Exactly this. You need deterministic serialization, because you need to be sure that when the _same_ object is constructed in different settings, it is going to hash consistently. In Hyper Hyper Space [1], the set of basic types as well as the composition primitives used to construct all data structures have built-in deterministic serialization, just for this reason (e.g. a set will serialize into a deterministically ordered list, etc.)
> Exactly this. You need deterministic serialization, because you need to be sure that when the _same_ object is constructed in different settings, it is going to hash consistently.
I can see how this might matter in some specific systems, but when we're talking about signatures only the signer constructs the object. Stuff like the "JWS/CT using JWS and JSON Canonicalization" recommended in a sibling comment is a complete misdesign for virtually all signing use cases. That's why "our signature scheme requires canonical representations" is a red flag.
But "the signer" here is a cryptographic identity, that may be present in more than one device. So, even when conceptually it is just one entity, in practice it may be several computers doing something independently, and one may need the result to be the same given identical inputs.
Offhand that sounds really cursed: Signing the "same" thing multiple times with the same key shared among multiple devices and also expecting the signatures to be identical. I honestly can't think of anything that would require something like this and it sounds like a really bad idea as well.
Just to clarify something I've seen a few siblings confused about: Saying your signature scheme should not rely on canonical representations doesn't mean you shouldn't parse the input properly before processing it (e.g. checking that the input corresponds to a schema). It just means those two things are separate things best done separately. It's a little like NATting and firewalling: often confused with each other, or people arguing one acts like the other, but they're really separate things, and everything is much easier once you understand and heed that.
[1] https://www.hyperhyperspace.org