If spellcaster A readies an action to cast time stop as soon as spellcaster B casts time stop, then on spellcaster B's turn they cast time stop, what happens? Who takes extra turns first? Also, can whoever goes first counterspell the other's time stop, and does doing so end their time stop early?
Can a caster interrupt another caster from casting Time Stop by readying a Time Stop of their own?
\$\begingroup\$ Related: "Is it possible to interrupt Spellcasting?" (not a duplicate as your question also asks about whether one can counterspell time stop) \$\endgroup\$– Exempt-MedicMar 27, 2020 at 6:55
1\$\begingroup\$ I wonder, what exactly is the problem you're trying to solve here? When are you ever going to ready time stop for when somebody else casts timestop, instead of just...casting it right away? \$\endgroup\$– TheikMar 27, 2020 at 15:33
Reaction generally occur after their trigger and so cannot interrupt the casting of a spell
The Ready action is no exception to this rule so when Bob casts time stop Alice cannot actually interrupt the casting. This means that Bob's time stop will go off and he will receive the 1+1d4 turns while Alice cannot do anything.
Immediately afterwards Alice's Readied action will trigger and she will cast time stop, granting her 1+1d4 turns.
After Alice's turns it will go back to the rest of Bob's turn (the turn on which he cast time stop). This seemingly odd outcome (Bob acts, Alice acts and then we're back to Bob) is in accordance with answers to the following questions:
- When do the extra turns from Time Stop happen?
- If a caster readies Time Stop and casts it as a reaction during another creature's turn, what happens to that creature's turn?
However, castings can be interrupted using counterspell
Unlike the Ready action, counterspell takes place before a spell's effects; it explicitly allows you to interrupt the casting of a spell.
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. [...]
As such Alice could interrupt Bob's time stop and Bob could interrupt Alice's time stop (also, either one could interrupt the other's counterspell assuming they hadn't already used their reaction in an attempt to interrupt time stop)
\$\begingroup\$ The rules you cite are correct, but I object to the resulting interpretation. The Ready action must indeed occur after the player observers the trigger happen, but the trigger can be "when they start casting the spell" instead of "when they cast a spell." You can also see this in the wording of counterspell's trigger - you can take it when you see someone casting a spell. In this way, you can cast your readied spell before the caster finishes their own casting. This makes sense if you imagine readied spells as already being 99% cast, while the other person is still casting from scratch. \$\endgroup\$– user62688Mar 27, 2020 at 17:39
3\$\begingroup\$ @user62688 However this question explains why you should never do that. Do not allow things to interrupt spellcasting unless they are designed to do so, such as counterspell \$\endgroup\$ Mar 27, 2020 at 18:01
\$\begingroup\$ To be fair, it doesn't interrupt spellcasting if you can get your attack or spell off before the caster does. There's no concentration save to make or anything like that, it's just a question of timing. After the readied spell is cast and its effects are resolved, the other caster can continue where they left off. \$\endgroup\$– user62688Mar 28, 2020 at 1:19
\$\begingroup\$ @user62688 It does interrupt the casting though. Here's an example: the caster dies. The problem is that the effects of the readied spell might impact the other caster's spell. Perhaps it teleports you out of range, or sends them flying off a cliff \$\endgroup\$ Mar 28, 2020 at 1:19
1\$\begingroup\$ I don't really want to drag this conversation out, so I'll end this by saying I'll agree to disagree. :) \$\endgroup\$– user62688Mar 28, 2020 at 1:32
This is pointless.
Medix2's answer addresses why this doesn't work under the reaction rules. But there's also no reason to try it.
When you Ready a spell, you're already committed to expending the spell slot and any consumed material components. So if you're Alice and your intent is to cast time stop before Bob gets a chance to, and your turn is up first, then just cast the spell now. That's the great thing about winning initiative.
Even if you could Ready it in a way that would trigger before Bob casts his spell, you're only setting yourself up to waste a spell slot if Bob decides to do something else.