My question pertains to how the Sentinel feat states;
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
And how Mobile states;
"When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
So my question is if two characters with these feats where fighting would the mobile feats attack and flee tactic be ignored by the sentinel feat or because its not technically a disengage action would the Mobile feat win out and the character would get to run away after making their attack?
Secondly if the mobile feat does win out would this also apply to swashbucklers Fancy Footwork feature which gives about the same benefit as the mobile feat saying;
"During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn."
I feel as though because sentinel specifically states that they take the disengage action that these other two features would ignore it but I thought i'd ask for clarification.
My question pertains to how the Sentinel feat states;
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
And how Mobile states;
"When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
So my question is if two characters with these feats where fighting would the mobile feats attack and flee tactic be ignored by the sentinel feat or because its not technically a disengage action would the Mobile feat win out and the character would get to run away after making their attack?
Secondly if the mobile feat does win out would this also apply to swashbucklers Fancy Footwork feature which gives about the same benefit as the mobile feat saying;
"During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn."
I feel as though because sentinel specifically states that they take the disengage action that these other two features would ignore it but I thought i'd ask for clarification.
dndbeyond world will correct next if I am wrong...
but in this instance. I believe they cancel out where applicable.
the mobile or swashbuckler. (If they can disengage as a bonus action after an attack) would still be able to disengage without AoO. But would not be able to just attack and walk by. Neither would the sentinel just be able to AoO just because they entered the reach range (provided they made an attack to cancel it out).
as you noticed reading mobile and sentinel. Not all the rules were well thought out or written, and there are contradictions or places where things just aren’t clear at all or make any sense.
dndbeyond world will correct next if I am wrong...
but in this instance. I believe they cancel out where applicable.
the mobile or swashbuckler. (If they can disengage as a bonus action after an attack) would still be able to disengage without AoO. But would not be able to just attack and walk by. Neither would the sentinel just be able to AoO just because they entered the reach range (provided they made an attack to cancel it out).
as you noticed reading mobile and sentinel. Not all the rules were well thought out or written, and there are contradictions or places where things just aren’t clear at all or make any sense.
I don't fully understand your position in this answer, but the correct answer is: Sentinel beats disengage (bonus) action, mobile and fancy footwork don't use disengage, so sentinel does not beat them.
As a leading advocate of "the rules are often unclear" myself, I don't think this is one of those times.
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
The way I read it is that Sentinel is there to beat Disengage if a character happens to take it, not that disengage must be used for Sentinel to work.
As a Feat, since feats are meant to be specialized areas of expertise (or however it's worded in the phb), its effect would supersede the specificity of a class feature for something like Swashbuckler's Fancy Footwork. However, since Mobile is also a feat Sentinel will win but you could rule that the opportunity attack should be made at disadvantage.
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
The way I read it is that Sentinel is there to beat Disengage if a character happens to take it, not that disengage must be used for Sentinel to work.
As a Feat, since feats are meant to be specialized areas of expertise (or however it's worded in the phb), its effect would supersede the specificity of a class feature for something like Swashbuckler's Fancy Footwork. However, since Mobile is also a feat Sentinel will win but you could rule that the opportunity attack should be made at disadvantage.
That sentence you quote provides an exception to the general rule about disengaging from opportunity attacks; it does not provide any other exceptions. I admit that disengage is not required for sentinel to work, but my point stands: sentinel's text says that it only defeats disengage as a way to avoid opportunity attacks. Sentinel does not provide any other exceptions. If you would avoid an opportunity attack by any means other than disengage, sentinel would not allow the opportunity attack.
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
The way I read it is that Sentinel is there to beat Disengage if a character happens to take it, not that disengage must be used for Sentinel to work.
As a Feat, since feats are meant to be specialized areas of expertise (or however it's worded in the phb), its effect would supersede the specificity of a class feature for something like Swashbuckler's Fancy Footwork. However, since Mobile is also a feat Sentinel will win but you could rule that the opportunity attack should be made at disadvantage.
