# Resource Cost Reduction stacks the same diminishing-returns way as Cooldown Reduction

Multiple Resource Cost Reduction sources combine multiplicatively (inverse-multiplicative, same shape as CDR), giving strong diminishing returns - not simple addition, despite the affix reading like a plain percentage.

Diablo IV patch 3.2.1. Category: breakpoint. Evidence: Likely.

## Detail
This resolves a gap the project's own damage_model.json had flagged as unconfirmed for 3.2.1: independent research now shows Resource Cost Reduction explicitly stacks the same way as CDR.

## Why it works
Each new Cost Reduction source multiplies the REMAINING resource cost after prior reductions, rather than subtracting a flat percentage from the original cost - identical mechanical shape to Cooldown Reduction.

## How to use it
Do not assume two 15% Resource Cost Reduction rolls give a flat 30% cheaper skills; expect noticeably less, and prioritize one large source over several small ones exactly as with CDR.