---

name: elysiajs description: Create backend with ElysiaJS, a type-safe, high-performance framework.

ElysiaJS Development Skill

Always consult elysiajs.com/llms.txt for code examples and latest API.

Overview

ElysiaJS is a TypeScript framework for building Bun-first (but not limited to Bun) type-safe, high-performance backend servers. This skill provides comprehensive guidance for developing with Elysia, including routing, validation, authentication, plugins, integrations, and deployment.

When to Use This Skill

Trigger this skill when the user asks to:

• Create or modify ElysiaJS routes, handlers, or servers
• Setup validation with TypeBox or other schema libraries (Zod, Valibot)
• Implement authentication (JWT, session-based, macros, guards)
• Add plugins (CORS, OpenAPI, Static files, JWT)
• Integrate with external services (Drizzle ORM, Better Auth, Next.js, Eden Treaty)
• Setup WebSocket endpoints for real-time features
• Create unit tests for Elysia instances
• Deploy Elysia servers to production

Quick Start

Quick scaffold:

bun create elysia app

Basic Server

import { Elysia, t, status } from 'elysia'

const app = new Elysia()
  	.get('/', () => 'Hello World')
   	.post('/user', ({ body }) => body, {
    	body: t.Object({
      		name: t.String(),
        	age: t.Number()
     	})
    })
    .get('/id/:id', ({ params: { id } }) => {
   		if(id > 1_000_000) return status(404, 'Not Found')
     
     	return id
    }, {
    	params: t.Object({
     		id: t.Number({
       			minimum: 1
       		})
     	}),
     	response: {
      		200: t.Number(),
        	404: t.Literal('Not Found')
      	}
    })
    .listen(3000)

Basic Usage

HTTP Methods

import { Elysia } from 'elysia'

new Elysia()
  .get('/', 'GET')
  .post('/', 'POST')
  .put('/', 'PUT')
  .patch('/', 'PATCH')
  .delete('/', 'DELETE')
  .options('/', 'OPTIONS')
  .head('/', 'HEAD')

Path Parameters

.get('/user/:id', ({ params: { id } }) => id)
.get('/post/:id/:slug', ({ params }) => params)

Query Parameters

.get('/search', ({ query }) => query.q)
// GET /search?q=elysia → "elysia"

Request Body

.post('/user', ({ body }) => body)

Headers

.get('/', ({ headers }) => headers.authorization)

TypeBox Validation

Basic Types

import { Elysia, t } from 'elysia'

.post('/user', ({ body }) => body, {
  body: t.Object({
    name: t.String(),
    age: t.Number(),
    email: t.String({ format: 'email' }),
    website: t.Optional(t.String({ format: 'uri' }))
  })
})

Nested Objects

body: t.Object({
  user: t.Object({
    name: t.String(),
    address: t.Object({
      street: t.String(),
      city: t.String()
    })
  })
})

Arrays

body: t.Object({
  tags: t.Array(t.String()),
  users: t.Array(t.Object({
    id: t.String(),
    name: t.String()
  }))
})

File Upload

.post('/upload', ({ body }) => body.file, {
  body: t.Object({
    file: t.File({
      type: 'image',              // image/* mime types
      maxSize: '5m'               // 5 megabytes
    }),
    files: t.Files({              // Multiple files
      type: ['image/png', 'image/jpeg']
    })
  })
})

Response Validation

.get('/user/:id', ({ params: { id } }) => ({
  id,
  name: 'John',
  email: 'john@example.com'
}), {
  params: t.Object({
    id: t.Number()
  }),
  response: {
    200: t.Object({
      id: t.Number(),
      name: t.String(),
      email: t.String()
    }),
    404: t.String()
  }
})

Standard Schema (Zod, Valibot, ArkType)

Zod

import { z } from 'zod'

.post('/user', ({ body }) => body, {
  body: z.object({
    name: z.string(),
    age: z.number().min(0),
    email: z.string().email()
  })
})

Error Handling

.get('/user/:id', ({ params: { id }, status }) => {
  const user = findUser(id)
  
  if (!user) {
    return status(404, 'User not found')
  }
  
  return user
})

Guards (Apply to Multiple Routes)

.guard({
  params: t.Object({
    id: t.Number()
  })
}, app => app
  .get('/user/:id', ({ params: { id } }) => id)
  .delete('/user/:id', ({ params: { id } }) => id)
)

Macro

.macro({
  hi: (word: string) => ({
    beforeHandle() { console.log(word) }
  })
})
.get('/', () => 'hi', { hi: 'Elysia' })

Project Structure (Recommended)

Elysia takes an unopinionated approach but based on user request. But without any specific preference, we recommend a feature-based and domain driven folder structure where each feature has its own folder containing controllers, services, and models.

src/
├── index.ts              # Main server entry
├── modules/
│   ├── auth/
│   │   ├── index.ts      # Auth routes (Elysia instance)
│   │   ├── service.ts    # Business logic
│   │   └── model.ts      # TypeBox schemas/DTOs
│   └── user/
│       ├── index.ts
│       ├── service.ts
│       └── model.ts
└── plugins/
    └── custom.ts

public/                   # Static files (if using static plugin)
test/                     # Unit tests

Each file has its own responsibility as follows:

• Controller (index.ts): Handle HTTP routing, request validation, and cookie.
• Service (service.ts): Handle business logic, decoupled from Elysia controller if possible.
• Model (model.ts): Define the data structure and validation for the request and response.

