Cap the Fan Out

One helper that can spawn more helpers turns a small job into a large one. Put a number on how deep the tree goes and stop spawning at the limit.

Count the depth on every task

Stamp each task with its depth and refuse to spawn once the maximum is reached.

def may_spawn(task, max_depth):
    return task.depth < max_depth

Let the leaf finish

A worker at the limit completes the work with the tools it has. Returning a plan instead of a result hides a task that never ran.