Does Doubling Season affect loyalty counters placed on planeswalkers when they enter the battlefield?

Doubling Season doubles counters placed on permanents you control, but a key rules distinction prevents it from applying to planeswalker loyalty counters placed as they enter the battlefield. CR 614.1d and the interaction with replacement effects are central here.

When a planeswalker enters the battlefield, it enters with loyalty counters equal to its printed loyalty. This is defined as part of a continuous effect that sets up how it enters — specifically, CR 306.5b states that a planeswalker's loyalty is established as it enters. Doubling Season's effect would need to apply as a replacement effect to the placement of those counters.

However, Doubling Season's oracle text (and CR 614.1c) specifies it applies when one or more counters 'would be placed' on a permanent you control. Because the planeswalker's loyalty counters are placed as part of the process of it entering the battlefield — not after it is already on the battlefield as a permanent you control — Doubling Season's replacement effect does apply. Wait — this is the crucial point: Wizards issued a ruling clarifying that Doubling Season does in fact double the loyalty counters placed on planeswalkers as they enter, because the replacement effect applies to the counters being placed, and the planeswalker is considered to be entering under your control.

So the correct answer is: Yes, Doubling Season does double loyalty counters placed on planeswalkers as they enter the battlefield. For example, if you control Doubling Season and play Jace, the Mind Sculptor (normally enters with 3 loyalty), it will instead enter with 6 loyalty counters, allowing you to immediately use his 0 ability without any risk.

This is confirmed by official Gatherer rulings on Doubling Season: 'If a planeswalker is entering the battlefield under your control and Doubling Season is already on the battlefield under your control, the planeswalker will enter with double the normal amount of loyalty counters.' This is because the replacement effect sees the placement of counters on a permanent that will be yours (CR 614.1c, 614.12).