This is a nice answer, but by the rules as written, the mouse is dead.

There are area effect spells that cause damage regardless of where you die. This explanation doesn't really work.

@ShadowKras Does the rules say the waterskin and spare jerkin is burned as well? Is the inside layers of a tied bedroll burned? Going by this rule would a person hiding inside a fire proof, sealed coffin also be hurt by the fireball as long as the coffin is inside the radius? I haven't GM'ed 5th edition so I'm a bit curious about this.

@ShadowKras I guess I wasn't just thinking about the fireball, but mostly the titular question: can a mouse survive combat? In the case of RAW + the example of fireball, you're right that the mouse should take damage. It'd also be true that we'd have to factor the mouse's HP into a sleep spell, probably give it disadvantage on a DEX save against faerie fire while snuggled in close and then track where it is to see whether the limning is visible... come to think of it, we'll need explicit declarations of the mouse's sleep cycle to know whether it's incapacitated for all these saves...

@ShadowKras Does the pet mouse have a stat block? The Fireball spell treats things without stat blocks differently than things with stat blocks. Per my memory of RAW, if you are in a fireball spell you do not calculate the damage done to every single item the player has on them, even if some would be destroyed if you did (say parchment stuck in the backpack).

Im going simply by the spell's description: "A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Each creature in a 20-foot-radius sphere centered on that point must make a dexterity saving throw. A target takes 8D6 fire damage on a failed save, or half as much damage on a successful one. The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried."

@Lawtonfogle Creatures and objects are treated differently by the spell's text, but the lack of an existing MM stat block doesn't make a mouse less of a creature. Goats and sheep, after all....

The mouse doesn't have to be on the urchin at all times. It likely vanishes into the underwood/... when combat starts and only peeks out after everything is quiet again...and it's tiny, it can hide inside things too. Lots of ways to escape danger.

Even by strict RAW, doesn't the fireball need line of sight to the mouse, which it won't have if the mouse is inside someone's clothing?

Again, let me say: I don't think this question's actually about a mouse surviving fireball. I think it's about a fragile creature surviving in a dangerous world, fireball being one such example. That said, on to your RAW question, @Erik. Fireball is an AoE spell whose target is a point in space. Caster can pick any point they can see within range and target that point. If the mouse is within the fireball's radius from that targeted point, it's taking damage.

@nitsua60 That would imply Fireball goes through walls? I don't think it does.

By that logic, if someone is behind the big tanky guy in full plate and heavy shield, he shouldn't take damage from a fireball?

@ShadowKras, if someone is completely buried under a pile of big tanky guys in full plate, than no, he shouldn't take any damage. From fire, I mean. Squashed, maybe, but not from fire.

@Erik A point on the other side of a wall wouldn't be a valid target for fireball, no. By the targeting rules of spellcasting.

If you're talking about the damage effect of the fireball, things are a bit murkier. There seem to be people here arguing that a mouse in a bedroll, a mummy in a sarcophagus, any "creature in a 20-foot radius sphere" takes damage, even if totally covered. I don't agree, but that's not this question.

Miniman for the win!