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Use MM Blocks

MM Blocks are the recommended way to build interactive messages. Instead of placing buttons and menus inside legacy message attachments, integrations send a block tree in props.mm_blocks and register action handlers in props.mm_blocks_actions.

Legacy attachment arrays (top-level attachments on webhooks and slash-command responses, or props.attachments on REST API posts), Slack Block Kit (props.blocks), and Microsoft Adaptive Cards (props.cards) are still accepted. Mattermost translates them into MM Blocks at render time, but new integrations should use native MM Blocks directly.

How it works

An interactive MM Blocks post has two parts:

props.mm_blocks — an array of block objects that define layout, text, images, buttons, and menus.
props.mm_blocks_actions — a map keyed by action ID. Each entry tells the server what to do when a user clicks a button, selects a menu option, or activates a markdown action button.

When the post is stored, the server validates that every referenced action ID has a matching registry entry (and no unused entries remain). It then encrypts the action registry into an opaque cookie string that clients send back when dispatching actions.

Example post payload

The following payload posts a message with text, a primary button, and a static select menu:

{
  "channel_id": "qmd5oqtwoibz8cuzxzg5ekshgr",
  "message": "Deployment #42 finished.",
  "props": {
    "mm_blocks": [
      {
        "type": "text",
        "text": "Deployed `main` to **staging**. Choose a follow-up action:"
      },
      {
        "type": "container",
        "flow": "horizontal",
        "gap": "small",
        "content": [
          {
            "type": "button",
            "text": "View logs",
            "style": "primary",
            "action_id": "view_logs"
          },
          {
            "type": "button",
            "text": "Rollback",
            "style": "danger",
            "action_id": "rollback"
          },
          {
            "type": "static_select",
            "action_id": "next_step",
            "placeholder": "Select next step…",
            "options": [
              {"text": "Promote to production", "value": "promote"},
              {"text": "Run smoke tests", "value": "smoke"}
            ]
          }
        ]
      }
    ],
    "mm_blocks_actions": {
      "view_logs": {
        "type": "external",
        "url": "https://integration.example.com/actions/view-logs",
        "context": {"deployment_id": "42"}
      },
      "rollback": {
        "type": "external",
        "url": "https://integration.example.com/actions/rollback",
        "context": {"deployment_id": "42"}
      },
      "next_step": {
        "type": "external",
        "url": "https://integration.example.com/actions/next-step",
        "context": {"deployment_id": "42"}
      }
    }
  }
}

You can send this payload using the create post REST API, an incoming webhook, a custom slash command, or from a plugin.

Submit using an incoming webhook

curl -X POST $MM_URL/hooks/$WEBHOOK_ID \
  -H "Content-Type: application/json" \
  -d '{
    "text": "Deployment #42 finished.",
    "props": {
      "mm_blocks": [
        {"type": "text", "text": "Deployed `main` to **staging**."},
        {
          "type": "button",
          "text": "View logs",
          "style": "primary",
          "action_id": "view_logs"
        }
      ],
      "mm_blocks_actions": {
        "view_logs": {
          "type": "external",
          "url": "https://integration.example.com/actions/view-logs",
          "context": {"deployment_id": "42"}
        }
      }
    }
  }'

For webhook payloads, place mm_blocks and mm_blocks_actions inside props. The top-level attachments array remains available for legacy integrations.

