizes a set of Streamable HTTP request headers (SEP-2243) so proxies and gateways can route MCP traffic without parsing the JSON body: Mcp-Method, Mcp-Name, and Mcp-Param-*. rmcp emits and validates these automatically once a connection negotiates 2026-07-28 or newer — no call-site changes are required, and older negotiated versions are untouched.

MCP Spec: Header standardization

Mcp-Method — the JSON-RPC method (e.g. tools/call).
Mcp-Name — the target name, sourced from params.name (tools/call, prompts/get), params.uri (resources/*), or params.taskId (tasks/*).
Mcp-Param-* — selected tools/call arguments, promoted from the tool's input schema.

To promote a tool argument into a routing header, annotate the top-level schema property with x-mcp-header:

// A `region` argument surfaces as the `Mcp-Param-Region` request header.
let schema = serde_json::json!({
    "type": "object",
    "properties": {
        "region": { "type": "string", "x-mcp-header": "Region" }
    }
});

Annotations must be non-empty RFC 9110 tokens, case-insensitively unique, and applied only to top-level primitive (string/integer/boolean) properties. Values that cannot travel as a bare header (leading/trailing whitespace, control/non-ASCII characters) are transparently Base64-wrapped as =?base64?<b64>?=.

Stateless Streamable HTTP

Per SEP-2567, rmcp serves the 2026-07-28 draft statelessly automatically: no Mcp-Session-Id, no standalone GET/DELETE stream, and no Last-Event-ID resumption. The legacy_session_mode flag below only controls behavior for legacy protocol versions (< 2026-07-28).

MCP Spec: Transports

Server-side

A default server already serves 2026-07-28 clients statelessly. To also serve legacy clients without sessions, disable legacy_session_mode (formerly stateful_mode; builder with_stateful_mode). Optionally set with_json_response(true) so simple request/response tools reply with a single application/json body instead of an SSE stream (the server still falls back to text/event-stream if a handler emits a notification or server request first):

use rmcp::transport::streamable_http_server::{
    StreamableHttpService, StreamableHttpServerConfig,
    session::local::LocalSessionManager,
};

let config = StreamableHttpServerConfig::default()
    .with_legacy_session_mode(false) // stateless for legacy versions too
    .with_json_response(true);       // plain JSON replies for simple tools

let service = StreamableHttpService::new(
    || Ok(Counter::new()),           // a fresh handler per request
    LocalSessionManager::default().into(),
    config,
);

// `StreamableHttpService` is a Tower service — mount it on any router.
let router = axum::Router::new().nest_service("/mcp", service);

Because there is no per-session state, the service_factory runs per request. Keep shared state (DB pools, caches) in a Clone handle captured by the closure; don't rely on in-memory state surviving between requests.

Client-side

The Streamable HTTP client transport allows stateless operation by default (allow_stateless: true), so no configuration is needed to talk to a stateless server — it simply omits the session header when the server doesn't issue one:

use rmcp::transport::StreamableHttpClientTransport;

// Defaults are stateless-friendly.
let transport = StreamableHttpClientTransport::from_uri("http://localhost:8000/mcp");
let client = ClientInfo::default().serve(transport).await?;

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

Examples

See examples.

OAuth Support

See Oauth_support for details.

Related Resources

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rmcp-actix-web - An actix_web backend for rmcp
rmcp-openapi - Transform OpenAPI definition endpoints into MCP tools

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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.

Name		Name	Last commit message	Last commit date
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.devcontainer		.devcontainer
.githooks		.githooks
.github		.github
conformance		conformance
crates		crates
docs		docs
examples		examples
scripts		scripts
.gitignore		.gitignore
Cargo.toml		Cargo.toml
LICENSE		LICENSE
README.md		README.md
ROADMAP.md		ROADMAP.md
SECURITY.md		SECURITY.md
client-metadata.json		client-metadata.json
clippy.toml		clippy.toml
justfile		justfile
release-plz.toml		release-plz.toml
rust-toolchain.toml		rust-toolchain.toml
rustfmt.toml		rustfmt.toml
typos.toml		typos.toml

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RMCP

Table of Contents

Usage

Import the crate

Third Dependencies

Build a Client

Client lifecycle modes

Build a Server

Tools

Server-side

Client-side

Resources

Server-side

Client-side

Notifications

Prompts

Server-side

Client-side

Notifications

Sampling

Server-side (requesting sampling)

Client-side (handling sampling)

Roots

Server-side

Client-side

Logging

Server-side

Client-side

Completions

Server-side

Client-side

Notifications

Progress notifications

Cancellation

Initialized notification

List-changed notifications

Subscriptions

Server-side

Client-side

Multi-Round-Trip Requests

Server-side

Client-side

Tasks (long-running tool invocations)

Caching

Standard HTTP Headers

Stateless Streamable HTTP

Server-side

Client-side

Examples

OAuth Support

Related Resources

Related Projects

Extending rmcp

Built with rmcp

Development

Tips for Contributors

Using Dev Container

About

Resources

Code of conduct

Contributing

Security policy

Stars

Watchers

Forks

Releases

Packages

Used by

Contributors

Languages

Extending `rmcp`

Built with `rmcp`