I know the title sounds impossible, give it a second. We have XSD schemas created by an open-standards industry group. In our service we'd like our customers to be able to use JSON or XML when supplying data. What's the best practice in a situation like this?

1. Convert JSON payloads to XML and validate against the XSD.

2. Convert the XSD to JSON Schema and validate each content-type against it's own schema

3. Convert the XSD to a class-based in language specification and do schema validation in code and then handle content negotiation in the api middleware

4. Secret super option

FWIW, these XSD schemas are not overly trivial: ~4000 lines long, multiple namespaces with a half-dozen common schemas shared between them. Changes are rare (one every 2 years) but new schemas are added much more frequently (10 per year)