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 m

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.