That sentence you quote provides an exception to the general rule about disengaging from opportunity attacks; it does not provide any other exceptions. I admit that disengage is not required for sentinel to work, but my point stands: sentinel's text says that it only defeats disengage as a way to avoid opportunity attacks. Sentinel does not provide any other exceptions. If you would avoid an opportunity attack by any means other than disengage, sentinel would not allow the opportunity attack.
Okay I need clarification because I can see this is going to go into a circle. I'm a little confused because you hit this from opposing points of view, and agreed but also disagreed with the sentiment of the knowledge concerning disengage with and without Sentinel. Sentinel does defeat disengage, yes, but any other means of voluntarily leaving someone's reach still provokes attack of opportunity, as per normal, with the exceptions provided like Fancy Footwork and Mobile. Are we or are we not agreed that it can and/or does defeat the class feature or Mobile?
If we're going to consider these effects as sans disengage, then how could we not classify Sentinel as superior in that regard, since reactions are provoked by a trigger and the reaction happens after the trigger, does Fancy Footwork and/or Mobile happen before the reaction can be had in a turn based value?
I would summarize it so: Sentinel works when someone provokes an attack of opportunity + it negates disengage action's ability to avoid attacks of opportunity.
Any other means of leaving the space without provoking attacks of opportunity are not covered by Sentinel.
Okay I need clarification because I can see this is going to go into a circle. I'm a little confused because you hit this from opposing points of view, and agreed but also disagreed with the sentiment of the knowledge concerning disengage with and without Sentinel. Sentinel does defeat disengage, yes, but any other means of voluntarily leaving someone's reach still provokes attack of opportunity, as per normal, with the exceptions provided like Fancy Footwork and Mobile. Are we or are we not agreed that it can and/or does defeat the class feature or Mobile?
Sentinel does not beat mobile or fancy footwork. Sentinel beats disengage. Mobile and fancy footwork are not disengage.
There shouldn't be any circles.
If we're going to consider these effects as sans disengage, then how could we not classify Sentinel as superior in that regard, since reactions are provoked by a trigger and the reaction happens after the trigger, does Fancy Footwork and/or Mobile happen before the reaction can be had in a turn based value?
I would summarize it so: Sentinel works when someone provokes an attack of opportunity + it negates disengage action's ability to avoid attacks of opportunity.
Any other means of leaving the space without provoking attacks of opportunity are not covered by Sentinel.
@DxJxC Pretty much was looking for a summation like this based on how a turn plays out, so I can wrap my head around how everyone is making sense of this. I'm not interested in the debate of it, per se, but I'd like a solid foundation to work from if something like this ever arises in the games I play.
The question was worded about as poorly as some features in the game (wife + kid aggro; happens a lot) but in short I was asking how we could apply Mobile or Fancy Footwork in the turn so that, essentially, it supersedes a reaction based feat in the order of combat as a step-by-step method. Like, what kind of action could we classify either of those as if not a type of disengage, if anything at all? I realize this isn't what the OP was asking, I'm curious by myself.
@DxJxC Pretty much was looking for a summation like this based on how a turn plays out, so I can wrap my head around how everyone is making sense of this. I'm not interested in the debate of it, per se, but I'd like a solid foundation to work from if something like this ever arises in the games I play.
Sentinel beats disengage (bonus) action, mobile and fancy footwork don't use disengage, so sentinel does not beat them.
Which I did post before your first post.
The question was worded about as poorly as some features in the game (wife + kid aggro; happens a lot) but in short I was asking how we could apply Mobile or Fancy Footwork in the turn so that, essentially, it supersedes a reaction based feat in the order of combat as a step-by-step method. Like, what kind of action could we classify either of those as if not a type of disengage, if anything at all? I realize this isn't what the OP was asking, I'm curious by myself.
Mobile and Fancy Footwork are static effects with conditional clauses. They are not any type of action let alone disengage. The only order that matters is that the condition is met before the AoO is provoked, then it simply denies the reaction.
