effect-stateless-mcp

A stateless MCP server (Model Context Protocol, spec 2026-07-28) that exposes a greet tool. Built and tested with Effect.ts v4 and run on Bun.

The server is fully stateless: a fresh McpServer instance is created per HTTP request, so no session state is retained between calls (no initialize handshake, no Mcp-Session-Id).

Requirements

• Bun (v1.3.14+)

• Node types are provided by Bun; no separate Node install needed

Related MCP server: Bun Fun MCP Server

Install

bun install

Run

bun run src/index.ts

By default the server listens on http://localhost:3000/mcp. Override the port with the PORT environment variable (read via Effect's Config):

PORT=8787 bun run src/index.ts

Calling the greet tool

Send a JSON-RPC tools/call request to the /mcp endpoint:

curl -s -X POST http://localhost:3000/mcp \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": { "name": "greet", "arguments": { "name": "Ada", "style": "formal" } }
  }'

The greet tool accepts:

Test

Tests use vitest with @effect/vitest:

bun run test

Type-check

bunx tsc --noEmit

Project structure

License

See repository for details.