TypeScript Error: Property 'config' does not exist on type 'FastifyInstance'

Question

I have created an environment plugin to load data from a .env file and another plugin to handle cookies in my Fastify application. However, when I try to use the environment configuration in the cookie plugin, TypeScript throws an error:

TSError: ⨯ Unable to compile TypeScript:
src/plugins/cookie.ts:7:34 - error TS2339: Property 'config' does not exist on type 'FastifyInstance

import fp from "fastify-plugin";
import Env from "@fastify/env";
import { FastifyInstance } from "fastify";

const jwtConfig = {
  type: "object",
  required: ["JWT_SECRET", "COOKIE_SECRET"],
  properties: {
    JWT_SECRET: { type: "string" },
    COOKIE_NAME: { type: "string", default: "token" },
    COOKIE_SECRET: { type: "string", default: "cookie-secret" },
  },
};

const serverConfig = {
  type: "object",
  properties: { PORT: { type: "string", default: "8080" } },
};

const envPlugin = fp(async (fastify: FastifyInstance, opts: any) => {
  fastify.register(Env, {
    confKey: "config",
    schema: {
      type: "object",
      properties: {
        ...jwtConfig.properties,
        ...serverConfig.properties,
      },
      required: [...jwtConfig.required],
    },
    dotenv: true,
  });
});

export default envPlugin;

Cookie Plugin:

import fastifyCookie from "@fastify/cookie";
import { FastifyInstance } from "fastify";
import fp from "fastify-plugin";

const cookiePlugin = fp(async (fastify: FastifyInstance, opts: any) => {
  const cookieSecret = (fastify as any).config.COOKIE_SECRET;

  fastify.register(fastifyCookie, { secret: cookieSecret, parseOptions: {} });
});

export default cookiePlugin;

That's how i register plugins:

import fastify from "fastify";
import envPlugin from "./plugins/env";
import cookiePlugin from "./plugins/cookie";

const server = fastify({ logger: true });

server.register(envPlugin);

server.register(cookiePlugin);

server.get("/ping", async (request, reply) => {
  return "pong\n";
});

export default server;

And that's how I added config type:

import "fastify";

declare module "fastify" {
  interface FastifyInstance {
    authenticate: (
      request: FastifyRequest,
      reply: FastifyReply
    ) => Promise<void>;
    config: {
      JWT_SECRET: string;
      COOKIE_NAME: string;
      PORT: string;
      COOKIE_SECRET: string;
    };
  }

  interface FastifyRequest {
    user?: { id: string; email: string };
  }
}