Callspec

Stop duct-taping your API stack.

One TypeScript route → typed SDK, docs, OpenAPI, and MCP.

Open source
MIT licensed — free to use and fork.
RPC routes
Methods like getProductById — not bikeshedding REST.
Structured errors
Errors as values with typed codes — no try/catch needed.
Test without HTTP
Unit-test handler(input, ctx) — no server setup.
MCP included
Same routes become Cursor/Claude tools — no second spec.

Most teams duct-tape the same API across REST routes, OpenAPI specs, DIY clients, docs, types, validators and MCP. That's a ton of repetitive work and brittle code that drifts.

Duct-taped tech stack — types, errors, OpenAPI, docs, SDKs, and more glued together

Instead, define each route once in TypeScript — inputs, outputs, and errors — and Callspec ships the server, SDK, docs, OpenAPI, and MCP from that contract. One source of truth, unit-testable, no drift.

How it works

Define

export const searchPosts = route({
  input,
  output,
  handler: async (inp, ctx) => {
    // your logic
  },
})

export const api = spec({
  routes: {searchPosts},
})

Serve
- MCP/mcp
- HTTP RPC/v1/*
- Docs UI/docs
- OpenAPI/openapi.json
Generate
npx callspec
api.searchPosts({…})generated/api.ts
Integrate
Generate multi-language SDKs and docs from your openapi.json with tools like Fern or Microsoft Kiota.
- Python
- Go
- Java
- C#

Built-in docs UI

Try-it panels, search, OpenAPI, and MCP connect — shipped with your API, no extra wiring. Whitelabel with your logo, accent, and nav links when you want; the defaults are simple and ready to go.

See the demo Run it locally

Chirp docs UI — routes sidebar, Connect MCP panel, Cursor MCP config

Typed SDK from the same spec

Docs, OpenAPI, and MCP share one contract — your frontend gets the same types and error codes, so nothing drifts when routes change.

const result = await api.getProductById({id});

if (!result.ok) {
  if (result.code === 'NOT_FOUND') return null;
  throw result;
}

return result.value; // typed Product

Built for coding agents

Agents call the same routes as your app — mountSpec exposes them as MCP tools with shared validation, auth, and error codes. The explorer ships connect snippets; the package ships a coding-agent skill.

MCP from your routes
Set mcp: true on a route — mountSpec exposes it as a tool with the same validation, auth, and error codes as RPC.
Connect from docs
The explorer home panel ships copy-paste snippets — server URL, bearer hints, and tool names ready for Cursor or Claude.
Agent skill included
The npm package ships SKILL.md so agents follow Callspec conventions — Result errors, callspec.json codegen, route layout.

We do file uploads too

Small files on the same RPC — avatars, CSVs, a PDF. Same handler, same generated client, same auth and errors. The client posts multipart for you.

const result = await api.upload({
  file: input.files[0],
  caption: 'hi',
});

Bring your own auth

Callspec reads the Bearer token from each request. Wire authenticate on your spec to verify it and return context — your existing provider or session logic.

Works with