TurboMCP Client

MCP client with complete MCP 2025-06-18 specification support and plugin middleware system.

Table of Contents

• Overview
• Supported Transports
• Quick Start
• Transport Configuration
• Advanced Features
• Plugin Middleware
• Sampling Handler Integration
• Handler Registration
• Error Handling
• Production Deployment

Overview

turbomcp-client provides a comprehensive MCP client implementation with:

• ✅ Full MCP 2025-06-18 compliance - All server and client features
• ✅ Bidirectional communication - Server-initiated requests (sampling, elicitation)
• ✅ Plugin middleware - Extensible request/response processing
• ✅ Sampling protocol support - Handle server-initiated sampling requests
• ✅ Transport agnostic - Works with STDIO, TCP, Unix, WebSocket transports
• ✅ Thread-safe sharing - Client is cheaply cloneable via Arc for concurrent async tasks

Supported Transports

Version 2.1.1: HTTP/SSE client transport with Client::connect_http() convenience API, OAuth 2.1 support, and universal proxy compatibility.

Quick Start

Basic Client (STDIO)

use turbomcp_client::Client;
use turbomcp_transport::stdio::StdioTransport;

#[tokio::main]
async fn main() -> turbomcp_protocol::Result<()> {
    // Create client with STDIO transport
    let transport = StdioTransport::new();
    let client = Client::new(transport);

    // Initialize connection
    let result = client.initialize().await?;
    println!("Connected to: {}", result.server_info.name);

    // List and call tools
    let tools = client.list_tools().await?;
    for tool in &tools {
        println!("Tool: {} - {}", tool.name,
            tool.description.as_deref().unwrap_or("No description"));
    }

    // Call a tool
    let result = client.call_tool("calculator", Some(
        std::collections::HashMap::from([
            ("operation".to_string(), serde_json::json!("add")),
            ("a".to_string(), serde_json::json!(5)),
            ("b".to_string(), serde_json::json!(3)),
        ])
    )).await?;

    println!("Result: {}", result);
    Ok(())
}

HTTP Client (One-Liner)

use turbomcp_client::Client;

#[tokio::main]
async fn main() -> turbomcp_protocol::Result<()> {
    // One-liner - connects and initializes automatically
    let client = Client::connect_http("http://localhost:8080").await?;

    // Ready to use immediately
    let tools = client.list_tools().await?;
    println!("Found {} tools", tools.len());

    Ok(())
}

TCP/Unix Clients

// TCP
let client = Client::connect_tcp("127.0.0.1:8765").await?;

// Unix socket
let client = Client::connect_unix("/tmp/mcp.sock").await?;

With ClientBuilder

use turbomcp_client::ClientBuilder;
use turbomcp_transport::stdio::StdioTransport;

let client = ClientBuilder::new()
    .with_tools(true)
    .with_prompts(true)
    .with_resources(true)
    .with_sampling(false)
    .build(StdioTransport::new())
    .await?;

Cloning Client for Concurrent Usage

use turbomcp_client::Client;
use turbomcp_transport::stdio::StdioTransport;

// Create client (cheaply cloneable via Arc)
let client = Client::new(StdioTransport::new());

// Initialize once
client.initialize().await?;

// Clone for multiple async tasks - this is cheap (just Arc clone)
let client1 = client.clone();
let client2 = client.clone();

let handle1 = tokio::spawn(async move {
    client1.list_tools().await
});

let handle2 = tokio::spawn(async move {
    client2.list_prompts().await
});

let (tools, prompts) = tokio::try_join!(handle1, handle2)?;

Transport Configuration

STDIO Transport (Default)

use turbomcp_transport::stdio::StdioTransport;

// Direct STDIO
let transport = StdioTransport::new();
let mut client = Client::new(transport);

HTTP Transport (New in 2.0!)

use turbomcp_client::Client;

// One-liner - connects and initializes automatically
let client = Client::connect_http("http://localhost:8080").await?;

Or with custom configuration:

use turbomcp_client::Client;
use std::time::Duration;

let client = Client::connect_http_with("http://localhost:8080", |config| {
    config.timeout = Duration::from_secs(60);
    config.endpoint_path = "/api/mcp".to_string();
}).await?;

TCP Transport

use turbomcp_client::Client;

// One-liner - connects and initializes automatically
let client = Client::connect_tcp("127.0.0.1:8765").await?;

Or using transport directly:

use turbomcp_transport::tcp::TcpTransport;
use std::net::SocketAddr;

let server_addr: SocketAddr = "127.0.0.1:8765".parse()?;
let bind_addr: SocketAddr = "0.0.0.0:0".parse()?;  // Any available port
let transport = TcpTransport::new_client(bind_addr, server_addr);
let mut client = Client::new(transport);
client.initialize().await?;

Unix Socket Transport

use turbomcp_client::Client;

// One-liner - connects and initializes automatically
let client = Client::connect_unix("/tmp/mcp.sock").await?;

Or using transport directly:

use turbomcp_transport::unix::UnixTransport;
use std::path::PathBuf;

let transport = UnixTransport::new_client(PathBuf::from("/tmp/mcp.sock"));
let mut client = Client::new(transport);
client.initialize().await?;

