I note that creating a new parameter type outside of core has many more requirements than just making it possible to create a widget in ApiSandbox. You'd also need to plug in to several places in the PHP code:

• ApiBase::getParameterFromSettings() for validation (T142080: HTTP API typechecking method is very confusing and intimidating may be related there)
• ApiStructureTest for parameter consistency checks in CI
• Possibly ApiHelp::getHelpInternal() to generate appropriate output for parameters using the new type
• Possibly ApiParamInfo::getModuleInfo() for similar data there
• Possibly ApiMain::modifyHelp() to add text to the datatypes documentation

I note there's some patches working towards much of the above at https://gerrit.wikimedia.org/r/434717 and https://gerrit.wikimedia.org/r/434718.

As for ApiSandbox, we'd need some way for SpecialApiSandbox to load the extension's ResourceLoader module for extending ApiSandbox, and then probably use of mw.hook() from createWidgetForParameter() and possibly also ApiSandbox.PageLayout.prototype.makeWidgetFieldLayouts() to get that code called from the right places.

Does that make sense to you?

@Anomie yes it does, I've updated the description to be more broad in what I was trying to do. :)

For some background, I was working on T209133 and I initially created a special page (and a non-localized custom route). But then I was thinking it might be better if it was a module in the action API (which would allow different return formats, etc.).

However, the UX on the Sandbox would be sub-optimal as the only allowed field for the query (GraphQL schema) and the variables (JSON) would be a text field. Ideally, I'd like to be able to create a graphql param type that would use something like https://github.com/graphql/codemirror-graphql in the sandbox and the extension would validate the param value and ensure that it's valid.

Anyways, feels like there are probably more use-cases than the one I have, so I abstracted the description. Ideally the Action API would expose a param type system that any extension would plugin to.