AI video clipping for social media: what to clip and where to post it

TL;DR

• An AI video clipping agent turns one long video into short clips, ranks the moments, adds captions and sends each clip to the platforms you connect.
• Clip moments that make sense without the rest of the video: a strong take, a complete answer, a story with a payoff, a number, a demo or a reaction.
• Limits differ: X caps non-Premium videos at 140 seconds, YouTube Shorts and TikTok run to 3 minutes, Instagram recommends Reels of 3 minutes or less to new audiences, and LinkedIn takes up to 15 minutes.
• A vertical 9:16 clip fits the aspect ratio rules on all six platforms, but length, captions, copy and schedule still change per platform.
• Marketing agencies, media teams and creator agencies can run this across many accounts, or build their own version on the FastPix API.

One strong moment from a webinar, podcast or keynote can run on YouTube Shorts, Instagram, Facebook, LinkedIn, X and TikTok. Each app handles it differently: X stops non-Premium uploads at 140 seconds, Instagram warns that a third-party watermark may cut a Reel's reach, and every app lays its buttons over a different part of the frame.

Posting one file everywhere wastes the moment. An AI video clipping agent finds the moments worth posting, and a few platform-specific choices turn each clip into five native formats.

How AI video clipping works for social media distribution

The FastPix AI video clipping agent runs the long-to-social job in one project:

1. Finds and cuts the moments that stand on their own, each with an engagement score out of 100, with detection tuned by genre.
2. Applies a caption style to every clip, transcribed in the video's spoken language.
3. Generates vertical 9:16 clips, with the aspect ratio adjustable per clip.
4. Drafts a title and description for each post with AI Copy Prompt.
5. Publishes or schedules to YouTube, Instagram, Facebook, LinkedIn and X, with every post on one calendar.

People keep the decisions that matter for reach: which clip, which platform, which words and when it goes out.

What to clip: moments that work on every platform

A social clip has to make sense to someone who never saw the full video. These six moment types hold up on their own:

In the agent, set the genre to match the source, such as Podcasts, Educational / Webinar, News or Sports, and name the moments you want in the Specific moments prompt, for example "the guest's strongest opinion" or "product demos with a visible result." Sorting by engagement score puts the strongest candidates first. Moments that lean on earlier context, like "as I mentioned earlier," belong in the long video.

What to post where: social media video specs by platform

The specs below come from each platform's help center, developer and ads documentation, and announcements, checked in September 2026.

YouTube Shorts

YouTube treats square or vertical videos up to 3 minutes as Shorts, and a Short over 1 minute with an active copyright claim is blocked worldwide. Complete answers and demos suit Shorts, and when the full video is on your channel, a related video link sends viewers to it. The guide to turning long videos into YouTube Shorts covers every step.

Instagram Reels

Instagram recommends Reels of 3 minutes or less to people who don't already follow you, and a 9:16 frame avoids cropping or blank space. The same creator guidance says a third-party watermark "may be impacting" a Reel's reach, and under Instagram's originality rules, accounts that mainly re-upload other people's work, even with subtitles or a credit added, stop appearing in recommendations. Post the clean file cut from your own video. Stories with a payoff, demos and reactions fit here.

Facebook Reels

In June 2025, Meta announced that all Facebook videos would be shared as reels, rolling out gradually, with no length or format restrictions in the app. Publishing from outside the app works differently: Meta's Reels publishing API takes 9:16 video between 3 and 90 seconds, posted to a Facebook Page. A vertical clip under 90 seconds covers both routes.

LinkedIn video

LinkedIn accepts videos from 3 seconds to 15 minutes, up to 5 GB, in aspect ratios from 1:2.4 to 2.4:1, and asks you to keep text and logos away from the edges of the frame. You can't post straight into LinkedIn's vertical video feed, but any video can appear there based on relevance and performance, and LinkedIn can generate captions automatically. Strong takes, complete answers and numbers suit it best.

X video

Without Premium, X accepts videos up to 140 seconds and 512 MB, in aspect ratios from 1:2.39 to 2.39:1. Premium accounts can upload videos up to 4 hours long and up to 16 GB. X lists a maximum resolution of 1200x1900 for vertical video. Keep the X version to the strongest stretch of the clip, with the take or the number in the first line of the post.

TikTok

TikTok lets every creator post videos up to 3 minutes, with 5- and 10-minute posting open to some accounts, and recommends a 9:16 frame. Under TikTok's For You feed standards, reused content showing someone else's watermark or logo isn't eligible for recommendation. From the AI video clipping agent, select Download HD and upload the file in TikTok, so the video carries only your own captions and branding.

One clip, five formats: adapting a single clip for every platform

Across these six platforms, one clip needs five formats: a YouTube Short, a Reel for Instagram and Facebook, a TikTok, a LinkedIn video and an X video. Four things change between them.

Length

Cut the master clip to the tightest limit you plan to use. A clip between 45 and 90 seconds fits the length limits on all six platforms, including X without Premium and Meta's Reels publishing API. Cuts up to 3 minutes still work on YouTube Shorts, Instagram and TikTok, and LinkedIn takes longer segments. In the agent, pick the 30 to 59 second or 60 to 89 second Clip length option before generating.

Frame and aspect ratio

Vertical 9:16 sits inside every platform's aspect ratio rules and is the default for clips from the agent, so one frame covers Shorts, Reels, TikTok and Facebook. LinkedIn and X also accept square and landscape video. Change the clip's aspect ratio in its details when a platform needs a different frame, and subject-aware reframing keeps the speaker in shot. [VERIFY: AI Reframe is beta; confirm the ratios available in the agent] For slide-heavy talks and screen demos, a square or landscape version keeps detail readable on LinkedIn and X, and the FastPix guide to video aspect ratios covers the trade-offs.

