Don't Mask the Environment: Observation Supervision Changes How Agents Explore Under RL
ActObs adds observation-token supervision to SFT, yielding higher pass@k for Qwen3 agents after GRPO on Terminal-Bench 2.0 and code editing.
Researchers introduce ActObs, an SFT variant that supervises environment-observation tokens in agent trajectories in addition to action tokens, without extra data, parameters, tokens, or forward passes. On Qwen3-4B, GRPO initialized from ActObs achieves higher pass@k at every sampling budget on Terminal-Bench 2.0, and on Qwen3-8B it trades some pass@1 for +3.4 pp at pass@16 while solving more distinct tasks. The benefit transfers to unseen code-editing tasks on aider-polyglot (+4.2 pp pass@1 at 4B scale). The authors trace the advantage to gradient analysis showing joint supervision preserves environment prediction and policy entropy, improving downstream RL exploration.