kelo-mcp

A small Model Context Protocol server that lets a Claude user ask what their business is owed, who to chase first, and get a chase email drafted in their own voice.

It cannot send anything. Every tool either reads or returns a draft. That is a product decision, not an oversight, and DESIGN.md explains what it costs and what it buys.

Extracted from Kelo, where it backs a live MCP endpoint. No SDK dependency: the JSON-RPC layer is hand-rolled, about 120 lines, and has no runtime dependencies at all.

What this demonstrates

A marketer who builds. Hand-rolled a spec-compliant MCP protocol layer with no SDK, a stateless HTTP transport with bearer-key auth and per-workspace scoping, a clean injected backend seam, and 18 tests that run against a stub in under a second, then wrote the judgment calls down in DESIGN.md. Stack: TypeScript, Web-standard Request/Response, Vitest.

Related MCP server: BizDesk MCP

Try it in 30 seconds

npm install
npm run example

curl -s localhost:8787 -H 'authorization: Bearer demo-key' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"get_weekly_number","arguments":{}}}'

{
  "verified_owed": "£6,050",
  "estimate_at_risk": "£9,600",
  "open_cases": 3,
  "note": "Verified is real overdue invoices. Estimate is quotes and enquiries, and is less certain."
}

That runs against an in-memory demo backend. No database, no Kelo account, no API key beyond the literal string demo-key.

The tools

Wiring it to your own data

The protocol layer knows nothing about invoices. Implement ToolBackend and pass it in:

import { createPostHandler, callTool, TOOL_DEFS, SERVER_INFO, INSTRUCTIONS } from 'kelo-mcp'

export const POST = createPostHandler({
  config: { serverInfo: SERVER_INFO, tools: TOOL_DEFS, instructions: INSTRUCTIONS, callTool },
  resolveKey: async token => {
    const row = await db.apiKeys.findByHash(sha256(token))
    return row && !row.revoked_at ? { workspaceId: row.workspace_id } : null
  },
  makeContext: workspaceId => ({ workspaceId, backend: myBackend }),
})

createPostHandler takes and returns Web-standard Request/Response, so it drops into Next.js route handlers, Cloudflare Workers, Deno, Bun or plain Node 18+ without adaptation.

Layout

src/protocol.ts      JSON-RPC and MCP methods. No product knowledge, no dependencies.
src/tools.ts         Tool definitions: the surface a model actually sees.
src/backend.ts       The seam. Dispatch, formatting, and the ToolBackend interface.
src/demo-backend.ts  In-memory data so the thing runs out of the box.
src/http.ts          Bearer auth and the stateless POST transport.

The split exists so the protocol can be tested without a database. All 18 tests run in well under a second and touch nothing external.

npm test

What is not here

The live Kelo server backs these tools with Postgres queries, a drafting service and a billing entitlement check. That is Kelo's business logic and it is not in this repo. What is here is the protocol layer, the transport, the tool surface, and a demo backend that satisfies the same interface.

Notes on the protocol

Three things the spec asks for that are easy to get subtly wrong, and how this handles them:

• Notifications get silence, not a response. A message with no id returns null internally and is dropped. If a batch was entirely notifications, the transport answers 202 with an empty body rather than 200 and [].

• An unknown tool is a tool error, not a protocol error. It comes back as a normal result with isError: true so the model can read it and pick another tool. Reserving -32601 for genuinely unknown methods keeps the two failure modes distinct.

• The client's protocol version is echoed when it sends one, rather than forcing the server's own.

Licence

MIT