THE PHYSICS OF A FABLE

Summary from the TLDR AI newsletter

Rafa Schwinger reverse-engineers Claude Mythos and Fable by arguing the moat is not architecture but the environment foundry, with capability decomposing as base foundation times gradeable signal extracted on top, and verifiable reward becoming the scarce decisive input now that text and raw compute no longer are. The recipe stacks dense pretraining, GRPO-style verifier RL where reward-hacking soundness is the actual binding constraint, long-horizon process rewards with learned context-folding that beats million-token windows at 32K active, plus best-of-N test-time compute exposed as an effort dial.