@8monkey/elysia-mcp

Turn your existing Elysia routes into MCP tools. No manual registration, no schema duplication, no handler rewrites.

Why?

The Model Context Protocol lets AI agents discover and call tools over a standard JSON-RPC interface. If you already have an Elysia API with typed schemas and handlers, you shouldn't have to rewrite all of that as MCP tool definitions.

@8monkey/elysia-mcp bridges the gap: add .use(mcp()) and every endpoint becomes a callable MCP tool, with its name, description, and input schema derived from what you already wrote.

Related MCP server: hono-mcp-server

Key highlights

• 🔄 Zero duplication — tool names, descriptions, and input schemas are derived from your existing route definitions. OpenAPI metadata such as detail.operationId and detail.summary also drive MCP tool discovery. Write once, serve both humans and AI agents.

• 🔌 Any schema library — works with TypeBox, Zod, Valibot, or any validation library supported by Elysia via Standard Schema.

• ⚡ Full lifecycle — MCP tool calls go through app.handle(), so derive, resolve, beforeHandle, afterHandle, error hooks, and all plugins run exactly as they do for normal HTTP requests.

• 📋 Schema flattening — params, query, and body schemas are merged into a single flat MCP input schema with property origins tracked for correct unflattening

• 🏷️ Smart naming — GET /users becomes list_users, GET /users/:id becomes get_user, POST /users becomes create_user, and nested paths like GET /users/:uid/posts become list_user_posts

How it differs from existing solutions

Compared to kerlos/elysia-mcp and keithagroves/Elysia-mcp, which require manual tool registration with separate Zod schemas and standalone handlers:

• Auto-discovery — routes become tools automatically; no registration callbacks

• Schema reuse — uses your existing Elysia schema definitions (TypeBox, Zod, Valibot, or any Standard Schema provider) instead of duplicating separately

• Full lifecycle execution — tool calls run through app.handle(), not standalone functions, so all middleware applies

• Streamable HTTP — full GET/POST/DELETE support over the MCP Streamable HTTP transport, including SSE streams for clients that open them via GET

Install

bun add @8monkey/elysia-mcp

Peer dependency: elysia >= 1.4.0

Basic usage

Add .use(mcp()) and all routes become MCP tools:

import { Elysia } from "elysia";
import { mcp } from "@8monkey/elysia-mcp";

const app = new Elysia()
  .use(mcp())
  .get("/users", () => db.users.findAll())
  .get("/users/:id", ({ params }) => db.users.find(params.id))
  .post("/users", ({ body }) => db.users.create(body))
  .listen(3000);

This exposes a /mcp endpoint that speaks the MCP Streamable HTTP transport — POST for JSON-RPC requests, GET (with Accept: text/event-stream) for SSE streams, and DELETE for session termination. An MCP client calling tools/list will see list_users, get_user, and create_user.

Descriptions matter — for agents and docs

Good descriptions are critical for AI agents to understand when and how to call your tools. The plugin uses Elysia's standard detail.summary as the MCP tool description, and TypeBox description on each property as the parameter description. These are the same fields that Elysia uses for OpenAPI/Swagger documentation, meaning there's zero duplication. Write them once and they serve both your API docs and your MCP tools.

The plugin warns at startup if any property is missing a description, since agents rely on these to choose the right tool and pass the correct arguments.

Route-level: detail.summary

Use detail.summary to describe what the tool does. This becomes the MCP tool description and the OpenAPI operation summary:

.get("/users", () => db.users.findAll(), {
  detail: {
    operationId: "list_users",
    summary: "List all users in the system",
    mcp: true,
  },
})

If operationId is omitted, the plugin falls back to generated names like list_users or get_user.

Property-level: schema description

Add description to each schema property. These become the MCP parameter descriptions and the OpenAPI property descriptions — the same metadata, no duplication:

import { t } from "elysia";

.get("/users/:id", ({ params }) => db.users.find(params.id), {
  params: t.Object({
    id: t.String({ description: "The user's unique ID" }),
  }),
  detail: { summary: "Get user by ID" },
})

MCP tools accept a single flat input object. The plugin merges params, query, and body into one schema:

Route: PATCH /users/:id  (params: { id }, query: { fields }, body: { name, email })
  ↓
MCP Tool Input: { id: string, fields?: string, name: string, email: string }

Property descriptions are preserved. The plugin warns at startup if properties collide across buckets or lack descriptions.

Opting routes out

By default, all routes are exposed. Opt out individual routes with mcp: false:

.get("/health", () => ({ status: "ok" }), {
  detail: { mcp: false },
})

Or flip the default — set allRoutes: false to require explicit opt-in:

.use(mcp({ allRoutes: false }))
.get("/users", () => db.users.findAll(), {
  detail: { mcp: true },  // only this route becomes a tool
})
.get("/health", () => "ok")  // not exposed

Overriding tool names and descriptions

Auto-generated names follow a {verb}_{resource} convention. If you want an explicit tool name, set detail.operationId:

.get("/items", handler, {
  detail: {
    operationId: "search_items",
    summary: "Full-text search across all items",
    mcp: true,
  },
})

Naming conventions

Configuration

mcp({
  name: "my-api", // MCP server name (default: "elysia-mcp")
  version: "1.0.0", // MCP server version (default: "1.0.0")
  path: "/mcp", // Endpoint path (default: "/mcp")
  allRoutes: true, // Expose all routes by default (default: true)
});

How tool calls work

When an MCP client calls a tool:

1. The flat args are unflattened back into params, query, and body

2. A synthetic HTTP request is built with the correct method, path, query string, body, and headers from the original MCP request

3. app.handle(request) runs the full Elysia lifecycle — derive, resolve, beforeHandle, the handler, afterHandle, and error hooks

4. The response body is parsed and returned as MCP text content

This means your auth middleware, rate limiting, validation, and every other plugin work exactly the same for MCP calls as they do for REST calls.

Connecting an MCP client

Point any MCP-compatible client at your /mcp endpoint. For example, with the MCP Inspector:

npx @modelcontextprotocol/inspector --transport http http://localhost:3000/mcp

Or configure it in Claude Desktop, Cursor, or any other MCP-enabled tool as an HTTP MCP server at http://localhost:3000/mcp.

Important notes

• Tools only (v1): This plugin exposes MCP tools. Resources and prompts are not supported yet.

• Stateless transport: Each request gets its own transport instance — no session tracking. POST returns JSON-RPC responses inline; GET with Accept: text/event-stream opens a server-initiated SSE stream; DELETE is accepted as a no-op terminator.