The trick they use is pretty clever. When you ask an AI to write code, it doesn't always get it right. Sometimes the code has bugs, sometimes it misunderstands the problem entirely. A naive way to address that is to generate a few solutions and test each one. The odds that at least one works go way up. ATLAS generates multiple attempts, running each through a test suite. Each retry also gets told what went wrong with the previous attempt, so it can try to avoid the same mistake.
But this can be pretty slow since you have to run the code in an isolated environment, check the outputs, wait for it to finish. Doing that for every candidate quickly adds up. So ATLAS has another shortcut for avoiding unnecessary testing. Instead of simply generating solutions and testing all of them, it tries to predict which one is most likely correct before running any tests.
ATLAS also asks the model for an embedding of what it just wrote which acts as a fingerprint. Two similar pieces of code will produce similar fingerprints. A well-written, confident solution will produce a different fingerprint than a confused, buggy one.
These fingerprints get fed into a separate, much smaller neural network called the Cost Field. This little network was trained ahead of time on examples where they already knew which solutions were correct and which were wrong. It learned to assign a score to each fingerprint. Correct solutions get a low score and incorrect ones get a high one.
So the process is to generate multiple solutions, get their fingerprints, score each one, and pick the lowest. Only that one gets tested. The Cost Field picks correctly about 88% of the time according to the repo.