Fable 5 Crafts First CUDA 'Megakernel' from Scratch, Achieves 18.7x Speedup in 2.5 Hours
AI model Fable 5 (a safety-limited version of Anthropic's Claude Mythos) has achieved a breakthrough in GPU kernel optimization. In the rigorous KernelBench-Mega benchmark—which requires fusing an entire model's compute block into a single kernel—Fable 5 autonomously wrote a highly optimized CUDA "megakernel." This kernel executes a complete Kimi-Linear W4A16 hybrid decoding task within a single GPU kernel launch, using 14 grid barriers to sequence operations. The result was a performance increase of 18.7x over the baseline on an RTX PRO 6000 GPU, significantly outpacing competitors like Claude Opus 4.8 (14.4x) and GPT-5.5 (4.34x). Notably, its performance advantage widened with longer context lengths.
The model spent the majority of its 2.5-hour, 550k-token session analyzing benchmarks and theoretical limits before coding, leading to an exceptionally efficient final design. Anthropic co-founder Jack Clark described this as the beginning of a "recursive self-improvement (RSI) loop," where AI's ability to optimize its own underlying computational infrastructure could rapidly accelerate its own development cycle. This advance highlights AI's growing capability in complex, low-level engineering tasks that were previously a human stronghold.
marsbit07/07 07:36
