J-bot is hosted by Firefly. J-bot runs the software called jevalbot, which was originally written by User:b_jonas. (This doesn't include the J interpreter, which was written mostly by Roger Hui.) Its source code is found at https://github.com/FireyFly/jevalbot and http://www.math.bme.hu/~ambrus/pu/jevalbot.tgz , and is currently unmaintained. At least four different people have ran instances of jevalbot in history, but of those, only j-bot is currently active. (There were at least two other bots that evaluated the J language: IJX, and buubot versions 2 and 3.) https://code.jsoftware.com/wiki/Community/IRC#bots has some very obsolete description of the bot.
J-bot can be invoked with the
[ shortcut, or using its full name followed by a space followed by a command name followed by a colon followed by optional arguments. The actual syntax is rather arcane and arbitrary, and b_jonas is sorry for it. J-bot can also be invoked in private message, where it takes a slightly different arcane syntax. Unlike most bots, j-bot can give several lines of output for one command, so you should be careful to avoid spamming channels when you use it.