Per-Platform Captions and Viral Hashtags: One AI Pass, Five Platforms, Each Within Its Limits

To write a different caption for each social platform without writing five captions, ReelMint reads the video once and writes the X post, the TikTok caption, the Instagram caption, the Facebook post and the YouTube title, description and tags in one AI pass, then fits each to that platform's real limits. "Find viral hashtags" is an optional second step, 5 credits, that researches what is ranking on your topic now and spreads the tags across those destinations.

We build ReelMint, an AI video studio that publishes to TikTok, YouTube, Instagram, Facebook and X. Pasting one caption everywhere is the most common way a cross-posted video underperforms, so this guide shows what each platform gets and why.

Why one caption does not fit five platforms

The limits differ, but the bigger difference is idiom. An Instagram caption whose first line is the only thing shown before "more" is a different piece of writing from an X post that has to land in one line, and YouTube's tags are search phrases, not hashtags. ReelMint gives each destination its own tab on the Share screen, and the writer fills each tab in that platform's style.

What each platform gets

X

• Hard limit: 280, counted the way X counts (emoji and CJK weigh 2, any link 23).
• Written to: one punchy post of at most about 240 characters including hashtags, a hook rather than a summary, no thread, no trailing ellipsis.
• Hashtags: 1 to 3, at most 3.

TikTok

• Hard limit: 2,200 characters including hashtags.
• Written to: one or two short lines, at most about 150 characters before the hashtags, the way TikTok creators talk, ending on a hook or a question.
• Hashtags: 3 to 6, one broad discovery tag plus niche ones.

Instagram

• Hard limit: 2,200 characters including hashtags, at most 25 tags.
• Written to: 300 to 900 characters before the hashtags. The first line is the hook, then two to four short paragraphs, then a call to action (save, share, follow, comment).
• Hashtags: 10 to 20, mixing broad, mid-size and niche community tags.

Facebook

• Hard limit: 5,000 characters.
• Written to: conversational and complete, since readers see the whole text; context and an invitation to discuss.
• Hashtags: 2 to 5 at most, because Facebook reach barely responds to them.

YouTube

• Hard limits: title 100 characters, description 5,000.
• Written to: a specific, searchable title with the key words near the front, no all caps and no clickbait the video cannot keep; a description whose first line is the hook, then what the video is about, then a call to action.
• Tags: 8 to 15 multi-word search phrases. These fill YouTube's tags field, not the caption.

How the fitting works

After the writer, a fitting step enforces each platform's limits. It counts characters, not bytes, so an Arabic or emoji caption is never cut through the middle of a character. Hashtags are de-duplicated and capped per platform, your own standing tags stay first, and when the caption plus tags is over the limit, whole hashtags are dropped before any words are cut, and words are cut on a word boundary. For X, the count is X's weighted count, and a link is never cut in half.

If the AI writer is unavailable, the generic caption is cut to each platform's limits instead, so a publish is never blocked waiting on copy. And you can always edit any tab by hand before you press Share.

How to use it on a manual post

1. Open the video in your Library and press Share.
2. Tick the destinations. Each gets a tab.
3. Press AI caption (on YouTube, AI title + description). Each run is 1 credit, and you can add a short brief, such as "playful, mention the Lisbon trip", to steer it.
4. Edit anything you like, then publish or schedule each destination.

Find viral hashtags: what the 5 credits buy

The amber Find viral hashtags button runs one research pass:

• It takes the topic from what the post already knows: the YouTube title if there is one, otherwise the caption, otherwise the file name. You can type a topic instead.
• It reads the top public YouTube videos on that topic from the last 30 days and ranks them by view velocity (views per day since posting), so a fast-rising video outranks an old one with more total views.
• An AI pass turns that evidence into tags shaped for each platform you selected.
• The results are spread across your destinations in each platform's way: YouTube gets multi-word phrases in its tags field, TikTok gets them appended to the caption, and the others get them as hashtags.
• The source videos are shown underneath, so you can see why each tag was suggested.

It costs 5 credits ($0.02 at one credit = $0.004), more than a 1-credit caption because it is a live search plus a writing pass, not just a writing pass. You are charged only once the request can actually run: if there is nothing to research, you see a message and pay nothing.

In Bot Studio

Every bot firing writes this same per-platform copy from what it generated, and posts each destination its own version. Hashtag research is also available to bots at the same 5 credits per pass, and appears as its own line in the bot's quote. See autopilot and the bot guide.

What it costs

AI caption: 1 credit a run. Viral hashtags: 5 credits a pass. Publishing: no credits, except an X post at 2. Plans start at Lite, $9.99 a month for 800 credits, and every plan runs every AI model. For what credits buy elsewhere, see what credits buy.

Does ReelMint write a different caption for each platform?

Yes. One pass writes the X post, TikTok caption, Instagram caption, Facebook post and YouTube title, description and tags, each in its platform's style and within its limits.

How many hashtags does each platform get?

X 1 to 3, TikTok 3 to 6, Instagram 10 to 20, Facebook 2 to 5, and YouTube 8 to 15 search phrases in its tags field.

Where do the viral hashtags come from?

From the top public YouTube videos on your topic in the last 30 days, ranked by views per day, then shaped per platform. The source videos are listed under the results.

Can I edit the AI captions?

Yes. Every tab is an editable box. The AI fills it; you decide what goes out.