agentkit-mcp

Model Context Protocol integration for agentkit, built on top of the official rmcp Rust SDK.

This crate covers:

• stdio and Streamable HTTP transports (driven by rmcp)
• multi-server lifecycle, discovery, and catalog diffing via McpServerManager
• adapters that surface MCP servers as agentkit tools and capabilities
• pluggable client-side responders for sampling/createMessage, elicitation/create, and roots/list
• a broadcast subscription for server-pushed events: progress, logging, resource updates, list-changed, cancellation
• auth replay for MCP operations that fail with an authentication challenge

The wire-protocol types — CallToolResult, ReadResourceResult, GetPromptResult, Content, RawContent, ToolAnnotations, Prompt, etc. — are re-exported from rmcp directly. There is no parallel agentkit-side type vocabulary to maintain.

Why this matters

Re-exporting rmcp::model keeps agentkit-mcp in lockstep with the MCP spec — new fields, content variants, capability flags, server-initiated requests, and notification payloads land in agentkit the moment rmcp ships them. No second source of truth to drift.

• Spec: modelcontextprotocol.io
• Rust SDK: rmcp on crates.io

The same applies to transports: any future rmcp transport is reachable through McpConnection::from_running_service_with_events without touching the built-in McpTransportBinding enum.

Configuring and connecting MCP servers

Register one or more MCP server configurations with McpServerManager, then connect them. Each connected server is represented by an McpServerHandle that holds the live connection and the discovery snapshot.

use agentkit_mcp::{
    McpServerConfig, McpServerManager, McpTransportBinding, StdioTransportConfig,
};

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
let mut manager = McpServerManager::new()
    .with_server(McpServerConfig::new(
        "filesystem",
        McpTransportBinding::Stdio(
            StdioTransportConfig::new("npx")
                .with_arg("-y")
                .with_arg("@modelcontextprotocol/server-filesystem"),
        ),
    ))
    .with_server(McpServerConfig::new(
        "github",
        McpTransportBinding::Stdio(
            StdioTransportConfig::new("npx")
                .with_arg("-y")
                .with_arg("@modelcontextprotocol/server-github")
                .with_env("GITHUB_TOKEN", "ghp_..."),
        ),
    ));

let handles = manager.connect_all().await?;
println!("connected {} MCP server(s)", handles.len());
# Ok(())
# }

Discovering tools

After connecting, each server's capabilities are available through its discovery snapshot. The tools/resources/prompts fields hold the raw rmcp types — pattern-match on them directly for output_schema, annotations, mime_type, and friends.

use agentkit_mcp::{
    McpServerConfig, McpServerManager, McpServerId, McpTransportBinding,
    StdioTransportConfig,
};

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
# let mut manager = McpServerManager::new().with_server(McpServerConfig::new(
#     "filesystem",
#     McpTransportBinding::Stdio(StdioTransportConfig::new("npx")
#         .with_arg("-y").with_arg("@modelcontextprotocol/server-filesystem")),
# ));
# manager.connect_all().await?;
let handle = manager.connected_server(&McpServerId::new("filesystem")).unwrap();
for tool in &handle.snapshot().tools {
    println!("  {} - {}", tool.name, tool.description.as_deref().unwrap_or(""));
}

let registry = manager.tool_registry();
for spec in registry.specs() {
    println!("{}", spec.name); // e.g. "mcp_filesystem_read_file"
}
# Ok(())
# }

Using MCP tools in an agent

The tool registry and capability provider produced by McpServerManager plug straight into the agentkit agent loop. Tools are namespaced as mcp_<server_id>_<tool_name> by default.

use agentkit_mcp::{
    McpServerConfig, McpServerManager, McpTransportBinding, StdioTransportConfig,
};

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
# let mut manager = McpServerManager::new().with_server(McpServerConfig::new(
#     "filesystem",
#     McpTransportBinding::Stdio(StdioTransportConfig::new("npx")
#         .with_arg("-y").with_arg("@modelcontextprotocol/server-filesystem")),
# ));
# manager.connect_all().await?;
let tool_registry = manager.tool_registry();
let capability_provider = manager.capability_provider();
# Ok(())
# }

Pick a different convention — strip the prefix, replace it with dots, anything — by installing an McpToolNamespace::Custom strategy:

use agentkit_mcp::{McpServerManager, McpToolNamespace};

let manager = McpServerManager::new().with_namespace(McpToolNamespace::custom(
    |server, name| format!("remote.{server}.{name}"),
));

Streamable HTTP transport

For modern remote MCP servers exposed over HTTP, use the Streamable HTTP transport. The bearer token (or any custom header) is set declaratively on the binding — rmcp drives the JSON/SSE response handling, session header propagation, and resumption with Last-Event-ID.

use agentkit_mcp::{
    McpServerConfig, McpServerManager, McpTransportBinding, StreamableHttpTransportConfig,
};

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
let mut manager = McpServerManager::new().with_server(McpServerConfig::new(
    "remote",
    McpTransportBinding::StreamableHttp(
        StreamableHttpTransportConfig::new("https://mcp.example.com/mcp")
            .with_bearer_token("tok_abc123"),
    ),
));

let handles = manager.connect_all().await?;
# Ok(())
# }

To rotate the bearer at runtime, drive an AuthResolution::Provided { credentials, .. } through McpServerManager::resolve_auth (or McpConnection::resolve_auth directly). The next operation reconnects with the new credentials.

