When you run out of climbing speed, can you still climb with your normal speed at a penalty?

Question

Let's say you're a 40 speed, 15 climbing speed creature. There's an initial ledge at 20 feet, but you're trying to reach the top at 25 feet.

From the Player's Handbook, page 182:

While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed. At the GM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check.

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However, on page 190:

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can’t use the new speed during the current move.

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Combined with this information, there may be an odd scenario where a character has moved their maximum climbing speed.

Can they:

1. Move the 15 feet up the cliff, and cannot reach neither the ledge or the top?
2. Move their normal speed at twice the cost, allowing them to climb up to 20 feet in this turn and reach the ledge? (40/2= 20 feet)
3. Move the 15 feet of climbing speed up the cliff, and are able to spend the rest of their speed (40-15 = 25 left) at the extra foot cost, since they "no longer have a climbing speed", putting them at the very top of the cliff (25/2=10 [rounded down]), 15+10 =25 feet)?

Answer 1

Yes, movement may continue [at a penalty] when special movement speed has run out

You've definitely called out the correct section on movement speeds found in the book, but it's worth noting that in the time since the original printing of the PHB, there was errata added to make this kind of movement more explicitly clear:

Climbing, Swimming, and Crawling

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you're climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb, or a swimming speed and use it to swim. At the DM's option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check.

—Movement, Player's Handbook, pg. 182 (with 2018 Errata)

So what we see here is that the movement cost only applies so long as the creature does not use their movement speed for that particular type of movement. It doesn't say that you're unable to use your walking speed simply because you have a climbing speed.

So if you're using your climbing speed, you'd use 1 foot of movement for each foot you move. Then, if you run out of climbing speed, you can keep going at a rate of 2 feet of movement for each foot you move. As a result, your third option, moving 15 feet with climbing speed, then moving 25 / 2 feet with their remaining walking speed is correct; though it should be noted that Rules-as-Written, there's no requirement to round distances, since 5th Edition does not "snap to a grid" by default. So it's perfectly legal for a character to move exactly 12.5 feet (or in this case, 27.5 feet in total) during their turn.

Answer 2

You can keep climbing if you have walking movement left

You have quoted the relevant text, which allows you to "switch back and forth between your speeds during your move". Therefore, the third option you present is accurate. You first move up to your climbing speed, then spend two feet for every one foot of your walking speed to keep climbing, leaving you at the top of the cliff with no remaining movement.

A strict RAW reading of "unless a creature has a climbing or swimming speed" would say that a creature with a climbing speed never has to spend extra movement to climb. A creature with a climbing speed less than their walking speed, like a Tabaxi PC, would be nonsensical - their walking speed could be used to climb without penalty, so why list a lower climbing speed? The only reasonable interpretation is that "unless a creature has a climbing or swimming speed" is intended to only apply while that climbing or swimming speed is being used; therefore, a PC can keep climbing with a penalty if they have walking movement left.

A stealth change was made to the paragraph in question around the time that the November 2018 errata came out. (Thanks to Carcer in the comments for pointing this out.) The updated wording confirms my interpretation (D&D Basic Rules 2018, p. 67; emphasis mine):

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling. You ignore this extra cost if you have a climbing speed and use it to climb or a swimming speed and use it to swim.

You may need to start making Athletics checks

If your DM allowed you to skip an Athletics check due to having a climbing speed, it may be reasonable for them to change their mind if you switch to using your walking speed. Mike Mearls says that you won't need an Athletics check if you have a climbing speed in the following Sage Advice tweet:

@JeremyECrawford @mikemearls Do creatures with a climb speed make Athletics checks? If so, Advantage would be nice-lets make that official.

— DnD Sphinx (@DnDSphinx)

no check needed to climb

— Mike Mearls (@mikemearls)

That said, even creatures with a climbing speed may need to make an Athletics check to climb "a slippery vertical surface or one with few handholds" (PHB, p. 182). Jeremy Crawford indirectly confirms this in the following Sage Advice tweet:

@JeremyECrawford Is it RAI that creatures with a climb speed need to make Athletics checks to climb? RAW are silent on the issue, thus yes?

