What is the character limit difference between Mastodon and Bluesky?

Mastodon allows 500 characters and Bluesky allows 300.

How do I post to Mastodon and Bluesky at the same time?

With PostWire, one API call publishes to both: POST /api/post with platforms set to ["mastodon","bluesky"]. Or one MCP tool call from an AI agent.

Works, with rewrites

Cross-posting Mastodon to Bluesky: what actually breaks

You can, but not by copying the text across: 2 things change between Mastodon and Bluesky, starting with you lose 200 characters. Below is exactly what breaks and what to do instead.

Side by side

	Mastodon	Bluesky
Characters	500	300
Media	optional	optional
Hashtags	2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers	1-2 specific hashtags
Links in body	fine	fine
Needs platform approval	no	no

What changes, one by one

You lose 200 characters

Mastodon allows 500 characters; Bluesky caps at 300. A post written to Mastodon's length gets truncated — usually mid-sentence — unless it is rewritten, not trimmed.

The hashtag rules are not the same

Mastodon: 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers. Bluesky: 1-2 specific hashtags. Carrying the tag block across is the single most obvious sign of an automated cross-post.

Why copy-paste cross-posting underperforms

Mastodon is community-first; alt text on media is expected, not optional. Bluesky is indie and anti-corporate — polished brand voice reads badly. Those are different jobs, so the same words cannot do both well — and every algorithm here demotes text that obviously was not written for it. The fix is not to skip cross-posting; it is to send a native version to each network.

Publish to both in one call

curl -X POST https://postwire.io/api/post \
  -H "Authorization: Bearer $POSTWIRE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "platforms": ["mastodon", "bluesky"],
    "text": "Your post",
    "per_platform": {
      "mastodon": { "text": "The Mastodon version" },
      "bluesky": { "text": "The Bluesky version" }
    }
  }'

PostWire's /api/generate writes those per-platform versions for you from a single prompt, respecting every limit in the table above. From an AI agent it is one MCP tool call.

Get an API key free →

Questions people actually ask

Can I cross-post from Mastodon to Bluesky?: You can, but not by copying the text across: 2 things change between Mastodon and Bluesky, starting with you lose 200 characters. Below is exactly what breaks and what to do instead.
What is the character limit difference between Mastodon and Bluesky?: Mastodon allows 500 characters and Bluesky allows 300.
How do I post to Mastodon and Bluesky at the same time?: With PostWire, one API call publishes to both: POST /api/post with platforms set to ["mastodon","bluesky"]. Or one MCP tool call from an AI agent.

Other pairs: Bluesky to Mastodon · all combinations · which APIs need approval