The Sandbox appears to be an island beach, surrounded by water with a decayed tree growing on it.
The tree's branches house several doors that each lead to bonus challenge levels. Additionally the island contains a hammock, a friendly Fetch called Luna and a beach ball. A jukebox can also be found here, and extra music tracks for it can be obtained via the bonus challenges.
The doors atop the tree branches lead to various bonus challenges that can be completed for more XP and music tracks for the jukebox. Initially only a couple of doors are available, but as the player progresses through the game, more of them will open and become available.
Each door houses a challenge, based around a specific theme (e.g. a Planning Test automatically drops the player into Turn() mode, and tasks them with destroying all Process on the field within that single Turn()). Every challenge completed will award XP and a music track, but it will also close it's corresponding door. However, the door can be re-opened a set number of times throughout the game, with each re-opening unlocking a new challenge. The only exception to this is the door leading to the Practice Test, which always remains open, and is designed so the player can experiment with different Function combos.
It is worth noting that it is practically impossible to unlock all challenge doors within a single playthrough - to unlock all of them would require a second playthrough via Recursion mode.
There are 6 doors to the decayed tree:
- Speed Test - Destroy all Process using certain Functions within the time limit
- Practice Test - A place to relax and experiment on unsuspecting Process in peace
- Stability Test - Survive the Process onslaught using set Functions for a specified amount of time
- Planning Test - Destroy all Process using certain Functions within a single Turn()
- Performance Test - Customise randomly selected Functions and destroy waves of Process
- Agency Test - Face off against a copy of Red with her own Transistor
- There is a bug that when all tests have been completed, if the performance test was completed last, it will not regain its name and will continue to state 'Access Denied'.
- If Red enters the Sandbox whilst the Transistor is under the effects of the Spine, the Transistor will return to normal. However it will fall back under the Spine's effects once Red exits the Sandbox and returns to Cloudbank.