What is Lyn?

Lyn is a small backend framework for Bun, written in TypeScript. You chain routes, describe their inputs with Zod, and Lyn gives you typed handlers, request validation and consistent JSON errors.

import { Lyn, z } from "@rouret/lyn";

await new Lyn()
  .get("/", () => "Hello World")
  .post(
    "/users",
    ({ body }) => ({ message: `Hello ${body.name}` }),
    { body: z.object({ name: z.string() }) }
  )
  .listen(3000);

Who it is for

Lyn is made for proofs of concept, solo developers and agencies: people who want a working, safe API quickly, without having to assemble a stack first.

It doesn't try to replace general-purpose frameworks. It makes a few choices for you so you can start writing routes right away.

What you get

Typed handlers	body, params and query are typed from your Zod schemas. Without a schema, they don't exist on the context.
Validation	Invalid requests are rejected with a 400 before your handler runs.
One error format	Every error, from 404 to a crash in your code, is the same JSON shape. Internal details never leak.
Checked configuration	Environment variables are validated and typed when the app starts, not when they are first used.
Postgres built in	Pass a Drizzle schema, get a typed db in every handler.
Logs	One log line per request, with a request id, ready for production.
Tests without a server	app.handle(request) runs a request through the app in memory.

What it doesn't do (yet)

• Bun only. Lyn does not run on Node.js or Deno.
• No authentication yet. It is the next big feature.
• No middlewares, route groups or plugins for now.

Each page ends with what is not supported yet on its topic.

Next steps

• Getting started: your first server in five minutes.
• Routing: how routes and paths work.