Best Practice

Elysia is unopinionated on design pattern, but if not provided, we can relies on MVC pattern pair with feature based folder structure.

• Controller:
  • Prefers Elysia as a controller for HTTP dependant controller
  • For non HTTP dependent, prefers service instead unless explicitly asked
  • Use onError to handle local custom errors
  • Register Model to Elysia instance via Elysia.models({ ...models }) and prefix model by namespace `Elysia.prefix('model', 'Namespace.')
  • Prefers Reference Model by name provided by Elysia instead of using an actual Model.name
• Service:
  • Prefers class (or abstract class if possible)
  • Prefers interface/type derive from Model
  • Return status (import { status } from 'elysia') for error
  • Prefers return Error instead of throw Error
• Models:
  • Always export validation model and type of validation model
  • Custom Error should be in contains in Model

Elysia Key Concept

Elysia has a every important concepts/rules to understand before use.

Encapsulation - Isolates by Default

Lifecycles (hooks, middleware) don't leak between instances unless scoped.

Scope levels:

• local (default) - current instance + descendants
• scoped - parent + current + descendants
• global - all instances

.onBeforeHandle(() => {}) // only local instance
.onBeforeHandle({ as: 'global' }, () => {}) // exports to all

Method Chaining - Required for Types

Must chain. Each method returns new type reference.

❌ Don't:

const app = new Elysia()
app.state('build', 1) // loses type
app.get('/', ({ store }) => store.build) // build doesn't exists

✅ Do:

new Elysia()
  .state('build', 1)
  .get('/', ({ store }) => store.build)

Explicit Dependencies

Each instance independent. Declare what you use.

const auth = new Elysia()
	.decorate('Auth', Auth)
	.model(Auth.models)

new Elysia()
  .get('/', ({ Auth }) => Auth.getProfile()) // Auth doesn't exists

new Elysia()
  .use(auth) // must declare
  .get('/', ({ Auth }) => Auth.getProfile())

Global scope when:

• No types added (cors, helmet)
• Global lifecycle (logging, tracing)

Explicit when:

• Adds types (state, models)
• Business logic (auth, db)

Deduplication

Plugins re-execute unless named:

new Elysia() // rerun on `.use`
new Elysia({ name: 'ip' }) // runs once across all instances

Order Matters

Events apply to routes registered after them.

.onBeforeHandle(() => console.log('1'))
.get('/', () => 'hi') // has hook
.onBeforeHandle(() => console.log('2')) // doesn't affect '/'

Type Inference

Inline functions only for accurate types.

For controllers, destructure in inline wrapper:

.post('/', ({ body }) => Controller.greet(body), {
  body: t.Object({ name: t.String() })
})

Get type from schema:

type MyType = typeof MyType.static

Reference Model

Model can be reference by name, especially great for documenting an API

new Elysia()
	.model({
		book: t.Object({
			name: t.String()
		})
	})
	.post('/', ({ body }) => body.name, {
		body: 'book'
	})

Model can be renamed by using .prefix / .suffix

new Elysia()
	.model({
		book: t.Object({
			name: t.String()
		})
	})
	.prefix('model', 'Namespace')
	.post('/', ({ body }) => body.name, {
		body: 'Namespace.Book'
	})

Once prefix, model name will be capitalized by default.

Technical Terms

The following are technical terms that is use for Elysia:

• OpenAPI Type Gen - function name fromTypes from @elysiajs/openapi for generating OpenAPI from types, see plugins/openapi.md
• Eden, Eden Treaty - e2e type safe RPC client for share type from backend to frontend

Resources

Use the following references as needed.

It's recommended to checkout route.md for as it contains the most important foundation building blocks with examples.

plugin.md and validation.md is important as well but can be check as needed.

references/

Detailed documentation split by topic:

• bun-fullstack-dev-server.md - Bun Fullstack Dev Server with HMR. React without bundler.
• cookie.md - Detailed documentation on cookie
• deployment.md - Production deployment guide / Docker
• eden.md - e2e type safe RPC client for share type from backend to frontend
• guard.md - Setting validation/lifecycle all at once
• macro.md - Compose multiple schema/lif