rust-sdk/README.md
Jack Amadeo 07abfdfd99
feat: Implement SEP-2663 Tasks Extension (#1020)
* feat!: implement SEP-2663 Tasks extension, removing the experimental 2025-11-25 tasks design

Reshape tasks from the experimental SEP-1319/1686 core-protocol feature into
the official io.modelcontextprotocol/tasks extension (SEP-2663):

Model:
- Re-model Task (statusMessage, ttlMs nullable, pollIntervalMs) and add
  DetailedTask with status-discriminated payloads (inputRequests/result/error
  inlined per spec)
- CreateTaskResult now flattens Task with resultType: "task";
  add ResultType::TASK and CallToolResponse::Task
- Add tasks/update (UpdateTaskParams with MRTR InputResponses); tasks/get and
  tasks/cancel reworked; remove tasks/list, tasks/result
- notifications/tasks now carries a full DetailedTask; removed from client
  notifications (SEP-2260)

Capabilities:
- Remove core TasksCapability et al; tasks are declared via the extensions
  map (enable_tasks() builders, supports_tasks() accessors)
- Remove tool-level execution.taskSupport and the _meta.task /
  TaskMetadata / with_task() opt-in: task creation is server-directed and
  gated on the per-request client capability

Runtime:
- Replace OperationProcessor with TaskManager: durable-before-response task
  creation, input_required round-trips via tasks/update, cooperative
  cancellation, TTL expiry
- Client peer helpers get_task/update_task/cancel_task
- Emit Mcp-Name routing header from params.taskId for tasks/* methods
  (SEP-2243/2663)

Macros:
- Remove #[task_handler] and the tool execution() attribute

Tests/examples/docs updated; schema snapshots regenerated.

BREAKING CHANGE: the 2025-11-25 experimental tasks API is removed without a
compatibility shim. Clients that do not declare the tasks extension always
receive synchronous results.

* feat: gate tasks/* methods on the client tasks-extension capability (SEP-2663)

When the server advertises io.modelcontextprotocol/tasks but the client did
not declare it (per-request _meta clientCapabilities, or initialize-time
capabilities in session mode), tasks/get, tasks/update, and tasks/cancel now
return -32021 Missing Required Client Capability with the required extension
in the error data, instead of falling through to the handler's -32601
default. Servers that do not advertise the extension keep returning -32601.

Also add regression tests confirming that unknown taskIds yield -32602 and
that a legacy 2025-11-25 'task' param on tools/call is silently ignored.

* feat: pass SEP-2663 Tasks extension conformance suite

Conformance fixtures (conformance/src/bin/server.rs):
- Add the required fixture tools: greet (sync-only), slow_compute,
  failing_job (task support: required), protocol_error_job, confirm_delete,
  multi_input, and test_tool_with_task (MRTR -> task escalation), all backed
  by TaskManager with server-directed task creation gated on the client's
  tasks-extension capability
- Advertise io.modelcontextprotocol/tasks in server capabilities and wire
  get_task/update_task/cancel_task handlers
- Task-required tools reject with -32021 when the client did not declare the
  extension

SDK wire-shape fixes surfaced by the suite:
- Add resultType: "complete" to GetTaskResult and introduce TaskAckResult
  so tasks/update and tasks/cancel acks carry the SEP-2322 discriminator
  (spec: every non-CreateTaskResult response on the tasks surface is
  resultType complete); dispatch now returns ServerResult::task_ack
- CreateTaskResult gets a strict deserializer requiring resultType: "task"
  so it does not shadow task-shaped results in untagged unions
- Client update_task/cancel_task accept both TaskAckResult and EmptyResult

All 9 runnable Tasks extension server scenarios now pass (35/35 checks;
tasks-status-notifications remains upstream-skipped), so the corresponding
entries are removed from conformance/expected-failures-extensions.yaml.
2025-11-25 server suite (40/40), 2026-07-28 server suite (114/114), draft
client suite, and extensions client suite all pass their baselines.

* fix: address PR 1020 review feedback on DetailedTask schema and cooperative cancel

- DetailedTask's JsonSchema now derives from the actual flattened wire shape
  (DetailedTaskWire: base Task + optional inputRequests/result/error) instead
  of approximating with the base Task schema, so generated schemas for
  GetTaskResult and notifications/tasks document the status-specific payload
  fields. Golden message schemas regenerated.

- TaskManager::cancel_task no longer aborts the underlying future. It records
  the observable cancelled state and acks immediately (spec's eventually
  consistent semantics), but lets the operation keep running so it can observe
  is_cancel_requested() or the error from a woken request_input() and perform
  cleanup; any late result is discarded. New unit tests cover both the
  cooperative-cleanup path and waking parked input requests.

