Block Kit is Slack's UI framework for creating rich, interactive messages, modals, and Home tabs. It uses a JSON-based structure with three main components:
| Messages | 50 | Notifications, bot responses, channel posts | | Modals | 100 | Forms, confirmations, multi-step workflows | | Home tabs | 100 | App dashboards, user-specific content |
| invalidblocks | Malformed JSON | Validate in Block Kit Builder | | toomanyblocks | Exceeds limit | Reduce to 50 (msg) or 100 (modal) | | invalidelement | Wrong element in block | Check element compatibility | | missingtext | Required text missing | Add text or fields to Section |
Build Slack Block Kit UIs for messages, modals, and Home tabs. Use when creating Slack notifications, interactive forms, bot responses, app dashboards, or any Slack UI. Covers blocks (Section, Actions, Input, Header), elements (Buttons, Selects, Date pickers), composition objects, and the slack-block-builder library. Source: linehaul-ai/linehaulai-claude-marketplace.