Bluesky Post Checker

What this checker checks

Each limit comes from one rules file that records its source and the date it was last verified. Post limits are kept separate from limits for other places, such as a profile picture, which has a different file-size limit.

• TextUp to 300 graphemes and 3,000 UTF-8 bytes. Both apply.
• ImagesUp to 4 images, each up to 2,000,000 bytes (about 2 MB). The official app also downsizes images larger than 4,000 px on a side.
• Alt textPresent or missing for each image. The official app limits alt text to 2,000 characters.
• VideoOne video up to 300 MB (decimal) and 10 minutes. Images and video can’t be combined in one post.
• LinkA well-formed web address, and complete campaign tags if it has any. Its preview isn’t fetched here.

What Pass, Warning, Fail and Not checked mean

• Pass The rule was checked and is met.
• Warning A recommendation, an official-app limit, or a SkyyCast-specific note. The post can still be published.
• Fail A hard requirement isn’t met: a Bluesky protocol limit, or the official app’s 10-minute video length.
• Not checked This tool can’t verify it from your browser, so it never counts it as a pass.

How to fix a problem

• Image over 2,000,000 bytes: compress it to a target size.
• Image larger than 4,000 px: resize it.
• Different image format: convert it to JPEG, PNG or WebP.
• Video too large or long: the Bluesky Video Upload Checker explains the limits. Trim or compress it in a video editor.
• Link preview or campaign tags: use the Social Preview Checker or the UTM Builder.
• Text too long: shorten it. There is nothing to compress; both the grapheme and the byte limit apply.

A worked example

To show what a report looks like, we ran the checker’s own rules on a made-up post: 312 characters of text, a link typed as example.com/launch (no https://), and two images. Image 1 was a 3000 × 2000 JPEG of 2,400,000 bytes with no alt text; image 2 was a 4200 × 3000 PNG of 900,000 bytes with alt text. The overall result was Fail, with 7 passes, 2 warnings, 3 fails and 1 not checked.

• Fail: 312 / 300 graphemes12 over the post limit. Shorten the text.
• Fail: image 1 is 2,400,000 bytes400,000 bytes over the 2,000,000-byte post-image limit, with a link to the target-size compressor.
• Fail: the link is not a valid web addressIt needs https:// and a domain.
• Warning: image 1 has no alt textA recommendation, not a Bluesky limit, with a link that jumps to the alt-text field.
• Warning: image 2 is 4200 × 3000 pxLarger than 4,000 px on a side. The official app downsizes it, which is app behaviour rather than a protocol limit, with a link to the Image Resizer.
• Not checked: links, mentions and hashtagsValidating them faithfully needs Bluesky’s own detection rules, so the checker says so instead of guessing.

Fix the three fails and the post can pass, while the warnings stay as recommendations. For the wider checklist behind a post, see our social media pre-publish checklist and the guide to adding alt text on Bluesky.

Sources and what was verified

Last verified 5 October 2026 against Bluesky’s public source files: app.bsky.feed.post, app.bsky.embed.images, app.bsky.embed.video and the official app’s constants.

• Protocol rules (the lexicons) are hard requirements. App limits (the image size in pixels, the 10-minute video length and the alt-text length) are enforced by the official app, not the protocol. Pixel size and alt-text length are reported as warnings; video length is reported as a failure because the official app would refuse the upload.
• Sources disagree on video formats: the schema lists only MP4, while the official app also accepts MPEG, WebM, QuickTime and GIF. This tool warns for those formats instead of failing them.
• Not verified by this tool: links, mentions and hashtags; link previews; whether an account or rate limit would block the post.

Privacy first

Your post text, link and files are read and measured by your own browser. They aren’t uploaded to SkyyCast, stored, or sent to another service, and no account is needed. The link is only checked for a valid format; it is never opened. The summary you can copy lists results only, never your post, link or file names. Closing or reloading the page discards everything.

Related tools and guides

See all free tools. The single-purpose Bluesky Post Validator, Image Size Checker and Video Upload Checker go deeper on one part of a post. For background, read how to add alt text on Bluesky and the social media pre-publish checklist.

About this tool

SkyyCast is an independent third-party service. It is not affiliated with, endorsed by or sponsored by Bluesky. The name “Bluesky” is used only to describe which service the tool checks posts for.