Slack Carousel, Card and Alert Blocks: Limits and Errors, Tested

A Slack carousel block is a horizontal row of 1 to 10 card blocks that the reader scrolls with an arrow. Each card can have a hero image, an icon, a title, a subtitle, a body, a subtext line and up to 3 buttons. A card also works on its own, outside a carousel. Alert blocks are different: chat.postMessage rejects them, and they render only in a modal. We tested all three on 5 October 2026 with a throwaway app and a bot token in our free-plan test workspace.

A carousel in a channel

This is one card from the carousel we posted. The full message had five of them in {"type": "carousel", "elements": [...]}:

{
  "type": "card",
  "icon": {"type": "image", "image_url": "https://picsum.photos/id/11/36/36", "alt_text": "icon"},
  "title": {"type": "mrkdwn", "text": "Room 1"},
  "subtitle": {"type": "mrkdwn", "text": "Floor 3, seats 6"},
  "hero_image": {"type": "image", "image_url": "https://picsum.photos/id/11/400/300", "alt_text": "room photo"},
  "body": {"type": "mrkdwn", "text": "Free 2-4 PM. *Projector* included."},
  "actions": [
    {"type": "button", "text": {"type": "plain_text", "text": "Book"}, "style": "primary", "action_id": "book1"}
  ]
}

In Slack's web app every card was 340 px wide, and the 400 x 300 hero image was drawn at 314 x 236 px. Three cards fit in our 1440 px wide window; the rest sat behind the arrow. The mrkdwn body rendered bold text. A carousel with one card posted fine and looked like a single card.

Limits and the exact errors

Each failing post returned "ok": false, "error": "invalid_blocks", and the reason in response_metadata.messages:

The slack_icon object takes its icon in name, for example {"type": "icon", "name": "calendar"}, and only names from Slack's fixed list such as calendar, rocket or warning. The Block Kit checker catches these limits before you post.

When we sent a carousel with no top-level text, the post still succeeded and Slack stored Room 1 Floor 3, seats 6 Free 2-4 PM. *Projector* included. as the message text, joined from the first card. That stored text is what clients without carousel support and notifications fall back to, so send your own text.

Button order and the slack_icon card

We sent three buttons in the order primary, plain, danger. Slack drew the danger button on the left and the other two on the right, with primary furthest right, as the reference says:

The second card above uses slack_icon instead of an image icon and has no hero image or buttons. subtext renders as a smaller grey line under the body. For a card that tracks a task with steps, see the task card and plan blocks instead.

Alert blocks only work in modals

Every alert block we sent to chat.postMessage failed, whatever its level:

{"ok": false, "error": "invalid_blocks", "response_metadata": {"messages": ["[ERROR] Block type is not supported in this container [json-pointer:/blocks/0]"]}}

So we added a slash command to the test app that opens a modal with views.open and one alert block per level:

Each level got its own colour and icon. An alert without level drew the same grey bar as "level": "default". Two modal tests failed in views.open with invalid_arguments:

• • 201 characters of alert text: must be less than 201 characters [json-pointer:/view/blocks/0/text/text].
• • "level": "critical": must be a valid enum value [json-pointer:/view/blocks/0/level].

When views.open fails, the user sees nothing. The slash command was acknowledged, no modal opened and Slack showed no error, so log the API response yourself.

FAQ

Do carousel and card blocks need a paid Slack plan?

No. Every post above ran on the Free plan.

How do I show a warning inside a channel message, since alerts are modal-only?

Use a context or section block with an emoji such as :warning:; see Slack context, header and divider blocks.