Claude Fable 5 Generates First Genuine Megakernel and Wins AI Decoding Benchmark
Claude Fable 5, Anthropic’s latest AI model, generated the first authentic single-fused megakernel submitted to KernelBench-Mega, according to a post Tuesday by benchmarking developer Elliot Arledge. In a hardware decoding test optimized for Nvidia’s RTX PRO 6000 Blackwell chip, Fable finished 18.7 times faster than a reference implementation, outpacing submissions from rival models including Opus 4.8, which trailed by 14.4 times, and GLM 5.2, which was 11.1 times slower.
Arledge's verification showed that Fable consolidated operations typically spread across multiple software layers into a single cooperative kernel launch per decoded token. The model demonstrated it recomputed directly on live data rather than relying on cached outputs, and its performance advantage scaled with longer context windows, extending from 17.8 times faster at 2,000 tokens to 19.5 times faster at 16,000 tokens.
From the sources (3 posts)
@elliotarledgeClaude Fable 5 [max] wrote the first genuine (and fastest) megakernel ever submitted to KernelBench-Mega. It was tested on: Kimi-Linear W4A16 batch-1 decode for RTX PRO 6000 Blackwell. Every prior model "won" it with a multi-kernel Triton
@teortaxestexMaturity.
@elliotarledgelook at the megakernel yourself