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chat.postMessage method

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Usage info​

This method posts a message to a public channel, private channel, or direct message (DM, or IM) conversation.

Consider reviewing our message guidelines, especially if you're using attachments or message buttons.

The text, blocks and attachments fields​

The usage of the text field changes depending on whether you're using blocks. If you're using blocks, this is used as a fallback string to display in notifications. If you aren't, this is the main body text of the message. It can be formatted as plain text, or with mrkdwn.

The text field is not enforced as required when using blocks or attachments. However, we highly recommended that you include text to provide a fallback when using blocks, as described above.

Accessibility considerations​

It is expected behavior that screen readers will default to the top-level text field of your post, and will not read the content of any interior blocks in the underlying structure of the message. Therefore, to make an accessible app, you must either:

  • include all necessary content for screen reader users in the top-level text field of your message, or
  • do not include a top-level text field if the message has blocks, and allow Slack attempt to build it for you by appending content from supported blocks to be read by the screen reader.

JSON POST support​

When POSTing with application/x-www-form-urlencoded data, the optional attachments argument should contain a JSON-encoded array of attachments.

Send JSON!

As of October 2017, it's possible to send a well-formatted application/json POST body to chat.postMessage and other Web API write methods. No need to carefully URL-encode your JSON attachments and present all other fields as URL encoded key:value pairs; just send JSON instead.

Response​

Formatting messages​

Messages are formatted as described in the formatting spec. The formatting behavior will change depending on the value of parse.

By default, URLs will be hyperlinked. Set parse to none to remove the hyperlinks.

The behavior of parse is different for text formatted with mrkdwn. By default, or when parse is set to none, mrkdwn formatting is implemented. To ignore mrkdwn formatting, set parse to full.

Unfurling content​

By default, we unfurl all links in any messages posted by users and Slack apps. We also unfurl links to media-based content within Block kit blocks.

If you want to suppress link unfurls in messages containing Block Kit blocks, set unfurl_links and unfurl_media to false.

For more detailed information about link unfurling, refer to unfurling links in messages.

Truncating content​

For best results, limit the number of characters in the text field to 4,000 characters. Ideally, messages should be short and human-readable. Slack will truncate messages containing more than 40,000 characters. If you need to post longer messages, please consider uploading a snippet instead.

If using blocks, the limit and truncation of characters will be determined by the specific type of block.

Threads and replies​

Provide a thread_ts value for the posted message to act as a reply to a parent message. Sparingly, set reply_broadcast to true if your reply is important enough for everyone in the channel to receive.

See threading message for a more in-depth look at message threading.

Channels​

You must specify a public channel, private channel, or an IM channel with the channel argument. Each one behaves slightly differently based on the authenticated user's permissions and additional arguments, as discussed in the sections below.

Post to a public channel​

Pass the channel name or the channel's ID (C123456) to the channel parameter and the message will be posted to that channel. The channel's ID can be retrieved through the conversations.list API method.

Post to a private channel​

As long as the authenticated user is a member of the private channel, pass the channel's ID (C123456) to the channel parameter and the message will be posted to that channel. The private channel's ID can be retrieved through the conversations.list API method.

Post to a multi-person direct message channel​

As long as the authenticated user is a member of the multi-person direct message (a "private group" or MPIM), you can pass the group's ID (G123456) and the message will be posted to that group. The private group's ID can be retrieved through the conversations.list API method.

Post to a direct message channel​

Posting to direct messages (also known as DMs or IMs) can be a little more complex, depending on what you actually want to accomplish.

If you want your app's bot user to start a 1:1 conversation with another user in a workspace, provide the user's ID as the channel value and a direct message conversation will be opened if it isn't open already. Resultant messages and associated direct message objects will have a direct message ID you can use from that point forward, if you'd rather.

Bot users cannot post to a direct message conversation between two users using chat.postMessage. If your app was involved in the conversation, then it would be a multi-person direct message instead. Apps can post to direct message conversations between users when a shortcut or slash command belonging to that app is used in the conversation.

