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Overview mcp-lite

A small, fetch-first implementation of the Model Context Protocol (MCP) server APIs.

mcp-lite is a ground-up rewrite of the TypeScript MCP SDK. It keeps only the pieces you need to stand up a server: JSON-RPC handling, typed tool definitions, and an HTTP + SSE transport that works anywhere Request and Response are available (Node, Bun, Cloudflare Workers, Deno, browsers with Service Workers).

What You Get

Section titled “What You Get”
  • Minimal core (packages/core) with zero runtime dependencies
  • Opt-in adapters for sessions and client calls so you can start without state and add storage when you need it
  • Plain TypeScript APIs that line up with the MCP spec and stay close to the wire format

Quick Start

Section titled “Quick Start”
Terminal window
npm install mcp-lite

Spin up a minimal MCP server with Hono and Zod:

import { Hono } from "hono";
import { McpServer, StreamableHttpTransport } from "mcp-lite";
import { z } from "zod";


const mcp = new McpServer({
  name: "example-server",
  version: "1.0.0",
  schemaAdapter: (schema) => z.toJSONSchema(schema as z.ZodType),
});


const WeatherInputSchema = z.object({
  location: z.string(),
});


const WeatherOutputSchema = z.object({
  temperature: z.number(),
  conditions: z.string(),
});


mcp.tool("getWeather", {
  description: "Gets weather information for a location",
  inputSchema: WeatherInputSchema,
  outputSchema: WeatherOutputSchema,
  handler: (args) => ({
    content: [
      {
        type: "text",
        text: `Weather in ${args.location}: 22°C, sunny`,
      },
    ],
    structuredContent: {
      temperature: 22,
      conditions: "sunny",
    },
  }),
});


const transport = new StreamableHttpTransport();
const httpHandler = transport.bind(mcp);


const app = new Hono();
app.all("/mcp", async (c) => {
  const response = await httpHandler(c.req.raw);
  return response;
});

Key Features

Section titled “Key Features”
  • No runtime dependencies and a single TypeScript entrypoint
  • Type-safe tool definitions with Standard Schema (Zod, Valibot, Effect, ArkType)
  • Structured outputs with runtime validation and schema exposure via tools/list
  • HTTP + SSE transport built on the Fetch API
  • Adapter interfaces for sessions, server-to-client requests, and persistence when you outgrow stateless mode
  • Middleware hooks and server composition via .group() for modular setups and namespacing

Next Steps

Section titled “Next Steps”