Threads long-form posts

Threads now supports a text attachment — a long-form block that appears as an expandable "Read more" section under a standard 500-character Threads post. posterly lets you write, format, and schedule these through the regular composer.

Character limits

Part	Limit
Main hook (visible in feed)	500 characters
Long-form text attachment	10,000 characters

Supported formatting

Select any text inside the long-form body and apply:

• Bold
• Italic
• <u>Underline</u>
• Strikethrough
• Highlight

Formatting renders on Threads exactly as it appears in the composer preview.

Rules (enforced before publish)

Meta restricts what long-form posts can contain. posterly blocks invalid posts before you schedule them, so nothing fails silently at publish time.

• Text-only posts. If you attach an image, video, or carousel, the long-form option is removed. Write long-form on its own post.
• Max 5 unique links. Counted across the main hook and the long-form body and any link attachment URLs.
• One link attachment. If the main post has a link attachment, the long-form body can't also have one. Pick one.
• Not part of a thread. Long-form posts are single-post only. Threads thread mode doesn't support text attachments.

Using it

1. Open the composer and select a Threads account.
2. Write your main hook (500 chars) in the caption box as usual.
3. Below the caption, click Long-form text (up to 10,000 chars).
4. Type your long-form body in the expanded textarea.
5. Highlight text and tap a toolbar button to format it.
6. (Optional) Add a link attachment URL.
7. Schedule or publish.

The live preview below the textarea shows exactly how the body will render once a reader taps "Read more".

Editing a published long-form post

Threads does not allow editing text attachments after publish — this is Meta's restriction, not posterly's. Proof the full post in the composer preview before scheduling; once it's live, it's final.

API & MCP

If you use the posterly API or MCP, pass the attachment through metadata.threads_text_attachment:

{
  "platform": "threads",
  "caption": "500-char hook that shows in the feed",
  "metadata": {
    "threads_text_attachment": {
      "plaintext": "Up to 10,000 characters of long-form body.",
      "link_attachment_url": "https://example.com/deep-link",
      "styles": [
        { "offset": 0, "length": 5, "styling_info": "BOLD" }
      ]
    }
  }
}

All the same rules apply — the API returns 400 with a clear message if something's off.