I am the developer of an anticrash, i can't add my own decoder before spigot's decoder cause of protocollib, when i try this protocollib or my plugin isnt working.

well, others manage to do that, so its not an general issue. whats your code?

i just add a decoder before the mc decoder (in players pipeline)

Best I can do is point you to where we do our injecting: https://github.com/dmulloy2/ProtocolLib/blob/master/src/main/java/com/comphenix/protocol/injector/netty/ChannelInjector.java#L265