> ## Documentation Index > Fetch the complete documentation index at: https://docs.darwin.so/llms.txt > Use this file to discover all available pages before exploring further. # AI Usage > Goal-level execution, runners, tools, and skills priced in dollars. AI usage covers billable work Darwin performs for a goal: AI execution rounds, runner time, paid tools, and skills. It appears as one expandable quote line with clear dollar-denominated subcharges. Clarification, scoping questions, quote review, approvals, status checks, and ordinary human input are free. Darwin snapshots the applicable public usage catalog into the approved quote. A later catalog change cannot alter an existing authorization. Usage is captured when billable work runs, even if no seller deal ultimately settles. Darwin reverses usage for Darwin-caused failures or another explicit billing correction. AI usage is not included in the Network-fee base. If expected usage would exceed the approved ceiling, execution pauses with `overage_approval_required`. Learn how reservations, captures, releases, and funding work.