live-site evaluation drifts and synthetic environments lack depth, so WebHarbor reconstructs 153 real web services as executable, dockerized environments under deterministic control, built by task-driven human–agent collaboration. The suite carries 3,518 tasks including post-authentication workflows, and a Qwen 4B-level GUI agent trained with RL inside it generalizes to unseen tasks and performs competitively with leading open-source GUI web agents across four benchmarks. Code and data are public.