I believe the features of Mobile and Fancy Footwork would be classified as an attack action as both features are dependent on if the character made a melee attack against the target that is trying to use the reactionary attack of opportunity against the original character as they strike and flee. So as it would go in turn based combat where player 1 has mobile or Fancy Footwork and player 2 has sentinel;
On player 1's turn they use their action to strike player 2 with melee, then exiting player 2's reach, player 2 then uses their reaction to attack player 1 but because mobile was activated by the melee attack player 1 avoids the reactionary attack or player 2 automatically misses either way you wanna spin it.
I don't know if that helps to clear it up but that's how I've come to understand it. It's that Mobile and Fancy Footwork are activated in the action and persist against the target until their turn ends. So by Player 1 using their action, if player 2 ends up trying to use their reaction against them they fail and miss.
I believe the features of Mobile and Fancy Footwork would be classified as an attack action as both features are dependent on if the character made a melee attack against the target that is trying to use the reactionary attack of opportunity against the original character as they strike and flee. So as it would go in turn based combat where player 1 has mobile or Fancy Footwork and player 2 has sentinel;
On player 1's turn they use their action to strike player 2 with melee, then exiting player 2's reach, player 2 then uses their reaction to attack player 1 but because mobile was activated by the melee attack player 1 avoids the reactionary attack or player 2 automatically misses either way you wanna spin it.
I don't know if that helps to clear it up but that's how I've come to understand it. It's that Mobile and Fancy Footwork are activated in the action and persist against the target until their turn ends. So by Player 1 using their action, if player 2 ends up trying to use their reaction against them they fail and miss.
I believe the features of Mobile and Fancy Footwork would be classified as an attack action as both features are dependent on if the character made a melee attack against the target that is trying to use the reactionary attack of opportunity against the original character as they strike and flee. So as it would go in turn based combat where player 1 has mobile or Fancy Footwork and player 2 has sentinel;
On player 1's turn they use their action to strike player 2 with melee, then exiting player 2's reach, player 2 then uses their reaction to attack player 1 but because mobile was activated by the melee attack player 1 avoids the reactionary attack or player 2 automatically misses either way you wanna spin it.
I don't know if that helps to clear it up but that's how I've come to understand it. It's that Mobile and Fancy Footwork are activated in the action and persist against the target until their turn ends. So by Player 1 using their action, if player 2 ends up trying to use their reaction against them they fail and miss.
I wouldn't classify it as an attack action since the condition is met by any melee attack, it can be a melee spell attack from the cast a spell action or a bonus action attack from 2 weapon fighting.
But what we call it doesn't really matter, because the 2 effects just don't directly interact.
I believe the features of Mobile and Fancy Footwork would be classified as an attack action as both features are dependent on if the character made a melee attack against the target that is trying to use the reactionary attack of opportunity against the original character as they strike and flee. So as it would go in turn based combat where player 1 has mobile or Fancy Footwork and player 2 has sentinel;
On player 1's turn they use their action to strike player 2 with melee, then exiting player 2's reach, player 2 then uses their reaction to attack player 1 but because mobile was activated by the melee attack player 1 avoids the reactionary attack or player 2 automatically misses either way you wanna spin it.
I don't know if that helps to clear it up but that's how I've come to understand it. It's that Mobile and Fancy Footwork are activated in the action and persist against the target until their turn ends. So by Player 1 using their action, if player 2 ends up trying to use their reaction against them they fail and miss.
Mobile fest's third bullet:
When you make a melee attack against a creature, you don’t provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not.
Opportunity Attacks:
In a fight, everyone is constantly watching for a chance to strike an enemy who is fleeing or passing by. Such a strike is called an opportunity attack. You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach. You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.
The movement out of reach without disengage is the normal trigger that allows the reaction for the AoO to take place. Mobile's third bullet doesn't allow that trigger to take place. This would forego any ability that requires AoO for interaction. Sentinel requires AoO.
