Without any major changes, struct tags cannot solve the majority of the problems, because they do not propagate. The v1 MarshalJSON/UnmarshalJSON interfaces are

Without any major changes, struct tags cannot solve the majority of the problems, because they do not propagate. The v1 MarshalJSON/UnmarshalJSON interfaces are too simplistic. At the very least, an alternative pair of interfaces should exist, which take options/flags/tags/whatever in the parameters. However, I agree this doesn't need to be in an entirely new package. It's not even hard to favor one interface over another: switch ifc := val.(type) { case MarshalJSONV2: // new interface, always used if present case MarshalJSON: // old interface, fallback default: // type isn't self-marshaling } The jsontext package is what's really revolutionary and needed here. The poor performance of the existing API is due primarily to the lack of a proper streaming API as the foundation. Using this as the basis for the new interfaces makes sense, and I agree that once this exists, the need for an entirely separate v2 package largely vanishes.