Captions and safe zones

Every app lays buttons, usernames and post text over the edges of a vertical video. Meta's Reels ads guidance puts numbers on it: leave at least 14% of the top, 35% of the bottom and 6% of each side free of text and logos. LinkedIn gives the same advice for organic video without percentages. Preview each clip at 9:16 with its caption style applied, and check that no caption line lands under the like and share buttons. LinkedIn, Meta and X all advise building video for viewers watching with the sound off, so captions are part of the clip, not an extra.

Copy, originality and a staggered schedule

Write the words for each platform separately. A LinkedIn post can carry a one-line takeaway and a link to the full video, an X post needs the take in its first line, and on Instagram and TikTok the on-screen captions carry the message while the post caption supports it. AI Copy Prompt drafts a title and description for each post, or you can write your own.

Post from your own source file every time. Instagram says a third-party watermark may limit a Reel's reach, and TikTok keeps reused clips showing someone else's watermark out of the For You feed, so upload each version natively instead of downloading a TikTok and re-posting it as a Reel.

Stagger the schedule instead of posting everywhere at once, and let the calendar show what has gone out and what is queued.

How to publish one clip to five platforms with an AI video clipping agent

The AI video clipping agent docs cover every setting. For social distribution, the flow runs in six steps.

Step 1: Generate clips from the long video

Create a project, upload the source or paste a link from Google Drive, Twitch, Instagram, Facebook or X, and trim it to the part worth clipping. Set the genre, clip length and Specific moments prompt, choose a caption style and select Generate clips.

Step 2: Pick the clip and set its frame

Sort by engagement score, open the strongest clip and read its summary and transcript to confirm it stands alone. Change the clip's aspect ratio in its details when a platform needs a different frame. Clips can also render in 9:16, 1:1, 4:5 and 16:9 in parallel. [VERIFY: marketing page only]

Step 3: Polish the clip in the AI editor

Select Edit Clip to trim the clip to each platform's length, move and arrange segments so the hook lands first, add transitions between cuts, and drop in B-roll where the speaker describes something the viewer should see. [VERIFY: AI Editor and B-roll are beta; confirm controls in the agent]

Step 4: Connect the accounts

Under Connect account, add a YouTube channel, a Facebook Page, an Instagram account, a LinkedIn account and an X account. Instagram only allows publishing from third-party tools to professional (creator or business) accounts. You can connect more than one account per platform, which lets an agency run several brands from one FastPix account.

Step 5: Write the copy for each platform

Publish the clip as a separate post per platform and use AI Copy Prompt to draft the title and description, or write your own, so each one fits: the takeaway first for LinkedIn and X, a short hook for Instagram and Facebook, and a title that matches the spoken hook for YouTube Shorts.

Step 6: Schedule each post and track it on the calendar

Choose Schedule for later with a date and time for each platform, and the Calendar shows Published and Scheduled posts by week or month. Turn on Auto-save so the project stays available past its 7-day window while the schedule runs. For TikTok, select Download HD and upload the file in TikTok.

AI video clipping for marketing agencies, media teams and creator agencies

Short clips get a campaign, a story or a creator found, and the full video is where that attention pays off. Three kinds of teams run this at volume.

Marketing and digital marketing agencies

Agencies turn a client's webinar, launch keynote or podcast into a run of clips built to trend, each one pointing viewers back to the full video. With several accounts connected per platform, one team schedules clips for many clients, tracks them on the calendar and keeps each client's caption style saved in My Templates. The full video can play on the client's own landing page in the FastPix Player, so the traffic the clips send lands on a page the client owns.

Media and publishing teams

Newsrooms and media brands clip the quote, the reveal or the key play, post it to social fast, and send viewers to the full video on their own site or app, where their ads and subscriptions run. For live coverage, FastPix can clip a moment from a live stream while the broadcast is still running.

Creator agencies

Agencies that manage creators run the same workflow across a roster: a project for each upload, each creator's accounts connected and each creator's caption template saved, so a creator's clips look like that creator on every platform. Brand templates add each creator's watermark, font and intro card automatically. [VERIFY: marketing page only]

Build a custom clipping workflow with the FastPix API

Agencies and media teams that want their own tools, such as a client approval screen, a creator portal or a newsroom desk, can build them on the FastPix API:

• Clips from any timestamp. Send a source media ID with start and end times, and each clip comes back as a new asset with its own playback ID, playable in your own player once it's ready.
• Every clip from one source. List all clips cut from a video, with duration, status, thumbnail and playback IDs, to power a review dashboard.
• Events on a webhook. video.media.ready fires when a clip is playable, and video.media.live_clip.created fires when a clip is cut from a live stream.
• AI clipping on upload. Trigger the AI video clipping agent from your own pipeline and receive ranked clips on a webhook. [VERIFY: marketing page only; docs cover the dashboard workflow]
• Full-video analytics. FastPix Video Data reports how viewers watch the full video once the clips send them there, through the API and dashboard.

curl -X GET 'https://api.fastpix.com/v1/on-demand/{MEDIA_ID}/media-clips' \
  --user '{ACCESS_TOKEN_ID}:{SECRET_KEY}'

Conclusion

A long video usually holds more social posts than a team has time to cut by hand. An AI video clipping agent finds the moments that stand alone, and a few choices on length, frame, captions and copy turn each one into five native formats across YouTube Shorts, Instagram, Facebook, LinkedIn, X and TikTok. Agencies and media teams run it at volume, with the FastPix API underneath when they want their own tools.

The simplest place to start is a video you already have. The AI video clipping agent runs on the Starter plan, $10 a month with $25 of usage credits included each month, on top of a free FastPix account, and the ranked clips from that first video show which moments are worth posting where.