Starting with the rules for the Ready action:
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
My question centers around the "choose the action you will take". How specific does this need to be? Are you literally just choosing one of the standard action choices ("Attack", "Help", "Hide", etc.), or do you need to specify any "parameters" or "targets" or the like for the action you will take?
To try to make this a bit more concrete, could I ready an action for "When the Goblin moves, I Attack", and then when the trigger happens I can choose whether to attack the Goblin or the Bugbear, and what weapon I'm using? Or do I have to give those details up front as part of specifying the Ready action? Other actions like Search and Help similarly have details that need to be given in order to really be, well, actionable, and it may be useful on occasion to defer the specifics until the trigger actually happens.
Shedding some light and some confusion on the issue is that Cast a Spell has its own special rules for being readied, but I don't know whether to treat that as a precedent for specifying details up front (since presumably you need to at least specify which spell to be able to "hold its energy", though I don't know about details like spell slot or targets), or whether it's a case where it's a specific rule specifically because because it's different from the general rule.
Related but different questions:
- How specific does the trigger for a readied action have to be? (But I am asking about the action instead.)
- Can the action in a Ready Action be conditional? (But that is about choosing a different action at the time of the trigger, whereas I am asking about a single Action choice but when the choices for the details of that action are made.)