Apply these conventions when working with Fastify code in node.js projects.

Server Setup

Create the Fastify instance with explicit options: logger: true (or Pino config object), trustProxy when behind a reverse proxy.
Separate app construction (app.ts) from server startup (server.ts / cluster.ts) to enable testing via inject() without starting the server.

Plugins & Encapsulation

Everything is a plugin. Register features with fastify.register(plugin, options).
Plugins are encapsulated by default — decorators, hooks, and routes registered inside a plugin are invisible to parent scopes.
Use fastify-plugin wrapper only when a plugin must expose decorators or hooks to the parent scope (e.g., database connection, authentication decorator).
Declare decorator dependencies in the dependencies array to fail fast at boot, not at runtime.
Plugin loading order matters — plugins registered first are available to later plugins in the same scope.

Use shorthand methods: fastify.get(), fastify.post(), etc.
Always use async handlers that return the response body. Do not mix return with reply.send() — pick one.
Group related routes in a plugin and apply a prefix via register options.
Use route-level schema for request validation (body, querystring, params, headers) and response serialization (response).