How does deflect work as a cost, and when is it paid for spells and reflexive triggers?

Ruling: Deflect is an additional cost paid when putting a spell or ability on the chain, not on resolution. You pay the full cost (base + deflect) all at once when casting.

Sequence:

• When casting a spell or putting a trigger on the chain, you pay both the base cost and deflect cost together
• For reflexive triggers, you can target a unit with deflect and then decline to pay the deflect cost
• If you decline the deflect cost, the trigger does not enter the chain

Nuances:

• At competitive REL, if deflect cost is not paid, it's a Mispayment for Costs error - rewind to before the card was played
• If rewinding is not possible because things already resolved, correct the underpaid cost by using runes as needed
• You are not forced to target non-deflect units; you can choose deflect units and decline to pay
• For "when I am targeted" effects, there is uncertainty - one interpretation is they would not trigger if deflect is declined since abilities never finalize into the chain, but another view is targeting is already complete when you decline the cost so the trigger should already be pending