| TikTok | ||
|---|---|---|
| Characters | 2,200 | 2,200 |
| Media | a photo or video (required) | a video (required) |
| Hashtags | max 5 hashtags at the END of the caption | max 5 hashtags, inside the caption (comments are invisible to the algorithm) |
| Links in body | fine | fine |
| Needs platform approval | yes | yes |
Instagram accepts photos, TikTok is video-only. Image posts have nowhere to land.
Instagram: max 5 hashtags at the END of the caption. TikTok: max 5 hashtags, inside the caption (comments are invisible to the algorithm). Carrying the tag block across is the single most obvious sign of an automated cross-post.
TikTok requires a separate title (max 150 characters). Instagram has no such field, so there is nothing to copy across — it has to be written.
Instagram expects 9:16 for Reels, 1:1 or 4:5 for feed. TikTok expects 9:16 vertical. The same file will be letterboxed or cropped.
Instagram is built for short captions — under 100 characters is rewarded. TikTok is hook-first, not SEO-first: the first 80-100 characters decide everything. 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", "tiktok"],
"text": "Your post",
"per_platform": {
"instagram": { "text": "The Instagram version" },
"tiktok": { "text": "The TikTok 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: TikTok to Instagram · all combinations · which APIs need approval