Fancy Footwork:
When you choose this archetype at 3rd level, you learn how to land a strike and then slip away without reprisal. During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn.
This one I'd probably rule that the AoO is still triggered, but can't be made. This means that sentinel couldn't apply because it requires the attack to hit, though, if the rogue that makes the attack tried to disengage for some reason, sentinel would still allow the trigger even if the AoO can't be made. This is only pertinent if the DM rules that Warcaster or something similar could allow for something that isn't an AoO to replace the AoO trigger.
My question pertains to how the Sentinel feat states;
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
And how Mobile states;
"When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
So my question is if two characters with these feats where fighting would the mobile feats attack and flee tactic be ignored by the sentinel feat or because its not technically a disengage action would the Mobile feat win out and the character would get to run away after making their attack?
Secondly if the mobile feat does win out would this also apply to swashbucklers Fancy Footwork feature which gives about the same benefit as the mobile feat saying;
"During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn."
I feel as though because sentinel specifically states that they take the disengage action that these other two features would ignore it but I thought i'd ask for clarification.
I didn't read other responses but I had the same question a while back and Sage Advice had an answer. Alvin has Mobile. Betty has Sentinel. Connor has nothing. Scenario 1: Alvin approaches Betty and Connor who are standing side by side; Alvin attacks Betty and then uses his movement to walk away. Mobile does not allow for Betty to make an Opportunity Attack, Mobile wins. Scenario 2: Alvin approaches Betty and Connor who are standing side by side; Alvin uses his first attack against Betty and then uses his Extra Attack against Connor. When Alvin attacks Connor, Betty can use Sentinel to attack Alvin. This is a reaction attack and not an Opportunity Attack, Sentinel wins.
If Mobile attacks Sentinel, Mobile wins and NO OPPORTUNITY ATTACKS. Sentinel 3rd benefit can always be used against Mobile.
Mobile trumps Sentinel. The text is clear, mobile does not proc an OA. Ever. When you make an attack, you do not provoke. Period, end of story. Nothing trumps this.
Sentinel allows you to hit some people, even if they use an action that normally allows them to leave a threatened square without provoking. It's not as definitive as mobile, therefore mobile wins. OAs are always conditional, and it's impossible for a character with the mobile feat to meet the conditions required to trigger an OA against them.
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Any time an unfathomably powerful entity sweeps in and offers godlike rewards in return for just a few teensy favors, it’s a scam. Unless it’s me. I’d never lie to you, reader dearest.
Mobile trumps Sentinel. The text is clear, mobile does not proc an OA. Ever. When you make an attack, you do not provoke. Period, end of story. Nothing trumps this.
Sentinel allows you to hit some people, even if they use an action that normally allows them to leave a threatened square without provoking. It's not as definitive as mobile, therefore mobile wins. OAs are always conditional, and it's impossible for a character with the mobile feat to meet the conditions required to trigger an OA against them.
The third bullet of sentinel reads as follows:
When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Ergo, the scenarios that Zach_Harrison3 laid out are correct outside of stating that Sentinel trumps Mobile in that scenario as Sentinel isn't interacting with the Mobile feat, just a character with the Mobile feat fulfilling the prerequisites to trigger the third bullet. I don't think many people were considering that aspect of it when responding from what I remember, I'm pretty sure that I didn't.
Edit: it should be noted that the attack made against the non sentinel character doesn't have to be made using extra attack, it could be made with a bonus attack. Neither feat establishes what kind of action must be used.
My question pertains to how the Sentinel feat states;
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
And how Mobile states;
"When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
So my question is if two characters with these feats where fighting would the mobile feats attack and flee tactic be ignored by the sentinel feat or because its not technically a disengage action would the Mobile feat win out and the character would get to run away after making their attack?
Secondly if the mobile feat does win out would this also apply to swashbucklers Fancy Footwork feature which gives about the same benefit as the mobile feat saying;
"During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn."
