A provider- or protocol-specific HTTP response header expressing limits or preferences about content use; not a universal, automatically enforced AI opt-out mechanism.
An opt-out header is a response header used within a particular policy or protocol to express a preference or reservation about content use. In AI discussions, the term is sometimes used loosely. It does not identify one universally recognized header that every crawler or model provider must interpret in the same way.
Before implementing one, identify the exact specification and intended recipient. For example, the TDM Reservation Protocol describes HTTP response fields for expressing text-and-data-mining reservations. That is a specific Community Group specification, not a universal W3C standard or proof that every AI service supports the mechanism.
A header is also different from access enforcement. Sending a preference with a publicly accessible response does not make the content confidential. The technical effect, provider behavior and any legal implications need separate evaluation; a glossary definition cannot establish a universal right, obligation or guarantee.
Takeaway: Do not install a vaguely named AI opt-out header without identifying its documented meaning and support. Use appropriate access controls when information must actually remain private.
Example
A publisher is offered a plugin claiming to block all AI use through a single header. It asks for the exact header name, specification, supported services and evidence of behavior before relying on the claim. The publisher keeps private customer documents behind authentication regardless of any public-content preference signal.
What to check
Inspect the actual HTTP response to confirm the intended field is present, then check the relevant provider or protocol documentation. Keep technical implementation separate from legal interpretation. Do not assume that a successfully delivered header proves compliance by every recipient.
For more detail, see TDM Reservation Protocol Community Group report.