Does casting a spell with a long casting time require the target to stay in range and line of sight the whole time?

Question

When casting a spell with a casting time greater than an action, do range and line of sight need to be maintained for the entire time?

For instance, does a cleric casting Prayer of Healing need the targets to be within 30 ft. and in line of sight for just the start of the spell, just the end or for the entire casting time.

Certain spells, such as Identify or Planar Binding, explicitly mention needing to stay within range for the casting time. Other spells like Magic Circle only imply you stay in range since the casting time represent drawing out the circle.

I'm fairly confident that one does indeed, need to stay in range at least. This line suggests that before a spell is cast, it remains limited by its range:

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.

I haven't found a rule requiring that one maintains line of sight.

The situation that this question arose from had an uncontrolled celestial being summoned into a magic circle. The wizard began to cast Planar Binding on it and realized that they could simply step into another room while they completed the spell, which seemed to trivialize what might otherwise be a very challenging task.

Answer 1

Yes, the target probably needs to stay in range.

The spellcasting rules say:

When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so.

So you are "casting the spell" from the turn you start until the turn you finish.

The target of a spell must be within the spell's range.

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.

"Once a spell is cast" rang doesn't matter, so it does matter before that. The only way for it to matter before that is if it matters while you are casting the spell, that is, for the entire time that you are casting it.

A typical spell requires you to pick one or more targets to be affected by the spell's magic.

To target something, you must have a clear path to it, so it can't be behind total cover.

Unless the particular spell is atypical, you have to "pick one or more targets" and "have a clear path to it". When? The only sensible answer is while you are casting the spell.

The alternative interpretation is open to abuse

If you adopt the idea that these rules only apply at a given instant (beginning? end? some point in the middle?) then it's possible to set up a situation where you spend 10 minutes or an hour casting a spell and then your colleague Teleports you to the target just before you complete it. I don't think that can be the intended outcome.