The pertinent ruling is apparently

608.2g: If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a-i, except no player receives priority after it’s cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.

EDIT: the above rule is why you can cast spells with Gix. There’s a ruling on Xanathar that says “Xanathar doesn’t change when you can cast spells […] You must still follow all normal timing rules”

  • LovesTha🦒@mtgjudge.social
    2·
    8 months ago

    @Worx There are two ways these abilities are worded:
    * Permission to play/cast a <set of cards> in a given <time period>
    * Instruction to (optionally) play/cast a <set of cards>.

    Technically neither grants you the ability to casts spells as an instant. The first doesn’t alter the timing of playing spells at all, just gives you the ability to play them from an unusual place (the usual place is from your hand). The second instructs you to cast the spells while the ability resolves, usually you can only play spells (or abilities) when you have priority (and no player has priority while a spell/ability is resolving).

    The two wordings are similar, except that the permission version lists a period of time (until the end of your next turn, while the cards are in exile, while ~ is in play, etc). Use the existence of a period of time to figure out which type of ability it is.