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Nine of twelve open VLA codebases train without an fp32 master copy, and at least 79 percent of their bf16 updates were silently erased

AspireAI community · arXiv
Fine-tuning a vision-language-action model in bfloat16 without an fp32 master copy makes AdamW write each update straight into bf16, which rounds away any update smaller than 1/512 of a weight. The authors found 9 of 12 open VLA codebases ship a default or documented recipe that does this, including LeRobot's SmolVLA default and its pi_0/pi_0.5 guides, and a probe every 100 steps of their LIBERO runs found at least 79 percent of bf16 weight updates erased. Keeping trainable weights in fp32 lifted LIBERO-Object success from 4.3 percent to 49.3 percent for SmolVLA and from 5.1 percent to 80.9 percent for pi_0.5 in the undertrained low-learning-rate setups, and from 89.0 percent to 97.7 percent for pi_0.5 at its default rate, with every run carrying the copy beating every run without it. The gaps are much smaller where the models were already well trained, so the headline effect concentrates on undertrained runs, but the fix is a one-line change.
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