yesss, and OpenAI tried this first when they were going to do a “GPT store”. But REST APIs tend to be complicated because they’re supporting apps. MCP, when it works, is very simple functions

in practice it seems like command line tools work better than either of those approaches

Command line tools are my preference just because they're also very useful to humans. I think providing agents function libraries and letting them compose in a repl works about as well but is higher friction due env management.