The Apple standard-positions in this was a pretty good read, noting indeed that this is a parallel new way to define accessibility. https://github.com/WebKit/standards-positions/issues/670

I'd also throw in schema.org Potential Actions as longstanding precedent for declaring what is on offer from the page. https://schema.org/docs/actions.html

It does get a bit tricky though. WebMCP allows the page to respond to the agent. Where-as these other systems the page itself has to update and respond.