Block Kit

Block Kit

Help the developer build a rich Block Kit layout. If $0 is provided, it specifies the target surface (message, modal, or home-tab).

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.

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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/-block.md
• Block elements: https://docs.slack.dev/reference/block-kit/block-elements/-element.md
• Composition objects: https://docs.slack.dev/reference/block-kit/composition-objects/.md
• The slug is not always the type name. For example datepicker maps to date-picker-element.md, and every *_select menu (static_select, users_select, multi_channels_select, and so on) is documented on select-menu-element.md. When unsure of a slug, follow the link from the master index rather than building the URL by hand.
• Surface payload structure lives in the surface guides: messages at https://docs.slack.dev/messaging/formatting-message-text.md, modals at https://docs.slack.dev/surfaces/modals.md, and Home tabs at https://docs.slack.dev/surfaces/app-home.md.

Never use a block type, element, or field you have not seen on a live page.

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Fast Path (for clear, specific requests)

If the developer's request is specific enough to determine both the target surface and the desired layout, collapse Steps 1-4 into a single pass:

1. Determine the surface from $0 or context
2. Fetch only the doc pages for the blocks and elements mentioned
3. Generate the JSON directly
4. Proceed to Step 5 (validation)

Fast-path indicators (skip the full workflow):

• Developer provides existing JSON to modify → use Modification Mode instead
• Developer names specific block types: "add an actions block with two buttons"
• Developer describes a well-known pattern: "approval message", "feedback form", "settings modal"
• Developer provides a complete description in one message with enough detail to build

Full-workflow indicators (use Steps 1-7):

• Vague requests: "make something cool", "build a dashboard"
• Exploratory: "what can Block Kit do?", "show me my options"
• Complex layouts: 10+ blocks, nested modals, conditional logic
• Developer asks for help deciding what to build

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Modification Mode

If the developer provides existing Block Kit JSON (pasted inline, in a file, or referenced from code), enter Modification Mode instead of the full creation workflow:

1. Parse the existing structure:

• List each block by index, type, and a short description of its content
• Infer the surface: "type": "modal" = modal, "type": "home" = home tab, bare blocks array = message

1. Ask what changes they want:

• Add blocks (where in the sequence?)
• Remove blocks (which ones?)
• Modify blocks (which block, what change?)
• Reorder blocks

1. Apply changes while preserving:

• All existing block_id values (these are referenced in app interaction handlers)
• All existing action_id values (these map to event listeners)
• Existing styles, text content, and structure for unchanged blocks

1. Validate the modified JSON: proceed to Step 5 (validation)

Detection: If the developer's message contains a JSON array starting with [{"type": or a view object with "blocks":, enter Modification Mode automatically. If they say "edit", "update", "modify", or "change" in reference to existing blocks, ask them to provide the current JSON.

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Step 1: Determine the Target Surface

If $0 is provided and matches one of message, modal, or home-tab, use it directly.

Otherwise, ask the developer using AskUserQuestion:

• Message: Conversational content posted to a channel or DM. Max 50 blocks.
• Modal: A dialog or form opened by a user action. Max 100 blocks.
• Home tab: A persistent, per-user dashboard in the App Home. Max 100 blocks.

Once the surface is determined, use the correct payload structure for it:

• Message: a { "text": "Fallback text", "blocks": [...] } object posted via chat.postMessage (and friends). The text field is the notification/accessibility fallback. For message text formatting (mrkdwn, mentions, dates), see https://docs.slack.dev/messaging/formatting-message-text.md.
• Modal: a view object ({ "type": "modal", "title": ..., "blocks": [...] }). For the full view object structure, lifecycle, and the rule that submit is required when the view contains any input block, see https://docs.slack.dev/surfaces/modals.md.
• Home tab: a view object ({ "type": "home", "blocks": [...] }) published via views.publish. For structure and behavior, see https://docs.slack.dev/surfaces/app-home.md.

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Step 2: Understand What to Build

Ask the developer to describe what they want their layout to look like or accomplish.

