Automatically generated is the only sane way (the metadata it is generated from will of course have to be hand-maintained to some extent, just like the OpenAPI specs are).

The biggest shortcoming of the hyperswitch/swagger-ui sandboxes compared to the action API sandbox is probably lack of internationalization (an architecture principle now!). Seems generally not well supported in the OpenAPI spec currently. Other than that they seem mostly on par (except the action API sandbox not documenting response formats, which is definitely a huge pain point, although once you are tightly integrated with MediaWiki, it probably becomes more complicated). The action API sandbox has better UX (autocompletion, permalinks etc.) but that can probably be added to any metadata format.

One thing to consider here is that we'll want to implement some APIs outside of MediaWiki, for security reasons (everything related to authentication, for starters), but we'll probably want the same sandbox look & feel for them. So either the there should be a way to host a sandbox in MediaWiki without hosting the API, or the sandbox should be librarized and reusable outside of MediaWiki.

Forgive me if I'm misunderstanding, but isn't this already provided at Special:ApiSandbox, i.e. https://en.wikipedia.org/wiki/Special:ApiSandbox

In T221742#5605824, @Mvolz wrote:

Forgive me if I'm misunderstanding, but isn't this already provided at Special:ApiSandbox, i.e. https://en.wikipedia.org/wiki/Special:ApiSandbox

That's for the action API.

This is a useful feature but not one that's needed for Parsoid/PHP.

SwaggerUI would give use this for free, see T323786: REST framework: Add support for outputting an OpenAPI (swagger) spec