Submit from a plugin

post := &model.Post{
    ChannelId: channelID,
    UserId:    p.botID,
    Message:   "Deployment #42 finished.",
    Props: model.StringInterface{
        "mm_blocks": []any{
            map[string]any{
                "type": "text",
                "text": "Deployed `main` to **staging**.",
            },
            map[string]any{
                "type":      "button",
                "text":      "View logs",
                "style":     "primary",
                "action_id": "view_logs",
            },
        },
        "mm_blocks_actions": map[string]any{
            "view_logs": map[string]any{
                "type":    "external",
                "url":     fmt.Sprintf("/plugins/%s/actions/view-logs", manifest.Id),
                "context": map[string]any{"deployment_id": "42"},
            },
        },
    },
}
_, err := p.API.CreatePost(post)

Block types

Each element of props.mm_blocks is an object with a required type field. The supported block types are:

Type	Purpose
`text`	Markdown-formatted text
`image`	Remote image with optional sizing and alignment
`divider`	Horizontal rule between blocks
`button`	Interactive button
`static_select`	Dropdown menu with static options or dynamic data sources
`container`	Groups blocks with optional border, accent bar, background, and layout flow
`collapsible`	Expandable section with separate header and content block arrays
`column_set`	Horizontal row of columns
`column`	Column inside a `column_set` (not valid as a top-level block)

Malformed blocks are omitted at render time; valid sibling blocks still display.

Text

{
  "type": "text",
  "text": "Hello **from** MM Blocks.",
  "is_subtle": false,
  "size": "default"
}

Field	Required	Description
`text`	yes	Markdown-formatted content. Supports @mentions.
`is_subtle`	no	When `true`, renders in a muted color. Does not change font size.
`size`	no	Typography scale: `small` or `default`. Omitted is equivalent to `default`.

Image

{
  "type": "image",
  "url": "https://example.com/logo.png",
  "alt_text": "Company logo",
  "title": "Logo",
  "size": "medium",
  "max_width": 400,
  "max_height": 300,
  "image_style": "default",
  "horizontal_alignment": "center"
}

Field	Required	Description
`url`	yes	Image URL (GIF, JPEG, PNG, BMP, or SVG).
`alt_text`	no	Accessible description.
`title`	no	Plain-text tooltip shown on hover.
`size`	no	Preset sizing: `auto`, `xsmall`, `small`, `medium`, `large`, or `stretch`. Defaults to `stretch`.
`max_width`	no	Maximum width in pixels.
`max_height`	no	Maximum height in pixels.
`image_style`	no	`default` or `person` (avatar-style crop).
`horizontal_alignment`	no	`left`, `center`, or `right`.

Divider

{"type": "divider"}

Button

{
  "type": "button",
  "text": "Approve",
  "action_id": "approve",
  "style": "primary",
  "tooltip": "Approve this change",
  "disabled": false,
  "query": {"ticket": "ISS-101"}
}

Field	Required	Description
`text`	yes	Button label. Supports Markdown.
`action_id`	yes	Must match a key in `mm_blocks_actions`.
`style`	no	Semantic color: `default`, `primary`, `danger`, `good`, `success`, or `warning`. Hex colors such as `#2d81ff` are also accepted.
`tooltip`	no	Help text shown on hover.
`disabled`	no	When `true`, the button renders but cannot be clicked.
`query`	no	Static query parameters merged into the action URL when clicked.

Static select

{
  "type": "static_select",
  "action_id": "pick_region",
  "placeholder": "Pick a region",
  "options": [
    {"text": "North", "value": "north"},
    {"text": "South", "value": "south"}
  ],
  "initial_option": "north",
  "disabled": false,
  "data_source": "channels"
}

Field	Required	Description
`action_id`	yes	Must match a key in `mm_blocks_actions`.
`placeholder`	yes	Placeholder text for the menu.
`options`	depends on	Array of `{text, value}` pairs. Required unless `data_source` is set.
`initial_option`	no	Pre-selected option value.
`disabled`	no	When `true`, the menu renders but cannot be used.
`data_source`	no	Dynamic option source: `channels` or `users`. When set, `options` is optional. Users can only select public channels in their teams.

When a user selects an option, the integration callback receives selected_option in the request context with the chosen value (or user/channel ID for dynamic data sources).

