During Sanity, an agent is installed on the manager's VM.
For that, the installation uses the current user as an agent user, and its private key as a private key path.
This imposes requirements:
Ability to SSH from the manager into itself.
Ability to run sudo through SSH without being prompted for a password and without a TTY.
For the functioning of the manager, these requirements are completely unnecessary.
In other words, there are cases in which sanity check would fail even though the machine is fully legitimate.