What exactly the end of "Legacy API" means?

[guilherme-gm]

Hello,
I am trying to understand the impact of the deprecation/future removal of Legacy API on my application, from here: https://vue-i18n.intlify.dev/guide/migration/breaking11.html#deprecate-legacy-api-mode

Overall, I do a quite simple use of Vue i18n, I would say ~80%+ of my code simply uses $t('key'), upgrading to v9/v10/v11 as we went to Vue 3 was not a major issue.

Back to the legacy API,

My understanding is that it will work fine in applications using Vue Options API, just how we interact with Vue i18n will change. is that correct?

Going further, I could not understand if this will simply means some overloads will work differently (like the changes in v10), or if there will be a whole set of functions that will no longer work, or that I will need to update all my components somehow...

A few other things I am assuming:

1. I can still use the global $t and etc
2. I don't need to add a setup function to my components calling useI18n (unless I need to customize something)
3. Switching to legacy: false on my createI18n, I would see the issues if I was still relying on the legacy api.

If I can use my application with legacy: false just fine, can I expect that the removal of the legacy api will have minor/no effect on it?

Thanks in advance

[M1CK431]

Your assumptions are mostly correct, but there's one important caveat worth knowing about.

The global $t does keep working with legacy: false — so for ~80% of your code you'd be fine. However, if you use component-scoped translations (via <i18n> SFC blocks or the i18n option in a component object), those will silently stop working when switching to composition mode with the Options API. The translations simply won't be picked up — no error, just missing strings.

So your point 2 is where things get tricky: if any of your components rely on component-scoped translations, you will need to add useI18n() in a setup hook for each of them — which can be quite tedious across a large codebase.

I ran into this exact situation and created a Vite plugin to solve it automatically: vite-plugin-vue-i18n-sfc-auto-import. The README explains the problem in detail and how the plugin handles it transparently at build time, so you don't have to touch your components at all.

Hope this helps!