You will receive a channel_not_found error if your app doesn't have permission to enter into a DM with the intended user.

Passing a "username" as a channel value is deprecated, along with the whole concept of usernames on Slack.

Please always use channel-like IDs instead to make sure your message gets to where it's going.

Getting a user's ID​

A list of user IDs can be retrieved via the users.list API method.

Begin a conversation in a user's App Home​

Start a conversation with users in your App Home.

With the chat:write scope enabled, call chat.postMessage and pass a user's ID (U123456) as the value of channel to post to that user's App Home channel. You can use their direct message channel ID (as found with coversations.open, for instance) instead.

Rate limiting​

chat.postMessage has special rate limiting conditions. It will generally allow an app to post 1 message per second to a specific channel. There are limits governing your app's relationship with the entire workspace above that, limiting posting to several hundred messages per minute. Generous burst behavior is also granted.

Channel membership​

New Slack apps do not begin life with the ability to post in all public channels.

For your new Slack app to gain the ability to post in all public channels, request the chat:write.public scope.

Sending messages as other entities​

Apps can publish messages that appear to have been created by a user in the conversation. The message will be attributed to the user and show their profile photo beside it.

This is a powerful ability and must only be used when the user themselves gives permission to do so. For this reason, this ability is only available when an app has requested and been granted an additional scope — chat:write.customize.

Your app should only use this feature in response to an inciting user action. It should never be unexpected or surprising to a user that a message was posted on their behalf, and it should be heavily signposted in advance.

To modify the appearance of the app, make calls to chat.postMessage while providing any of the following parameters:

  • username to specify the username for the published message.
  • icon_url to specify a URL to an image to use as the profile photo alongside the message.
  • icon_emoji to specify an emoji (using colon shortcodes, eg. :white_check_mark:) to use as the profile photo alongside the message.

If the channel parameter is set to a User ID (beginning with U), the message will appear in that user's direct message channel with Slackbot. To post a message to that user's direct message channel with the app, use the DM ID (beginning with D) instead.

Legacy concerns​

Information in the section below applies only to classic apps.

This feature works differently for classic apps.

Legacy authorship​

Classic apps using the umbrella bot scope can't request additional scopes to adjust message authorship.

Legacy as_user parameter​

For classic apps, the best way to control the authorship of a message was to be explicit with the legacy as_user parameter. If you didn't use the as_user parameter, chat.postMessage would guess the most appropriate as_user interpretation based on the kind of token you were using. If as_user was not provided at all, the value was inferred based on the scopes granted to the caller: If the caller could post with as_user passed as false, then that was how the method behaved; otherwise, the method behaved as if as_user were passed as true.

When the as_user parameter was set to false, messages were posted as "bot_messages", with message authorship attributed to the user name and icons associated with the classic app.

Effect on identity​

Token types provide varying default identity values for username, icon_url, and icon_emoji.

  • Test tokens inherits the icon and username of the token owner.
  • Slack App user token with chat:write:user inherits the icon and username of the token owner.
  • Slack App bot user token inherits Slack App's icon and app's bot username.

Legacy identity rules in DMs​

If using icon_url, icon_emoji, or username with chat.postMessage and a direct message, some special rules apply to ensure the receiver is crystal clear about who is sending the message:

  • If the legacy as_user argument was false:
    • Pass the DM channel's ID (D123456) as the value of channel to post to that DM channel as the app, bot, or user associated with the token. You can change the icon and username that go with the message using the icon_url and username parameters. The IM channel's ID can be retrieved through the conversations.list API method.
  • If the legacy as_user parameter was true:
    • Pass the DM channel's ID (D123456) or a user's ID (U123456) as the value of channel to post to that DM channel as the app, bot, or user associated with the token. The IM channel's ID can be retrieved through the conversations.list API method. When as_user is true, the caller may not manipulate the icon and username on the message.