Can you play new spells as items on the chain resolve?

Yes, you can play new spells as items on the chain resolve.

According to Rule 340.4 and supported by various RiftJudge FAQs (such as FAQ #3479, #6477, and #4385), after an individual item on the chain finishes resolving, there is a priority window before the next item resolves. During this window, players have the opportunity to add new spells (provided they are [Reaction] speed) or activated abilities to the chain.

How it works:

1. Chain Resolution: The current top item on the chain resolves completely (Rule 340.1).
2. Priority Window: After the item resolves and is cleared from the chain, a priority window opens (Rule 340.4).
3. Adding New Items: If you have priority, you may play a spell or activate an ability. Because this adds to the existing chain, the spell must have the [Reaction] keyword to be played during this time (Rule 158.2.b).
4. Resuming Resolution: Once both players pass priority without adding anything new, the next item currently on the chain resolves, and the process repeats (Rule 340.4).

Important Note: While you can add new items between resolutions, you cannot play [Action] spells during this time because [Action] spells require an Open State (meaning the chain must be empty) to be played (Rule 158.2.a.1). You must use [Reaction] spells to add to a chain that is still mid-resolution.