I'm trying to use the Slack API to mention/tag a user in a canvas so others can click the name to reach out (open profile or DM). Manually typing @UserName in the canvas UI works, creating a clickable mention, but I need to create or edit canvases dynamically using conversations.canvases.create or canvases.edit. I've tried using <@U1234567890> (the standard format for messages), but it renders as plain text in the canvas (e.g., "<@U1234567890>"). Other formats like @<U1234567890>, @[U1234567890], [@U1234567890], @UserName, ![user:U1234567890], and <user:U1234567890> also render as plain text. I also tried rich_text document_content, but got an invalid_arguments error.
My bot has a valid token with scopes like channels:write, users:read, canvases:write, and can tag users in channel messages without issue. The API accepts my payload, returning { "ok": true }, but the mention isn’t clickable, and is just the user ID in plain text. Here’s my JSON body for canvases.edit (using placeholder IDs):
{
"canvas_id": "F1234567890",
"changes": [
{
"operation": "replace",
"document_content": {
"type": "markdown",
"markdown": "User: <@U1234567890>"
}
}
]
}
This is a POST request to https://slack.com/api/canvases.edit with headers: Content-Type: application/json;charset=utf-8. Without charset=utf-8, I get a missing_charset error. I’m using a Node.js app with @slack/web-api@7.9.2 (latest as of June 2025). The Slack Canvas API docs don’t mention user mention syntax.
Is there a supported format for programmatic user mentions in Slack canvases, or does the Canvas API Markdown parser not recognize mention syntax? Any insights or workarounds?
The Slack Canvas API docs don’t mention user mention syntax.
This here appears to be the applicable part, https://api.slack.com/surfaces/canvases#markdown-formatting:
For Slack-specific elements, the markdown syntax for canvases is a bit different. For example, a channel mention, where the channel ID is C123ABC456:
Why not join And a user mention, where the user ID is U123ABCDEFG:
