Anthropic is the more common of the two in our index: it runs on 5,931 websites against 3,842 for OpenAI, and 2,332 sites use both. Counts come from real scans, not vendor claims.
Anthropic is an AI safety company that develops Claude, a family of large language models, and provides an API for building AI-powered applications. The anthropic-domain-verification TXT record confirms domain ownership for Anthropic services.
OpenAI provides the GPT family of models and the ChatGPT product line.
2,332 sites run both Anthropic and OpenAI. That is 61% of the sites using OpenAI, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.