I feel as though because sentinel specifically states that they take the disengage action that these other two features would ignore it but I thought i'd ask for clarification.
dndbeyond world will correct next if I am wrong...
but in this instance. I believe they cancel out where applicable.
the mobile or swashbuckler. (If they can disengage as a bonus action after an attack) would still be able to disengage without AoO. But would not be able to just attack and walk by. Neither would the sentinel just be able to AoO just because they entered the reach range (provided they made an attack to cancel it out).
as you noticed reading mobile and sentinel. Not all the rules were well thought out or written, and there are contradictions or places where things just aren’t clear at all or make any sense.
Blank
In this case 7bitbrian>8pack.
Sentinel relies on disengage, but mobile doesn't use that mechanic.
I don't fully understand your position in this answer, but the correct answer is: Sentinel beats disengage (bonus) action, mobile and fancy footwork don't use disengage, so sentinel does not beat them.
As a leading advocate of "the rules are often unclear" myself, I don't think this is one of those times.
"Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
The way I read it is that Sentinel is there to beat Disengage if a character happens to take it, not that disengage must be used for Sentinel to work.
As a Feat, since feats are meant to be specialized areas of expertise (or however it's worded in the phb), its effect would supersede the specificity of a class feature for something like Swashbuckler's Fancy Footwork. However, since Mobile is also a feat Sentinel will win but you could rule that the opportunity attack should be made at disadvantage.
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That sentence you quote provides an exception to the general rule about disengaging from opportunity attacks; it does not provide any other exceptions. I admit that disengage is not required for sentinel to work, but my point stands: sentinel's text says that it only defeats disengage as a way to avoid opportunity attacks. Sentinel does not provide any other exceptions. If you would avoid an opportunity attack by any means other than disengage, sentinel would not allow the opportunity attack.
I agree with what 7bit, Wolf, and I said.
Okay I need clarification because I can see this is going to go into a circle. I'm a little confused because you hit this from opposing points of view, and agreed but also disagreed with the sentiment of the knowledge concerning disengage with and without Sentinel. Sentinel does defeat disengage, yes, but any other means of voluntarily leaving someone's reach still provokes attack of opportunity, as per normal, with the exceptions provided like Fancy Footwork and Mobile. Are we or are we not agreed that it can and/or does defeat the class feature or Mobile?
If we're going to consider these effects as sans disengage, then how could we not classify Sentinel as superior in that regard, since reactions are provoked by a trigger and the reaction happens after the trigger, does Fancy Footwork and/or Mobile happen before the reaction can be had in a turn based value?
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I would summarize it so: Sentinel works when someone provokes an attack of opportunity + it negates disengage action's ability to avoid attacks of opportunity.
Any other means of leaving the space without provoking attacks of opportunity are not covered by Sentinel.
Sentinel does not beat mobile or fancy footwork. Sentinel beats disengage. Mobile and fancy footwork are not disengage.
There shouldn't be any circles.
What even is this question?
@DxJxC Pretty much was looking for a summation like this based on how a turn plays out, so I can wrap my head around how everyone is making sense of this. I'm not interested in the debate of it, per se, but I'd like a solid foundation to work from if something like this ever arises in the games I play.
The question was worded about as poorly as some features in the game (wife + kid aggro; happens a lot) but in short I was asking how we could apply Mobile or Fancy Footwork in the turn so that, essentially, it supersedes a reaction based feat in the order of combat as a step-by-step method. Like, what kind of action could we classify either of those as if not a type of disengage, if anything at all? I realize this isn't what the OP was asking, I'm curious by myself.
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Successfully completed the Tomb of Horrors module (as part of playing Tomb of Annihilation) with no party deaths!
A summation like:
Which I did post before your first post.
Mobile and Fancy Footwork are static effects with conditional clauses. They are not any type of action let alone disengage. The only order that matters is that the condition is met before the AoO is provoked, then it simply denies the reaction.
