Bluesky to Mastodon is the rare cross-posting direction where the destination is roomier than the source: a 300-grapheme Bluesky post always fits inside Mastodon’s 500-character default, so nothing you wrote gets cut. The two networks also share more culture than any other pair in this matrix — alt text as a norm, substance over engagement-bait, threads as native behavior.
The work hides in the plumbing instead: Mastodon’s hashtags do the discovery job Bluesky’s custom feeds do, its servers accept 16 MB images where Bluesky compresses to ~1 MB, and an opt-in bridge called Bridgy Fed can mirror your posts across the protocol gap automatically. Here’s what transfers, what needs attention, and both workflows.
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Everything that changes between the two composers, side by side.
| Spec | Bluesky (from) | Mastodon (to) |
|---|---|---|
| Primary canvas | 1200 × 675 px · 16:9 (post image) | 1200 × 675 px · 16:9 (post image) |
| Caption limit | 300 graphemes (visual characters — emoji count once) | 500 characters by default — instance admins can raise it |
| Video length | Short clips — Bluesky has been raising limits (around 3 minutes as of early 2026) | Instance-dependent; default upload caps are modest, so keep clips short |
| Hashtags | Plain-text hashtags count toward the 300-grapheme limit | Count toward the limit and drive most discovery (most instances have no algorithmic feed) |
| Protocol | AT Protocol — bridges to the fediverse only via opt-in Bridgy Fed | ActivityPub — native to the fediverse |
| Image file size | Compressed to ~1 MB (976.56 KB) per image | Up to 16 MB per image on default servers |
| Quote posts | Native, routine behavior | Added in Mastodon 4.5 (late 2025) with author-permission controls |
The good news
The fine print
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Tip 1
Treat the extra 200 characters as room for context, not padding — un-abbreviate the jargon, add the one-line background, write out what the link is. The Bluesky squeeze often cuts exactly what Mastodon’s slower, read-everything culture rewards.
Tip 2
If you only want a mirror, Bridgy Fed can do it for you: follow @ap.brid.gy from your Bluesky account and the opt-in service republishes your public posts to a bridged fediverse handle (as of June 2026). Just know Mastodon users will be interacting with a mirror, not a native account.
Tip 3
Don’t lean on quote posts as the format: with Mastodon quote support still spreading across servers since 4.5, a post that only makes sense as a quote can render as a plain link for part of your audience.
FAQ
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Yes — Bridgy Fed, a free opt-in bridge, mirrors public Bluesky posts to a bridged fediverse handle once you follow @ap.brid.gy on Bluesky (as of June 2026). It’s a mirror rather than a real account: replies and discovery behave differently, which is why brands usually post natively to a Mastodon account they control instead.
Always. Bluesky caps posts at 300 graphemes while standard Mastodon allows 500 characters — and Mastodon counts more generously still, billing links at a flat 23 characters, with instance admins free to raise the limit further.
Yes — two or three specific tags. Most Mastodon servers have no algorithmic feed, so hashtag follows and tag timelines are how new readers find you. On Bluesky tags mostly spend graphemes; on Mastodon they are the distribution.
Re-upload from your original exports rather than saving images off Bluesky — its pipeline compresses every image to roughly 1 MB, while default Mastodon servers accept up to 16 MB and re-serve your file across the fediverse. Carry the alt text over and set Mastodon’s focal point on each image.
Compose once, customize the caption per network, and let SocialKit publish to Bluesky, Mastodon, and 9 more platforms on schedule — no re-uploading, no copy-paste.
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