Sampling, elicitation, and roots

Servers can issue requests back into the client: sampling/createMessage (ask the host LLM to generate), elicitation/create (ask the user for input), roots/list (enumerate workspace roots in scope). Wire in trait implementations to handle each:

use std::sync::Arc;
use async_trait::async_trait;
use agentkit_mcp::{
    McpCreateMessageRequestParams, McpCreateMessageResult, McpError, McpHandlerConfig, McpRoot,
    McpRootsProvider, McpSamplingMessage, McpSamplingResponder, McpServerManager,
};

struct HostSampling;
#[async_trait]
impl McpSamplingResponder for HostSampling {
    async fn create_message(
        &self,
        _params: McpCreateMessageRequestParams,
    ) -> Result<McpCreateMessageResult, McpError> {
        Ok(McpCreateMessageResult::new(
            McpSamplingMessage::assistant_text("(host LLM response)"),
            "host-model".into(),
        ))
    }
}

struct StaticRoots;
#[async_trait]
impl McpRootsProvider for StaticRoots {
    async fn list_roots(&self) -> Result<Vec<McpRoot>, McpError> {
        Ok(vec![McpRoot::new("file:///workspace").with_name("workspace")])
    }
}

let manager = McpServerManager::new().with_handler_config(
    McpHandlerConfig::new()
        .with_sampling_responder(Arc::new(HostSampling))
        .with_roots_provider(Arc::new(StaticRoots)),
);

McpElicitationResponder follows the same shape. The handler advertises the corresponding ClientCapabilities entry only when a responder is installed — servers that probe client.capabilities.sampling will see the host opt in.

Subscribing to server events

McpConnection::subscribe_events returns a tokio::sync::broadcast::Receiver<McpServerEvent> that surfaces every push notification the server sends:

• Progress — keyed by the progress_token the client issued in a request
• Logging — notifications/message; throttled by set_logging_level
• ResourceUpdated — for URIs the client subscribed to via subscribe_resource
• ToolListChanged / ResourceListChanged / PromptListChanged
• Cancelled — server-initiated cancellation of an in-flight request

use agentkit_mcp::{McpConnection, McpServerEvent};

# async fn watch(connection: &McpConnection) -> Result<(), Box<dyn std::error::Error>> {
let mut events = connection.subscribe_events();
connection.subscribe_resource("memo:welcome").await?;
while let Ok(event) = events.recv().await {
    match event {
        McpServerEvent::Progress(progress) => println!("progress: {}", progress.progress),
        McpServerEvent::Logging(message) => println!("log: {message:?}"),
        McpServerEvent::ResourceUpdated(updated) => println!("updated: {}", updated.uri),
        other => println!("event: {other:?}"),
    }
}
# Ok(())
# }

Catalog list-changed events are also delivered through the legacy McpServerNotification mpsc receiver consumed by McpServerManager::refresh_changed_catalogs. The two channels coexist: events for live UI/observability, mpsc for re-discovery.

Lifecycle management

use agentkit_mcp::{
    McpServerConfig, McpServerManager, McpServerId, McpTransportBinding,
    StdioTransportConfig,
};

# #[tokio::main]
# async fn main() -> Result<(), Box<dyn std::error::Error>> {
# let mut manager = McpServerManager::new().with_server(McpServerConfig::new(
#     "filesystem",
#     McpTransportBinding::Stdio(StdioTransportConfig::new("npx")
#         .with_arg("-y").with_arg("@modelcontextprotocol/server-filesystem")),
# ));
let server_id = McpServerId::new("filesystem");

let handle = manager.connect_server(&server_id).await?;
let snapshot = manager.refresh_server(&server_id).await?;
println!("now has {} tools", snapshot.tools.len());
manager.disconnect_server(&server_id).await?;
# Ok(())
# }

manager.refresh_changed_catalogs() drains pending list-changed notifications across every connection and re-runs discovery for each affected server, returning the diffs as McpCatalogEvents.

Custom transports

When you need a transport rmcp supports but McpTransportBinding does not (in-memory pipes, websockets, custom IO), build the rmcp RunningService directly and adopt it:

use agentkit_mcp::{McpConnection, McpHandlerConfig, McpServerId};
use rmcp::ServiceExt;

# async fn adopt(client_io: tokio::io::DuplexStream) -> Result<(), Box<dyn std::error::Error>> {
let (handler, channels) = McpHandlerConfig::new().build();
let service = handler.serve(client_io).await?;
let connection = McpConnection::from_running_service_with_events(
    McpServerId::new("in-memory"),
    service,
    channels.notifications,
    channels.events,
);
# Ok(())
# }