Where the LinkedIn Alt Text Character Limit Is Actually Written

What is the LinkedIn alt text character limit?

Two pages in LinkedIn's API reference set the LinkedIn alt text character limit at 4,086 characters, and both add a recommendation to stay under 120. The Images API schema row for altText reads, word for word: "The alternate text of this thumbnail. Used for screen reader accessibility. Maximum length is 4,086 characters, recommended length is less than 120 characters."

That is the whole rule. One hard ceiling, one soft target, and a gap of nearly four thousand characters between them.

The field travels with the post body, not with the upload. You attach it inside content.media when you create the post:

POST https://api.linkedin.com/rest/posts
Linkedin-Version: 202608
X-Restli-Protocol-Version: 2.0.0

{
  "author": "urn:li:organization:5515715",
  "commentary": "test strings!",
  "visibility": "PUBLIC",
  "content": {
    "media": {
      "altText": "testing for alt tags",
      "id": "urn:li:image:C5610AQFj6TdYowm17w"
    }
  },
  "lifecycleState": "PUBLISHED"
}

The id there is the image URN you get back from the initializeUpload call that registers an image upload. Alt text and image binary arrive on LinkedIn through two different calls, and only the second one carries the description.

Recommended versus maximum

Recommended length120 characters

Maximum length4,086 characters

Which LinkedIn pages publish a limit, and which stay silent?

Four places document altText or its equivalent. Two carry the number. Two do not.

Page	Field	What it says about length
Images API schema	altText	"Maximum length is 4,086 characters, recommended length is less than 120 characters"
MultiImage API schema	altText	The identical sentence, word for word
Post API Schema, Media record	altText	"The alternate text for the media. None if empty. Optional." No limit
LinkedIn Help, alt text for accessibility	The Alt.text box	No character count published

That third row is the one that trips people up. The Posts API is where you actually send the field, and the schema page for the Posts API describes altText without a ceiling. If you read only the page that matches the call you are making, you come away believing the field is unbounded. The number lives on the Images API page, which is the page about uploading, not about posting.

The MultiImage repeat matters for a different reason. The 4,086 number sits on the MultiImage record's own altText field, the one description that covers the whole set. Beside it runs an images array with a "Minimum of 2 images and maximum of 20 images," and every entry in that array is a Media record carrying an altText of its own, documented on the Post API Schema page with no ceiling at all.

What does the LinkedIn API return when altText is too long?

LinkedIn does not publish the answer. That is the honest version, and it is worth saying plainly rather than guessing.

Here is what the documentation does contain. The Posts API error table lists a length error:

HTTP status	Error code	Documented description
400	FIELD_LENGTH_TOO_LONG	"{field} length exceeds the allowed maximum. Reduce the length of the commentary or other text fields."

The template names commentary and then waves at "other text fields." It never names altText. Meanwhile the Images API has its own error table, and every entry in it is about identity rather than length: INVALID_IMAGE_ID ("This Image ID is invalid"), INVALID_URN_ID, INVALID_URN_TYPE and NOT_FOUND, all at 400. No length error appears there at all.

There is a third error page, the Marketing API error response reference, which spells out FIELD_LENGTH_TOO_LONG as "{field} length {length} can't exceeded maximum {max} length" with the field, length and max filled in. That page scopes itself in a note: the standardized schema is "available for adCampaignGroups, adCampaigns and adAccounts endpoints only." So the one error message precise enough to quote comes from endpoints that have nothing to do with images.

Build your composer to validate at 4,086 on your side. The alternative is discovering LinkedIn's actual response in production, and no page tells you in advance what it looks like.

Does the LinkedIn app enforce the same limit?

LinkedIn publishes no character limit for the alt text box in the app. The help article "Add alternative text to images for accessibility" walks through both flows and never mentions a count: on desktop you "click on Alt.text below the image to write a description of the image/photo," on mobile you "tap ALT below the image." That article was last updated three years ago.

The same article adds a behaviour the API has no equivalent for: "LinkedIn may automatically add alt-text to images that don't have it. When uploading an image from a desktop computer, you'll be alerted if alt-text is automatically assigned. You won't be alerted if you're uploading an image from a mobile device."

The advertising side is the sharper example of the silence. LinkedIn's single image ads specification page publishes a character maximum for four separate fields: the ad name at 255 characters, introductory text at a 3,000 character maximum with 150 recommended to avoid truncation, the headline at a 200 character maximum with 70 recommended, and the description at a 300 character maximum with 100 recommended. The row immediately below those reads "Image alt text (optional)" and describes what alt text is for, with no number attached.

