Slack Markdown File Preview: How to Read .md Files in Slack

Slack does not render markdown files. When you upload a .md file, Slack shows it as a text file, with the #, ** and - characters visible, the same as a .txt. To read a markdown file formatted inside Slack, put it in a canvas: copy the rendered text from a preview (GitHub or VS Code) and paste it into a canvas, or have an app create the canvas from the markdown with Slack's canvases.create API. For a bot message, the Block Kit markdown block formats standard markdown up to 12,000 characters.

HTML files and Mermaid diagrams do not render in Slack either. For those, share a link or an image.

We build Slack Green. For a markdown snippet going into a normal message, our free Markdown to Slack converter rewrites it in Slack's own syntax.

Where markdown renders in Slack

Snippets are Slack's way to share code, config files and logs. Slack's help says you choose a file type for syntax highlighting, so a snippet set to Markdown gets colors, not formatting.

Read a .md file formatted in a canvas

• Open a rendered preview of the file. GitHub shows .md files formatted. In VS Code, press Cmd+Shift+V (Ctrl+Shift+V on Windows) for the Markdown preview.
• Select the rendered text and copy it. You copy formatted text, not the raw symbols.
• Create a canvas: open Canvases in the sidebar, or the canvas icon at the top of a channel, and paste.
• Share the canvas in the channel or DM.

The same copy-the-rendered-text trick works in the message box for short notes, because Slack keeps bold, links and lists from formatted text you paste. Tables and headings do not exist in messages, so long documents belong in a canvas.

Create a canvas from a markdown file with the API

Slack's canvas API takes markdown directly. canvases.create accepts a document_content object with "type": "markdown", up to 1 MiB of text. The supported elements include headings h1 to h3, bold, italic, strikethrough, bulleted and ordered lists, checklists, code blocks, quotes, dividers, links, tables and @mentions.

# pip install slack_sdk ; needs a bot token with the canvases:write scope
from pathlib import Path
from slack_sdk import WebClient

client = WebClient(token="xoxb-your-bot-token")
text = Path("README.md").read_text()

resp = client.canvases_create(
    title="README",
    document_content={"type": "markdown", "markdown": text},
)
print(resp["canvas_id"])

Canvas tables are limited to 300 cells. Block Kit does not work inside canvases, and Slack's docs note that mentions use a slightly different syntax there. To attach the canvas to a channel instead of creating a standalone one, use conversations.canvases.create with the channel ID.

Send markdown as a bot message

For shorter text, such as a changelog or an AI answer, send it in a markdown block and Slack formats it:

{
  "channel": "C0123456789",
  "blocks": [
    { "type": "markdown", "text": "## Release 3.4\n- Faster search\n- **Fixed:** login loop" }
  ]
}

The limit is 12,000 characters for all markdown blocks in one message. The syntax differences between this block and Slack's older mrkdwn are in Slack Block Kit markdown.

Can Slack render HTML or Mermaid?

No. An uploaded .html file shows its source code, and Slack does not run or display the page. A mermaid code block shows as text. The options:

Paste markdown into a normal message

For a few lines of markdown, paste into the message box and press Cmd+Shift+F (Ctrl+Shift+F), Apply markdown formatting. Slack converts bold, links and lists. In the composer's markup mode you can also type [text](link); app and bot messages still need <url|text>. The full walkthrough, with what survives and what does not, is in how to paste ChatGPT into Slack, and the reasons markdown sometimes shows raw are in Slack markdown not working.

FAQ

Can Slack preview markdown files? It previews them as plain text. The markdown is not formatted.

How do I view a markdown file formatted in Slack? Copy the rendered text from a preview into a canvas, or create the canvas from the file with canvases.create.

Can Slack render HTML? No. Share a link to the hosted page, or a screenshot.

Can Slack render Mermaid diagrams? No. Export the diagram as an image and upload it.

Does a Slack canvas support markdown? Yes. Canvases support headings, lists, tables, checklists and code, and the canvas API takes markdown directly.