Well, Zod isn't just for validating JSON. It supports validating objects that can't be represented in JSON without transformation (dates, class instances, and so on). You can also use it as your JSON transformer if you want to - you can write your schema so it accepts strings, validates them as ISO date strings, and outputs Date objects, for instance.

> Well, Zod isn't just for validating JSON. It supports validating objects that can't be represented in JSON without transformation (dates, class instances, and so on).

Thanks, this was the missing piece for me. I'd been thinking about using Zod for an old client's Node codebase, but I'd only need it to validate the shape of json objects received by http endpoints. Zod was looking like overkill, but I know it's popular so I wasn't sure what I was missing.

Zod 4 supports converting a Zod schema to JSON-Schema (natively, this has always been possible with 3rd-party libs).

One key difference is preprocessing/refine. With Zod, you can provide a callback before running validation, which is super useful and can't be represented in JSON. This comes in handy more often than you'd think - e.g converting MM/DD/YYYY to DD/MM/YYYY before validating as date.

Does it support the other way around too? I'd live to ditch AJV.

It is a good choice and in that case TypeBox is a decent way to generate it.

And you can use AVJ or its own schema validation system (which is much faster, but only sync where avj has asynchronous validation options in case you want to check against a database or something).

JSON Schema is the way for cross-language validation. Wrap it in OpenAPI and you also get nice API docs for free.