WebSocket Transport

use turbomcp_transport::websocket_bidirectional::{
    WebSocketBidirectionalTransport,
    WebSocketBidirectionalConfig,
};

let config = WebSocketBidirectionalConfig {
    url: Some("ws://localhost:8080".to_string()),
    ..Default::default()
};

let transport = WebSocketBidirectionalTransport::new(config).await?;
let mut client = Client::new(transport);

Advanced Features

Robust Transport with Retry & Circuit Breaker

use turbomcp_client::ClientBuilder;
use turbomcp_transport::stdio::StdioTransport;

// Configure retry and health checking
let client = ClientBuilder::new()
    .with_max_retries(3)      // Configure retry attempts
    .with_retry_delay(100)    // Retry delay in milliseconds
    .with_keepalive(30_000)   // Keepalive interval
    .build(StdioTransport::new())
    .await?;

Additional Configuration Options

use turbomcp_client::ClientBuilder;
use turbomcp_transport::stdio::StdioTransport;
use std::time::Duration;

let client = ClientBuilder::new()
    // Configure capabilities needed from the server
    .with_tools(true)
    .with_prompts(true)
    .with_resources(true)
    // Configure timeouts and retries
    .with_timeout(30_000)          // 30 second timeout
    .with_max_retries(3)           // Retry up to 3 times
    .with_retry_delay(100)         // 100ms delay between retries
    .with_keepalive(30_000)        // 30 second keepalive
    // Build the client
    .build(StdioTransport::new())
    .await?;

Plugin Middleware

use turbomcp_client::ClientBuilder;
use turbomcp_client::plugins::{MetricsPlugin, PluginConfig};
use std::sync::Arc;

let client = ClientBuilder::new()
    .with_plugin(Arc::new(MetricsPlugin::new(PluginConfig::Metrics)))
    .build(StdioTransport::new())
    .await?;

Sampling Handler Integration

Handle server-initiated sampling requests by implementing a custom sampling handler:

use turbomcp_client::sampling::SamplingHandler;
use turbomcp_protocol::types::{CreateMessageRequest, CreateMessageResult, Role, Content, TextContent};
use async_trait::async_trait;
use std::sync::Arc;

#[derive(Debug)]
struct MySamplingHandler {
    // Your LLM integration (OpenAI, Anthropic, local model, etc.)
}

#[async_trait]
impl SamplingHandler for MySamplingHandler {
    async fn handle_create_message(&self, request_id: String, request: CreateMessageRequest)
        -> Result<CreateMessageResult, Box<dyn std::error::Error + Send + Sync>>
    {
        // Forward to your LLM service
        // Use request_id for correlation/tracking
        // Return the generated response
        Ok(CreateMessageResult {
            role: Role::Assistant,
            content: Content::Text(TextContent {
                text: "Generated response".to_string(),
                annotations: None,
                meta: None,
            }),
            model: Some("your-model".to_string()),
            stop_reason: None,
        })
    }
}

// Register the handler
let handler = Arc::new(MySamplingHandler { /* ... */ });
client.set_sampling_handler(handler);

Note: TurboMCP provides the sampling protocol infrastructure. You implement your own LLM integration (OpenAI SDK, Anthropic SDK, local models, etc.) as needed for your use case.

Handler Registration

use turbomcp_client::handlers::{ElicitationHandler, ElicitationRequest, ElicitationResponse};
use async_trait::async_trait;
use std::sync::Arc;

#[derive(Debug)]
struct MyElicitationHandler;

#[async_trait]
impl ElicitationHandler for MyElicitationHandler {
    async fn handle_elicitation(&self, request: ElicitationRequest)
        -> Result<ElicitationResponse, Box<dyn std::error::Error + Send + Sync>>
    {
        // Prompt user for input based on request.schema
        let user_input = collect_user_input(request.schema)?;
        Ok(ElicitationResponse {
            action: ElicitationAction::Accept,
            content: Some(user_input),
        })
    }
}

let client = ClientBuilder::new()
    .with_elicitation_handler(Arc::new(MyElicitationHandler))
    .build(StdioTransport::new())
    .await?;

MCP Operations

Tools

// List available tools
let tools = client.list_tools().await?;
for tool in &tools {
    println!("{}: {}", tool.name, tool.description.as_deref().unwrap_or(""));
}

// List tool names only
let names = client.list_tool_names().await?;

// Call a tool
use std::collections::HashMap;
let mut args = HashMap::new();
args.insert("text".to_string(), serde_json::json!("Hello, world!"));
let result = client.call_tool("echo", Some(args)).await?;

Prompts

use turbomcp_protocol::types::PromptInput;

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

// Get prompt with arguments
let prompt_args = PromptInput {
    arguments: Some(std::collections::HashMap::from([
        ("language".to_string(), "rust".to_string()),
        ("topic".to_string(), "async programming".to_string()),
    ])),
};

let result = client.get_prompt("code_review", Some(prompt_args)).await?;
println!("Prompt: {}", result.description.unwrap_or_default());
for message in result.messages {
    println!("{:?}: {}", message.role, message.content);
}

