If I use the Temporal Portal and Rocket Barrage combo, having Temporal Portal in base, I'll recycle a rune to tap Temporal Portal then tap 4 runes and recycle a blue to play Rocket Barrage next. Does Temporal Portal discount the repeat of Rocket Barrage?

To address your question about whether Temporal Portal discounts the Repeat cost of Rocket Barrage, we must look at how costs are calculated and how the Repeat keyword functions.

The Ruling

No, Temporal Portal does not discount the Repeat cost of Rocket Barrage.

According to the official rulings on cost calculation (see RiftJudge FAQ #8937 and #8418):

1. Printed Cost: Anything that references a card's "cost" looks at its printed (base) cost.
2. Repeat Cost: The cost of a Repeat is determined by the card's printed cost.
3. Cost Reductions: Cost reductions are passive abilities that apply during the cost determination step to the total cost (Base Cost + Repeat Cost). They do not change the card's printed cost, nor do they apply separately to the Repeat cost.

Why they don't "stack" in the way you described

While you can have multiple instances of Repeat (one printed on Rocket Barrage and one granted by Temporal Portal), they are paid for separately as additional costs when you play the spell.

• Rocket Barrage has a printed cost of [4][C]. Its printed Repeat cost is also [4][C].
• Temporal Portal grants a second instance of Repeat equal to the spell's cost ([4][C]).
• If you choose to pay for both instances of Repeat, you are paying the base cost plus two separate Repeat costs.
• Any cost reduction effects you have would apply to the total sum of these costs, not to the individual Repeat costs themselves.

Sequence of Events

When you play Rocket Barrage with Temporal Portal active:

1. Declare Play: You announce you are playing Rocket Barrage and declare which (if any) Repeat costs you are paying.
2. Calculate Total Cost:
   • Base Cost: [4][C]
   • Printed Repeat Cost: [4][C]
   • Temporal Portal Repeat Cost: [4][C]
   • Total: [12][3C] (minus any applicable cost reductions).
3. Pay Costs: You pay the total calculated energy and power.
4. Finalize: The spell is placed on the chain as a single item.
5. Resolution: Upon resolution, the instructions of Rocket Barrage are executed three times (the initial play + two paid Repeat executions).

Important Note: Because Rocket Barrage is only "played" once (Rule 820.3.a), you make choices for each of the three executions separately at the time you play the card. Once the spell is on the chain, you cannot add or change your decision to pay for Repeat after your opponent has had an opportunity to react.

This is my interpretation based on the provided rules and FAQs. No official FAQ exists for this specific interaction between Temporal Portal and Rocket Barrage.