Direct answer
Each AI assistant reads the web through its own crawler or a partner index: ChatGPT uses OAI-SearchBot plus partner search providers, Claude searches with Brave, Copilot uses Bing, Gemini and AI Overviews use Google, and Perplexity runs its own index. If robots.txt or a firewall blocks one of these crawlers, that assistant cannot read or cite your pages. This checker requests your pages as each crawler and reads your robots.txt rules for each one.