SlowMCP

Blazingly adequate.

A framework for building, testing, and shipping MCP servers, on top of the official MCP TypeScript SDK.

Written in CoffeeScript.

FastMCP already exists. It is excellent. Unfortunately, it is fast.

AI is writing so much code these days, so I built this one in a dead language AI doesn't know: CoffeeScript. Let's see if THAT cuts down on the AI-generated pull requests.

Then the test harness found a real protocol bug on day one, and the joke got a lot more expensive.

Install

SlowMCP is not on npm. To run it today:

git clone https://github.com/LeeFlannery/slowmcp.git
cd slowmcp
pnpm install
pnpm test

Related MCP server: My Awesome MCP

Write a tool

import { createServer, text } from 'slowmcp'
import * as z from 'zod'

export const app = createServer({ name: 'hello-tool', version: '1.0.0' })

app.tool({
  name: 'greet',
  description: 'Greet someone by name.',
  input: z.object({ name: z.string().min(1) }),
  handler: ({ name }) => text(`Hello, ${name}!`)
})

export default app

Test it against a real client

import { testServer } from 'slowmcp/testing'
import app from './server.mjs'

const mcp = testServer(app)

await mcp.protocolVersion()          // '2026-07-28', asserted, not assumed
await mcp.tools()                    // [{ name: 'greet', ... }]
await mcp.call('greet', { name: 'Lee' })

await mcp.close()

testServer is a real official MCP Client over the official Streamable HTTP transport. There is no in-process shortcut and no fake handler. Connection is lazy, and the harness asserts SlowMCP's protocol policy before any of your assertions run.

Serve it

import { createHttpHandler } from 'slowmcp/http'
export default createHttpHandler(app)

Why not just use the official SDK

Because of what that harness caught.

The first spike passed everything it asserted: right server, right tool, right result. It was also speaking 2025-11-25 to a server that fully supported 2026-07-28, because the official client's negotiation mode defaults to 'legacy'. Nothing errors. Nothing warns. Your tests go green against a protocol generation you did not mean to test.

Defending against it by hand does not work unless you already know the shape of the problem. The SDK exports LATEST_PROTOCOL_VERSION as the older revision and leaves 2026-07-28 out of SUPPORTED_PROTOCOL_VERSIONS entirely, so the obvious assertion passes while you sit on the old one.

So slowmcp/protocol is the single place SlowMCP states what it speaks, testServer asserts what was actually negotiated before handing you the connection, and a regression test pins the whole thing: default client, same handler, lands on the old revision, passes discovery and invocation, gets caught by the policy.

That is the wedge. Not renamed SDK methods. Full write-up in docs/bootstrap-2026-08.md.

Versus FastMCP

Measured on 9 August 2026, by commands in this repository. Reproduce the first two rows with pnpm spike:baseline.

No cold start, no throughput, no time-to-first-tool, because none have been measured yet. The parity harness that produces those rows is roadmap work, and the numbers get published whether or not SlowMCP wins. Method and caveats in VERTICAL_SLICE_FINDINGS.md.

Categories that actually matter:

Public API

slowmcp            createServer, text, SlowMcpError, version
slowmcp/http       createHttpHandler
slowmcp/testing    testServer
slowmcp/protocol   protocolPolicy, satisfiesProtocolPolicy, assertProtocolPolicy

Nine exports across four subpaths. The root is the authoring API and deliberately does not re-export the others for convenience: what you import should say what it does.

Ship it

pnpm slowmcp:check

SLOWMCP CHECK

PASS package
PASS protocol
PASS tools
PASS invocation
PASS snapshot
PASS types
PASS coffeescript-containment

7/7 checks passed.

Blazingly adequate.

It packs the package, installs the tarball into a clean project outside the monorepo, and verifies it from the outside: every public subpath enumerated from the packed export map, protocol negotiation, discovery, invocation, snapshot semantics, the shipped TypeScript declarations, and the total absence of CoffeeScript.

You consume compiled ESM with hand-written .d.ts declarations. CoffeeScript never leaves this repository, and the check is what proves it every time.

Status

Vertical slice, and honest about it. Tools, Streamable HTTP, and the test harness work end to end from the packed artifact, verified in a clean external project. Resources, prompts, stdio, and the CLI are not built yet.

175 significant lines of CoffeeScript ship as a 14 KB tarball, described by 219 lines of hand-written declarations. A framework in a language with no type emission pays for every public promise by hand, which is why the declarations are larger than the implementation. That ratio is a real cost and it belongs in the open rather than in a footnote.

Design in ARCHITECTURE.md.

Roadmap

• Tools, resources, and prompts

• stdio and Streamable HTTP

• Protocol-backed test harness

• slowmcp check as an installable command

• Reference implementations

• GitHub Pages documentation

• TypeScript support: never.

License

MIT