I believe the features of Mobile and Fancy Footwork would be classified as an attack action as both features are dependent on if the character made a melee attack against the target that is trying to use the reactionary attack of opportunity against the original character as they strike and flee. So as it would go in turn based combat where player 1 has mobile or Fancy Footwork and player 2 has sentinel;
On player 1's turn they use their action to strike player 2 with melee, then exiting player 2's reach, player 2 then uses their reaction to attack player 1 but because mobile was activated by the melee attack player 1 avoids the reactionary attack or player 2 automatically misses either way you wanna spin it.
I don't know if that helps to clear it up but that's how I've come to understand it. It's that Mobile and Fancy Footwork are activated in the action and persist against the target until their turn ends. So by Player 1 using their action, if player 2 ends up trying to use their reaction against them they fail and miss.
Comment redacted.
Blank
D. That last post of yours bottom paragraph makes sense to me.
Blank
I wouldn't classify it as an attack action since the condition is met by any melee attack, it can be a melee spell attack from the cast a spell action or a bonus action attack from 2 weapon fighting.
But what we call it doesn't really matter, because the 2 effects just don't directly interact.
Mobile fest's third bullet:
When you make a melee attack against a creature, you don’t provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not.
Opportunity Attacks:
In a fight, everyone is constantly watching for a chance to strike an enemy who is fleeing or passing by. Such a strike is called an opportunity attack. You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach. You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.
The movement out of reach without disengage is the normal trigger that allows the reaction for the AoO to take place. Mobile's third bullet doesn't allow that trigger to take place. This would forego any ability that requires AoO for interaction. Sentinel requires AoO.
Fancy Footwork:
When you choose this archetype at 3rd level, you learn how to land a strike and then slip away without reprisal. During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn.
This one I'd probably rule that the AoO is still triggered, but can't be made. This means that sentinel couldn't apply because it requires the attack to hit, though, if the rogue that makes the attack tried to disengage for some reason, sentinel would still allow the trigger even if the AoO can't be made. This is only pertinent if the DM rules that Warcaster or something similar could allow for something that isn't an AoO to replace the AoO trigger.
I didn't read other responses but I had the same question a while back and Sage Advice had an answer.
Alvin has Mobile.
Betty has Sentinel.
Connor has nothing.
Scenario 1: Alvin approaches Betty and Connor who are standing side by side; Alvin attacks Betty and then uses his movement to walk away. Mobile does not allow for Betty to make an Opportunity Attack, Mobile wins.
Scenario 2: Alvin approaches Betty and Connor who are standing side by side; Alvin uses his first attack against Betty and then uses his Extra Attack against Connor. When Alvin attacks Connor, Betty can use Sentinel to attack Alvin. This is a reaction attack and not an Opportunity Attack, Sentinel wins.
If Mobile attacks Sentinel, Mobile wins and NO OPPORTUNITY ATTACKS. Sentinel 3rd benefit can always be used against Mobile.
Hope this helps. ;)
Mobile trumps Sentinel. The text is clear, mobile does not proc an OA. Ever. When you make an attack, you do not provoke. Period, end of story. Nothing trumps this.
Sentinel allows you to hit some people, even if they use an action that normally allows them to leave a threatened square without provoking. It's not as definitive as mobile, therefore mobile wins. OAs are always conditional, and it's impossible for a character with the mobile feat to meet the conditions required to trigger an OA against them.
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The third bullet of sentinel reads as follows:
When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Ergo, the scenarios that Zach_Harrison3 laid out are correct outside of stating that Sentinel trumps Mobile in that scenario as Sentinel isn't interacting with the Mobile feat, just a character with the Mobile feat fulfilling the prerequisites to trigger the third bullet. I don't think many people were considering that aspect of it when responding from what I remember, I'm pretty sure that I didn't.
Edit: it should be noted that the attack made against the non sentinel character doesn't have to be made using extra attack, it could be made with a bonus attack. Neither feat establishes what kind of action must be used.