So on one table LinkedIn commits four limits in writing and skips the fifth. Anyone quoting an in-app alt text figure is quoting something LinkedIn has not published.

How does LinkedIn's alt text limit compare to Instagram's?

Four times longer, and scoped differently.

Platform	Field	Documented maximum	Recommended	Scope
LinkedIn	altText	4,086 characters	Under 120	Image media on posts and multi-image posts
Instagram	alt_text	1,000 characters	None published	Single images and images in a carousel

Meta's number and its exclusions are covered in detail in the breakdown of Meta's 1,000 character alt_text limit on Instagram, including the reels and stories carve-out, so there is no point repeating it here. The structural difference is what matters: Meta publishes a ceiling and no recommendation, LinkedIn publishes both, and neither publishes a limit for the field their own app shows to ordinary users.

If you are writing one description and pushing it to several networks through a workflow that posts to multiple accounts at once, the binding constraint is whichever platform documents the smallest number. For a LinkedIn and Instagram pair, that is 1,000.

How long should LinkedIn alt text actually be?

Under 120 characters, by LinkedIn's own recommendation, and the 4,086 ceiling exists for the images where 120 is not enough.

That is the useful way to read the two numbers together. A photograph of two people at a conference needs a sentence. A screenshot of a dashboard, a chart with labelled axes, a price table, an event schedule, a slide with dense on-image text: those need the transcription, and the recommendation is not a rule, it is a hint about what most images need.

Lead with the subject and the action, add the setting, then transcribe any text visible in the frame rather than paraphrasing it. Skip "image of" and "photo of," since screen readers already announce that an image is present. Keep the marketing voice in the commentary field, where it belongs, and let the alt text describe what is in the frame.

One practical note on sizing, since alt text and image specs get planned together: the cropping rules that decide what survives in the feed are a separate question, covered in the guide to LinkedIn image dimensions for every post type. Alt text describes the image LinkedIn renders, not the file you uploaded.

Frequently asked questions

What is the maximum length of altText on the LinkedIn API?

4,086 characters. The Images API and MultiImage API schema rows both describe altText as "the alternate text of this thumbnail," with "Maximum length is 4,086 characters, recommended length is less than 120 characters."

Does the Posts API documentation state the alt text limit?

No. The Post API Schema describes altText in its Media record as "The alternate text for the media. None if empty. Optional," with no ceiling. The 4,086 figure appears on the Images API and MultiImage API pages instead.

What error does LinkedIn return when alt text exceeds the limit?

LinkedIn does not document one for altText. The Posts API error table lists FIELD_LENGTH_TOO_LONG at 400 with the description "{field} length exceeds the allowed maximum. Reduce the length of the commentary or other text fields," and the Images API error table carries no length error at all.

Is there a character limit on the alt text box in the LinkedIn app?

LinkedIn publishes none. The help article on adding alternative text describes the desktop and mobile flows without stating a count, and the single image ads specification page lists character maximums for four ad fields while leaving the alt text row without one.

Can you set alt text on each image in a LinkedIn multi-image post?

Yes. The MultiImage API takes an images array with a "Minimum of 2 images and maximum of 20 images," each one a Media record carrying its own altText field. The 4,086 character maximum is documented on the MultiImage record's own altText, the description for the whole set, while the per-image ones are documented with no limit.

At which step do you send altText, the upload or the post?

The post. The initializeUpload call and the binary upload move the file, and altText travels in the content.media object of the POST /rest/posts request that references the resulting image URN.

Does LinkedIn add alt text automatically if you skip it?

LinkedIn may assign alt text on its own when an image ships without any. Uploads from a desktop trigger an alert when this happens, but uploads from a mobile device do not, so the description can end up written by LinkedIn rather than by you.

How many alt text fields does a full LinkedIn multi-image post need?

A full 20-image post carries up to 21. One altText sits on the MultiImage record itself, covering the set, and that is the field LinkedIn caps at 4,086 characters. Each of the 20 Media entries then carries an altText of its own, which the Post API Schema documents without a limit.

Does LinkedIn's ad specification page set a character limit for alt text?

The single image ads specification lists character maximums for the ad name, introductory text, headline, and description, four fields in total. The row for image alt text sits right below those limits and describes what alt text is for without attaching a number to it.

Should LinkedIn alt text describe the original file or the cropped version?

It should describe what LinkedIn actually renders in the feed, not the file as uploaded. Cropping rules decide what survives from the original image, and alt text needs to match that final, visible frame.