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Elysia is an ergonomic web framework for building backend servers with TypeScript. It is designed with simplicity and type safety in mind — the framework infers types from your route definitions automatically, so you can focus on business logic instead of writing boilerplate types.

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What is Elysia?

Elysia is optimized for Bun, a fast JavaScript runtime, but it also runs on Node.js, Deno, Cloudflare Workers, and any WinterTC-compliant platform. It gives you a familiar, chainable API with deep TypeScript support baked in from the start. Here is a minimal Elysia server: 
import { Elysia } from 'elysia'

new Elysia()
  .get('/', 'Hello Elysia')
  .get('/user/:id', ({ params: { id } }) => id)
  .post('/form', ({ body }) => body)
  .listen(3000)
Elysia automatically infers the type of params.id and body — no explicit type annotations required.

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Who is Elysia for?

Elysia is a good fit if you want to:
  • Build REST APIs or full-stack services with TypeScript
  • Share types between your frontend and backend without code generation
  • Generate OpenAPI documentation directly from your types
  • Deploy to Bun for maximum performance, or target Node.js and serverless platforms
TypeScript is not required to use Elysia, but the framework is designed around it and provides its greatest benefits when TypeScript is enabled.

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Key capabilities

Automatic type inference — Elysia’s type system derives route parameter, body, query, and response types from your code. You define a schema once and get runtime validation, TypeScript types, and OpenAPI schema from the same source. End-to-end type safety — The Eden client library lets your frontend call Elysia routes with full type safety and autocomplete, without any code generation step. It works similarly to tRPC but over standard HTTP. Standard Schema support — Alongside its built-in TypeBox-based validator (Elysia.t), Elysia supports Zod, Valibot, ArkType, Effect Schema, and other Standard Schema-compliant libraries. OpenAPI out of the box — Because Elysia tracks schema metadata, adding the OpenAPI plugin generates complete, accurate API documentation with a single line of code. Plugin system with encapsulation — Plugins compose cleanly. Lifecycle hooks and decorators are scoped to the instance they are registered on unless explicitly shared, preventing unintended side effects. Platform agnostic — Elysia’s WinterTC compliance means the same code runs on Bun, Node.js, Deno, Cloudflare Workers, Vercel, and serverless framework integrations like Next.js, Astro, and SvelteKit.

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Where to go next

Elysia at a glance

A deeper look at type inference, validation, Standard Schema, OpenAPI, and Eden.

Quick start

Get a working server running on Bun or Node.js in under five minutes.

Routing and handlers

Learn how to define routes, path parameters, query strings, and response types.

Eden client

Connect a frontend to your Elysia server with end-to-end type safety.

Official plugins

Browse CORS, JWT, static files, OpenAPI, OpenTelemetry, and more.

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