| Mastodon | ||
|---|---|---|
| Characters | 2,200 | 500 |
| Media | a photo or video (required) | optional |
| Hashtags | max 5 hashtags at the END of the caption | 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers |
| Links in body | fine | fine |
| Needs platform approval | yes | no |
Instagram allows 2,200 characters; Mastodon caps at 500. A post written to Instagram's length gets truncated — usually mid-sentence — unless it is rewritten, not trimmed.
Instagram: max 5 hashtags at the END of the caption. Mastodon: 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers. Carrying the tag block across is the single most obvious sign of an automated cross-post.
Instagram is built for short captions — under 100 characters is rewarded. Mastodon is community-first; alt text on media is expected, not optional. 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.
curl -X POST https://postwire.io/api/post \
-H "Authorization: Bearer $POSTWIRE_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"platforms": ["instagram", "mastodon"],
"text": "Your post",
"per_platform": {
"instagram": { "text": "The Instagram version" },
"mastodon": { "text": "The Mastodon 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.
Other pairs: Mastodon to Instagram · all combinations · which APIs need approval