Every byte counts: ARCv3 and the case for cross-region RL

Fireworks announced ARCv3, a lossless compressor designed to reduce the BF16 model-weight updates transferred from reinforcement learning trainers to geographically distributed rollout machines. Based on 1,000 production RL update deltas, ARCv3 achieved an average compressed size of 0.19% of the original weights, compared with 0.36% for ARCv2, 0.35% for PULSESync, and 0.67% for a draft Hugging Face TRL implementation. The approach exploits the observation that only about 2% of weights change between checkpoints and that most changes affect mantissa bits rather than a weight’s exponent or sign, omitting unchanged values and separately encoding common and rare update types. By shrinking transfers while reconstructing weights bit-for-bit identically, the system is intended to help rollout fleets remain aligned with the latest policy across multiple regions using shared object storage and standard network links rather than a single co-located cluster. ARCv3 is the default for Fireworks Trainer SDK rollouts and is also available through the fireworks-delta-compression package for users operating their own trainers.

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