Slack message preview generator

Messages from a Slack app are built from Block Kit blocks. This preview draws three of them: a header block for the title, a section block for the body text, and a row of buttons underneath.

Slack documents the limits for each. A header takes plain text up to 150 characters. A section's text runs from 1 to 3,000 characters. A button label can be up to 75 characters, although Slack notes that it may truncate at around 30. The fields in this tool stop earlier (80 characters for the header, 600 for the body, 24 for a button) to keep the mockup readable.

Block Kit buttons take an optional style. Slack's reference describes primary as green and says to use it on one button in a set, and danger as red, for destructive actions only and even more sparingly. A button without a style gets the default look.

The preview draws the first button as the primary action and the second in the default style.

Courier's Slack integration sends through a Slack app you create, using its bot token. You can address a message to a channel, a user ID, or an email address, and design it in Courier's Slack template editor. For a channel, use its ID rather than its name.

The bot token needs the chat:write, im:write, users:read, and users:read.email scopes. Courier's docs walk through creating the app, and point to Slack's Block Kit Builder for building the block JSON itself.