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Site Spy’s detection engine is built on top of changedetection.io, an excellent open source website change detection tool. The core functionality — page fetching, diffing, and change detection — comes from their project. What Site Spy adds on top:
  • Browser extension with a visual element picker
  • Browser-first workflow (install, connect, click what you want to watch)
  • Hosted multi-user infrastructure with per-user API keys
  • Billing and plan management
  • MCP server for AI agent integrations
  • Extended notification system (email, Telegram, browser push)
changedetection.io is actively maintained and free to self-host. If you prefer running your own instance, check out their project directly.