If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.

If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.

Shield counters don't prevent players from sacrificing creatures.

The shield counter on Agent's Toolkit won't prevent its controller from sacrificing it.

If Agent's Toolkit somehow ends up with counters on it other than the ones it entered with, those may also be moved from it by its triggered ability.

“Shield” is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.

Removing a shield counter in this way isn't the same as regenerating a creature.

A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.