# block-kit

Builds and validates Block Kit JSON for Slack messages, modals and Home tabs. Useful when you want a working layout without digging through the docs by hand.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 500 installs · ★ 155
- Author: slackapi
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/slackapi/slack-skills-plugin/tree/HEAD/skills/block-kit/SKILL.md
- Updated: 2026-10-24
- HTML: https://affpapa.org/en/nevibecode/skills/block-kit

## What it does

The skill walks through surface selection, layout planning, JSON generation and validation. Block types and elements come from live `docs.slack.dev` pages rather than memory, so field schemas and examples stay current.

Two modes. A specific request collapses the workflow into one pass: pick the surface, fetch the relevant pages, generate the JSON, validate. If you paste existing JSON, modification mode kicks in — the agent lists blocks by index, asks what to change, and preserves existing `block_id` and `action_id` values, since your app's interaction handlers depend on them.

The surface comes from the `[message | modal | home-tab]` argument. Messages cap at 50 blocks, modals and Home tabs at 100. Messages also need a top-level `text` fallback: `blocks.validate` won't flag a missing one, but that field is what notifications and screen readers show.

## Requirements
- Works with: Slack

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add slackapi/slack-skills-plugin --skill block-kit -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/block-kit/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/block-kit/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add slackapi/slack-skills-plugin --skill block-kit -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/block-kit/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add slackapi/slack-skills-plugin --skill block-kit -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/block-kit/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install slackapi/slack-skills-plugin block-kit
```

2. **Manually: put the files in place**
- File: `.github/skills/block-kit/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add slackapi/slack-skills-plugin --skill block-kit -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/slackapi/slack-skills-plugin --path skills/block-kit
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/block-kit/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add slackapi/slack-skills-plugin --skill block-kit -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/slackapi/slack-skills-plugin/tree/HEAD/skills/block-kit/SKILL.md

## When to use it
- build a message layout from sections and buttons
- create a modal form with input fields
- publish a Home tab dashboard for a user
- edit existing Block Kit JSON without breaking handlers
- validate blocks before posting to Slack

## Example prompts
- build a feedback modal with block-kit
- add an actions block with two buttons to this message
- validate my Block Kit JSON and show the errors

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 43045dcad83c7636.

BEGIN THIRD-PARTY FILE 43045dcad83c7636
```text
---
name: block-kit
description: 'Use when a developer wants to build or validate Block Kit layouts for Slack messages, modals, or Home tabs: message layouts, modals/forms/dialogs, Home tab interfaces, interactive buttons or menus, or modifying existing Block Kit JSON. Also trigger on any Slack UI component (sections, actions, inputs, headers, alerts, tables, carousels), the words "blocks" or "Block Kit Builder", a request to preview rendered blocks, or pasted JSON like "type": "section". Validates via the blocks.validate API method.'
argument-hint: "[message | modal | home-tab]"
---

# Block Kit

Help the developer build a rich Block Kit layout. If `$0` is exactly `message`, `modal`, or `home-tab`, treat it as the target surface. Otherwise (including if it's empty or any other value), ignore it and determine the surface in Step 1.

This skill walks through surface selection, layout planning, JSON generation, and validation. Block types, elements, and fields come from the live docs (see **Source of Truth** below) — discover them and read each component's schema there, never from memory.

> **Common Block Kit mistakes (and why):** A few errors recur often enough to flag up front. Most others are caught by `blocks.validate` in Step 5, so lean on validation rather than memorizing rules.
>
> - **`"type": "text"` is not a thing.** Text is a composition object: `{ "type": "plain_text", "text": "..." }` or `{ "type": "mrkdwn", "text": "..." }`.
> - **`markdown` is a _block_ type, not a text type.** A `markdown` block holds standard markdown; text objects inside other blocks use `mrkdwn` (see **mrkdwn vs. the `markdown` block** in Step 4). Slack's `mrkdwn` is `*bold*` / `_italic_` / `~strike~`, not `**bold**`.
> - **Messages need a top-level `text` fallback.** `blocks.validate` won't flag a missing one, but notifications and screen readers display it instead of the blocks — so summarize what the layout conveys rather than leaving it empty.

---

## Source of Truth: the Live Docs

Every block, element, and composition object is documented on `docs.slack.dev`. Append `.md` to any reference URL to fetch it as markdown with WebFetch (no auth required).

- **Master index**: the authoritative list of every block, block element, and composition object, each linking to its own page: `https://docs.slack.dev/reference/block-kit.md`. WebFetch it to confirm a type exists and to get the link to its page.
- **Per-component pages** carry the full field schema (a fields table with required/optional flags and constraints, plus JSON examples):
  - Blocks: `https://docs.slack.dev/reference/block-kit/blocks/<slug>-block.md`
  - Block elements: `https://docs.slack.dev/reference/block-kit/block-elements/<slug>-element.md`
  - Composition objects: `https://docs.slack.dev/reference/block-kit/composition-objects/<slug>.md`
```
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---
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