Can a wizard transcribe prepared spells from multiclassing? [duplicate]

Question

Say you're a 1st-level wizard and also a 1st-level druid. Could you, as a druid, prepare spells that both classes have, like Detect Magic and Ice Knife, then, since they're now "prepared" for you, write them in your spellbook so that you can cast them as a wizard, allowing you to cast with intelligence without needing to choose them on level-up or find them in a book/scroll?

Also, would it cost 10 gp per level or 50 gp?

As far as I can tell RAW seems to say yes but I just wanna make sure I'm not missing anything.

Answer 1

Wizards cannot transcribe prepared spells from multiclassing

The "Your Spellbook" sidebar is clear that Wizards need to find a spell in a spellscroll or spellbook to transcribe it into their spellbook, then only exception given in the sidebar is for lost spellbooks:

If you lose your spellbook, you can use the same procedure to transcribe the spells that you have prepared into a new spellbook. Filling out the remainder of your spellbook requires you to find new spells to do so, as normal. For this reason, many wizards keep backup spellbooks in a safe place.

However, even a multiclassed Wizard who lost his spellbook cannot transcribe prepared spells from his other class for the following reason.

The Wizard's spellcasting states:

At 1st level, you have a spellbook containing six 1st-level wizard spells of your choice. Your spellbook is the repository of the wizard spells you know, except your cantrips, which are fixed in your mind.

According to the multiclass rules for spellcasting:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

Transcribing spells into a spellbook determines what spells the Wizard knows and can prepare, because the spellbook is the repository of the Wizard's known spells. Therefore, when a Wizard transcribes spells into his spellbook, he must do so as though he were not multiclassed.

However, Xanathar's Guide to Everything has rules for scribing spell scrolls as a downtime activity. If the DM gives you the opportunity, you can scribe a spell scroll for any of your multiclass spells and, if that spell is also a wizard spell, then you can copy the spell scroll into your spellbook.

Answer 2

Not directly, no

You can't scribe a prepared spell into a wizard's spellbook as the rules state you must copy the spell from a scroll:

Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it. Copying that spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation. For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.

However, you can scribe a spell that you have prepared into a scroll, see this posting for more details. After you do that, you can scribe it into your spellbook provided it is a spell on the wizard spell list.