Magic action: resolve an interrupted long casting

SRD 5.2.1 · Magic

At the table

1. For an action-time spell or feature requiring the Magic action, follow its specific activation text.
2. For a spell with casting time of one minute or longer, take the Magic action each turn of casting.
3. Maintain Concentration during that casting; if it breaks, the spell fails without expending a spell slot.

Example decision

A caster begins a spell with a one-minute casting time and loses Concentration before finishing. Under this long-casting rule, what happens to the spell slot?

The spell fails, but no spell slot is expended.

This rule says concentration failure during the longer casting makes the spell fail without expending a slot. That differs from Ready, where a spell is cast and its energy held. Identify which procedure is happening before assigning a resource loss. The caster must also use the required action on each turn of the ongoing casting.

Watch for this mistake

Do not apply this no-slot-loss rule to every failed spell or broken concentration after a spell is already active. It concerns the specified ongoing long casting.

Ask the table

Is this a spell still being cast, a readied spell, or an already active concentration effect?

Source rule

When you take the Magic action, you cast a spell that has a casting time of an action or use a feature or magic item that requires a Magic action to be activated. If you cast a spell that has a casting time of 1 minute or longer, you must take the Magic action on each turn of that casting, and you must maintain Concentration while you do so. If your Concentration is broken, the spell fails, but you don’t expend a spell slot. See also “Concentration.”

Source: SRD 5.2.1, 185. Online guide.