You can do something like that, but I need a more generic way, so I don't have to relay on if let, custom decoders, custom types, casting or similar. I have ended up using the extension mentioned above, which let me. class Jumps: Decodable { var value: Bool CodingKeys... required init(from decoder: Decoder) throws { let container = decoder.container... let value = try decodeWithUnknowType(Bool.self, for: .value) } } Not the most elegant, but usable and it cleans up a lot of my decoders.

Not exactly, I know what type to expect, I just don't know how it will be formatted when I get it. So in principle you are right, I don't know if it will be a string or bool but I know to expect a bool. So I want to tell the decoder that "true" is also a bool value without having to declare a custom decoder.