Modal tests mixed-precision, mixed-accelerator RL samplers on Qwen3.6-35B-A3B

Modal trained Qwen3.6-35B-A3B on agentic coding with samplers spread across six accelerator types (B200, B300, H100, H200, A100, RTX PRO 6000) at NVFP4, FP8 and BF16 precision, feeding one BF16 trainer on B200. Plain GRPO collapsed on both mixed and homogeneous samplers, but adding masked importance sampling plus score centering, with or without top-p mask replay, trained stably to the end and reached SWE-bench Pro V2 pass@4 on par with homogeneous samplers, at roughly 15% lower sampler cost per update. The mixed samplers ran with an average sampler-trainer KL 4.7x that of the homogeneous setup, and the authors argue stability depends on keeping the mismatch bounded rather than low.