Start a comparison job. Free up to 40 pages a day per client address when the request carries no payment credential and no prepaid key: the response is the normal 201 with sample: true, pagesCharged: 0 and allowanceRemaining, and a job above the remaining allowance, or any request once it is used up, is challenged as usual with nextSteps in the 402 body. A sample report over three pages, produced by this API, is at https://difficat.com/agents/sample/report.json. Then 0.02 USD per compared pair (2 captured pages at 0.01 USD each) for the first 1000 pairs, 0.01 USD per pair beyond; a Pro account’s key draws 2 captured pages per pair from its monthly allowance instead. Compares Site A against Site B on page existence, content and visual regression. Visual captures cover the first 10,000 px of each page; a page cut there carries checks.visual.truncatedAt in the report. The job is priced on the pages it compares: the urls pairs, or the pages discovery finds, at most maxPages when given (an optional ceiling, not a reservation); there is no page limit, and the 402 challenge names the count and the exact amount. A single-page comparison is a urls list with one pair (preferred), or maxPages: 1 with the page URLs as sourceUrl and targetUrl; either is priced as one pair and runs no discovery. The discovery that prices a job without urls is not charged; POST /discover is a separate, optional purchase for callers who want the page list itself. Send dryRun: true to get the page count and exact amount without charging, using the free allowance or starting a job; without a credential or key the quote also carries allowanceRemaining and freeCovers, true when the same body would run free. Set maxPriceUsd to refuse a job priced above your budget before any challenge; a job the free allowance covers costs nothing and is not refused by it. A settled charge is final: cancelling, a failed job or partial results do not reduce it. POST /jobs/recover returns a job or screenshot whose create response was lost, given the same payment credential (a /discover response is not recoverable). A body names at most 20 distinct hosts, and a caller runs at most 3 jobs at once. Then GET /jobs/{id}?wait=<seconds> holds the response until the job ends (at most 300 s), or give callbackUrl to receive one JSON POST when it does; a terminal status read carries summary.issuesBySeverity, the verdict. Read /jobs/{id}/report?failedOnly=true first for the pages that failed, then the rest in pages.