Alt text on every social network: support and limits

Alt text is the description of a picture that a screen reader speaks and that shows when the picture does not load. Most networks have a field for it, and they differ in how long it may be, which pictures take it and what happens to text that is too long. A scheduler adds one more question: whether the alt text you write once travels with the picture to every network. This page lists, network by network, whether alt text is supported, how long it can be, and where it is not accepted at all, from what Preflight sends to each network's API, checked on 28 October 2026.

Alt text on every network

Where the table says "sent as written", Preflight does not shorten the text, and the network's own limit decides. Where it gives a number, a longer alt text is cut to that length before sending, so the post goes out instead of failing.

One picture, several networks

In Preflight the alt text belongs to the picture, not to a network. Write it once on the picture and it goes to every network in the post that takes alt text: the same description reaches Instagram, Facebook, Threads, Bluesky, Mastodon, LinkedIn, Pinterest, Tumblr and Discord, each within its own limit. Networks that take none, such as TikTok and Telegram, simply do not receive it, and nothing is refused for that.

The one trap is the shortest limit in the list. A description written for LinkedIn's 4,086 characters is cut at 500 on Pinterest, and one written for Pinterest fits everywhere. If a picture needs a long description, give the key part first, because that is the part that survives the cut.

What a good description does

These are practice, not network rules. A description says what is in the picture and why it is in the post, in the order a listener needs it, and leaves out "image of" and "picture of", which a screen reader announces anyway. Text that is part of the picture should be written out. A purely decorative picture can have a short description or none. Charts need their conclusion in the description, not just their title.

Where alt text is not available

• Reels and stories on Instagram and Facebook. The picture field exists only for feed pictures, so a video or a story gets no alt text. Put the description in the caption or in the video's own captions.
• Video on most networks. Bluesky takes alt text on a video; Threads, LinkedIn and Tumblr take it on pictures only.
• Documents and polls on LinkedIn. They are alone in their post and take none.
• YouTube and TikTok. Preflight sends no alt text to either, for thumbnails or photo posts.

Checking and scheduling

Add the description on the picture in the composer; the limit of each network is applied when the post is sent, from the app, the API and the MCP server alike, and the field is the same in all three. The per-network walkthroughs are Instagram, Threads, Bluesky, LinkedIn, Pinterest and Tumblr, and the rest of each network's limits are on the limits page.