# Can you Ready an action with someone's reaction as a trigger?

The rule for readied action says:

First, you decide what perceivable circumstance will trigger your reaction.

Of course, it's rather hard to be ready for someone's else reactions, especially first-met enemies. I'm also aware that what got me thinking about this (the trigger being someone's attack of opportunity) is very much a meta-thinking (I, as a player, can be well aware of what actions will almost always trigger AoO's. Characters in game - not so much). However, I didn't see anybody asking this question, so what do you think?

Here's a specific example: a wizard readies a Magic MissileFire Bolt with a trigger "when my familiar gets attacked" and then on the familiar's turn it sends it within 5ft. of an enemy to do a Help action to aid himself in attacking, and then moves it out of the enemy's melee reach, which will in turn trigger an AoO, hence a triggering attack.

I posted a follow-up question here

• @z33k I’m sure we have a Q&A or five about reaction resolution, see if we already have something that answers your question about that. – Thomas Markov Nov 9 '20 at 21:20
• For resolving reaction order, a look at this question, this one, or this one might be helpful. – Upper_Case Nov 9 '20 at 21:22
• I've already posted the question. Feel free to close it, if you deem it uneeded. – z33k Nov 9 '20 at 21:26

### You can do this.

Your cat getting slapped is obviously a perceivable circumstance. Nothing wrong here.

"Perceivable" just means a circumstance you can perceive. You name your trigger. If you perceive it happen, you can use your held action.

### Be aware, enemies may not be interested in slapping cats.

I wouldn't count on this working too often. Imagine a tense, heated battle between the party and some well armed bandits. You have this idea, so you send in your cat to harrass the bandit leader. Your cat runs around a bit then runs away. If I'm DMing, the bandit leader isn't wasting her reaction slapping your cat, and you never use your held action.

Remember: opportunity attacks are not mandatory.

• Thx, great response! What about the resolving order? If trigger is interrupted then maybe it won't be ever finished (Magic Missile can kill a bandit that uses his AoO to swat the cat). – z33k Nov 9 '20 at 21:03
• Clarify for me, what is the cat helping with? – Thomas Markov Nov 9 '20 at 21:04
• Also, this answer is good but would read more clearly (to me) if it emphasized not just that a perceivable circumstance is a valid trigger for readied actions but also that the specific rule which allows that perceivable circumstance to arise doesn't matter: an attack on the familiar is an equally valid reaction trigger whether it is an off-turn readied action or an on-turn regular Attack action. – Upper_Case Nov 9 '20 at 21:12
• @Upper_Case Good thoughts, I’ll work that in a bit, wrapping up at work now. – Thomas Markov Nov 9 '20 at 21:16
• It seems like it would be better to set the trigger on "When my familiar attempts to leave melee range of the target" or "Once my familiar has suitably distracted the target with his Help Action." – notovny Nov 9 '20 at 22:15