Does Tunnel Fighter turn an Opportunity Attack into a non-action?

Question

The Tunnel Fighter fighting style from Unearthed Arcana: Light, Dark, Underdark! allows you to make Opportunity Attacks without using a reaction.

Tunnel Fighter: You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5ft while within your reach.

Since you are no longer using a reaction, does this mean that when in this stance Opportunity Attacks no longer take an action?

Opportunity Attacks: ... 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. ...

This becomes important because there are spells that take an action to cast. eg

Booming Blade:
Casting Time: 1 action As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range ...

A tunnel fighter in their stance certainly meets the 'you must make a melee attack with a weapon' criteria. My question is does it still meet the 'Casting Time: 1 action' criteria.

The Specific beats general rubrik makes me think that the Opportunity Attack is now no longer an action of any sort. But that seems weird.

Somewhat related questions:

Tunnel Fighter & Polearm Master : Indefinite Attacks of Opportunity?

How does Tunnel Fighter interact with Paladin's Relentless Avenger?

Answer 1

Tunnel Fighters don't need any action, reaction, or bonus action to make an opportunity attack RAW

You are correct in your interpretation that, as it is currently written, tunnel fighters do not need any sort of action to take their opportunity attacks that meet the requirements.

You can't cast Booming Blade with any opportunity attack

To cast booming blade you must take the Cast a Spell action. The casting time of 1 action that you point out simply indicates that casting the spell only takes 1 action. And, as part of that action, you can make a melee attack. So there is no way for this spell to interact with either Polearm Fighter or opportunity attacks in general (at least in any way that you describe).

Tunnel Fighter is playtest material and not released or finalized

This might explain why this seems weird and out of place with the rest of 5e's rules. If and when such a feature ever gets released in official published material, then we may see a change in the wording it bring it better in line.

This would be the exact type of issue you would expect to work out in public playtesting. However...

There aren't many (if any) situations in which this seems to matter

I can't think of a single situation in which something interacts with an opportunity attack only if that opportunity attack takes a reaction. So, in all likelihood the fact that no reaction is spent by the tunnel fighter is rather meaningless (aside from the obvious action economy implications for said tunnel fighter).

War Caster does not change anything about this

When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack.

• War Caster allows you to cast a spell instead of an opportunity attack. It does not allow you to cast a spell as an opportunity attack.

• No matter what, War Caster specifically says it requires a reaction to cast the spell.

In the end you are simply using the Cast a Spell action as a reaction which triggers when an opponent provokes an OA from you. You are trading a potential OA for the opportunity to cast the spell in this way. But War Caster does not enable the cast of a spell as an opportunity attack, so it does not interact with tunnel fighter in any unusual way.

Answer 2

No, and it makes no difference if you use the feat or not; an opportunity attack is not an action to begin with, it is a reaction, and you could not cast Booming Blade as part of an opportunity attack in normal circumstances.

Answer 3

This wouldn't work.

Booming Blade is a spell. The fact that it contains a melee attack as part of its action to cast doesn't automatically mean that it can be subbed in as an attack of any sort. You still need to cast the spell to perform the attack.

Whether or not the opportunity attack costs your reaction doesn't matter.

But there's a feat for it!

War Caster, which states:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

Whether you'd be able to use it for Tunnel Fighter's unlimited OAs is more iffy, as War Caster does specify that it takes a reaction, but you can still use it for the first.