The sanity.io page writes:

> serving agents a bunch of HTML might just bloat their context window.

That's only true if you assume the the agent can't extract the useful text before it goes into the model as tokens. Your browser's reader mode uses heuristics to identify what the actual content is in a large HTML response and strips away the rest.

To me this is a far better approach than worrying about an llms.txt files or looking at HTTP headers to see if markdown is preferred. Such efforts could easily be directed at ensuring the useful content on your site carries the appropriate markup for an agent or any other tool to extract it. And it would require less work to implement for the publisher of the content.