There are a lot of ways to slice the problem of getting the agents to complete a goal without getting lost, and the right solutions are somewhat problem-specific. For the projects I've done 35B Qwen is about the smallest that seems to make useful progress in a general purpose harness while 4B Qwen is workable with a task-specific harness. At the lower end the plan has to be traditional search/planner techniques in code not something the model has any control over, of course that limits the kinds of problems that fit. The high end coding models are perfectly capable of making a functional 1-off harness for those jobs so it ends up not being that bad to implement.