Does Solemnity prevent planeswalkers from gaining loyalty counters when their abilities resolve?

Solemnity reads: 'No counters can be placed on permanents you control.' Planeswalkers are permanents, and loyalty counters are counters. Therefore, if you control Solemnity, no loyalty counters can be placed on planeswalkers you control — including when you activate a loyalty ability that would add counters.

Activating a loyalty ability requires you to put loyalty counters on or remove them from the planeswalker as part of the cost (CR 606.3). Even abilities that would add loyalty counters when they resolve are blocked by Solemnity. This means the counters simply cannot be placed; Solemnity creates a replacement or prevention effect that stops the event entirely (CR 614.1).

Note that Solemnity also prevents your opponent's planeswalkers from receiving counters only if they are under YOUR control. If an opponent controls a planeswalker, Solemnity does not interfere with that planeswalker's counters.

Example: You control Solemnity and Jace, the Mind Sculptor. You activate Jace's +2 ability. Even though the ability resolves, the two loyalty counters cannot be placed on Jace due to Solemnity. Jace's loyalty remains unchanged after the ability resolves.

Additionally, because loyalty costs involve adding or removing counters (CR 606.3), an ability that costs loyalty counters to remove can still be activated — you remove the counters as a cost — but abilities that add counters as part of resolution are fully blocked by Solemnity.