Not sure if this expected, when running the add command (for example npx nuxi@latest module add nuxt-viewport) in a project using nuxt.config.js a new file is created nuxt.config.ts with the new module:

// https://nuxt.com/docs/api/configuration/nuxt-config
export default defineNuxtConfig({
  modules: ["nuxt-viewport"]
})

I would expect it to add the new module to the exisiting config file.

Reproduce this issue on StackBlitz starter: https://stackblitz.com/fork/github/nuxt/starter/tree/v3-stackblitz

Apologies if this is an upstream issue.