mcp-host

Production-grade Rust crate for building Model Context Protocol (MCP) servers.

Features

• MCP Protocol 2025-11-25: Full JSON-RPC 2.0 with protocol version negotiation
• Tools, Resources, Prompts: All three core MCP primitives
• Elicitation: Request structured user input with JSON Schema validation
• Tasks: Async task execution with status tracking
• Bidirectional Communication: Server→Client requests (roots/list, sampling/createMessage)
• Contextual Visibility: Per-session tool/resource/prompt filtering
• Transport Abstraction: STDIO and HTTP transports
• Middleware Chain: Extensible request processing pipeline
• Session Management: Thread-safe multi-session support with lifecycle state machine
• Background Notifications: Send notifications from background tasks
• Fluent Builder API: Ergonomic server construction

Resilience Patterns

• Circuit Breakers: Per-tool failure protection using breaker-machines
• Retry with Backoff: Exponential backoff with jitter via chrono-machines
• Rate Limiting: GCRA algorithm from throttle-machines
• Structured Logging: MCP-compliant notifications/message for LLM visibility

Requirements

• Rust 1.92+ (edition 2024)
• Tokio runtime

Installation

[dependencies]
mcp-host = "0.1"

Quick Start

use mcp_host::prelude::*;
use async_trait::async_trait;
use serde_json::Value;

// Define a tool
struct EchoTool;

#[async_trait]
impl Tool for EchoTool {
    fn name(&self) -> &str { "echo" }

    fn description(&self) -> Option<&str> {
        Some("Echoes back the input message")
    }

    fn input_schema(&self) -> Value {
        serde_json::json!({
            "type": "object",
            "properties": {
                "message": { "type": "string" }
            },
            "required": ["message"]
        })
    }

    async fn execute(&self, ctx: ExecutionContext<'_>) -> Result<Vec<Box<dyn Content>>, ToolError> {
        let msg = ctx.params["message"].as_str().unwrap_or("?");
        Ok(vec![Box::new(TextContent::new(format!("Echo: {}", msg)))])
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = Server::new("my-server", "0.1.0");
    server.tool_registry().register(EchoTool);

    server.set_capabilities(ServerCapabilities {
        tools: Some(mcp_host::protocol::capabilities::ToolsCapability {
            list_changed: Some(false),
        }),
        ..Default::default()
    });

    server.run(StdioTransport::new()).await
}

Architecture

Core Components

Traits

// Implement custom tools
#[async_trait]
pub trait Tool: Send + Sync {
    fn name(&self) -> &str;
    fn description(&self) -> Option<&str>;
    fn input_schema(&self) -> Value;

    // Optional: contextual visibility (default: always visible)
    fn is_visible(&self, ctx: &VisibilityContext) -> bool { true }

    // Execute with full session context
    async fn execute(&self, ctx: ExecutionContext<'_>) -> Result<Vec<Box<dyn Content>>, ToolError>;
}

// Implement custom resources
#[async_trait]
pub trait Resource: Send + Sync {
    fn uri(&self) -> &str;
    fn name(&self) -> &str;
    fn description(&self) -> Option<&str>;
    fn mime_type(&self) -> Option<&str>;

    fn is_visible(&self, ctx: &VisibilityContext) -> bool { true }

    async fn read(&self, ctx: ExecutionContext<'_>) -> Result<Vec<Box<dyn Content>>, ResourceError>;
}

// Implement custom prompts
#[async_trait]
pub trait Prompt: Send + Sync {
    fn name(&self) -> &str;
    fn description(&self) -> Option<&str>;
    fn arguments(&self) -> Option<Vec<PromptArgument>>;

    fn is_visible(&self, ctx: &VisibilityContext) -> bool { true }

    async fn get(&self, ctx: ExecutionContext<'_>) -> Result<GetPromptResult, PromptError>;
}

Bidirectional Communication

Request data from clients:

