Scala 3 pickle format is not compatible with Scala 2 format about boopickle HOT 5 CLOSED

I think you should provide a class hierarchy, with a description of how you create the "picklers".
Otherwise, no one can help you.
My first guess would be, that macro-generated picklers behave differently (if they still work with scala3).

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I think the difference is how value classes are serialized (extends AnyVal).

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Figured it out. Minimal example:

case class B(p1: Seq[Int])
Pickle.intoBytes(B(Seq(10, 20, 30, 40)))

Scala3 does not output the extra zero at the begining:

Scala 2: 0, 4, 10, 20, 30, 40
Scala 3: 4, 10, 20, 30, 40

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Actually, I changed my mind.

I find the Scala 3 format much better. It is about 20-30 percent more efficient, due to skipping of those useless zero bytes.

Let's keep the Scala 3 format, as it is more efficient and easier to write custom decoders. Scala 2 is going to be phased out soon enough.

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One more difference. For sealed trait hierarchies:

Scala 2 encodes with a starting tag "1" the first child that comes alphabetically by name.
Scala 3 uses a tag "1" for the class that appears in the file first.

Scala 3 again much better, since it is common to rename things, but not so common to move them around.

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