If they need inspiration, suggest examples — several map directly onto a ready-made template in references/common-patterns.md (named in parentheses), which you can start from in Step 3:

• "A feedback form with a text input and a category selector" (Simple Form Modal)
• "A notification message with an alert banner, description, and Approve/Reject buttons" (Notification Alert / Approval Message)
• "A dashboard home tab with a welcome header, key metrics in fields, and quick-action buttons" (Dashboard Home Tab)
• "A settings modal with dropdowns, checkboxes, and a time picker" (Settings Modal with Multiple Input Types)
• "A table of sprint tasks with status and points" (Data Table)

Get enough detail to plan the layout before generating any JSON.

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Step 3: Plan the Block Layout

Based on the developer's description:

1. Fetch only what you need from the live docs:

• WebFetch the master index (https://docs.slack.dev/reference/block-kit.md) to confirm the block and element types you plan to use exist and to grab links to their pages.
• Check references/common-patterns.md (the one local reference file) if the request matches a common pattern; start from the template instead of building from scratch.
• Defer reading individual component pages until Step 4, when you build each block's fields.

1. Propose a numbered block outline. For example:

```text

1. header: "Weekly Report"
2. section: Summary text with a datepicker accessory
3. divider
4. section: Status fields (Name, Role, Team)
5. actions: "Approve" button (primary) and "Reject" button (danger)

```

1. Present the outline to the developer and ask for approval or changes before generating JSON.

Surface constraints to check:

• Block count limit: 50 for messages, 100 for modals/home tabs
• Modal-specific: if using input blocks, the modal payload must include a submit field
• Table: only one table block per message
• Surface compatibility (whether a block is valid on the chosen surface) and element compatibility (whether an element is allowed inside a given block) are not always spelled out on a component's doc page. Build the layout from the docs, and let blocks.validate in Step 5 confirm it. It is the authoritative check.

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Step 4: Generate the Block Kit JSON

Once the layout is approved, build each block from its live doc page, fetching each page's fields table (required vs optional, constraints) and JSON example with WebFetch. The URL patterns are in Source of Truth above; the one slug to remember is that every *_select menu (static_select, users_select, multi_channels_select, …) lives on select-menu-element.md. Fetch pages as you need them and reuse what you have already fetched — don't re-fetch the same page for every block of the same type. Then build the payload block-by-block and wrap it in the surface structure from Step 1.

Guidelines:

• Use descriptive action_id values (e.g., "approve_report_btn" not "action_1") — they identify the element in your interaction handlers
• Include block_id values where the developer will need them for interaction handling
• For modals, include title, submit, close, and callback_id; for home tabs, the type: "home" wrapper
• Use mrkdwn text for rich formatting, plain_text where required (headers, labels, modal title)

mrkdwn vs. the markdown block: section and context blocks format text with Slack's mrkdwn (*bold*, _italic_, ~strike~, ` code ) — use these for short, interactive layouts. The separate markdown block (Messages only) renders _standard_ markdown (bold, headings, tables, numbered lists) and is meant for AI/LLM-generated or long-form content that already exists in standard markdown. Reach for it when the developer has such content or needs those features in the message body; there is a cumulative 12,000-character limit across all markdown` blocks in one message.

Accessibility is easy to skip and hard to retrofit, so build it in now:

• Give images descriptive alt_text (what the image shows, not just "image"), and make sure image-heavy layouts also carry the key information as text
• Summarize the layout in the message's text fallback (notifications and screen readers show it instead of the blocks)
• Use header blocks for logical section headings — they convey document structure to assistive tech

Present the complete payload to the developer in the Step 1 surface structure.

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Step 5: Validate with blocks.validate

Always validate. blocks.validate is a public Web API method, so no auth token is required.

The authoritative reference for this method (its parameters, auth requirements, and response/error shape) is the live doc. WebFetch it before relying on any detail here: https://docs.slack.dev/reference/methods/blocks.validate.md. It documents the accepted parameters (blocks for a message's blocks array, view for a modal/home-tab view, message for a full message payload; send exactly one) and the response shape.

