Slack Block Kit vs Microsoft Adaptive Cards: The Cross-Platform Formatting Problem

Why Message Formatting Matters for Interoperability

A basic bidirectional message bridge that passes plain text between Slack and Teams is technically achievable in a weekend project. The real engineering challenge is rich formatting: bold text, code blocks, bullet lists, file attachments, @mentions, hyperlinks, and interactive elements.

Each enterprise messaging platform has a different JSON schema for representing formatted content. Slack uses Block Kit. Microsoft Teams uses Adaptive Cards. Google Chat uses Cards V2. Zoom Team Chat uses its own markdown-adjacent format.

When a Slack message with a code block, a bullet list, and an @mention to a user arrives at the bridge and needs to be delivered to a Teams channel, the bridge must translate from Block Kit JSON to Adaptive Card JSON while preserving the semantic meaning of each formatting element. This translation is where low-quality integration tools fail — they either drop all formatting (producing unreadable plain text) or fail silently on edge cases.

Slack Block Kit Architecture

Block Kit is Slack's structured message layout system, introduced in 2019. A Block Kit message is a JSON array of "blocks," each of which represents a layout component.

Core block types

Section block — the primary text content block:

{
  "type": "section",
  "text": {
    "type": "mrkdwn",
    "text": "This is *bold* text with a `code span`"
  }
}

Divider block — a horizontal rule:

{ "type": "divider" }

Context block — smaller, secondary text (author attribution, timestamps):

{
  "type": "context",
  "elements": [
    { "type": "mrkdwn", "text": "Posted by *Jordan Hayes* at 2:30 PM" }
  ]
}

File block — represents a shared file:

{
  "type": "file",
  "external_id": "ABCD1234",
  "source": "remote"
}

Slack mrkdwn vs standard markdown

Slack uses a proprietary markdown variant called mrkdwn that differs from CommonMark in important ways:

The @mention and hyperlink formats are particularly important for cross-platform translation — they contain user IDs and channel IDs that must be resolved to the destination platform's equivalent.

Microsoft Adaptive Cards Architecture

Adaptive Cards is Microsoft's cross-platform card specification, used in Teams, Outlook, and other Microsoft 365 products. Like Block Kit, it represents formatted content as a JSON structure — but with a different schema and different design philosophy.

Core element types

TextBlock — the primary text element:

{
  "type": "TextBlock",
  "text": "This is **bold** text in Adaptive Cards markdown",
  "wrap": true
}

Adaptive Cards TextBlock uses CommonMark-compatible markdown (double asterisks for bold), not Slack's mrkdwn format.

ColumnSet and Column — for multi-column layouts (used for sender attribution):

{
  "type": "ColumnSet",
  "columns": [
    {
      "type": "Column",
      "width": "auto",
      "items": [
        { "type": "TextBlock", "text": "Jordan Hayes", "weight": "bolder" }
      ]
    },
    {
      "type": "Column",
      "width": "stretch",
      "items": [
        { "type": "TextBlock", "text": "via Slack", "color": "accent", "size": "small" }
      ]
    }
  ]
}

Image — for inline images and thumbnails:

{
  "type": "Image",
  "url": "https://example.com/image.png",
  "size": "medium"
}

The Translation Matrix

Every formatting element that exists in Slack must have a corresponding representation in Adaptive Cards, and vice versa. The translation matrix for a production-grade bridge:

The @mention translation problem

@mention translation is the most complex element to handle correctly. In Slack, a mention is encoded as <@U01234567>, where U01234567 is the Slack user ID. To translate this to a Teams @mention:

1. Resolve the Slack user ID to an email address via the Slack Users API
2. Resolve the email address to a Teams AAD object ID via the Microsoft Graph Users API
3. Construct the Teams mention in the Adaptive Card:

{
  "body": [
    {
      "type": "TextBlock",
      "text": "Hello <at>Jordan Hayes</at>, the build is complete."
    }
  ],
  "msteams": {
    "entities": [
      {
        "type": "mention",
        "text": "<at>Jordan Hayes</at>",
        "mentioned": {
          "id": "29:aad-object-id-here",
          "name": "Jordan Hayes"
        }
      }
    ]
  }
}

The mention entity must appear both in the text content (as <at>Name</at>) and in the msteams.entities array. If either is missing, Teams will not render the mention with a notification — it will appear as plain text.

What Gets Lost in Translation

Some formatting elements have no equivalent on the other platform:

• Slack's Action blocks (interactive buttons) — Teams can render Action Sets, but the interaction model is different. Action blocks in bridged messages are typically rendered as plain text links.
• Teams' Adaptive Card Facts — key/value pairs with no Block Kit equivalent; best translated as a bullet list
• Slack's App surfaces (Home tab, modals) — these are not message content and are not bridged
• Custom emoji — platform-specific emoji (:slack-emoji:) have no cross-platform equivalent; substitute with nearest Unicode emoji or strip

The Rendering Fidelity Tradeoff

A production messaging bridge must make an explicit architectural decision about rendering fidelity: how much formatting complexity to preserve vs. how much to simplify for reliability.

SyncRivo's translation layer makes the following tradeoffs:

• Plain text, bold, italic, code spans, and hyperlinks: full fidelity
• @mentions: full fidelity (with identity resolution)
• File attachments: full fidelity (with file transfer)
• Complex multi-column layouts: simplified to linear format
• Interactive elements (buttons, selects): stripped to link text

This tradeoff ensures that the bridge never fails on a message it cannot render perfectly — it degrades gracefully to simpler formatting rather than silently dropping the message.

Read the Teams Graph API deep dive → | See Slack ↔ Teams integration →