Container

{
  "type": "container",
  "content": [
    {"type": "text", "text": "Container title"},
    {"type": "divider"},
    {"type": "text", "text": "Body copy", "is_subtle": true, "size": "small"}
  ],
  "border": true,
  "accent_color": "primary",
  "background": "gray",
  "flow": "vertical",
  "gap": "small",
  "max_height": "medium"
}

Field	Required	Description
`content`	yes	Array of nested blocks.
`border`	no	When `true`, draws a border around the container.
`accent_color`	no	Left accent bar color. Semantic values: `default`, `primary`, `good`, `warning`, or `danger`. CSS colors such as `#439FE0` are also accepted.
`background`	no	`none` (default) or `gray`.
`flow`	no	Child layout direction: `horizontal` or `vertical`. Defaults to `vertical`.
`gap`	no	Spacing between children: `none`, `small`, `medium`, `large`, or `xlarge`. Defaults to `none`.
`max_height`	no	Maximum height preset: `none`, `small`, `medium`, or `large`. Overflowing content scrolls inside the container. On mobile, users can open scrollable content in a dedicated full-screen view.

Collapsible

{
  "type": "collapsible",
  "collapsed": false,
  "header": [
    {"type": "text", "text": "**Details**"}
  ],
  "content": [
    {"type": "text", "text": "Expanded content goes here."}
  ]
}

Field	Required	Description
`header`	yes	Blocks shown in the always-visible header row.
`content`	yes	Blocks shown when expanded.
`collapsed`	no	Initial collapsed state. Defaults to `false`.

Column set and column

{
  "type": "column_set",
  "gap": "medium",
  "columns": [
    {
      "type": "column",
      "width": "stretch",
      "gap": "small",
      "items": [
        {"type": "text", "text": "Left column"}
      ]
    },
    {
      "type": "column",
      "width": "auto",
      "items": [
        {"type": "text", "text": "Right column"}
      ]
    }
  ]
}

column blocks are only valid inside a column_set. Each column has an items array of nested blocks.

The `mm_blocks_actions` registry

The mm_blocks_actions post prop is a map keyed by action ID. Each entry describes how the server handles clicks on that action.

{
  "mm_blocks_actions": {
    "<action_id>": {
      "type": "<action_type>",
      "url": "...",
      "context": { ... },
      "query": { ... }
    }
  }
}

Field	Required	Description
`type`	yes	Action type. See Action types below.
`url`	depends on type	Target URL. Required for `external` and `openURL`.
`context`	no	Server-side context forwarded to the integration in the post-action request body. Not visible to clients.
`query`	no	Static `string → string` map merged into the target URL’s query string. Combined with any per-control `query` on the block — block values win on key conflict.

Every action ID referenced by interactive content — MM Blocks controls, markdown mmaction:// links, Block Kit actions, or Adaptive Card actions — must have a matching registry entry. Unused registry entries are rejected at post-create time.

After the post is stored, clients receive an encrypted cookie string in place of the plaintext registry map.

Action types

Type	Behavior
`external`	The server sends an HTTP POST request to `url` with the standard post-action request body. The integration responds with a post-action response (update, ephemeral message, or navigation). Relative plugin paths such as `/plugins/myplugin/action` are supported.
`openURL`	Navigates the user without calling an integration. Relative paths (for example `/myteam/channels/off-topic`) navigate inside Mattermost. `http://` and `https://` URLs open in a new browser tab. Plugin paths are not allowed.

Additional action types may be introduced in future releases. Entries with an unknown type value are rejected at post-create time.

Action dispatch flow

Integration creates a post with mm_blocks controls and matching mm_blocks_actions entries.
Mattermost server validates the pairing, encrypts the action registry, and stores the post.
Client renders the blocks. When the user clicks a button or selects a menu option, the client sends POST /api/v4/posts/{post_id}/actions/{action_id} with the encrypted cookie, optional query, selected_option (for menus), and integration_format: "mm_block".
Mattermost server decrypts the cookie, resolves the action, merges query parameters, and either navigates (openURL) or POSTs to the integration endpoint (external).
Integration responds with a standard post-action response.

