What happens if you jump while using the spider climb spell?
Can you jump across a surface upside down, or do you fall?
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The spider climb spell states:
Until the spell ends, one willing creature you touch gains the ability to move up, down, and across vertical surfaces and upside down along ceilings, while leaving its hands free. The target also gains a climbing speed equal to its walking speed.
Spells only do what they say and nothing in this spell description gives you the ability to remain in the air after breaking contact with the surface you are climbing on. The spell allows you to climb without using your hands, it still requires you to use your feet.
There are falling rules in both the basic rules and Xanathar's Guide to Everything which help determine what happens after you start falling. Nowhere however does it clarify what it takes to begin falling, other than a brief section on flying creatures falling. It is assumed that any creature in the air is falling, unless they have a way to remain aloft. Since spider climb doesn't grant a way to remain aloft, you fall.
By RAW you fall, however this DM would likely allow you to attempt it. I would call for a Dexterity (Acrobatics) or Strength (Athletics) check, likely with disadvantage if you were upside down. On a success you manage to get you feet back on the wall quickly enough to prevent your fall. On a failure, you don't and plummet to your doom.
The DC for this check would depend on the distance of the jump and the difficulty of the situation. You should reference the rules for jumping as a guideline and increase the DCs accordingly.