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OK, looks like I got this wrong. We did have the right version of asm on the build classpath, but the latest version of Jackson embeds Java 17 and 21 classes under its META-INF, which couldn’t be parsed by shadow when running Java 11. Excluding those has prevented the error from being thrown.

It should be pointless which version you use to run. The point is, that shadow for the remapping asks ASM to parse a class. That class was compiled with Java 21 (class version 65), but the ASM version that was used does not support reading Java 21 files.