* fix: make tasks/cancel truly cooperative per SEP-2663 (FEEDBACK_2)

- TaskManager::cancel_task no longer forces terminal 'cancelled'. It records
  the cancellation intent, acks immediately, and wakes parked request_input
  awaits, but the operation decides its own terminal state: a post-cancel
  error settles as 'cancelled', while an operation that finishes its work
  settles as 'completed' — per the spec, 'the task may still reach a
  non-cancelled terminal status'.
- Add TaskContext::cancelled(), a watch-based await for use with
  tokio::select! as the cooperative cancellation exit path.
- Correct the unsupported-notification method string from
  'notifications/tasks/status' to 'notifications/tasks' and document that
  task status notifications are not yet routable through subscriptions/listen
  (SubscriptionFilter has no taskIds field; upstream check still skipped).
- Update conformance slow_compute fixture, task_demo example, and tests to
  honor cancellation via ctx.cancelled(); lifecycle conformance still 8/8
  (all 9 scenarios remain green, 35/35 checks).

* fix: sweep and evict expired tasks from TaskManager (FEEDBACK_3)

TTL handling previously only ran from get_task and only flipped overdue
non-terminal tasks to 'failed', never removing entries — an unbounded leak
for long-lived servers, and it made ttl_ms: None = 'unlimited retention'
meaningless since everything was retained forever.

- Rename expire_overdue to sweep_expired and run it from every entry point
  (spawn, get_task, update_task, cancel_task).
- Track terminal_at on each entry; terminal tasks are evicted after being
  retained for one further ttl_ms window past their terminal transition, so
  well-behaved pollers can observe the final state before late tasks/get
  calls return -32602 (spec: servers may delete expired tasks at any time,
  and returning task-not-found for purged tasks is compliant behavior).
- ttl_ms: None entries are never evicted (spec: unlimited retention);
  document the retention model on TaskManager, including that there is no
  background sweeper.
- New tests: retention-window eviction, sweep of abandoned tasks via other
  entry points, unlimited-TTL retention, and error-code assertions
  (-32602) for unknown ids across get/update/cancel.

Reviewed the second feedback item (dedicated task-not-found error code)
against SEP-2663 §Protocol Errors and it is incorrect: the SEP explicitly
specifies -32602 (Invalid params) for invalid or nonexistent taskIds, which
is what unknown_task already returns. Kept -32602; strengthened tests to
assert the code.

* fix: address Copilot review feedback on PR #1020

- TaskManager: drop the JoinHandle as soon as the operation settles instead
  of retaining it for the whole retention window, and only store it at spawn
  time if the task is still non-terminal (avoids keeping a completed handle
  that the completion path could never clear).
- Use RequestContext::client_capabilities() (which applies the initialize-time
  fallback for session peers) instead of raw context.meta.client_capabilities()
  in the task_demo example, the README snippet, and the test server — the
  meta-only form would wrongly treat session-declared tasks clients as
  unsupported.

All 9 Tasks extension conformance scenarios remain green (35/35 checks).

* fix: enforce SEP-2663 task-result gating in tools/call dispatch

A handler could return CallToolResponse::Task without checking whether the
request declared the io.modelcontextprotocol/tasks extension, sending a
CreateTaskResult to a client that cannot parse it. The SDK dispatch now
rejects that case with -32021 Missing Required Client Capability before the
response leaves the server, using RequestContext::client_capabilities()
(per-request _meta with initialize-time fallback).

Adds a regression test with a deliberately misbehaving handler that always
materializes a task; all Tasks extension conformance scenarios remain green
(35/35 checks).

* fix: align TTL boundary comparisons in sweep_expired

Copilot review: ttlMs: 0 never expired immediately because phase 1 used a
strict 'elapsed > ttl_ms' comparison, and phase 2 retained terminal tasks
at exactly the TTL boundary ('elapsed <= ttl_ms'). Treat elapsed >= ttl_ms
as expired in phase 1 and evict when elapsed >= ttl_ms in phase 2, so both
phases agree at the boundary and ttlMs: 0 expires/evicts on the first sweep.

* fix: strict TaskAckResult deserializer and TASK_REQUIRED_TOOLS consistency

- TaskAckResult carried only resultType (+ optional _meta) with a derived
  Deserialize, so inside the untagged ServerResult union it greedily matched
  any result object containing a resultType key, shadowing CustomResult and
  losing data (verified with a probe). Replace with a strict deserializer:
  deny_unknown_fields and require resultType == "complete". Regression
  tests cover both the non-matching shapes and the genuine ack shape.

- Conformance fixtures: confirm_delete and multi_input rejected non-tasks
  clients inline, contradicting the TASK_REQUIRED_TOOLS doc/constant. Move
  them into TASK_REQUIRED_TOOLS (they park on in-task elicitation and have
  no synchronous fallback) so the upfront -32021 gate is the single source
  of truth, and drop the now-unreachable inline checks.

