%:url and $:title are wrong, they should be %:link and %:description
respectively.
Check out the source code of org-protocol-do-capture.
The valid placeholders are
type
link
description
annotation
I don't use org-protocol, but I'd just like to understand how you derived that.
Source code:
(defun org-protocol-do-capture (info)
"Perform the actual capture based on INFO."
(let* ((temp-parts (org-protocol-parse-parameters info))
(parts
(cond
((and (listp info) (symbolp (car info))) info)
((= (length (car temp-parts)) 1) ;; First parameter is exactly one character long
(org-protocol-assign-parameters temp-parts '(:template :url :title :body)))
(t
(org-protocol-assign-parameters temp-parts '(:url :title :body)))))
(template (or (plist-get parts :template)
org-protocol-default-template-key))
(url (and (plist-get parts :url) (org-protocol-sanitize-uri (plist-get parts :url))))
(type (and url (if (string-match "^\\([a-z]+\\):" url)
(match-string 1 url))))
(title (or (plist-get parts :title) ""))
(region (or (plist-get parts :body) ""))
(orglink (if url
(org-make-link-string
url (if (string-match "[^[:space:]]" title) title url))
title))
(org-capture-link-is-already-stored t)) ;; avoid call to org-store-link
(setq org-stored-links
(cons (list url title) org-stored-links))
(org-store-link-props :type type
:link url
:description title
:annotation orglink
:initial region
:query parts)
(raise-frame)
(funcall 'org-capture nil template)))
From that, we have:
(org-store-link-props :type type
:link url
:description title
:annotation orglink
:initial region
:query parts)
but that is an internal call.. :link key gets its value from let-bound url and :description gets its value from let-bound title.
And further up, url gets assigned value from a plist with key :url:
(url (and (plist-get parts :url) (org-protocol-sanitize-uri (plist-get parts :url))))
and title gets assigned value from a plist with key :title.
So that seems to match the documented: "The template refers to the data through %:url and %:title placeholders.".
So I am surprised why %:link and %:description worked..
I didn't dig deeper as to where the connection with org-protocol and org-capture happens.