Threads made an interesting decision: one topic tag per post, rendered as a link rather than an inline hash. It is a direct response to what tag stacking did to Instagram, and it forces a genuine choice.
Choosing the one
With a single slot, the calculus is simple — pick the most specific tag that still has an active audience. Broad tags waste the slot entirely, because your post will be buried within seconds. Ultra-specific tags with no readership waste it too.
The sweet spot is a tag where you can open it, scroll, and see posts from the last few hours that look like conversations rather than broadcasts.
Topic tags behave like rooms
Because there is only one per post, Threads tags function much more like forum topics than like classification labels. People genuinely browse them. That means the social norms of the tag matter — a promotional post in a discussion-oriented tag stands out badly.
Multi-word tags are allowed
Threads permits spaces in topic tags, which is unusual and useful. You can use a readable phrase rather than a compressed compound, and readable phrases get typed into search more often.
What actually drives reach
Replies. Threads weights conversation heavily and rewards posts that generate back-and-forth. The single tag helps the right people find you; whether they reply is down to whether you gave them something to reply to.
Cross-posting from Instagram
Resist it. Content written for a visual feed reads flat in a text-first conversation platform, and a caption full of Instagram tags cannot even carry over. Write for Threads or skip it.