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A dedicated Slay the Spire 2 fan popped the hood on the game to inspect its inner workings, and soon emerged from its underbelly, face and fingers smeared with grease, with a head-turning report: the RNG is rigged. Kind of.
As PCGamesN spotted, Slay the Spire 2 mathematician Tckmn streamed and explained – in some 6,600 words – how he accidentally discovered correlated RNG in developer Mega Crit's hit roguelike deckbuilder.
The core claim is simple: certain events in Slay the Spire 2 influence and tell you about the outcomes of other events, many of which have a deceptively small pool of outcomes with unequal weighting. Or in Tckmn's words: "Knowing the first output of one of the game's RNGs gives information that helps predict the first output of all of the others."
Pertinently, this is described as a much bigger problem in Slay the Spire 2 compared to the original because the sequel apparently has a much narrower range of checks for outcomes.
This is similar in concept to the game's seed system. Each seed delivers a predetermined series of encounters, but you're still able to make branching decisions within that seed – the path you take, the cards you play in what order, the items you buy, and so on. Tckmn convincingly argues that there's almost a sub-seed of sorts that quietly creates runs where, as a very crude example, Outcome A in Event 1 indicates Outcome A is also highly likely in Event 2, even if Outcome B appears equally likely.
The relics you choose from at the start of a run help explain this. Tckmn reckons that the Neow relic Neow's Bones, which gives you a random curse card, has a 54% chance of giving the Debt curse in the Underdocks map, but a flat 0% chance of the same curse in Overgrowth, the other Act 1 map. Instead, Neow's Bones in Overgrowth massively favors the Writhe curse at 73%.
The same logic applies to the Neow capsules that give you other random relics. In Underdocks, Tckmn found that the large capsule had a 23.75% chance of giving you Precarious Shears; in Overgrowth, that drops to 1.35%.
This is a pretty clear-cut example of how this knowledge could warp your decisions and strategies. If you're choosing an RNG event hoping for a specific outcome, that desired outcome may actually be incredibly rare behind the scenes, which makes that RNG event less appealing.
"All of these give some information about all the others," Tckmn says of a remarkably long list of RNG events, including the Act 1 map, the last card you draw in your first combat, the first card you generate in combat, and whether your first fight rewards you with a potion. This can even impact which enemies the Defect's default lightning orb will 'randomly' hit, it seems (75% chance to aim left in the first fight of Underdocks, if you're wondering).
Tckmn says he only discovered this after building a tool that could search for seeds containing specific combinations of cards and events. Adding Overgrowth as a condition totally warped the otherwise normal search results, and that sent him down this rabbit hole.
With this in mind, Tckmn makes a "plea to the developers" at Mega Crit. "Of course, I do think that [correlated RNG] in Spire 2 is a bug and ought to be fixed, and I think it would be pretty bad for the game if it wasn't," he says. "However, I am confident that Mega Crit will address this issue. For one thing, Spire 2 is still in Early Access, much earlier in its development cycle than when CRNG was discovered in Spire 1."
Assuming that every assumption here is true, this isn't game-breaking or anything – obviously, Slay the Spire 2 is and has been fun – but could skew RNG in an unintuitive way. Tckmn suggests the fix is simple, and even mocked up some simple "drop-in" code that could apparently address it by decoupling that correlation. I would love to read Mega Crit's response to all this, in part to confirm how much of it is intended.
