most agent training fixes the model
Google just open-sourced EnvHarness, which fixes the environment instead. it wraps an existing setup and adds three pieces: setup (change the starting state), rule (constrain what happens next), and link (change what the agent sees). the agent, the base environment, and the eval stay untouched
on WebArena, SWE-bench Verified, and ALFWorld it lifted several models by 2.9 to 3.7 points, including Qwen3.6 27B and Claude Sonnet 4.6
the useful part is the split. if the agent keeps failing the same way, you do not always need a bigger model. you can make the environment show it the failure more clearly
open-source stacks can do this without retraining. the loop is the environment, not just the weights