Apps like that (a customizable system prompt with a token api endpoint) would become more common if the cost of tokens decreases. Current AI subscription providers have been trying to crack that nut with GPTs, skills, extensions, etc, but they have the n+1 problem. The scope is too much to do it securely and get value other than entertainment from it. If one could make a link to take you to a Claude.ai interface for “generate a legit LinkedIn post”, next people are gonna want to connect it to their email and documents, and LinkedIn account then build a workflow for making posts and addressing comments, messaging leads and cross posting to Twitter or Reddit.