I'm writing a client app using InSim.NET.

I initialise InSim using the IS_ISI with a prefix. When I type a message using this prefix, the IS_MSO packet's user type is MSO_PREFIX as expected on some servers, while on others, it's MSO_USER.

Is there some sort of setting on hosts that disables the prefix? If not, why am I seeing different behaviours?

I've never tried using a prefix on a client InSim app, but I guess this is what's happening:

If the server's prefix is the same as yours, you get an MSO_PREFIX
If the server's prefix is different (or it doesn't use one) you get an MSO_USER and your prefixed message is shown to other players

If you want to send hidden messages to a client InSim, you probably should to use /O <message>, which will send an MSO_O

Thanks for that.

I guess those servers where the prefix seems to work must have a host app running with that same prefix.

Would have been nice to be able to configure the prefix for a client app too. Would have allowed for cleaner looking messages to control it. Oh well...