Does a continuous effect that sets a creature's power to a specific value override +1/+1 counters on it?

Magic resolves characteristic-defining effects in a sequence of sub-layers within Layer 7. A continuous effect that sets a creature's power and toughness to specific values (e.g., 'this creature's power and toughness are each equal to the number of creatures you control') applies in layer 7b (or 7a if it's a characteristic-defining ability). This sets the base value, overriding the creature's printed power/toughness.

However, +1/+1 counters on a creature are applied in layer 7c (CR 613.3c — counters). Because 7c comes after 7b, the counters are still added on top of whatever the effect set the power/toughness to. The counters are not 'overridden' — they simply add their bonus after the set-value effect resolves in the layer system.

So the practical result is: the set-value effect replaces the printed stats, but any +1/+1 counters still contribute their bonus afterward. A 3/3 with two +1/+1 counters that gets affected by 'base power and toughness become 1/1' ends up as a 3/3 (1+2 / 1+2), not a 1/1.

Example: You control a Grizzly Bears (2/2) with three +1/+1 counters on it. An opponent casts an effect that sets its power and toughness to 1/1. In layer 7b the stats become 1/1; then in layer 7c the three counters are applied, making the final result a 4/4 — not a 1/1.

Reference rules: CR 613.3 describes the layer system and its sub-layers; CR 613.3b covers set-power/toughness effects (layer 7b); CR 613.3c covers counter modifications (layer 7c). The counters survive because they operate in a later sub-layer.