Prompt injection in HTTP headers: what 217M web responses show

tl;dr We read the HTTP response headers of 217,097,223 page responses from Common Crawl, 10% of its September 2026 crawl of 2.17 billion pages, and looked for any header written for an AI system. 7,959 sites sent one. Almost all of them point agents to another resource, such as an llms.txt file or an MCP server. A small number try to give the agent orders. Whether any of it reaches your model depe...