According to
https://wiki.project1999.com/Magician#Chain_Casting, it seems that chain casting does not work the way we could expect.
"Unfortunately, the way agro current works on P99, this strategy is not very effective. When a pet dies, all of the hate that it built up seems to be applied directly to the caster (instead of "lost"). This means that a newly summoned pet will have very little chance of peeling the mob off of the caster. This has been reported as a bug, but has yet to be acknowledged as one (as of 6/23/20)."
Any news on this point ? Is it really true ? It's very hard for me to remember how it could work in 1999 especially because I was exclusively using an earth pet during this period.