All 9 Tasks extension conformance scenarios remain green (35/35 checks).

* fix: let the operation decide its terminal state via TaskExit

After tasks/cancel, any operation error was coerced to terminal
'cancelled', masking real failures (e.g. an unrelated error landing just
after a late cancel request) and contradicting the documented 'operation
decides its own terminal state' contract.

Change TaskFuture's error type from McpError to a new TaskExit enum:

- TaskExit::Cancelled — an explicit cooperative-cancellation exit;
  settles as terminal 'cancelled'.
- TaskExit::Error(McpError) — a real failure; settles as terminal
  'failed' with the error inlined, even after tasks/cancel was received.

From<McpError> for TaskExit keeps '?' ergonomic in task bodies, and
request_input() now returns TaskExit directly (its wake-on-cancel path
yields TaskExit::Cancelled), so parked operations that propagate it with
'?' settle as 'cancelled' automatically.

Update the conformance fixtures, task_demo example, and tests; add a
regression test asserting a post-cancel unrelated error settles as
'failed' with its error payload preserved.

All 9 Tasks extension conformance scenarios remain green (35/35 checks).

* fix: allow TaskExit enum to be exhaustive

* fix: close spawn/shutdown race, sweep in running_task_count, clarify TTL retention docs

Addresses branch-review findings:

- spawn() raced with shutdown(): if shutdown drained the task map between
  the entry insert and the JoinHandle store, the handle was dropped and the
  operation kept running detached. If the entry is gone at store time, the
  handle is now aborted instead.
- running_task_count() now runs the TTL sweep like every other entry point,
  so it no longer reports overdue tasks as running (matching the documented
  sweep-on-every-entry-point behavior). Regression test added.
- Document that terminal-task retention intentionally extends one ttl_ms
  window past the terminal transition (observation grace period) beyond the
  creation-based lifetime ttlMs advertises on the wire — compliant since
  SEP-2663 allows deleting expired tasks at any time after the TTL.
2026-07-22 14:16:52 -04:00

35 KiB

RMCP

Crates.io Version docs.rs CI License

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

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:

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.

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.


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.

Examples

See examples.

OAuth Support

See Oauth_support for details.

Extending rmcp

Built with rmcp

  • goose - An open-source, extensible AI agent that goes beyond code suggestions
  • apollo-mcp-server - MCP server that connects AI agents to GraphQL APIs via Apollo GraphOS
  • rustfs-mcp - High-performance MCP server providing S3-compatible object storage operations for AI/LLM integration
  • containerd-mcp-server - A containerd-based MCP server implementation
  • rmcp-openapi-server - High-performance MCP server that exposes OpenAPI definition endpoints as MCP tools
  • nvim-mcp - A MCP server to interact with Neovim
  • terminator - AI-powered desktop automation MCP server with cross-platform support and >95% success rate
  • stakpak-agent - Security-hardened terminal agent for DevOps with MCP over mTLS, streaming, secret tokenization, and async task management
  • video-transcriber-mcp-rs - High-performance MCP server for transcribing videos from 1000+ platforms using whisper.cpp
  • NexusCore MCP - Advanced malware analysis & dynamic instrumentation MCP server with Frida integration and stealth unpacking capabilities
  • spreadsheet-mcp - Token-efficient MCP server for spreadsheet analysis with automatic region detection, recalculation, screenshot, and editing support for LLM agents
  • hyper-mcp - A fast, secure MCP server that extends its capabilities through WebAssembly (WASM) plugins
  • rudof-mcp - RDF validation and data processing MCP server with ShEx/SHACL validation, SPARQL queries, and format conversion. Supports stdio and streamable HTTP transports with full MCP capabilities (tools, prompts, resources, logging, completions, tasks)
  • MCPMate - Desktop app for progressive MCP management: start with guided server import, then grow into multi-client profiles and Unify meta tools to keep tool exposure, token use, and runtime state under control, with more options for efficiency, cost, and reliability
  • McpMux - Desktop app to configure MCP servers once at McpMux, connect every AI client (Cursor, Claude Desktop, VS Code, Windsurf) through a single encrypted local gateway with Spaces for project organization, FeatureSets to switch toolsets per client, and a built-in server registry
  • systemprompt-template - Single-binary Rust runtime providing MCP governance — authentication, authorisation, rate-limiting, audit trails, and cost tracking for AI agents. Self-hosted, air-gap capable, 3,300+ req/s with sub-5ms governance overhead
  • jilebi-mcp - an extensible MCP server through plugins in Javascript with a secure permissions model

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.