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Let's say a spellcaster is fighting a dragon. Can they ready their action to cast a Wall of Stone with a trigger of "If the dragon uses its breath weapon I block it with Wall of Stone"? If so, how high would the wall have to be made?

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No, this won't work

The method for readying an action is that you use your action to hold the spell at the ready (pre-casting it), then use your reaction to cast it when a given trigger occurs.

But here's the catch (PHB, p. 193):

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.

If you have readied a spell to cast if a dragon uses its breath weapon, the only time you may unleash the spell is after the dragon has used its breath weapon and the outcome of that has been resolved (finished). Meaning you will cast Wall of Stone after the dragon uses its breath weapon, and thus after you've already saved/taken damage from it.

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    \$\begingroup\$ couldn't you just use "when the dragon shows signs that he's about to use his breath weapon" or something like that as the trigger? \$\endgroup\$ Commented Mar 27, 2018 at 18:39
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    \$\begingroup\$ @PixelMaster Yes, but how confident are you that you will correctly interpret the "signs"? You might end up casting the spell after the dragon sharply inhales (not because it was going to use a breath weapon: it was just breathing. But it sure looked like it was showing signs that it would use it). If your trigger is that it looks like something might happen, it'll get triggered pretty often. \$\endgroup\$ Commented Mar 27, 2018 at 19:21
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    \$\begingroup\$ Additionally, the reason this doesn't work while a spell like Shield does is that Shield explicitly defines the exception that it blocks against the triggering attack. \$\endgroup\$
    – Mwr247
    Commented Mar 27, 2018 at 20:58
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    \$\begingroup\$ Shield also explicitly uses a reaction. Other spells that require the Ready action only do so because of that, so these are 2 completely different situations \$\endgroup\$ Commented Mar 27, 2018 at 21:56
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    \$\begingroup\$ Also, if the dragon doesn't use its breath weapon that round, you've wasted your spell slot. \$\endgroup\$
    – nick012000
    Commented Jul 8, 2021 at 1:56
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Yes

reactions can be taken after a trigger which you state, just use the same trigger for counterspell but reword it, which is just "when you see a creature within (range of spell you're casting) of you, starts casting a breath attack"

I don't see why you can only take your reaction after the thing already happens because almost all reactions this is false and you are actively doing something right after your trigger.

Another Example would be can you hold action to cast globe of invulnerability to save a friend from a breath attack. if you couldn't that would just be dumb and screw support classes

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    \$\begingroup\$ With that phrasing the dragon would still use its breath weapon before your character casts the spell. \$\endgroup\$
    – Akixkisu
    Commented Jul 6, 2021 at 22:01
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    \$\begingroup\$ "Starts" its the same phrasing as counterspell. \$\endgroup\$
    – Heyz
    Commented Jul 7, 2021 at 15:07
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    \$\begingroup\$ Unfortunately, Counterspell is a rules exception and the Ready action doesn't include the ability to use phrasing from other more specific options. The related links listed under the question go into some detail here. \$\endgroup\$
    – NotArch
    Commented Jul 7, 2021 at 15:29
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    \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. \$\endgroup\$
    – V2Blast
    Commented Jul 8, 2021 at 5:39
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    \$\begingroup\$ “Starts casting a breath attack” is not a perceptible trigger. Triggers for the ready action must be perceptible to the character, and as explained in this three year old comment, the trigger you have proposed here is based on a future event that hasn’t happened yet, not what your character is perceiving at the time of the trigger. \$\endgroup\$ Commented Jul 8, 2021 at 7:24

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