Watch one public page for 7 days. Checks hourly. When the content hash changes, POST {watch_id,url,old_fingerprint,new_fingerprint,changed_at} to your https callback (5s timeout, one retry). POST JSON {"url":"https://example.com","callback_url":"https://example.com/hook"}. Returns {watch_id,url,expires,status_url}. Free status is GET /v1/watch/:id and does not show the callback. At 500 active watches, POST returns 503 before payment. $0.10 USDC on Base (eip155:8453). No API key.

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