duckweb_header_value

scalar · LOAD duckweb

duckweb_header_value(VARCHAR, VARCHAR) -> VARCHAR

Reads one header out of the request’s header block.

duckweb_cookie(duckweb_header_value(headers, 'Cookie'), 'theme')

headers is the third argument render receives: every request header line verbatim, exactly as it arrived. The lookup is case-insensitive, per HTTP.

A header that is not present reads as the empty string, not NULL — so a comparison against it is always a comparison, never a three-valued surprise in the middle of a route gate:

WHERE duckweb_header_value(headers, 'X-Duckweb-Client') = ''

is how a branch says “only when this is a real browser navigation, not the client renderer asking for the same route”.