Starcraft AI blog

Your bot got 5 pooled and lost. Well, you can fix that, you just need a strong reaction to the rush. Maybe scout early so you can see it coming (Iron does this), and if so, get the barracks earlier and have a bunker up in time. Then stay at home until you’ve built up enough forces to move out against the ever-increasing number of zerglings at your doorstep. And it works! The early pool set your enemy back, and with your reaction you smash those cheap rushbots!

Then along comes the bot named 5 Pool. (It last played on SSCAIT over a year ago, and not in its most sophisticated version.) It opens with 5 pool as its name says, you see the rush coming, you are completely safe—but there is no stream of zerglings looking for a way to break in. After a handful of zerglings to scare your bot into reacting, 5 Pool switched into drones and tech while you unnecessarily built up your forces to safely move out. Marines and medics step into the map and... mutalisks annihilate the force and move on to pick apart your base. Somehow your reaction to being rushed set you further back than playing 5 pool set your enemy back! How did that happen?

Zergling rushes are successful in bot world because they are easier to code than to defend against. In fact, rushes like that are very difficult to defend accurately against; it is much easier to overreact to the rush and be safe for the moment than to react with exactly enough defense, and especially so if your bot is new and lacking in skills. If you see the rush coming in time and react accurately, you are ahead because the early pool set your opponent back. If you overreact and spend too much on defense, you may fall behind. The bot 5 Pool exploited the fact that most opponents either didn’t react and lost, or overreacted and fell behind. Exploiting overreaction is a fancier trick, but it is also easier to code than to defend against, and 5 Pool ranked high.

I remember the struggle of Steamhammer versus 5 Pool. As zerg, it is relatively easy to avoid overreacting to a rush: Just go 9 pool, and if your worker defense is sub-par, add one sunken. You are in the lead. At first, 5 Pool could not make headway against Steamhammer’s strong ZvZ (well, strong by bot standards). Eventually, the author developed a sunken turtle into 3 hatch ling strategy that overwhelmed Steamhammer’s fixed middlegame plan. Later came a tougher strategy, sunken turtle into mutalisks, that beat Steamhammer reliably. Steamhammer understood what was happening and tried to react correctly, but overreacted to the enemy sunkens, went overboard on drones, and got its mutalisks too late. It is very difficult to react perfectly. More recently, CasiaBot adopted an optimized version of the same opening to beat Steamhammer, and CherryPi can play a similar strategy. It was months before Steamhammer’s openings and reactions were crisp enough to avoid overreaction.

Overreactions are exploitable weaknesses, and exploiting them is generally easier to code than to defend against. Bots overreact often, and to various events.

Steamhammer was possibly the first zerg bot to make a small number of zerglings to keep tabs on the enemy while teching to mutalisks (it’s standard in human play). In early months, Steamhammer won a lot of games when opponents overreacted to the zerglings, seeming to assume that they were facing a mass zergling threat. Even Iron often made an unnecessary bunker. Today most opponents have become more circumspect, but I think LetaBot overreacts to the early zerglings even now.

Locutus's pylon harassment skill (which is so strange that I could not understand it at first) is another example of exploiting overreactions. The skill amounts to simply building pylons in plain sight in the enemy base. It was first used against Iron, which overreacts extremely to proxy buildings and sets itself far back. Many other bots overreact to a lesser degree, including Steamhammer (I made a change recently to reduce the overreaction, but it’s not gone yet). How is your bot supposed to know that it’s only a pylon, and no cannons or gateways are going to appear in your base? You have to react, and it is difficult to react just enough to be safe against a proxy while not so much that you fall behind when it turns out to be a bluff.

Most forms of deception do not work against bots, because bots are not smart enough to fall for them. Exploitation of overreactions is a form of deception that often does work, at least against bots that react at all. I think bot authors should be on the lookout for other cases of overreaction to exploit. Since eliminating overreactions is difficult, exploiting overreactions will force a lot of progress.