Does Slack Collapse Long Messages? When Show More Appears

Slack does not collapse long messages that people type. In the Slack web app on October 28, 2026, we posted messages of 5 to 40 lines and of 1,000 to 3,900 characters, and every one showed in full, with no Show more link. The only messages that collapsed were legacy attachments sent by an app: Slack cut those at 5 lines, or at about 700 characters, and added Show more. You cannot make a collapsible section in a normal message, and none of the Block Kit messages we sent folded.

What we tested

We posted each message into a test channel in a Free workspace and then checked every message in the page for a Show more or Show less link. Messages from a person were posted through the web app's own session; app messages came from a test bot.

The 40-line message from a person took about 890 pixels of height in the channel. The Block Kit section with the same 40 lines took about 910. Slack let both run the full height.

Where Show more does appear

Legacy attachments are the older way apps format messages: a colored bar on the left with text next to it. Many alert and CI integrations still use them. This is what our 5-, 6- and 7-line attachments looked like:

Clicking Show more opened the rest, and the link turned into Show less:

For text length, an attachment of 680 characters still showed in full, and 700 collapsed, so the cut sits between those two. Slack's docs do not give the number; this is what we measured.

How to make a collapsible message

There is no collapse option in the message box, and no markdown for a folded section. What you can do:

• • Put the long part in a thread. Post a short summary, then the details as the first reply. The channel shows one line, "1 reply", and people open it if they want.
• • From an app, use a legacy attachment. If you build an integration and want Slack to fold long output, send it as an attachment with text longer than 5 lines. Slack's docs call attachments a legacy feature and recommend Block Kit, which did not fold in our test.

{
  "channel": "C0123456789",
  "text": "Nightly build report",
  "attachments": [
    {
      "color": "#22c55e",
      "text": "Line 1: build passed\nLine 2: build passed\nLine 3: build passed\nLine 4: build passed\nLine 5: build passed\nLine 6: build passed"
    }
  ]
}

This payload, sent with chat.postMessage, produced the 6-line attachment above with its Show more link.

A message that is too long to post at all is a different limit. Slack splits text over about 4,000 characters into several messages; the Slack message character limit has those numbers. To see how a Block Kit payload will look before you send it, paste it into the Block Kit checker.

Can you collapse a thread or a channel message you received?

You can collapse images and files: each has a small arrow next to its name that hides the preview for you. Text has no such arrow. A Reddit thread in r/Slack asks for a way to fold a long thread parent, and in our test there was none.

FAQ

Why do I see Show more on some messages and not others?

The ones with Show more are app messages built as legacy attachments, with a colored bar on the left. Messages from people and Block Kit messages from apps show in full.

Does the mobile app collapse long messages?

We tested the web app only, so we cannot say how the phone app folds attachments.