.NET 7 introduced Int128 / UInt128.

How to implement those formatters?

Option 1 - Bin Array

BigIntegerFormatter serializes BigInteger as byte[].
This option seems most performant because sizeof(Int128) is always 16.

Option 2 - Msgpack Way?

There are 2 proposals for big integer/float.
Both proposals are denied by messagepack author.
This option seems not good because other messagepack implementations do not adopt this option.

Option 3 - Original Ext

Original extension.

Option 4 - Do Nothing

Do not support Int128/UInt128 officially.
There was no issue about these types and no one suffers. (Except #1593, but this issue opener seems not in trouble.)