I'm under the impression that's what the Remote Network Interaction clause of the the Affero GPL license is supposed to do. The "boundary" is then if someone is

I'm under the impression that's what the Remote Network Interaction clause of the the Affero GPL license is supposed to do. The "boundary" is then if someone is interacting with the AGPL code at all, so when you use the AGPL-licensed code behind a REST API, even if that's on someone else's server, the use of that code in producing any response to the API request requires publishing the AGPL'ed code/modifications.