A major challenge when making the Stellaris AI has been the randomized nature of the game. With thousands of different combinations of ethoses and traits, there's a risk that every AI Empire ends up feeling the same to the player, or fall into a very basic categorization of 'aggressive aliens' and 'peaceful aliens'. I as the AI programmer might know that an AI with Fanatic Collectivism makes their decisions differently from with plain old vanilla Collectivism, but it might all look the same to a player who doesn't have this foreknowledge.
In order to address this problem, we've implemented a system of AI Personalities that govern almost every aspect of how they behave, such as who they'll pick a fight with, which trade deals they are interested in and how they budget and utilize the resources available to them. This personality is determined by their ethos, government form and traits, and will be shown to the player when diplomatically interacting with that Empire. To feel recognizeable to the player, all of the personalities are rooted in sci-fi tropes, so that you'll immediately know who the Klingons are to your United Federation of Planets.
Personalities naturally have a bigger impact on diplomacy than anything else - if your goal is to form a Federation, it'll be much easier to do so with an Empire of Federation Builders than a bunch of Ruthless Capitalists, and forget getting Xenophobic Isolationists to agree to any such proposal unless they have a very pressing reason. You can tell how an Empire feels about you from their Attitude, which is primarily driven by opinion, and affects factors such as what diplomatic offers they'll consider and how fair a shake they will give you in trade deals.
In addition to the regular personalities, there is also a special set of personalities for Fallen Empires. Instead of the usual mix of Ethoses, each Fallen Empire has only a single Fanatic Ethos - the single remaining ideal they hold to after centuries of seeing what the galaxy has to offer. This Ethos determines their personality, which in turn affects how they view your actions. For example, a Xenophobic Fallen Empire will want nothing to do with you or anyone else and will be very upset if you start encroaching on their borders, while a Spiritualist Fallen Empire will consider themselves the protectors of the galaxy's holy sites, and will not look kindly on your colonists trampling all over their sacred planets. If you think angering a Fallen Empire is harmless because they won't conquer you - think again. Fallen Empires get a special wargoal to force you to abandon planets, and will be more than happy to cut your upstart species down to size if you don't show sufficient respect for your elders.
Threats and Rivals
So what then, is a pressing reason for an AI to go against their personality? Well, one such reason is Threat. Threat is a mechanic somewhat similar to Aggressive Expansion in Europa Universalis 4. Conquering planets, subjugating other Empires and destroying space installations will generate Threat towards other Empires. The amount of Threat generated depends both on how far away the Empire is from what's happening and on their Personality. Xenophobic Isolationists won't care if you're purging aliens half a galaxy away, but if all the planets around them being swallowed up by an expanionistic Empire, they'll definitely take note. Empires that are threatened by the same aggressor will get an opinion boost towards each other, and will be more likely to join in Alliances and Federations - if you go on a rampage, you may find the rest of the Galaxy uniting to take you down, and while Threat decays naturally over time, there's no guarantee that the alliances formed by your imperialism will break up even if you take a timeout from conquering... so expand with care.
Another feature borrowed from EU4 to drive AI behaviour is Rivals. Any independent Empire that are you not allied to can be declared a Rival, up to a maximum of 3 Rivals at the same time. Having an Empire as a Rival will give you a monthly increase of Influence, with the amount gained based on how powerful they are relative to yourself - having a far weaker Empire as your antagonist will not overly impress your population. It is further modified by Ethos, with Militarist Empires benefitting significantly more from Rivalries than Pacifist ones (but paying more influence to be part of an Alliance). Naturally, Empires won't be particularly happy about being declared a Rival, and are pretty likely to rival you right back. Having a Rival will improve relations with their enemies and worsen relations with their friends, so the Rivalry system will act as a primary driver of conflict and alliance in the galaxy.
Finally, I wanted to cover the topic of the AI's bookkeeping. While it may be far less exciting and far less visible to the player than its diplomatic behaviour, having solid economics is one of our biggest priorities for the Stellaris AI, for multiple reasons. Firstly, so that the AI is able to compete reasonably with the player without resorting to outright cheating. True, the AI will never be as good as an experienced player, but there is a big difference between the player being able to outproduce one AI Empire and the player being able to outproduce five of them together. Secondly, because of the Sector mechanic that was covered in DD 21, the AI will actively be making construction and management decisions on the player's planets, and while - again - it will never be as good as an experienced player making the decisions themselves, it needs to be good enough that the player doesn't feel like the AI is actively sabotaging their Empire.
In order to accomplish all this, a huge amount of time has been put into the AI's budgeting system. Every single mineral and energy credit that the AI takes in is earmarked for a particular budget post such as navies or new colonies, with the division between the posts being set according to the AI's personality and what it needs at the time. The AI is only permitted to spend appropriately budgeted resources, so it'll never fail to establish new colonies because it's too busy constructing buildings on its planet, or miss building a navy because mining stations are eating up its entire mineral income. In times of dire need, it can move resources from one budget post to another - if it's at war and its navy gets destroyed, expect it to pour every last mineral into building a new one.
When making decisions about what to construct, the AI looks primarily at what resources it has a critical need for (such as Energy if it's running a deficit), secondarily at what resources it's not producing a lot of compared to what it expects an Empire of its size to produce, and lastly at whatever it deems useful enough for the mineral investment. Sectors have additional logic to ensure they produce more of the resource you've set them to focus on, so an Energy sector will naturally overproduce Energy - you told it to, after all.
Alright, that's all for today. Next week we'll be talking about debris and the fine art of reverse engineering.