# Compose for each channel

One idea, with the right fields for its destination.

## Caption and media

Select connected accounts, add captions and inspect channel previews. Current uploads accept JPEG, PNG, WebP and MP4 with a 20 MB per-file limit; provider constraints may be stricter.

## Channel-specific fields

Pinterest needs a selected board and Pin metadata. Instagram needs suitable media and a professional account. Threads sequences use ordered replies. TikTok consent and completion depend on publishing mode.

## Approval

Review all destinations and schedule before submitting. Previews illustrate content but do not guarantee identical native rendering. A successful preview is not approval or publication.

## Current caption limits

Postroom validates X at 280 characters, LinkedIn at 3,000, Pinterest descriptions at 800, TikTok and Instagram at 2,200, Bluesky at 300, Threads at 500 and Facebook at 10,000. These are application limits; provider rules and Unicode counting can impose additional restrictions. Bluesky also checks a 3,000-byte UTF-8 limit.

## Media and accessibility

The workspace uploader accepts JPEG, PNG, WebP and MP4, up to 20 MB per file. Add alt text for each image where supported. Review the actual crop in each destination preview. A file uploading successfully does not guarantee every platform supports its dimensions, duration or format.