https://app-api.forgeintel.co/api/sdk/v2 by default. Advanced deployments may override either URL; never derive a public URL from an incoming Host header.
Node also offers
describeChallenges, challengeExtension, receiptExtension, injectBody, injectText, rateHint, and flushIntervalMs. Python uses their snake_case equivalents where supported. Read the Node package README or Python package README for the full beta API.