Can you use reactions at the start of an encounter?

[Ravingdork]

In a recent game, I had several zombies unearth themselves underneath the PCs and attack.

The party fighter asked me if he could make an attack of opportunity against the biggest zombie as it rose up next to him. I said that he hadn't acted yet, and I wasn't sure if he could or not. So we looked at the attack of opportunity text and it didn't say anything about being flat-footed or other relevant limitations, so I allowed it for the time being, but said I'd look into it further.

So, can you use reactions outside of encounter mode? Can you make attacks of opportunity even though you have not yet acted in an encounter? Are there any general rules that would have made our zombie situation play out differently than it had?

[Githzilla]

I don't think you can. For reference page 468-469 that details what you do on your turn. It suggests that on your turn is when you gain your 3 actions and your reaction. To me that suggests you don't start an encounter with a reaction - you gain it at the start of your turn.

[Fuzzy-Wuzzy]

Up to circumstance by way of the GM.

CRB page 472, Reactions in Encounters wrote:

The GM determines whether you can use reactions before your first turn begins, depending on the situation in which the encounter happens.

[ShieldLawrence]

There are some reactions that react to initiative rolls, so reactions before your turn exist.

[Ravingdork]

Thanks everyone. This looks like GM adjudication territory to me then, based on the quoted texts.

Since they were exploring a haunted mansion, had been attacked once already, and were clearly on edge, I think I probably handled it just fine.

Had the same thing happened while they were relaxing and drinking pinacoladas on the beach, I likely wouldn't have allowed it. :P

[sherlock1701]

My thought is that, since you gain reactions on your turn, you can't react before your first turn. Exceptions for reactions that trigger off of rolling initiative or a similar circumstance where you could not possibly have acted yet.

[David knott 242]

One point worth noting is that you are not automatically flat-footed when combat begins, and there are no other restrictions stated that prevent you from using reactions whose triggers occur before your turn. You just have to have that reaction as a character ability, as you normally still can't ready an action before your turn.