Does the item you apply the Replicate Magic item infusion change its shape to fit the magic item you chose?

Question

Does the type of item you apply the Replicate Magic item infusion matter? I mean, if I infuse the replica of Bag of Holding in a dagger, does the dagger turn into a Bag of Holding, or it stays like a dagger with the same effects?

The artificer's Infuse Item feature states, under the "Infusing an Item" subheader (E:RftLW, p. 57):

Whenever you finish a long rest, you can touch a nonmagical object and imbue it with one of your artificer infusions, turning it into a magic item. An infusion works on only certain kinds of objects, as specified in the infusion's description. If the item requires attunement, you can attune yourself to it the instant you infuse the item.

The Replicate Magic Item infusion states (E:RftLW, p. 63):

Using this infusion, you replicate a particular magic item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables below. A table's title tells you the level you must be in the class to choose an item from the table.

If yes, can the common armour variant magic items found in Xanathar's Guide to Everything, such as Cast-Off Armor, be created? Can I use this to get a set of Half Plate Armor from this infusion?

Answer 1

To infuse an item, you need the nonmagical version of that item.

The artificer's Infuse Item feature states (TCoE p. 12, E:RftLW p. 57, WGtE p. 179; emphasis mine):

At 2nd level, you gain the ability to imbue mundane items with certain magical infusions. [...] Whenever you finish a long rest, you can touch a non-magical object and imbue it with one of your artificer infusions, turning it into a magic item. An infusion works on only certain kinds of objects, as specified in the infusion’s description.

The Artificer Infusions section at the end of the class description states (TCoE p. 20, E:RftLW p. 61, WGtE p. 181; emphasis mine):

The description of each of the following infusions details the type of item that can receive it, along with whether the resulting magic item requires attunement.

And finally, the description of the Replicate Magic Item infusion states (TCoE p. 22, E:RftLW p. 63, WGtE p. 182; emphasis mine):

[...] Alternatively, you can choose the magic item from among the common magic items in the game, not including potions or scrolls.

In the tables, an item’s entry tells you whether the item requires attunement. See the item’s description in the Dungeon Master’s Guide for more information about it, including the type of object required for its making.

Each of these portions I've bolded make it clear that you need a nonmagical version of the item to infuse, whether you're using Replicate Magic Item or one of the other artificer infusions.

Even though the Dungeon Master's Guide doesn't actually say "[X] is the type of item needed to make [Y]" for any listed magic item (as far as I know), this sentence seems to refer to the fact that each magic item that can be replicated is essentially an enhanced version of a clearly identifiable mundane item.

For instance, boots of elvenkind are boots, so the clear implication is that you would need mundane boots to infuse if you want to use Replicable Magic Item to make boots of elvenkind. This follows the same logic as the Boots of the Winding Path infusion (TCoE p. 21, E:RftLW p. 62, WGtE p. 181), which does explicitly state:

Item: A pair of boots [...]

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In the case of your example, the description of cast-off armor states (XGtE, p. 136; emphasis mine):

Armor (light, medium, or heavy), common

Thus, you need a nonmagical version of whatever kind of armor you want to turn into cast-off armor. If you want cast-off half plate armor, you need a nonmagical set of half plate armor to imbue with that infusion.

The Replicate Magic Item infusion doesn't create a magic item from thin air - it gives you the ability to imbue a certain mundane version of that item with the necessary magic to function as that magic item.

Answer 2

No. The infusion simply adds traits to an item as per what the infusion itself does. The item it must be applied to is a normal version of that item. So a bag of holding could be made out of a regular belt pouch, a +1 dagger out of a normal dagger, and cast-off chain mail out of normal chain mail. I can see where the question here is coming from due to a lack of clarification in the rules, but based on the wording we can infer that it is intended to mean that any infusion simply adds properties to an item instead of changing the item into something completely different.

Answer 3

While all given answers here are absolutely correct, I want to add something from Tasha's Couldron of Everything p.116

Personalizing Spells

Just as every performer lends their art a personal flair and every warrior asserts their fighting styles through the lens of their own training, so too can a spellcaster use magic to express their individuality. Regardless of what type of spellcaster you're playing, you can customize the cosmetic effects of your character's spells. Perhaps you wish the effects of your caster's spells to appear in their favorite color, to suggest the training they received from a celestial mentor, or to exhibit their connection to a season of the year. The possibilities for how you might cosmetically customize your character's spells are endless. However, such alterations can't change the effects of a spell. They also can't make one spell seem like another- you can't, for example, make a magic missile look like a fireball.

Tho this paragraph is explicitly about spells, I don't see any reason why you can't "reskin" everything as long as it doesn't influence the game's mechanics. That means that you still need some kind of container or item that works mechanically like a container tho to replicate a Bag of Holding.

The Artificer is pretty much made for reflavouring. And it doesn't change anything if your replicated Boots of Levitation are actually a Jet Pack that is still affected by effects that target footwork for example. The Artificer is an arcane inventor and even if the rule aren't too clear about it, it doesn't hurt anyone, if your Bag of Holding is actually some technical device that is about as big as a belt pouch and weighs 15lbs.; like a material convertor that turns hold items into a hologram and stores them digitally in an artificial extraplanar space. As long as it needs an action to get something out of it... and as long as the device can break like a normal pouch... or in short: As long as it works like a normal Bag of Holding everything's fine. I think sticking to the rules too much at this point, diminishes the Artificer's possibility to give the players room for creativity.