I don't think it's deliberate RNG manipulation they worry about. It's a single player (or coop) game after all.

However, one of their design goals is that people playing on the same seed should have roughly the same game, it should feel "fair". Some things you probably want to be fairly random, for instance your card choices can depend on what cards you chose before. But it's also important that people choosing the exact same cards (and taking the same path, maybe?) should be offered the same options.

In STS1, the order of relics was fixed from the start as I recall. So if you skipped a shop, you'd get exactly the same relics in the next shop as you would have in the one you skipped. Good for seed fairness, but a little odd.

For a singular seed, they wanted the resulting run to be stable in the sense that small deviations in decision making does not result in a vastly different result (as far as random events are concerned)

Imagine the game of two players having the same state X. While combat, one player would trigger a random action, the other doesn't. After the combat, both should still get the same randomized reward options. This wouldn't work with just a singular RNG.

The game stores and allows you to see the RNG seed that controls the run's events and layout. The developers want players to be able to share seeds that produce interesting runs.

That requirement is what made this problem difficult for the devs to solve.

Some degree of stability in seeds is desirable, partly because of the compatitive elements (multiple players playing the same seed should have roughly the same game), but also when updating the game is means that if they tweak one area the rest of the seed will stay roughly the same. (Maybe less important for games with short runs compared to sandbox games like Minecraft where the world might be generated by very different versions if the game and you don't want blatent seams when it happens)

> With a good RNG it should not be possible to predict future numbers based on past numbers so players cannot manipulate card rewards in their favour based on combat actions, right?

I think you're overlooking the distinction between seeded and unseeded runs. An unseeded run is a run in which the player enters the game not knowing what the seed of the RNG is and not being able to pick it (this is the default mode). A seeded run is where the player provides the RNG seed. Generally, things like unlocks and steam achievements can only be earned on unseeded runs, but players want to be able to play seeded runs anyway. Of course all runs have an RNG seed: an unseeded run is when the seed is itself chosen at random, a seeded run is when the player specifies the seed.

Imagine a game with a standard deck of cards played over several rounds, where your opponent performs actions in response to your actions. The deck of cards is shuffled pseudo-randomly between every round. Every time you make a move, your opponent has N valid moves, and picks between them pseudo-randomly.

Players play a seeded run because they want to retry the same set of challenges, because they are asking themselves "if I had done this, would I have won" or "if I had done this, what would have happened".

So in this example, given a known seed: in round 1, my cards are shuffled this way, and in round 2, my cards a shuffled this other way, regardless of which moves I make in round 1.

If the opponent picks its response using the same RNG that shuffles the deck, the players actions in round 1 would change the shuffling of the deck in round 2. This would change the design parameters of what a seeded run means: it's no longer giving the guarantee "the deck is shuffled in the same way in round 2 regardless of what you do in round 1", which is the experience the designer wants to create and what the players want to play. Players might, e.g., say "who can get the highest score on this seed", they might search for the easiest or most difficult seeds, or they might search for seeds where particularly unlikely sequences of events are guaranteed to occur, because perhaps this sequence of events is so unlikely that a human would have to play a hundred million games to witness that sequence organically, and people want to see that sequence of events because it's interesting in some way. It's designed this way so that if you play the same seed, certain random events play out the same way, i.e., non-randomly.

Because they appear to have a curious way of doing their saves. From the article:

>The way Slay the Spire allows you to save and resume runs is by storing the total number of times each RNG has been called, and then calling each RNG that many times (throwing away the result) whenever a save file is loaded.

Depending on what the game is like (I know nothing about it), that could make sense, even if it is inelegant.

You're confusing RNG manipulation (really clearly bad, basically cheating by removing the randomness from the game) from RNG prediction (less bad but still unfun, being able to predict future random states).

You can be safe from RNG manipulation while still suffering from RNG prediction. Players can't modify the events that are going to happen, but if they can predict them, it's still a problem.

The situation is like there's a bug in the blackjack table where, instead of shuffling the whole shoe together, each deck in it was shuffled on its own in the same way and then the identical decks are stacked together. Once you've seen 52 cards, you know the repeating pattern and can play with perfect or near-perfect knowledge of what's about to be dealt.

> With a good RNG it should not be possible to predict future numbers based on past numbers

Since they are using the built-in RNG, it is trivial to predict if you know (or can guess) the seed: just run the same RNG a few steps ahead.

For something like a tool-assisted speed run, this is very exploitable to setup optimal runs

You could observe future random numbers by taking combat actions, and then reset to the start of the fight and play a line which consumes fewer random numbers in order to manipulate your card rewards. Maybe you could generate the card reward at the start of the fight, but what if they play a card which impacts the card reward, e.g. by creating an extra card reward.

For things like daily runs / seeded runs part of the fun is getting the same card rewards.

I guess it's mainly a limit to savescumming.