— Shawn Dry (@ProfSiccus)

A trait like Spider Climb, not a climbing speed, lets some creatures forgo ability checks when climbing.

— Jeremy Crawford (@JeremyECrawford)

Answer 3

You quoted the PHB:

From the Player's Handbook, page 182:

While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed. At the GM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check.

PBH Pg 190

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can’t use the new speed during the current move.

Since you do have a climbing speed in this case, you don't suffer any halving of any of your movements. It seems that, RAW, possession of the climbing speed (any amount of climbing speed) makes climbing no longer cost you any extra movement, and you can just climb up the wall with no penalty, thus 15 feet of climb, plus the remaining ten feet of regular movement speed up the wall, plus fifteen more feet after that.

This is clearly ridiculous and not RAI, but by the usual degree of picking apart the text for exact meaning, it's RAW. (I think if they'd said 'Unless a creature is using their climbing or swimming speed' it would reflect what they really meant, but as written, having the other types of speed removes all the extra movement costs.

So RAW says you can dart up that entire 25 foot cliff without any movement penalty, and continue on for 15 more feet after that.

And reason (and RAI) says if you have 40 feet of total movement, 15 of which can be used without extra cost of speed, then you can climb the additional ten feet of the cliff at a 50% movement cost, ending on the very top (25/2 rounded down to nearest multiple of 5 = 10 feet, as you said).

So using RAI: Case 1 is not right, because PHB pg 190 says:

If the result is 0 or less, you can’t use the new speed during the current move.

It says nothing about preventing you from using the first speed, only the new one (climbing) once it hits 0

Case 2 is correct but you don't have to choose to do it that way. If you burn it all on regular movement, your 15 feet of climbing speed cannot be used, since all your movement is burned climbing the 20 feet

Case 3 works just fine, RAI

RAW: You "Have a climbing speed" So your movement is not penalized by climbing, and you can climb the 25 foot cliff and move an additional 15 feet subsequently. But this is (as already recognized) absurd

Answer 4

No, when you run out of climb speed, you're done moving.

In the introduction to the Monster Manual (page 8), it has this to say about climb speed:

CLIMB

A monster that has a climbing speed can use all or part of its movement to move on vertical surfaces. The monster doesn't need to spend extra movement to climb.

And you quoted the other relevant text (emphasis mine):

While climbing or swimming, each foot of movement costs 1 extra foot, unless a creature has a climbing or swimming speed.

Based on those, my interpretation is that having a climb speed completely overrides the normal climb rules and replaces them; you climb at the listed speed instead of being able to spend 2 feet of movement per foot of climbing -- in other words, the climb speed replaces the '1 extra foot' rule and you no longer have the option of climbing by using additional base speed. You must use that climb speed instead of your walking speed when climbing, and once that's used up, you're done climbing. You can't convert your normal speed into additional climbing, even if it would be better.

But it won't be better. In every case I can find, a creature's climb speed is more than half its walking speed, so using their climb speed is the strictly superior choice. In most cases, the climb speed is equal to move speed, but in the few where it isn't (such as the Panther), the climb speed is still greater than half their movement.

So, for example, if the aforementioned panther climbed a 40-foot cliff, it could then switch to normal movement, and move an additional 10 feet (by the 'switch back and forth' rule), but if the cliff is 45 feet tall, the panther is going to have to just hang out there until next turn (unless it took a Dash action, which essentially doubles all your speeds).

As a DM, I would allow a character who has managed to acquire a movement speed in excess of twice their climb speed to either use their climb speed or use their normal movement at 2-for-1, but I wouldn't allow them to mix the two in the same round. The rule about mixing movement modes is clearly meant to deal with changing the kind of movement you're making -- such as going from flying to running -- not to allow you to climb, and then climb some more only in a different way.