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Approximate rendering. Use Slack's own Block Kit Builder for pixel-exact checks.
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Block Kit tips
- Always send a top-level text field as well: it is used in notifications and by screen readers.
- Limits: 50 blocks per message, header 150 characters, section text 3000, up to 10 fields and 25 elements in an actions block.
- Button presses arrive at your Interactivity Request URL as block_actions payloads. Verify the Slack signature and acknowledge within 3 seconds.
- Input blocks only work inside modals and Home tabs, or in messages with dispatch_action.
Related: Webhook signature generator, Slack bot guide.