Complete multiple org-mode tags with helm

helm-org-tag-completion.el

;;;;;; Fix Helm org tag completion
;; From Anders Johansson <https://groups.google.com/d/msg/emacs-helm/tA6cn6TUdRY/G1S3TIdzBwAJ>

;; This works great!  He posted it on 3 Mar 2016, on a thread that was
;; started in Oct 2013.  He also posted this message on 2 Apr 2014,
;; maybe an earlier attempt at a solution:
;; <http://article.gmane.org/gmane.emacs.orgmode/84495> I've just
;; tidied it up a bit and adjusted the prompt.

(add-to-list 'helm-completing-read-handlers-alist '(org-capture . aj/org-completing-read-tags))
(add-to-list 'helm-completing-read-handlers-alist '(org-set-tags . aj/org-completing-read-tags))

(defun aj/org-completing-read-tags (prompt coll pred req initial hist def inh)
  (if (not (string= "Tags: " prompt))
      ;; Not a tags prompt.  Use normal completion by calling
      ;; `org-icompleting-read' again without this function in
      ;; `helm-completing-read-handlers-alist'
      (let ((helm-completing-read-handlers-alist (rassq-delete-all
                                                  'aj/org-completing-read-tags
                                                  helm-completing-read-handlers-alist)))
        (org-icompleting-read prompt coll pred req initial hist def inh))
    ;; Tags prompt
    (let* ((initial (and (stringp initial)
                         (not (string= initial ""))
                         initial))
           (curr (when initial
                   (org-split-string initial ":")))
           (table (org-uniquify
                   (mapcar 'car org-last-tags-completion-table)))
           (table (if curr
                      ;; Remove current tags from list
                      (cl-delete-if (lambda (x)
                                      (member x curr))
                                    table)
                    table))
           (prompt (if initial
                       (concat "Tags " initial)
                     prompt)))
      (concat initial (mapconcat 'identity
                                 (nreverse (aj/helm-completing-read-multiple
                                            prompt table pred nil nil hist def
                                            t "Org tags" "*Helm org tags*" ":"))
                                 ":")))))

(defun aj/helm-completing-read-multiple (prompt choices
                                                &optional predicate require-match initial-input hist def
                                                inherit-input-method name buffer sentinel)
  "Read multiple items with `helm-completing-read-default-1'. Reading stops
when the user enters SENTINEL. By default, SENTINEL is
\"*done*\". SENTINEL is disambiguated with clashing completions
by appending _ to SENTINEL until it becomes unique. So if there
are multiple values that look like SENTINEL, the one with the
most _ at the end is the actual sentinel value. See
documentation for `ido-completing-read' for details on the
other parameters."
  (let ((sentinel (or sentinel "*done*"))
        this-choice res done-reading)
    ;; Uniquify the SENTINEL value
    (while (cl-find sentinel choices)
      (setq sentinel (concat sentinel "_")))
    (setq choices (cons sentinel choices))
    ;; Read choices
    (while (not done-reading)
      (setq this-choice (helm-completing-read-default-1 prompt choices
                                                        predicate require-match initial-input hist def
                                                        inherit-input-method name buffer nil t))
      (if (equal this-choice sentinel)
          (setq done-reading t)
        (setq res (cons this-choice res))
        (setq prompt (concat prompt this-choice ":"))))
    res))

@clemera Thanks. For my own use, I'm satisfied with the code in this gist, but maybe I'll look at the CRM version again sometime.

As this is currently one of the top search results for 'org-mode helm select tag multiple', I'd like to point to

emacs-helm/helm-org#3

The mentioned fix works for me on GNU Emacs 27.0.50 and Org 9.3.6:

(require 'helm-org)
(add-to-list 'helm-completing-read-handlers-alist '(org-set-tags-command . helm-org-completing-read-tags))