Unable to @ mention external collaborators in comments within Office online files

In Microsoft 365 / Office Online, you can’t @mention external users in comments.

@mentions trigger identity + notification workflows tied to your tenant. It only works for users inside your tenant.

Workaround: share the file and notify them separately (email/Teams).

No confirmed roadmap yet from Microsoft.

https://learn.microsoft.com/en-us/answers/questions/5320247/future-plans-for-%40mention-functionality-for-extern

https://www.microsoft.com/en-us/microsoft-365/roadmap?filters=&searchterms=%40mentions