What is the character limit difference between Mastodon and Reddit?

Mastodon allows 500 characters and Reddit allows 10,000.

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

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

Works, with rewrites

Cross-posting Mastodon to Reddit: what actually breaks

You can, but not by copying the text across: 4 things change between Mastodon and Reddit, starting with you have 9,500 characters you are not using. Below is exactly what breaks and what to do instead.

Side by side

	Mastodon	Reddit
Characters	500	10,000
Media	optional	optional
Hashtags	2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers	none — Reddit has no hashtags
Links in body	fine	fine
Needs platform approval	no	yes

What changes, one by one

You have 9,500 characters you are not using

Mastodon caps at 500 characters, Reddit allows 10,000. Posting the short version verbatim wastes the room Reddit gives you to actually explain the thing.

The hashtag rules are not the same

Mastodon: 2-4 hashtags in CamelCase (#SocialMedia, not #socialmedia) for screen readers. Reddit: none — Reddit has no hashtags. Carrying the tag block across is the single most obvious sign of an automated cross-post.

Reddit needs a title you never wrote

Reddit requires a separate title (max 300 characters). Mastodon has no such field, so there is nothing to copy across — it has to be written.

Reddit needs approval, Mastodon does not

You can post to Mastodon today with a credential you generate yourself. Reddit gates posting behind a platform review — see our API approval notes for what that actually takes.

Why copy-paste cross-posting underperforms

Mastodon is community-first; alt text on media is expected, not optional. Reddit is value-first; it must read like a community member, not an ad. 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", "reddit"],
    "text": "Your post",
    "per_platform": {
      "mastodon": { "text": "The Mastodon version" },
      "reddit": { "text": "The Reddit 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 Reddit?: You can, but not by copying the text across: 4 things change between Mastodon and Reddit, starting with you have 9,500 characters you are not using. Below is exactly what breaks and what to do instead.
What is the character limit difference between Mastodon and Reddit?: Mastodon allows 500 characters and Reddit allows 10,000.
How do I post to Mastodon and Reddit at the same time?: With PostWire, one API call publishes to both: POST /api/post with platforms set to ["mastodon","reddit"]. Or one MCP tool call from an AI agent.

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