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I don't know a tool for it, but I often find from experience that dynamic API's are usually not the right tool for the job. However if it is required and you have a declarative API, it will make the process much easier I would imagine.

EDIT: `declarative API` as in some kind of proxy layer which you can better isolate.

OpenAPI v3 can handle what you describe, although it's a pain to write it by hand and tooling in your language may be missing support for edge cases (i.e. incomplete spec implementation).

My suggestion would be to define your APIs in a neutral, flexible format (such as Spot[1]) and then generate OAS 3 or whatever other formats you need. That helps you avoid lock-in and any tedium required to handle less-capable formats.

1. https://github.com/airtasker/spot

If I'm understanding you correctly, this is what discriminators are supposed to solve, but I've never used them and don't know the state of the tooling for them.