When during your movement does Tenser's Floating Disk start to follow you and does it do so exactly?

Question

Let's assume the caster of Tenser's Floating Disk has a speed of 6000ft (for no particular reason), that is more precisely, they can cover 6000ft in 6 seconds.

However there are limitations on the disc's movement:

If you move more than 20 feet away from it, the disk follows you so that it remains within 20 feet of you.

and

If you move more than 100 feet from the disk (typically because it can’t move around an obstacle to follow you), the spell ends.

as well as

It can move across uneven terrain, up or down stairs, slopes and the like, but it can’t cross an elevation change of 10 feet or more.

Does the spell text of Tenser's Floating Disk, or other rules tell us enough, to say when the disk's movement/following happens? Instantly? At the end of turns? End of movement chunks?

Say that if the caster moved 200ft east as one part of their movement, then separately moved 250 ft west, the spell ends part way through the first part of the movement, even though you end up less than 100 ft away from the disk? If the spell doesn't end part way through the movement, does it attempt to travel east first then west, or just 50ft west.

Can the caster make the journey in 95 ft bursts, so as to not risk ending the spell early, letting the disk catch up each time?

Can the caster complete a 5900ft maze in a single movement, and end up ~100ft from the disk, and if so what path if any does it take?

Can the caster run round an obstacle course with disk following it like a prize dog, so long as it's not forced to traverse inclines of more than 10ft? Does it skirt around such obstacles (hypothetically losing points) or does it attempt a 12ft high bridge (with slopes either side) even though it would be crossing over an elevation change of more than 10 ft thus ending the spell, or does it 'lazily' go around? Does it matter if the disk is teased up in 9ft increments? We know it can 'take corners', but is that it? We know it likely can't do 'path finding': Can magical constructs pathfind?

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In short, how closely can the disk follow the caster's path based on when it starts to follow the caster, vs. just staying within 20ft?

Answer 1

The disk follows you as you move.

Does the spell text of Tenser's Floating Disc, or other rules tell us enough, that if the caster moved 200ft east as one part of their movement, then separately moved 250 ft west, the spell ends part way through the first part of the movement, even though you end up less than 100 ft away from the disc?

When the caster is 21 feet east of the disk, the disk moves one foot east. That's what "the disk follows you so that it remains within 20 feet of you" means. So in the circumstance you describe (moving 200 feet east then 250 feet west), there's no point during that movement where the disk isn't within 20 feet of you. The disk doesn't have its own turns or take actions; it just follows you as you move.

As for other details of how the disk moves, it's up to the DM. As long as it is (a) trying to stay within 20' of the caster, (b) no more than 100' from the caster, and (c) not traversing obstacles it can't pass, it could be as smart or dopey as the DM chooses. Maybe the disk has personality and acts like a loyal dog. Or a truculent mule. Maybe it just optimally implements the A-star route-finding algorithm and does so using spatial information not available to the spellcaster.

Answer 2

You can't outrun the disc; it has no movement rate at all.

(This is inherently a frame challenge; the 6000' movement rate limit is not in the spell)

Think of the disc being attached to the caster by an 20' incorporeal rope, which can stretch if it must to a maximum extension of 100', beyond which it breaks, ending the spell. The rope can pass through any matter, but the disc itself cannot, and can traverse any vertical 'step' up or down less than 10'. The 'rope' never 'stretches' if it doesn't have to; only if it cannot physically maintain the 20' distance does it 'stretch'. And, per the spell, the only thing that can move the disc is the 'rope'.

The disc doesn't move under its own power; the caster 'drags it along'.

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The disc moves, along the ground, simultaneously with the caster unless physically blocked from doing so, no matter when, how, or how fast the caster is moved. The total distance the caster moves doesn't matter, nor does his speed, unless the disc is blocked from following, or the caster exceeds the distance limit at any instant (such as via a teleportation type effect, which isn't movement). It's got no physical limitation: it's Conjuration magic; real-world limitations don't apply. The spell does what it says it does, no more or less.

The disc follows the caster as soon as the caster moves farther from the disc than 20'. It moves to maintain the 20' maximum distance whenever it's able. The caster can move at any rate, and the disc can keep up, if it has a path available to do so.

In the case of the caster climbing to a 10' high bridge, the disc would follow beside the bridge if able, in an attempt to maintain the 20' maximum distance. Unless the terrain the bridge spans has a vertical cliff greater than 10', the disc will follow along the ground as closely to the 20' maximum as possible in its attempt to maintain that distance, until it either got within 20', or farther than 100' due to terrain (the bridge spans a 110' deep gulf that is walkable).

If the disc were blocked from moving, the caster moved more than 20' from it, and the disc were then released, there's no information to definitively determine how fast it would reestablish the 20' maximum distance. The players (including DM) would need to decide what they like best in that case.

Answer 3

Marq's reasoning about how the disk follows you is a good reading of the rules, but we disagreed in the comments about how sudden teleportation would affect the disk.

Reading their comments and the quotes in my question I realized the movement and spell ending would attempt to happen simultaneously.

As per Xanathar's Guide to Everything, because of how the limitations are worded, when two things would happen simultaneously, you (or the GM) get to decide which happens first:

If two or more things happen at the same time on a character or monster's turn, the person at the game table-whether player or DM-who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

If you ever find yourself more than 100ft away from the disk and you were previously with 20ft, you can then decide for the disk to follow you first before the 'spell ending' thing happens. I'm not confident that the spell doesn't end after it's gotten within 20ft of you however; that would require the GM's arbitration.

Otherwise, as per Marq's answer, the disk follows you without waiting for you to stop.

When the caster is 21 feet east of the disk, the disk moves one foot east. That's what "the disk follows you so that it remains within 20 feet of you" means.

Answer 4

It's up to the DM, but the PHB seems to suggest the disc moves once per round

PHB 181 states

In situations where keeping track of the passage of time is important, the DM determines the time a task requires.

and

In combat and other fast-paced situations, the game relies on rounds...

"The disc follows you as you move" means that whenever you move, the disc moves whenever it is able to. It is up to the DM to determine when the disc is able to move. Whether that be immediately, or once per round, is up to the DM.