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There is no universal signal a website visitor can check to prove that an AI agent is acting on their behalf. Website operators can inspect request logs for clues such as the User-Agent header and browser request-context headers, but those clues do not prove the client is an AI agent—or that a person authorized it. Cryptographic request signatures can provide stronger evidence of an automated client’s identity, where supported, but still do not establish user authorization.
What a website can—and cannot—tell from a request
Every visit reaches a site as an HTTP request. Site operators can review request logs and headers to learn what software appears to be making a request and how the request relates to other web activity. Those observations can help distinguish patterns, but they are not a reliable way to confirm that an AI agent is involved or that it was instructed by a particular person.
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Two questions must remain separate: Which client or agent made this request? and Which user, if any, authorized it? A technical identity signal may help answer the first. It does not, by itself, answer the second.
Start with the User-Agent, but treat it as a claim
The User-Agent request header is intended to identify the requesting application, operating system, vendor, or version. A value may name a browser, software library, or bot. But a User-Agent string is metadata supplied by the client, not cryptographic proof of identity. It can be altered, combine information associated with multiple browsers, or expose less detail than expected. MDN explains the header’s purpose in its User-Agent header guide and warns that identification based on user-agent strings is unreliable in its browser detection guide.
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For a website operator, a bot-like label is a reason to examine the request in context, not a verdict. Compare the claimed User-Agent with other available log details and observed request behavior. A matching label cannot establish that the client really is that agent, and it cannot establish that a user asked it to act.
Use Fetch Metadata for request context, not identity
Browsers can send Fetch Metadata headers that describe a request’s context. They do not name an AI agent:
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Sec-Fetch-Sitedescribes the relationship between the initiating site and the target. Its values can includesame-origin,same-site,cross-site, andnone.Sec-Fetch-Modedescribes the request mode, whileSec-Fetch-Destdescribes its destination.Sec-Fetch-Usercan indicate user activation for a request when supported. It is not sent for every request and is not supported everywhere.
These browser-generated fields can help a site understand request context and apply security policies. They do not identify an agent, prove that a request came from AI, or demonstrate that a human delegated the action. MDN describes the fields in its Fetch Metadata guide and covers the support limitation for Sec-Fetch-User in its header reference.
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The IETF Internet-Draft HTTP Message Signatures for automated traffic proposes a stronger way for automated clients to identify themselves. It describes signing HTTP requests with HTTP Message Signatures and using a Signature-Agent header to support discovery of the key needed for verification. If a site supports the scheme and successfully verifies a signature and its association with a claimed agent identity, it has stronger evidence about which signing identity made the request than a User-Agent string alone provides.
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The document is an Internet-Draft, not evidence that all agents or websites support the proposal. Its cover identifies revision 00 with an expiry date of 5 March 2027; check the Datatracker entry for a newer revision or current status before relying on it.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A verified agent is not the same as an authorized user
A valid signature can help verify the identity of the agent that signed a request. It does not prove who directed the agent, whether that person consented to this particular action, or whether the agent is still acting within the user’s wishes. The draft addresses agent signing and verification; how a user directs an agent is outside its scope.
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Sites and users should therefore treat agent identity and human authorization as distinct checks. A site that needs to know whether a person approved an action requires an authorization mechanism or evidence appropriate to that action; the agent’s name, headers, or signature alone do not supply it.
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Practical checks for website operators
- Inspect the request logs. Note the User-Agent and available request-context fields, but treat them as clues rather than verified identity.
- Compare signals with observed behavior. Check whether the claimed client, request context, and activity are consistent with one another. No one header settles the question.
- Verify signatures when a supported scheme is available. For a client using Web Bot Auth, validate the message signature and the key’s association with the claimed agent identity rather than trusting its label.
- Handle user authorization separately. Decide what evidence of human permission a sensitive action requires; do not infer permission from successful agent verification.
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