engine?
compose the Slay the Spire 2 run start you want and find seeds that match, or inspect a seed you already have to see how it unfolds. works for the public beta (v0.111.0) and the game's main branch (v0.107.1).
SearchTheSpire is a free browser-based seed explorer for Slay the Spire 2: search for seeds that match a run start, or inspect one you already have. the board covers most defining characteristics of a run: Neow blessings, card rewards, relics, events, and ancient offerings. compose the parts you care about and it searches the seed space for runs that match. paste a seed into the inspect box and it shows how the run unfolds. all in your browser, with no server involved.
to play a seed you found, copy it from the results, then in the game go to singleplayer and start a custom run, which has a seed field; paste the seed there. you must have beaten act 3 three times to unlock custom mode.
randomness in games like Slay the Spire uses a concept called a random number generator (RNG) to produce a series of random numbers. when you "seed" a random number generator, you alter it such that it always produces the same values in the same order given the same seed. the series of numbers produced is often referred to as a "stream"; taking a number from a stream is often called a "roll" or a "draw".
Slay the Spire 2 has multiple RNG streams which dictate how your run plays out, like which cards you are offered, which ancients you encounter, what each ancient offers you, etc.
the most important such stream is the rewards stream, and it dictates every combat reward you are offered: potion chance, potion dropped, gold amount, card reward rarity, card rewards, and even relic rarity.
this means that anything that pulls from that stream along your path changes what you'll see next. whether you take 4 combats or 5 before your first elite, or whether you gorge or search in the room full of cheese, affects every successive combat reward. as a result, predicting some kinds of rewards (e.g. the act boss card rewards) would require you to take a precise set of actions, including making specific event choices as you play the game. in my opinion, that's not very fun, and so this tool mostly only supports things which largely stay fixed independently of the actions you take.
the two branches generate runs from seeds differently. on the main branch, the alphanumeric seed is hashed down to a 32-bit number, so there are exactly 4,294,967,296 distinct runs. that is small enough to search the entire space, so a full search finds every match there is.
the beta draws 12-character seeds from a 34-letter alphabet and hashes each down to a 64-bit number, giving about 2.4 quintillion possible runs. that would take close to two decades to sweep even at GPU speed, and centuries on CPU, so beta searches cover a large slice at a time instead, and keep searching picks up where the last pass stopped.
the same seed therefore produces a different run on each branch, and the branches ship different card, relic and event pools on top of that. a seed found for one branch will not work on the other, so set the version toggle to the branch you actually play before searching or inspecting a seed.
small portions of StS2's RNG engine were re-implemented in Rust, using the game's own code as a reference. given a seed, this engine can reproduce certain rewards the game would grant without actually running the game.
that engine is compiled to WebAssembly and runs entirely in your browser. there are two search engines: one is purely CPU, and the other uses WebGPU to massively parallelize the search across GPU cores. the CPU engine gets through tens of millions of seeds per second, the GPU engine billions.
the same search can be instant on one machine and slow on another; the difference is almost always whether the GPU engine could take over. it needs two things. your browser and hardware have to expose WebGPU, which older devices and some graphics drivers do not. and your filters have to be selective enough that discarding seeds in bulk beats simply checking them; a loose search turns up matches almost immediately on CPU, so it stays there.
a filter the GPU cannot evaluate does not send the whole search back to CPU. the GPU pre-filters on the parts it can and passes the survivors on. every seed it keeps is re-checked by the CPU engine before it reaches the results, so both engines return the same seeds and the GPU only changes how fast they arrive.
when you hit search, a pool of workers splits the seed space into chunks, shuffles them, and checks run starts against your filters. because the chunks are shuffled, a search that stops early at its match cap holds a random sample of the matches, not the first ones by seed; searching again turns up different seeds. a main branch sweep has provably found every match only when it finishes without hitting the cap.
checking a seed doesn't simulate a whole run; the engine only makes exactly as many RNG calls as your filters need.
the engine is pinned against tests generated from the game's own code, and i personally check seeds in-game :)
no, the opposite: it's the one slot on the board where empty is a promise rather than a wildcard. leave it empty and seeds must match no matter which bonus you take. here's why.
every run starts with a neow bonus, and several neow bonuses draw from the shared rewards stream (see how does RNG work). this means that your choice of neow bonus can change every reward you see for the whole run.
the stance of this tool is that it does not want to dictate your choices; i don't want to return a seed that only works if you pick a certain neow bonus.
that means that when your search does not specify a neow bonus, only seeds which match regardless of which bonus you take will appear. the rng can work out so that a bonus that moves the stream still lands your cards, and those seeds do come back, but they are the minority: about three quarters of the results are seeds where all three offers happen to be from the list that leaves the stream alone.
pinning a neow bonus is the way to search the other side of that. it does not widen the results, since the bonus you pin also has to be offered, but it asks a different question: these rewards, given you take this bonus.
these are the bonuses that can move rewards. an unpinned search has to hold even when they're offered, so when a card reward search comes back empty, one of these sitting in the seed's offers is often the reason:
massive scroll is co-op only. it is never offered in a solo run, so it only matters when a seat is pinned in the inspector. the three cards it offers are the co-op-only cards from your character's pool and the colorless pool.