How to hide without triggering opportunity attacks?

Question

All halflings have the Halfling Nimbleness feature:

You can move through the space of any creature that is of a size larger than yours.

Lightfoot Halflings have the Naturally Stealthy ability:

You can attempt to hide even when you are obscured only by a creature that is at least one size larger than you.

Halflings are sized small, while Dragonborn, for instance, are medium.

A rogue has a Cunning Action:

You can take a bonus action on each of your turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.

And our group has been playing with a Barbarian Dragonborn and the Lightfoot Halfling Rogue. In combat, let's say, facing a single boss, the rogue usually hits and hides behind the Dragonborn (moving through him). A stealth check is rolled against the boss's perception, and if the boss wins, he can go around the barbarian and hit the rogue. He does not trigger attacks of opportunity because he is not leaving the barbarian's melee range.

However, for the rogue to go behind the dragonborn, he should trigger an opportunity attack from the boss, right? Or can the hide action include this 5 feet move to hide behind the dragonborn? Or does the nimbleness feature prevent attacks of opportunity, since the rogue can simply stand in the dragonborn's space, only behind him? Does standing in the same space as a bigger creature prevent attacks against you because the creature shields you?

In terms of RP and realism, we've been describing this as such: the halfling is hiding in the back of the barbarian, pops out to strike, and hides back there. The idea here is to have the rogue safely out of harm's way (dumb enemies won't understand where he is, others will attack with disadvantage, and others will actually have to move to him him (unless they're grappled).

Answer 1

It seems the problem you're trying to solve, is how to get consistent Advantage on attacks, without provoking opportunity attacks. This depends on a number of things.

What is the boss's reach? If the boss's reach is longer than 5 ft, then you will not provoke an opportunity attack to move behind your Dragonborn friend, if he is within 5 ft of the boss.

Must you use melee attacks? Using ranged weapons in a different location would be much safer, keeping you farther from the Boss, protecting you from Area of Effect spells or abilities, and giving you Advantage consistently without having to worry about disengaging.

You're also making a few assumptions:

1. Hiding means you are completely undetectable. This is false. He can't currently see you, and you have advantage to hit the boss, but for an intelligent creature, it's pretty obvious you're behind the Dragonborn somewhere, and he can still move around your friend to spot you. Additionally, barring the Skulker feat, attacking will reveal you. Also, simply stepping out from behind the Dragonborn will reveal you if the Boss is looking in your direction. (Also note there are no facing rules in fifth edition by default, so this is subject to GM rulings) Note this makes hiding in melee rather impractical without specialized abilities.
2. Nimbleness means you can stand in another creature's space. You may pass through another creature's space, but you may never end your turn in another creature's space, unless they are multiple size categories larger than you.

As it stands, the Rogue does not have enough actions to Disengage, Hide, AND Attack in one turn, so he would either provoke an opportunity attack, or not be able to attack. The alternative that would allow Advantage attacks every turn, is to use a ranged weapon, and hide at a distance.

Note, if all you're concerned with is getting your sneak attack bonus, all that is required is either having advantage or an allied creature is within 5 ft of your target. Keep in mind that applies to ranged or melee attacks: so as long as the Dragonborn is next to the boss, the Rogue will still get sneak attack against it.

Unseen Attackers and Targets (Fifth Edition SRD)

Combatants often try to escape their foes’ notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the GM typically just says that the attack missed, not whether you guessed the target’s location correctly.

When a creature can’t see you, you have advantage on attack rolls against it. If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

Answer 2

Use Cunning Action to take the Disengage action

The set up:

• The Barbarian is standing in front of a Boss
• The Rogue is standing behind the Barbarian
• The Rogue has already taken the Hide action, and the DM has determined that the Rogue's Stealth check has beaten Boss's Passive Perception.

The play by play:

1. The Rogue moves forward 5ft into the Barbarian's square
2. The Rogue attacks the Boss with advantage since they are Hidden
3. The Rogue uses Cunning Action to take the Disengage action
4. The Rogue moves back behind the Barbarian, and does not provoke an Attack of Opportunity

At the end of this play the Rogue is back where they started, behind the Barbarian, and is still Hidden. If at some point the Boss takes the Search action and discovers the Rogue, the Rogue would need to back off and Hide again, breaking their rhythm.

Note the following potential mechanics misunderstandings:

• A creature taking the Hide action is actively trying to be sneaky. They are not revealed unless their Stealth check is beaten by an enemy's Passive Perception or an active Perception check (usually via the Search action).
• You do not have to constantly re-Hide unless you have been detected. Hide lasts indefinitely.
• Attacking does not reveal you, it reveals your location. This is the same as when a Hidden creature makes noise, for example if they knock over a vase you will hear a crash and be able to tell where it came from, but this doesn't let you magically see the Hidden creature. It remains hidden.
• Crawford's rulings are not RAW, and you do not have to abide by their modifications to RAW. If it is troubling to you that Crawford says you can't attack while using Halfling Nimbleness, then move 5ft to the left/right of the Barbarian instead. It doesn't change the outcome.
• You do not have to remain out of line of sight to be hidden. "Being seen" doesn't mean "in line of sight", it means that the enemy can clearly see you. Darkness, fog, being 1000ft away, are all things that enable you to Hide in line of sight. Once you are Hidden, only an Active Perception check beating your Stealth check will reveal you. You can also stop being Hidden by choice.