5a. Build the validation request

Prefer the Slack CLI when it's available, since it reuses the slack-cli skill's CLI detection and needs no token wrangling. If the CLI isn't installed, fall back to curl. Both call the same public method and return the same response, so Step 5b applies either way.

Path A: Slack CLI (preferred).

Use the slack:slack-cli skill, Step 1: Detect the Slack CLI, to check whether the public CLI is installed and resolve its command (SLACK_CMD).

If the CLI is available, use the slack:slack-cli skill, Step 4: Calling Web API Methods (slack api), to invoke it. That step covers the SLACK_CMD api key=value … syntax. Run SLACK_CMD api --help first to confirm the syntax and the flag that skips authentication. blocks.validate needs no token, so call it without authentication. Don't hard-code that flag from memory; read it from the help output so this stays correct if it's ever renamed. Pass the payload as a positional key=value argument: blocks= for messages, or view= for modals and home tabs.

Path B: curl (fallback, when the CLI isn't installed).

POST to the endpoint with the Bash tool. The API uses form-urlencoded encoding, so pass the JSON directly as the parameter value.

For messages, send the blocks array as a form-encoded parameter:

curl -s -X POST 'https://slack.com/api/blocks.validate' \
  -d 'blocks=[ ... the blocks array ... ]'

For modals and home tabs, send the complete view object in the view field:

curl -s -X POST 'https://slack.com/api/blocks.validate' \
  -d 'view={ "type": "modal", "title": ..., "blocks": [...] }'

For large payloads, if the JSON is complex or contains special characters, write it to a temp file to avoid shell escaping issues:

curl -s -X POST 'https://slack.com/api/blocks.validate' \
  --data-urlencode "blocks@/tmp/blocks.json"

5b. Handle the response

Success:

{ "ok": true }

Tell the developer their blocks are valid.

Failure:

{
  "ok": false,
  "error": "invalid_blocks",
  "errors": [
    {
      "code": "missing_field",
      "message": "missing required field: type",
      "field": "type",
      "pointer": "/0"
    }
  ]
}

When validation fails:

1. Read each error. pointer is a JSON pointer to the offending node (e.g., /0 = first block, /2/elements/1 = second element of the third block, /0/text/type = the type field of the first block's text object). message describes the problem, and constraint (when present) names the rule that failed and its expected values.
2. Explain the error to the developer in plain language.
3. Fix the JSON. For the authoritative meaning of an error code and the field requirements behind it, consult the live method doc (https://docs.slack.dev/reference/methods/blocks.validate.md) and the relevant block/element/composition-object page you fetched in Steps 3-4.
4. Re-validate. Repeat until "ok": true.

Escape Hatch

If validation fails 3 times on the same error:

1. Present the raw error and current JSON to the developer
2. Offer to open the JSON in Block Kit Builder for visual debugging: https://app.slack.com/block-kit-builder
3. Ask if they want to simplify the layout to avoid the constraint

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Step 6: Deliver the Final Output

Present the validated payload, then help the developer put it to use.

Send it

Building the payload is this skill's job; sending it (chat.postMessage, views.open, views.publish, and the token/scope handling around them) belongs to the Web API layer. To call the right method — via the Slack CLI, raw curl, or a Bolt SDK — use the slack:slack-api skill, Step 4: Call the Method (Manage), passing this payload as the method's blocks argument (messages) or view argument (modals and home tabs). That skill matches the argument names to the SDK or HTTP call so we don't duplicate them here.

Preview it

Offer a Block Kit Builder link so the developer can see a live preview and tweak visually: https://app.slack.com/block-kit-builder. Builder needs an object ({ "blocks": [...] } or a full view object), not a bare array. Suggest this proactively when the developer says something is "not quite right" but can't articulate what, when the layout has 10+ blocks, or when iteration isn't converging.

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Step 7: Iterate

Ask whether the developer wants to add, modify, remove, or reorder blocks, or build a l

…

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: slackapi
• Source: slackapi/slack-skills-plugin
• License: MIT
• Homepage: https://docs.slack.dev

Install and usage instructions live in the source repository linked above.