The endpoint /z also allows you to determine a tag zettel, i.e. a zettel that documents a given tag.
The query parameter tag allows you to specify a value that is interpreted as the name of a tag. Zettelstore tries to determine the corresponding tag zettel.
A tag zettel is a zettel with the role value tag and a title that names the tag. If there is more than one zettel that qualifies, the zettel with the highest zettel identifier is used.
For example, if you want to determine the tag zettel for the tag #api, your request will be:
# curl -i 'http://127.0.0.1:23123/z?tag=%23api'
HTTP/1.1 302 Found
Content-Type: text/plain; charset=utf-8
Location: /z/00001019990010
Content-Length: 14

00001019990010
Alternatively, you can omit the # character at the beginning of the tag:
# curl -i 'http://127.0.0.1:23123/z?tag=api'
HTTP/1.1 302 Found
Content-Type: text/plain; charset=utf-8
Location: /z/00001019990010
Content-Length: 14

00001019990010
If there is a corresponding tag zettel, the response will use the HTTP status code 302 (Found), the HTTP response header Location will contain the URL of the tag zettel. Its zettel identifier will be returned in the HTTP response body.
If you specified some more query parameter, these will be part of the URL in the response header Location:
# curl -i 'http://127.0.0.1:23123/z?tag=%23api&part=zettel'
HTTP/1.1 302 Found
Content-Type: text/plain; charset=utf-8
Location: /z/00001019990010?part=zettel
Content-Length: 14

00001019990010
Otherwise, if no tag zettel was found, the response will use the HTTP status code 404 (Not found).
# curl -i 'http://127.0.0.1:23123/z?tag=notag'
HTTP/1.1 404 Not Found
Content-Type: text/plain; charset=utf-8
Content-Length: 29

Tag zettel not found: #notag
To fulfill this service, Zettelstore will evaluate internally the query role:tag title=TAG, there TAG is the actual tag.
Of course, if you are interested in the URL of the tag zettel, you can make use of the HTTP HEAD method:
# curl -I 'http://127.0.0.1:23123/z?tag=%23api'
HTTP/1.1 302 Found
Content-Type: text/plain; charset=utf-8
Location: /z/00001019990010
Content-Length: 14
HTTP Status codes
302
Tag zettel was found. The HTTP header Location contains its URL, the body of the response contains its zettel identifier.
404
No zettel for the given tag was found.