Boom -- I got it.

I used helm's emacs-helm.sh and got to the "minimal" config necessary
to run helm.  The org-set-tags-command worked correctly when running
in the minimal config.

I moved some initialization code from my org-mode-config.org file
to my init.el file and I got helm to work with org-set-tags-command
correctly!

init.el

(setq package-load-list '((helm-core t) (helm t) (async t) (popup t)))
(package-initialize)
(require 'helm-config)
(helm-mode 1)






On Fri, Jul 26, 2019 at 4:47 PM Nathan Neff <nathan.neff@gmail.com> wrote:
Hello all,

I would like to use helm to select tags for my headings,
and I'm having a tough time getting something to work.

I would like the following behavior:

1) org-set-tags-command would show a list of all tags
in my agenda files using the helm interface

2) I could set one tag for each call to org-set-tags-command.  I've 
seen posts that want to be able to multi-select tags, but I don't want to
over-complicate things.

Currently, I'm using helm-core 2019-05-27 and org-mode
9.2.3

I have this set:
;; Always offer all agenda tags in capture and in normal operations
(setq org-complete-tags-always-offer-all-agenda-tags 't)

org-set-tags-command works fine if a headline does *not* have a tag.  I get helm
mode with all of the tags from my agenda-files shown.  However, after I assign
a tag to a headline, I no longer see the list of tags when I call org-set-tags-command.  Instead I see: "Tags: :currtag:" prompt and no suggestions from helm.  If I remove the tag, then my
tags show up again in helm, but selecting another tag will then *replace* the current tag
which is not what I want.

Any ideas?

I have removed all #+TAGS: "fast selection" from my org-files.

I have this code on my machine, but it appears not to be working:

Thanks,
--Nate