Action IDs in the URL path must match [A-Za-z0-9_-]+.

Receiving action callbacks

When a user activates an external action, the Mattermost server sends an HTTP POST request to the configured url. The request body uses the same PostActionIntegrationRequest shape as legacy message attachment buttons:

{
  "user_id": "rd49ehbqyjytddasoownkuqrxe",
  "user_name": "alice",
  "channel_id": "j6j53p28k6urx15fpcgsr20psq",
  "channel_name": "town-square",
  "team_id": "5xxzt146eax4tul69409opqjlf",
  "team_domain": "myteam",
  "post_id": "gqrnh3675jfxzftnjyjfe4udeh",
  "trigger_id": "...",
  "type": "button",
  "context": {
    "deployment_id": "42",
    "selected_option": "promote"
  }
}

For static select menus, the selected value is added to context.selected_option.

Integrations respond with the same post-action response format used by attachment actions:

{
  "update": {
    "message": "Updated!",
    "props": {
      "mm_blocks": [
        {"type": "text", "text": "Deployment promoted to production."}
      ]
    }
  },
  "ephemeral_text": "Promotion started.",
  "goto_location": "/myteam/channels/releases"
}

Response field	Description
`update`	Replaces the original post message and props. Use `update.props.mm_blocks` to refresh the block layout.
`ephemeral_text`	Sends a private message visible only to the user who clicked.
`goto_location`	Navigates the user to a URL after the action completes. Supports in-app paths and external URLs.
`error`	Returns a custom error message displayed below the interactive content.
`skip_slack_parsing`	Set to `true` to bypass Slack-compatibility parsing of `ephemeral_text`.

See interactive messages for error handling details and update.props semantics.

Legacy format compatibility

Mattermost continues to accept these older payload formats:

Prop	Format	Notes
`attachments`	Legacy message attachments	Attachment `actions` arrays are translated into MM Blocks buttons and selects at render time.
`blocks`	Slack Block Kit	Translated into MM Blocks. Interactive Block Kit elements require matching `mm_blocks_actions` entries keyed by `action_id`.
`cards`	Microsoft Adaptive Cards	Translated into MM Blocks. Interactive card actions require matching `mm_blocks_actions` entries keyed by action `id`.

New integrations should prefer native mm_blocks for full control over layout and action registration.

Validation limits

Posts that exceed any of the following limits are rejected at create or update time.

Limit	Value
Maximum entries in `mm_blocks_actions`	50
Maximum length of an action ID (map key)	64 characters
Action ID character set	`[A-Za-z0-9_-]+`
Maximum entries in `query` (link, registry, or block)	50
Maximum length of a query key	128 characters
Maximum length of a query value	2048 characters
Maximum entries in `context` per action	50
Maximum length of a context key	128 characters

Security considerations

MM Blocks follow the same security model as legacy interactive messages:

Integration url values are invoked server-to-server, never directly from the client.
context values are server-only and are not exposed to rendering clients.
After create, the plaintext mm_blocks_actions map is replaced with an encrypted cookie.
Action IDs are validated and must match referenced interactive content exactly.
openURL actions reject plugin paths and path-traversal segments.

Use MM Blocks

How it works

Example post payload

Submit using an incoming webhook

Submit from a plugin

Block types

Text

Image

Divider

Button

Static select

Container

Collapsible

Column set and column

The `mm_blocks_actions` registry

Action types

Action dispatch flow

Receiving action callbacks

Legacy format compatibility

Validation limits

Security considerations

See also

Tell us more

Use MM Blocks

How it works

Example post payload

Submit using an incoming webhook

Submit from a plugin

Block types

Text

Image

Divider

Button

Static select

Container

Collapsible

Column set and column

The mm_blocks_actions registry

Action types

Action dispatch flow

Receiving action callbacks

Legacy format compatibility

Validation limits

Security considerations

See also

Tell us more

The `mm_blocks_actions` registry