Fastify Plugin Shape

Fastify plugin shape

> If it doesn't hold up in production, it doesn't make the cut.

I publish Fastify plugins (fastify-orama, fastify-at-mysql, lyra-impact). The shape below is what every one of mine looks like. It's small, it's deliberate, and it doesn't drift between repos.

When this skill is active

You are about to:

• Write a new Fastify plugin (in-repo or standalone package)
• Add a fastify.decorate(...) call
• Configure or modify a fastify-plugin (fp) wrapper
• Add a TypeScript module augmentation (declare module 'fastify' {})
• Validate plugin options or decide what to expose

The shape

'use strict'

const fp = require('fastify-plugin')

async function fastifyMyPlugin (fastify, options) {
  if (fastify.myPlugin) {
    throw new Error('fastify-my-plugin is already registered')
  }

  const { /* my-plugin options */ ...rest } = options

  // ... build api, open connections, restore state ...

  function withMyPlugin () {
    return this
  }

  fastify.decorate('myPlugin', api)
  fastify.decorate('withMyPlugin', withMyPlugin)
}

module.exports = fp(fastifyMyPlugin, {
  fastify: '5.x',
  name: 'fastify-my-plugin'
})

// Re-export raw function + helpers as named exports
module.exports.fastifyMyPlugin = fastifyMyPlugin

That's the spine. Eight non-negotiables:

1. 'use strict' at the top, CommonJS, JS source — TS lives in index.d.ts.
2. fastify-plugin wrapper. Without it, decorators are encapsulated and your plugin "doesn't work" outside its register scope. fp is the de-encapsulation primitive.
3. async (fastify, options) signature. Even if you don't await — keep it async for symmetry.
4. Idempotency guard. First line of the body checks if (fastify.myPlugin) throw. Stops the foot-gun where two registers silently clobber each other.
5. Destructure plugin-only options before forwarding. Your plugin's options are not the underlying library's options — separate them at the boundary, don't leak them.
6. withMyPlugin() helper that returns this. No-op at runtime; pure TypeScript narrowing tool. Lets users write fastify.withMyPlugin().myPlugin.foo() in handlers where the decorator's type isn't yet visible.
7. module.exports = fp(plugin, { fastify, name }) — pin the Fastify major ('5.x') and the plugin name. The name is what shows up in fp warnings.
8. Re-export named. Default-export the wrapped plugin, named-export the raw async function and any side classes (persistence adapters, helpers). Lets consumers compose.

TypeScript module augmentation

Ship types in index.d.ts. Augment Fastify, don't subclass it:

import 'fastify'

declare module 'fastify' {
  interface FastifyInstance {
    myPlugin: MyPluginApi
    withMyPlugin: () => FastifyInstance & { myPlugin: MyPluginApi }
  }
}

export interface MyPluginOptions { /* ... */ }
export interface MyPluginApi { /* ... */ }

declare const fastifyMyPlugin: FastifyPluginAsync
export default fastifyMyPlugin

Module augmentation is the only correct way — it composes with other plugins, it's discoverable, it works with Fastify's type inference.

Options validation

Validate at the entry point, fail fast:

if (!options.schema && !options.persistence) {
  throw new Error('You must provide a schema or a persistence adapter')
}

Don't reach for ajv for plugin options — overkill. Plain checks at the top of the function are enough; the schema for request/response is what ajv is for.

Decorator naming

• Singular noun for the resource: fastify.orama, fastify.mysql, fastify.redis. Not fastify.oramaClient, not fastify.dbConnection.
• with() helper for the type-narrowing trick. Always returns this.
• No verbs as decorator names. fastify.search() is wrong — that lives on the API object: fastify.orama.search().

Anti-patterns

• Plugin without fastify-plugin. Decorators get encapsulated and disappear in handlers. The bug is silent and infuriating to debug.
• fastify.decorate('orama', () => api) — passing a function instead of the value. Now every callsite invokes it (fastify.orama().search). Pass the API object directly.
• Leaking the underlying library's instance instead of an API surface. Don't fastify.decorate('orama', oramaDb) and let consumers call Orama.search(fastify.orama, …). Wrap the methods so the db instance is implicit: fastify.orama.search(...).
• No idempotency guard. Two register calls silently overwrite the decorator and you spend an afternoon hunting it down.
• Pinning Fastify with ^5.0.0 instead of '5.x'. The fp peer constraint is a major-version range, not a npm range.
• TS via subclassing or extension instead of module augmentation. Won't compose with other plugins.
• No withX() helper. Users have to cast fastify as FastifyInstance & { myPlugin: ... } everywhere. Annoying. The helper costs three lines.

When the plugin needs cleanup

Use fastify.addHook('onClose', async () => { /* close db, drain queue, persist state */ }). Don't expose a manual close() decorator — the framework already has the lifecycle hook.

Cross-references

• Real example: [deprecated/fastify-orama/references/core-plugin-api.md](../../deprecated/fastify-orama/references/core-plugin-api.md)
• Source: github.com/mateonunez/fastify-orama

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: mateonunez
• Source: mateonunez/skills
• License: MIT

Install and usage instructions live in the source repository linked above.