Alt Text Limits on X, Threads, Bluesky, Mastodon, LinkedIn

Every network takes alt text, with limits from 1,000 characters per image on X and Threads to 10,000 on a standard Mastodon server, and none of it counts toward the post's own limit. Most images need one or two sentences: what the image shows that matters to the post, plus any text in it.

Alt text limits on each network

These come from each network's own help pages, API docs or source code, checked on October 19, 2026.

A Mastodon server reports its own limit as description_limit at https://your.server/api/v2/instance. On October 19, 2026, mastodon.social, fosstodon.org and infosec.exchange all reported 10,000. Our Mastodon character limit guide shows how to read that page.

How to add alt text on each network

• X: attach the image, tap +Alt on it, type the description and tap Done. Each image in a post gets its own.
• Threads: Threads writes a description for photos and videos automatically, and you can edit it in the composer before you post. Meta added that in August 2023.
• Bluesky: tap ALT on the image in the composer. For a video, it's Alt text in the app and Captions & alt text on the web. Under Settings, Accessibility, you can turn on Require alt text before posting, and the composer then flags any image, GIF or video that's missing one.
• Mastodon: select Edit on the attachment's thumbnail and fill in the description.
• LinkedIn: after you upload a photo, click ALT to add alternative text.

How to write alt text that helps

Alt text is for people who can't see the image: screen reader users, people on slow connections where images don't load, and anyone who wants the detail. X's own guide adds that on X everyone can read it. The advice from X's help center and WebAIM, the accessibility nonprofit, comes down to a few habits:

1. Say what matters to the post. The same photo needs a different description in a post about the coffee than in a post about the café. WebAIM's guide shows one portrait needing very different alt text depending on how it's used.
2. Write out text in the image. If the image is a screenshot, quote, slide or sign, the text is usually the point. X's guide says to write it out, and to summarize it if it runs past the limit.
3. Skip "image of" and "photo of". Screen readers already announce an image. Do say when it's a screenshot, a close-up or an illustration: X's guide counts that as useful context.
4. Describe, don't judge. X's guide gives the example of writing "smiling, close-lipped into the camera" instead of an opinion about someone's face.
5. Don't repeat the post. If the post names the book, the description doesn't need to.
6. Keep it short, but complete. WebAIM says alt text should be as short as it can be "without sacrificing accuracy". X's guide notes that some screen readers once stopped reading at 200 to 250 characters, and while that's no longer the case, short still reads better.

Examples

All three fit every network's limit, and two are under the 120 characters LinkedIn's API recommends. The chart description gives the numbers, not just the shape, because the numbers are why the chart is there.

Images that need more than a sentence

Some images carry more than a sentence can hold:

• A detailed chart, map or infographic. Describe the trend and the key numbers. X's guide says to describe findings, not the research behind them, and to link the source in the post. WebAIM's advice is the same: put the full data somewhere else, like a table or page.
• A screenshot of a long text. Mastodon's 10,000 fits almost anything. On X and Threads, 1,000 is about a long paragraph, so summarize the part that matters and link the source in the post.
• Several images in one post. Each one gets its own description. Don't describe all four in the first image's alt text.

Alt text on GIFs and video

Video is where the networks differ most:

• X takes descriptions on images only, so a video on X needs its context in the post text.
• Threads takes alt text on videos through its API, and writes one automatically in the app.
• Bluesky takes alt text on GIFs and videos, up to the same 2,000 characters. On the web, the same dialog takes a captions file in .vtt format.
• Mastodon takes a description on any attachment, video and audio included.

Alt text and captions do different jobs. Alt text describes what's on screen, and captions write out what's said, so a video with speech needs both.

Alt text when you post to several networks

The lowest published limit is 1,000 characters, on X and Threads. LinkedIn doesn't publish one for its app, and its API recommends under 120 characters, so if LinkedIn is in the mix, one or two sentences is the safe length.

In Shaflex, the alt text you add to an image goes out with it to Threads, X, Bluesky, Mastodon and LinkedIn, in each network's own alt text field. Our cross-posting guide covers the rest of what changes from one network to the next.

Sources

Checked on October 19, 2026.

• X Help: How to write great image descriptions (1,000 characters, writing advice)
• X Help: How to make images accessible for people (+Alt, one per image, not on videos)
• Threads API docs: Publishing reference (alt_text up to 1,000 characters, images and videos)
• Meta: Introducing Threads (August 9, 2023 update: edit the automatic alt text before posting)
• Bluesky app source: constants (MAX_ALT_TEXT 2,000), image gallery (ALT button), video alt text, accessibility settings (Require alt text before posting)
• AT Protocol lexicons: app.bsky.embed.images and app.bsky.embed.video (alt text has no maximum length)
• Mastodon source: MediaAttachment (MAX_DESCRIPTION_LENGTH 10,000)
• Mastodon docs: Posting toots (Edit on the attachment)
• LinkedIn Help: Share photos on LinkedIn (ALT button)
• LinkedIn API docs: Images API (altText up to 4,086 characters, under 120 recommended)
• WebAIM: Alternative text