// Request workspace roots (requires client roots capability)
let roots = server.request_roots("session-id", None).await?;
for root in roots {
    println!("Root: {} ({})", root.name.unwrap_or_default(), root.uri);
}

// Request LLM completion (requires client sampling capability)
let params = CreateMessageParams {
    messages: vec![SamplingMessage {
        role: "user".to_string(),
        content: SamplingContent::Text { text: "Hello!".to_string() },
    }],
    max_tokens: 1000,
    ..Default::default()
};
let result = server.request_sampling("session-id", params, None).await?;

Visibility Context

Filter tools/resources/prompts based on session state:

struct GitCommitTool;

impl Tool for GitCommitTool {
    // Only visible when in a git repo with uncommitted changes
    fn is_visible(&self, ctx: &VisibilityContext) -> bool {
        ctx.environment
            .map(|e| e.has_git_repo() && !e.git_is_clean())
            .unwrap_or(false)
    }
    // ...
}

struct AdminOnlyResource;

impl Resource for AdminOnlyResource {
    // Only visible to admin users
    fn is_visible(&self, ctx: &VisibilityContext) -> bool {
        ctx.is_admin()
    }
    // ...
}

Server Builder

use mcp_host::prelude::*;

let server = server("my-server", "1.0.0")
    .with_tools(true)
    .with_resources(true, false)
    .with_prompts(true)
    .with_logging()
    .with_logging_middleware()
    .with_validation_middleware()
    .build();

Resilience Configuration

use mcp_host::prelude::*;

let server = server("resilient-server", "1.0.0")
    .with_tools(true)
    .with_resources(true, false)
    // Circuit breaker: opens after 3 failures in 60s
    .with_circuit_breaker(ToolBreakerConfig {
        failure_threshold: 3,
        failure_window_secs: 60.0,
        half_open_timeout_secs: 10.0,
        success_threshold: 2,
    })
    // Retry: exponential backoff with full jitter
    .with_retry(ResourceRetryConfig {
        max_attempts: 3,
        base_delay_ms: 100,
        multiplier: 2.0,
        max_delay_ms: 5000,
        jitter_factor: 1.0,
    })
    // Rate limit: 100 req/s with burst of 20
    .with_rate_limit(100.0, 20)
    .build();

MCP Logging (LLM-visible)

use mcp_host::prelude::*;

// Create logger with notification channel
let logger = McpLogger::new(notification_tx, "my-tool");

// Log messages are visible to the LLM via notifications/message
logger.info("Tool initialized successfully");
logger.warning("Rate limit approaching threshold");
logger.error("External API unavailable");

Notifications

Send notifications from background tasks:

let server = Server::new("server", "1.0.0");
let notification_tx = server.notification_sender();

// Spawn background task
tokio::spawn(async move {
    notification_tx.send(JsonRpcNotification::new(
        "notifications/progress",
        Some(serde_json::json!({ "progress": 50 })),
    )).ok();
});

Feature Flags

# Minimal (STDIO only)
mcp-host = "0.1"

# With HTTP transport
mcp-host = { version = "0.1", features = ["http"] }

# With macros for ergonomic tool definition
mcp-host = { version = "0.1", features = ["macros"] }

# Everything
mcp-host = { version = "0.1", features = ["full"] }

Supported MCP Methods

Client→Server

• initialize / ping
• tools/list / tools/call
• resources/list / resources/read / resources/subscribe / resources/unsubscribe
• prompts/list / prompts/get
• completion/complete
• tasks/list / tasks/get / tasks/cancel

Server→Client (Bidirectional)

• roots/list - Request workspace roots from client
• sampling/createMessage - Request LLM completion from client
• elicitation/create - Request structured user input with schema validation

Examples

# Run the comprehensive PROMETHEUS example with all features
cargo run --example prometheus_project

# Run with HTTP transport (requires http feature)
cargo run --example prometheus_project --features http -- --http --port 8080

License

BSD-3-Clause