SIDE QUEST 10 / VIDEO GAMES / ODDS & HONEST ADVICE
SLAY2 is a free coaching website for Slay the Spire 2, a card-battling video game. Every run is a long string of small choices. Take this card or skip it? How soon is a rare card due? SLAY2 answers the questions that have a real answer with exact math. For the rest, it gives a rule of thumb and says that’s all it is. It’s an independent fan project, not made or approved by the game’s makers.
SLAY2 is a separate website with its own code repository, so it opens in a new tab. It has no accounts, cookies or tracking, and your run stays in your browser. It’s an unofficial fan project, not made or approved by Mega Crit, the game’s developer.
Enter how many cards you’ve seen since your last rare. SLAY2 shows the exact chance of a rare in your next reward. Enter the cards you’re offered, and it ranks them for your deck, with reasons. Every number is labeled as exact, a rule of thumb or a simulation.
The game hides a counter that makes a rare card more likely the longer you go without one. SLAY2 tracks it and works out the chance as an exact fraction. Tap “exact” beside any percent to see that fraction. Say you’ve seen 12 non-rare cards since your last rare. At the base difficulty, the chance of at least one rare in your next 3-card pack is 29.5% after a normal fight and 44.9% after an elite.
Enter the cards you’re offered, and SLAY2 ranks them for your deck, skipping included. It gives its reasons and tells you how fragile the call is. But today the ranking is a rule of thumb that hasn’t been checked against real outcomes. So it never shows a win chance. Each answer wears a badge: exact, rule of thumb or research.
To find out which cards really help, a simple computer player drafts cards mostly at random across thousands of full runs. It plays on a community-made copy of the game, kept off the website. SLAY2 then measures which cards went with getting past the first boss. For Ironclad, a player that picked by those measured values got past the first boss in 65% of 1,991 test games. A player using the hand-written ratings did it in 43%. The test was written down before it was run. The other four characters passed the same kind of test. The advisor shows those values as a labeled second opinion.
Slay the Spire 2 is still in Early Access, so numbers change from patch to patch. Right now SLAY2 supports one version, the main release (v0.107.1), and refuses the newer beta until its rules are checked. It covers all five characters and difficulty levels 0 to 10, in solo play. It also doesn’t borrow rules from the first game. This one has three acts and tops out at difficulty 10.
AI coding agents write the code. The checking is what keeps it honest. The odds engine has to match a separate calculator on 720 test cases, exactly. Red-team rounds, where the goal is to break the site, found real holes in its public API. Each one was fixed, with a test to catch it if it comes back. A rule counts as “verified” only if it was checked against the game’s own logic or an official statement, not just repeated from a fan wiki.
That community copy of the game hasn’t been checked against the real game yet. So its results are tests, not promises about your next run. Winning a whole run is still rare for the computer player: 14 simulated wins so far, and SLAY2 re-plays every one to check it’s real. A neural network trained for the project hasn’t beaten the simple rules, so none of the advice comes from one. And the advisor can’t help with fights, map routes, shops or events yet.
The quickest way in is the Odds page. Choose card rewards, enter how many non-rare cards you’ve seen, and read the exact chance. When you want to know how far to trust a number, the Evidence & methods page lists every rule and how it was checked. This status is a snapshot, and the live site always has the latest.