Banger paper on evolving agent interfaces. Pay close attention to this new trend if you build with agents. Like optimizing harnesses, it's remarkable what you can achieve with agents that have access to better interfaces. Computer-use agents usually act through interfaces that people design by hand, so an agent may type a spreadsheet cell by cell when one line of code would do. Simular's Agent S4 has a meta-agent that builds those interfaces instead. It reads the task agent's trajectories, with no success labels, and writes new tools and notes on when to use them. With the evolved interfaces, an open 27B model reaches 21.3% on OSWorld V2, 3.8 times that of the same model with hand-built interfaces. That is above GPT-5.5 (13.0%) and Claude Opus 4.8 (20.6%), at about 4.3% and 1.5% of their cost. Population-based selection keeps the interface repositories that do best on validation tasks the meta-agent never sees, so the changes carry over to new tasks. On ScienceBoard, the evolved interfaces raise success from 41.4% to 58.6% with the same model. The code is open source. https://arxiv.org/abs/2610.28601 Chat with Paper: https://academy.dair.ai/papers/evolving-agent-interfaces-2610.28601