Microsoft Paper Co-Evolves RL Harness and Qwen3.5 Model
Co-evolving the harness and the model appears to unlock gains that optimizing either alone cannot reach, suggesting the harness itself is a first-class lever in RL training. Teams training coding agents can copy the loop to squeeze more performance from mid-size open models like Qwen3.5-35B-A3B. The distillation trick of using Opus 4.8 as the harness proposer also offers a cheaper route to borrow frontier-model judgment.
A Microsoft-led paper introduces a method that alternates between optimizing the agent harness and training the model with reinforcement learning. On Qwen3.5-35B-A3B, average resolve rate across three coding benchmarks rises from 57.8% to 64.8%. In each round, the policy reads its own failures, proposes harness changes, and is then trained with RL under the harness it selected; tasks that remain hard become the search set for the next harness round. The joint gain exceeds the harness-only and policy-only gains combined, and each optimized component keeps its improvement when paired with the other's original version. Using Claude Opus 4.8 as the harness proposer adds further gains, transferring a frontier model's harness judgment to a smaller policy.
Great work from Microsoft and colleagues.
This one is on the hot new topic of co-evolving the harness and the model.
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They raise Qwen3.5-35B-A3B's average resolve rate on three coding benchmarks from 57.8% to 64.8% by alternating the two.
In each round, the policy reads its own failures and proposes harness changes. Then it is trained with RL under the harness it picked. Tasks that stay hard during training become the search set for the next harness round.
The joint gain is larger than the harness-only gain and the policy-only gain added together. Each optimized part also keeps its improvement when paired with the other's original version.
Using Opus 4.8 as the harness proposer in the same loop adds more on top, which makes this a practical way to transfer a frontier model's harness judgment into a smaller policy.
https://t.co/FlYU8zQBCz
Chat with Paper: https://t.co/PrpXkk1qey