| Mastodon | ||
|---|---|---|
| Characters | 500 | 2,200 |
| Media | optional | a photo or video (required) |
| Hashtags | 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers | max 5 hashtags at the END of the caption |
| Links in body | fine | fine |
| Needs platform approval | no | yes |
Mastodon posts are frequently text-only. Instagram requires a photo or video on every post, so there is no direct equivalent — you have to attach media before this cross-post is even possible.
Mastodon caps at 500 characters, Instagram allows 2,200. Posting the short version verbatim wastes the room Instagram gives you to actually explain the thing.
Mastodon: 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers. Instagram: max 5 hashtags at the END of the caption. Carrying the tag block across is the single most obvious sign of an automated cross-post.
You can post to Mastodon today with a credential you generate yourself. Instagram gates posting behind a platform review — see our API approval notes for what that actually takes.
Mastodon is community-first; alt text on media is expected, not optional. Instagram is built for short captions — under 100 characters is rewarded. 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": ["mastodon", "instagram"],
"text": "Your post",
"per_platform": {
"mastodon": { "text": "The Mastodon version" },
"instagram": { "text": "The Instagram 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: Instagram to Mastodon · all combinations · which APIs need approval