[stephen_hn]

I am not totally sure what your API's look like but if you know the API only has a certain type (or subset of types), you would only validate that type on the method. Don't use a single union for everything.

Here is some pseudocode.

Person = { name: string, height: number }

Animal = {name: string, capability: string}

A = { post: object, methodType: string, person: Person }

ModeratorA = { post: object, moderatorField1: string, moderatorField2: string, person: Person }

UnionA = A && ModeratorA (There's probably a better way of defining A and ModeratorA to share the shared fields)

B = { post: object, animal: Animal }

endpoint person parses UnionA

endpoint animal parses B

You don't put all of your types in one big Union.