Which parser? That’s the problem: if you’re using JSON as a data interchange format, you’ll need to carefully control both the serializers and deserializers, and whatever libraries you use, they will need to (at least internally) hold onto the number in a lossless way — I am not aware of any libraries that do this. They all parse the number as an f64 before any deserializers run. If your input JSON contains a u128, then you’ll have a loss of precision when your type is deserialized.

If you can set up (de)serialization to work the way you need it, then there’s no problem. But if you share your JSON serialized data with other parties, then you/they may be in for a bit of a surprise.

You might find it a worth while exercise to try parsing JSON containing an arbitrary unsigned 128 bit integer in your language of choice.