Resources

// List resources
let resources = client.list_resources().await?;

// Read a resource
let content = client.read_resource("file:///etc/hosts").await?;

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

Completions

use turbomcp_protocol::types::CompletionContext;

// Complete a prompt argument
let completions = client.complete_prompt(
    "code_review",
    "framework",
    "tok",  // Partial input
    None
).await?;

for value in completions.completion.values {
    println!("Suggestion: {}", value);
}

// Complete with context
let mut context_args = std::collections::HashMap::new();
context_args.insert("language".to_string(), "rust".to_string());
let context = CompletionContext { arguments: Some(context_args) };

let completions = client.complete_prompt(
    "code_review",
    "framework",
    "tok",
    Some(context)
).await?;

Subscriptions

use turbomcp_protocol::types::LogLevel;

// Subscribe to resource updates
client.subscribe("file:///config.json").await?;

// Set logging level
client.set_log_level(LogLevel::Debug).await?;

// Unsubscribe
client.unsubscribe("file:///config.json").await?;

Health Monitoring

// Send ping to check connection
let ping_result = client.ping().await?;
println!("Server responded: {:?}", ping_result);

Bidirectional Communication

Processing Server-Initiated Requests

use turbomcp_client::Client;
use turbomcp_transport::stdio::StdioTransport;

// Create and initialize client
let client = Client::new(StdioTransport::new());
client.initialize().await?;

// Message processing is automatic! The MessageDispatcher runs in the background.
// No need for manual message loops - just use the client directly.

// Perform operations - bidirectional communication works automatically
let tools = client.list_tools().await?;

Error Handling

use turbomcp_protocol::Error;

match client.call_tool("my_tool", None).await {
    Ok(result) => println!("Success: {}", result),
    Err(Error::Transport(msg)) => eprintln!("Transport error: {}", msg),
    Err(Error::Protocol(msg)) => eprintln!("Protocol error: {}", msg),
    Err(Error::BadRequest(msg)) => eprintln!("Bad request: {}", msg),
    Err(e) => eprintln!("Error: {}", e),
}

Examples

For working client examples, see the parent turbomcp crate examples:

• elicitation_client.rs - Interactive elicitation handling
• basic_client.rs - Basic client usage
• stdio_client.rs - STDIO transport example
• tcp_client.rs - TCP transport example
• http_client_simple.rs - HTTP transport example
• websocket_client_simple.rs - WebSocket transport example

Run examples from the workspace root:

cargo run --example elicitation_client
cargo run --example basic_client
cargo run --example stdio_client

Feature Flags

Enable features in Cargo.toml:

[dependencies]
turbomcp-client = { version = "3.0.0-exp", features = ["tcp", "websocket"] }

Architecture

┌─────────────────────────────────────────────┐
│            Application Code                 │
└─────────────────────────────────────────────┘
                    ↓
┌─────────────────────────────────────────────┐
│           Client API (Clone-able)           │
│  ├── initialize(), list_tools(), etc.      │
│  ├── Handler Registry (elicitation, etc.)  │
│  └── Plugin Registry (metrics, etc.)       │
└─────────────────────────────────────────────┘
                    ↓
┌─────────────────────────────────────────────┐
│       Protocol Layer (JSON-RPC)             │
│  ├── Request/Response correlation          │
│  ├── Bidirectional message routing         │
│  └── Capability negotiation                │
└─────────────────────────────────────────────┘
                    ↓
┌─────────────────────────────────────────────┐
│       Transport Layer                       │
│  ├── STDIO, TCP, Unix, WebSocket           │
│  ├── RobustTransport (retry, circuit)      │
│  └── Connection management                 │
└─────────────────────────────────────────────┘

Development

Building

# Build with default features (STDIO only)
cargo build

# Build with all transport features
cargo build --features tcp,unix,websocket,http

# Build with robustness features
cargo build --all-features

Testing

# Run unit tests
cargo test

# Run with specific features
cargo test --features websocket

# Run examples
cargo run --example sampling_client

Related Crates

• turbomcp - Main framework with server macros
• turbomcp-protocol - Protocol types and core utilities
• turbomcp-transport - Transport implementations

Resources

• MCP Specification - Official protocol docs
• MCP 2025-06-18 Spec - Current version
• TurboMCP Documentation - Framework docs

Roadmap

Version 2.0.4 Features

• HTTP/SSE Client Transport - Client-side HTTP/SSE with Client::connect_http()
• Convenience Constructors - One-liner client creation for all transports
• Ergonomic Config Builders - Simplified configuration APIs

Planned Features

• Connection Pool Management - Multi-server connection pooling
• Session Persistence - Automatic state preservation across reconnects
• Roots Handler - Complete filesystem roots implementation
• Progress Reporting - Client-side progress emission
• Batch Requests - Send multiple requests in single message

License

Licensed under the MIT License.

Part of the TurboMCP Rust SDK for the Model Context Protocol.