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Provide custom unfurl behavior for user-posted URLs


Method URL:
Preferred HTTP method: POST
Accepted content types: application/x-www-form-urlencoded, application/json
Rate limiting: Tier 3
Works with:
Token type Required scope(s)
workspace links:write
user links:write

This method attaches Slack app unfurl behavior to a specified and relevant message. A user token is required as this method does not support bot user tokens.

The first time this method is executed with a particular ts and channel combination, the valid unfurls attachments you provide will be attached to the message. Subsequent attempts with the same ts and channel values will modify the same attachments, rather than adding more.


Argument Example Required Description
token xxxx-xxxxxxxxx-xxxx Required

Authentication token bearing required scopes.

channel C1234567890 Required

Channel ID of the message

ts   Required

Timestamp of the message to add unfurl behavior to.

unfurls   Required

URL-encoded JSON map with keys set to URLs featured in the the message, pointing to their unfurl message attachments.

user_auth_message   Optional

Provide a simply-formatted string to send as an ephemeral message to the user as invitation to authenticate further and enable full unfurling behavior

user_auth_required true Optional, default=0

Set to true or 1 to indicate the user must install your Slack app to trigger unfurls for this domain

user_auth_url Optional

Send users to this custom URL where they will complete authentication in your app to fully trigger unfurling. Value should be properly URL-encoded.

This method supports application/json via HTTP POST. Present your token in your request's Authorization header. Learn more.

The provided ts value must correspond to a message in the specified channel. Additionally, the message must contain a fully-qualified URL pointing to a domain that is already registered and associated with your Slack app.

The unfurls parameter expects a URL-encoded string of JSON. Unlike chat.postMessage's attachments parameter, it does not expect a JSON array but instead, a hash keyed on the specific URLs you're offering an unfurl for. Each URL can have a single attachment, including message buttons.

Consult the unfurling docs for more guidance on this parameter.

The user_auth_required parameter is optional. By providing a 1 or true value, it will require the user posting the link first authenticate themselves with your app. See also the authenticated unfurling docs.

To point users to a specific page on your server to authenticate, pass a valid URL using the user_auth_url parameter. When sending this parameter via application/x-www-form-urlencoded GETs or POSTs, values must be URL-encoded such that becomes

Or, you can send an ephemeral message to that user by providing a simple user_auth_message value. Simple slack message formatting like *bold*, _italics_, and linking is supported, so you can wrap your custom URLs in a blanket of situationally accurate, actionable text.

Specifying user_auth_url or user_auth_message will automatically imply user_auth_required being set to true. If both user_auth_url and user_auth_message are provided, user_auth_message takes precedence.


Typical, minimal success response

    "ok": true

Typical error response

    "ok": false,
    "error": "cannot_unfurl_url"

As you can see, we provide a minimal positive response when your unfurl attempt is successful. When it is not, you'll receive one of the errors below and ok will be false.


This table lists the expected errors that this method could return. However, other errors can be returned in the case where the service is down or other unexpected factors affect processing. Callers should always check the value of the ok params in the response.

Error Description

The specified channel could not be located for this token.


The URL cannot be unfurled. This error may be returned if you haven't acknowledged a link_shared event tied to the same URL. It is also returned when the domain appears in a workspace's administrative blacklists.


The ts value in the request does not match a message.


A record of your app being allowed to unfurl for this workspace could not be found.


The request is missing the unfurls parameter.


The current user has already interacted with and dismissed a prompt for this application.


The provided unfurls argument could not be parsed or understood.


No authentication token provided.


Some aspect of authentication cannot be validated. Either the provided token is invalid or the request originates from an IP address disallowed from making the request.


Authentication token is for a deleted user or workspace.


Authentication token is for a deleted user or workspace or the app has been removed.


The workspace token used in this request does not have the permissions necessary to complete the request. Make sure your app is a member of the conversation it's attempting to post a message to.


The workspace is undergoing an enterprise migration and will not be available until migration is complete.


This method cannot be called by a bot user.


The method was passed an argument whose name falls outside the bounds of accepted or expected values. This includes very long names and names with non-alphanumeric characters other than _. If you get this error, it is typically an indication that you have made a very malformed API call.


The method was passed a PHP-style array argument (e.g. with a name like foo[7]). These are never valid with the Slack API.


The method was called via a POST request, but the charset specified in the Content-Type header was invalid. Valid charset names are: utf-8 iso-8859-1.


The method was called via a POST request with Content-Type application/x-www-form-urlencoded or multipart/form-data, but the form data was either missing or syntactically invalid.


The method was called via a POST request, but the specified Content-Type was invalid. Valid types are: application/x-www-form-urlencoded multipart/form-data text/plain.


The method was called via a POST request and included a data payload, but the request did not include a Content-Type header.


The workspace associated with your request is currently undergoing migration to an Enterprise Organization. Web API and other platform operations will be intermittently unavailable until the transition is complete.


The method was called via a POST request, but the POST data was either missing or truncated.


The server could not complete your operation(s) without encountering a catastrophic error. It's possible some aspect of the operation succeeded before the error was raised.


This table lists the expected warnings that this method will return. However, other warnings can be returned in the case where the service is experiencing unexpected trouble.

Warning Description

The method was called via a POST request, and recommended practice for the specified Content-Type is to include a charset parameter. However, no charset was present. Specifically, non-form-data content types (e.g. text/plain) are the ones for which charset is recommended.


The method was called via a POST request, and the specified Content-Type is not defined to understand the charset parameter. However, charset was in fact present. Specifically, form-data content types (e.g. multipart/form-data) are the ones for which charset is superfluous.