Ready cast spell when an enemy cast a spell vs Counterspell

Question

Inspired by Can I deny a counterspell by readying my spell behind full cover?

What happens when I ready to cast a spell when an enemy cast a spell, when an enemy has Counterspell prepared?

According to this post, readied action can be triggered on your own turn.

So the scenario going on my mind is:

1. I ready a spell to cast when an enemy is casting a spell (the readied spell is irrelevant)

2. When you ready a spell, you cast it as normal but hold its energy

   So the enemy sees you cast a spell, and cast Counterspell on you.

3. Resolve the Counterspell, and if it doesn't fail, I can cast the spell in response to the Counterspell.

Is my interpretation correct? Or can the spell go first in response to the enemy casting Counterspell, thus wasting his Counterspell?

Answer 1

No, at the time counterspell is cast, there is no ready action to trigger.

This answer assumes that the enemy can see you, that you are not under full cover, and that the enemy can tell you are casting a spell.

You attempt to interrupt a creature in the process of casting a spell. 

You must completely and successfully cast a spell before it can be considered prepared. However, this is an issue because counterspell actually interrupts the casting of the spell. And because it interrupts the casting of the spell it also interrupts the readying action itself. So, counterspell is actually cast before there is any readied action completed. Without any readied actions complete, there is nothing for counterspell to trigger.

I'll break down the progression for the case in which counterspell fails and the one where it succeeds.

Counterspell succeeds

If the counterspell succeeds, you have lost the spell and the potential readied action like one would expect.

1. Wizard A begins to ready action by beginning to cast a spell.
2. Wizard B sees the spell starting to be cast and casts counterspell in response.
3. Counterspell succeeds and the Wizard A's spell is lost. The action used to try to ready the spell is also lost.

Counterspell fails

If counterspell fails to interrupt your spell, your readied action would then become complete, because the casting of the spell was completed.

However, because the counterspell was cast before the readied action was complete, it would not trigger in response to the counterspell.

1. Wizard A begins to ready action by beginning to cast a spell.
2. Wizard B sees the spell starting to be cast and casts counterspell in response.
3. Counterspell fails so the spell completes.
4. The spell is now readied, but nothing happens because counterspell was cast before the readied action was complete.
5. If Wizard B casts another spell before the start of your next turn, you may use your readied action against that in accordance to how you set the trigger.

Answer 2

The readied spell is Counterspelled, and nothing happens.

When you choose to cast a spell as a ready action, the PHB says the following:

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. (PHB, 193)

If you take the Ready action, your character casts the spell as normal. The enemy would then cast Counterspell, interrupting that spell. As a result, you no longer have a spell readied and have consumed your action for that turn. You get your reaction as a response to the enemy casting Counterspell, but since you no longer have a spell readied, nothing happens.