How can a damaged magic item be repaired?

Question

All objects, including magic items, have hit points. Some magic items, such as Ioun Stones and the Apparatus of Kwalish, specify how many hit points they have in their description, while the rest of the magic items can be assigned hit points based on the Object Hit Points table (DMG p.247).

Moreover, as explained in Magic Item Resilience (DMG p.141), many magic items even have resistance to all damage, so even though in my experience it's rare for magic items to take damage, it is nonetheless possible. In particular, the Apparatus of Kwalish is likely to take damage at some point while being used.

However, I can't find any guidelines to repair magic items, hence the question.

If a magic item takes damage, how can it be repaired? Is it any different from repairing mundane items?

A good answer should be based on existing rules, not arbitrary suggestions. Pointing out that no such rules exist is also an acceptable answer, should that be the case.

Answer 1

You can do it with smiths tools and a forge

In Xanathar's Guide to Everything in chapter 2 under the Tool Proficiencies section, you can find rules for Smith tools that will allow you to repair damaged metal objects.

Repair. With access to your tools and an open flame hot enough to make metal pliable, you can restore 10 hit points to a damaged metal object for each hour of work.

For magic Items not made out of metal you may be out of luck, perhaps tinkers tools

Repair. You can restore 10 hit points to a damaged object for each hour of work. For any object, you need access to the raw materials required to repair it. For metal objects, you need access to an open flame hot enough to make the metal pliable.

These are the only completely raw methods of healing it that I could find, every other method has some level of ambiguity to it.

Answer 2

The Mending spell is probably your best bet.

Its description is as follows:

This spell repairs a single break or tear in an object you touch, [...]. As long as the break or tear is no larger than 1 foot in any dimension, you mend it, leaving no trace of the former damage.

This spell can physically repair a magic item or construct, but the spell can't restore magic to such an object.

I believe logic dictates that a magic item doesn't lose its magic in the first place until it drops to 0 hit points. The alternative would be that a kobold throwing a stone at an Apparatus of Kwalish and dealing 1 damage would render the Apparatus nonmagical, which is obviously not intended.

Anyways, as long as the damage suits the criteria mentioned by the Mending spell, it will be removed entirely.

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Unfortunately, in the case of an Apparatus of Kwalish, for example, it's not unlikely that the magic item will get damaged more severely, in a way that it cannot be fixed by Mending (by default).

However, the (currently) latest version of the Artificer has the ability to create Homunculi (different from those made with the Create Homunculus spell) and magical turrents. If these get damaged, one can restore 2d6 hit points to them by casting the Mending spell on them.

I believe this amount of healing is appropriate, and I, therefore, suggest you ask your DM to adopt the Artificer's 2d6 Mending to other magical objects as well.

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In the end, as per RAW, you can only repair damage that suits the regular Mending criteria. If an object has e.g. a tear bigger than 1 foot, then, to my knowledge, there are no official rules on how to repair it.

Answer 3

The Wish Spell seems to be a viable option

The only reason on why I state this is because of Daern's Instant Fortress, in which each wall, roof and door are made of adamantium, and each have 100 HP. In the DMG, it states:

Only a wish spell can repair the fortress (this use of the spell counts as replicating a spell of 8th level or lower). Each casting of wish causes the roof, the door, or one wall to regain 50 hit points.

So, one could assume that this rule would apply to other magical items that inherently have hit points. If it would work for Daern's Instant Fortress, I do not see why it would not work for Apparatus of Kwalish, or anything else that is magical of nature, and has hit points. I would recommend making it 50 hit points for the heal, just like how the wish spell interacts with Daern's instant fortress.

As @PixelMaster pointed out, you might be able to try the Mending cantrip, but I honestly think that Wish would be a better option simply because of a better healing option (since Wish is instantaneous and it would heal 50 points of HP, and because of it being instantaneous, it outclasses the Mending cantrip when you need a quick heal. Now, I am not naysaying that idea, because outside of battle, if I had to choose, I would choose Mending if I could. But, sometimes, you need to patch up something bigger then a foot. Thus, I believe that Wish is a viable option.