Can you cast a stone wall as a readied action?

[SangoProduction]

Or held action. Or whatever the term is to do an action in response to another action, rather than using it on your turn.

Essential idea: You have some defensive barrier you can throw up (whether it's stone or wall of force or invincible sphere, or whatever line-of-effect-denial effect you please). And the either the enemy has a really dangerous attack, or you just prefer being obnoxious to being effective.
So you ready the action for when they go to attack. Assuming you can, you then interrupt the attack by shoving the wall between them and their target (and preferably all applicable targets as well).

Can you do that? Are there any particular complications?
Do conjurations need an entire 5 foot square cleared in order to conjure stuff into it? If so, does the wall not have a problem with blocking reach weapons?

[hamishspence]

Spellcasting is a Standard Action (normally). You can Ready one of those. And Readied actions are taken in response to certain conditions, and go off before the thing that actually "triggers" them does.

https://www.d20srd.org/srd/combat/specialInitiativeActions.htm#ready

Ready
The ready action lets you prepare to take an action later, after your turn is over but before your next one has begun. Readying is a standard action. It does not provoke an attack of opportunity (though the action that you ready might do so).

Readying an Action
You can ready a standard action, a move action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, any time before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.

[KillianHawkeye]

Wall spells generally have a casting time of 1 Standard Action, so yes you can use a readied action to cast one to interrupt an opponent's line of sight or line of effect (usually both, depending on which wall you use).

Most of the wall spells in the PHB are only a few inches thick at most, so there should be no issue putting them in between creatures' spaces. The big exceptions are wall of ice which must be unbroken and can thus be disrupted by an adjacent creature (but still works against a ranged threat), and wall of thorns which has a minimum width of five feet.

[Gruftzwerg]

as others said this works and is one of the intended ways to use such wall spells.

trigger examples:
- when the enemy casters starts/wants to cast
- when the enemy starts moving in my direction

simple said, put a wall in front of the enemy caster if he wants to cast anything.

[Aracor]

as others said this works and is one of the intended ways to use such wall spells.

trigger examples:
- when the enemy casters starts/wants to cast
- when the enemy starts moving in my direction

simple said, put a wall in front of the enemy caster if he wants to cast anything.

That does work, but since Readied Actions specifically say the occur before the action, the caster can arguably change their action based on the existence of the newly created wall. Either cast a different spell, or cast the same spell but target something different.

[Crake]

That does work, but since Readied Actions specifically say the occur before the action, the caster can arguably change their action based on the existence of the newly created wall. Either cast a different spell, or cast the same spell but target something different.

Not according to the very same rules: "Assuming he is still capable of doing so, he continues his actions once you complete your readied action." No mention of changing actions if the new situation isn't favourable.

[SangoProduction]

Not according to the very same rules: "Assuming he is still capable of doing so, he continues his actions once you complete your readied action." No mention of changing actions if the new situation isn't favourable.

Now that's an interesting debate topic.
It would be positively hilarious to have a wizard one shot themselves with an unintentionally point blank fireball.

[Khedrac]

Now that's an interesting debate topic.
It would be positively hilarious to have a wizard one shot themselves with an unintentionally point blank fireball.

Now that you can do (if you can predict that the spell will be a fireball) as you can ready to put the wall up after the cast and before the fireball has travelled from the wizard to the target.

More to the point - if you read a 5' step and attack when an opponent comes in range and someone with a reach weapon moes up to attack you they potentially get embarrassed. You are no longer in a targettable square so unless they have a backup non-reach weapon, they need to either attack someone else, or keep moving (and provoke an AoO) to attack you.
Chargers can get even more stuk as a charge has to be a straight line movement - so a 5' step at the wrong time can leave them having charged without anyone to attack!

[Ramza00]

Yes.

Nice tricks with this is Synchronicity from Complete Psionic and Wall of Ectoplasm as defensive, or an offensive spell for Synchronicity at 3pp allows you to pick what spell / power you are using in response to, nor do you need to specify the trigger.

3rd Party Pathfinder does not have Wings of Cover, but it has several maneuvers and veils that allow you to create walls at will within limits, thus you can do all the defensive stuff you already mentioned, like a warlock.

[Jack_Simth]

Yes.

Nice tricks with this is Synchronicity from Complete Psionic and Wall of Ectoplasm as defensive, or an offensive spell for Synchronicity at 3pp allows you to pick what spell / power you are using in response to, nor do you need to specify the trigger.

3rd Party Pathfinder does not have Wings of Cover, but it has several maneuvers and veils that allow you to create walls at will within limits, thus you can do all the defensive stuff you already mentioned, like a warlock.

Well, Pathfinder does have Emergency Force Sphere (https://www.d20pfsrd.com/magic/all-spells/e/emergency-force-sphere/), although it does stick around for a while if it's not obliterated, which is mixed.

[Crake]

Now that you can do (if you can predict that the spell will be a fireball) as you can ready to put the wall up after the cast and before the fireball has travelled from the wizard to the target.

More to the point - if you read a 5' step and attack when an opponent comes in range and someone with a reach weapon moes up to attack you they potentially get embarrassed. You are no longer in a targettable square so unless they have a backup non-reach weapon, they need to either attack someone else, or keep moving (and provoke an AoO) to attack you.
Chargers can get even more stuk as a charge has to be a straight line movement - so a 5' step at the wrong time can leave them having charged without anyone to attack!

Depending on how you define an action, they may not even have the option to attack someone else. If you define the action as simply "Attack", then yes, they could pick a different target, but if you define the action as "Attack XYZ target" then if you move out of range, then they simply cannot complete the action and it's lost.

Personally I see it as the latter, as it makes the most sense.