Code change:
The value of the header is encoded as described in encoding. The "Client-Cert" header field is only for use in HTTP requests and <del> MUST NOT be used in HTTP responses.  It is a single HTTP header field value as defined in Section 3.2 of RFC7230, which MUST NOT have a </del> <ins> MUST NOT be used in HTTP responses.  It is a singleton header field value as defined in Section 5.5 of RFC9110, which MUST NOT have a </ins> list of values or occur multiple times in a request. 2.3. In the context of a TLS terminating reverse proxy deployment, the <del> proxy MAY make the certificate chain used for validation of the end- entity certificate available to the backend application with the Client-Cert-Chain HTTP header field.  This field contains certificates used by the proxy to validate the certificate used by the client in the TLS handshake.  These certificates might or might not have been provided by the client during the TLS handshake. </del> <ins> proxy MAY make the certificate chain available to the backend application with the Client-Cert-Chain HTTP header field. </ins> Client-Cert-Chain is a List Structured Header RFC8941.  Each item in the list MUST be a Byte Sequence (RFC8941) encoded as described in <del> encoding. </del> <ins> encoding.  The order is the same as the ordering in TLS (such as described in TLS). </ins> The header's ABNF is:

Related discussion:
text: attempt NAME "simple alternative" in to say the chain is in the same order as it appears in TLS rather than copying the language from TLS
There's a bunch of text in the draft about putting the certificate chain in the right order. This tries to crib from TLS and ends up being somewhat tricky to get right as TLS has a bunch of carefully worded and - frankly - ambiguous rules. As a simple alternative, how about we just state that the certificates need to appear in the same order as the client sent them in TLS.
Makes sense. Will look at adjusting the text accordingly.
I think this is a bit more fundamental than just the order. It's really a question of whether this is a certificate path, or a pile of input certificates to run verification over. That is, either: The origin runs path building, taking the unordered/unverified pile of input certificates from the header, in TLS order. The proxy runs path building and passes the result in the header. The origin assumes a path has been built and is merely applying ACLs based on some aspect of the resulting path. A mismatch here could result in security issues. For instance, suppose the origin wants to only allow chains issued by intermediate certificate I1 to access . If the header means (1) but the origin expects (2), this ACL is trivially bypassable: Client has a certificate chain EE -> I2 -> CA. Client connects to proxy and sends {EE, I2, CA, I1}. (Or perhaps {EE, I2, I1, CA}. Or whatever else. Different verifiers are differently permissive about path-building. Perhaps it omits I2 altogether and the path-builder picks it up some other way.) Even if the proxy runs certificate verification, a path-building verifier will build the EE -> I2 -> CA path and just consider I1 as a random unused certificate. , per the TLS order rule, contains {EE, I2, CA, I1} Origin server checks this and concludes "aha, this was issued by I1 and can access the resource". Conversely, the semantics are (2), the proxy needs to have enough information to validate the certificate, up to a valid trust anchor for the application. If you all intended to punt that to the origin, that's a different protocol.