chat.postMessage

Facts

This method posts a message to a public channel, private channel, or direct message/IM channel.

Arguments

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

Please note that the default value of the as_user parameter varies depending on the kind of token you're using. It's best to be explicit with this value. Read more about Authorship to understand how its default value may vary.

A message must have either text or attachments or both. The text parameter is required unless you provide attachments. You can use both parameters in conjunction with each other to create awesome messages.

JSON POST support

As of October 2017, it's now 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.

Now you can send messages lovingly authored with the message builder to chat.postMessage without further modification.

Learn more about this support in the Web API docs or this changelog.

Response

{
    "ok": true,
    "channel": "C1H9RESGL",
    "ts": "1503435956.000247",
    "message": {
        "text": "Here's a message for you",
        "username": "ecto1",
        "bot_id": "B19LU7CSY",
        "attachments": [
            {
                "text": "This is an attachment",
                "id": 1,
                "fallback": "This is an attachment's fallback"
            }
        ],
        "type": "message",
        "subtype": "bot_message",
        "ts": "1503435956.000247"
    }
}

{
    "ok": false,
    "error": "too_many_attachments"
}

The response includes the "timestamp ID" (ts) and the channel-like thing where the message was posted. It also includes the complete message object, as parsed by our servers. This may differ from the provided arguments as our servers sanitize links, attachments, and other properties. Your message may mutate.

Formatting messages

Messages are formatted as described in the formatting spec. You can specify values for parse and link_names to change formatting behavior.

When POSTing with application/x-www-form-urlencoded data, the optional attachments argument should contain a JSON-encoded array of attachments. Make it easy on yourself and send your entire messages as application/json instead.

For more information, see the attachments spec. If you're using a Slack app, you can also use this method to attach message buttons.

By default links to media are unfurled, but links to text content are not. For more information on the differences and how to control this, see the the unfurling documentation.

Use the Message Builder to preview your message formatting and attachments in real time! It's easy to translate your JSON examples to the parameters understood by chat.postMessage.

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.

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

Authorship

How message authorship is attributed varies by a few factors, with some behaviors varying depending on the kinds of tokens you're using to post a message.

The best way to realize your intended result is to be explicit with the as_user parameter.

chat.postMessage wants your message posting to succeed and may attempt to guess the most appropriate as_user interpretation based on the kind of token you're using.

If as_user is not provided at all, then the value is inferred, based on the scopes granted to the caller: If the caller could post with as_user passed as false, then that is how the method behaves; otherwise, the method behaves as if as_user were passed as true.

When as_user is false

When the as_user parameter is set to false, messages are posted as "bot_messages", with message authorship attributed to the user name and icons associated with the Slack App.

With as_user set to false, you may also provide a username to explicitly specify the bot user's identity for this message, along with icon_url or icon_emoji.

Effect on identity

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

• test tokens
  • generic user icon and "bot" username
• Slack App user token with chat:write:bot
  • inherits Slack App's icon, with generic "bot" username (see below)
• Slack App bot user token
  • inherits Slack App's icon, with generic "bot" username (see below)

Note: In the Slack App cases above, it would certainly make more sense for your application's name to be the default username associated with your app. This inconsistent behavior will be corrected. Of course, you can still name your bot "bot," if that is your bot's name.

When as_user is true

Set as_user to true and the authenticated user will appear as the author of the message, ignoring any values provided for username, icon_url, and icon_emoji. Posting as the authenticated user requires the client or the more preferred chat:write:user scopes.

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 icon and username of the token owner
• Slack App bot user token
  • inherits Slack App's icon and app's bot username

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 message threading 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:

Post to a public channel

You can either pass the channel's name (#general) or encoded ID (C024BE91L), and the message will be posted to that channel. The channel's ID can be retrieved through the channels.list API method.

Post to a private group

As long as the authenticated user is a member of the private group, you can either pass the group's name (secret-group) or encoded ID (G012AC86C), and the message will be posted to that group. The private group's ID can be retrieved through the groups.list API method.

Post to an IM channel

Warning: here be dragons. Posting to an IM channel is a little more complex depending on the value of as_user and the type of token associated with your app.

• If as_user is false:
  • Pass the IM channel's ID (D023BB3L2) as the value of channel to post to that IM 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 im.list API method.
• If as_user is true (for workspace apps, this is always the case):
  • Pass the IM channel's ID (D023BB3L2) or a user's ID (U0G9QF9C6) as the value of channel to post to that IM channel as the app, bot, or user associated with the token. The IM channel's ID can be retrieved through the im.list API method. When as_user is true, the caller may not manipulate the icon and username on the message. You might receive a channel_not_found error if your app doesn't have permission to enter into an IM with the intended user.

To send a direct message to the user owning the token used in the request, provide the channel field with a conversation/IM ID value found in a method like im.list.

We are phasing out support for ambiguously passing a "username" as a channel value. Please always use channel-like IDs instead.

Begin a conversation in a user's App Home

Workspace apps can use chat.postMessage as a one-stop shop for conversing with users. Your app can initiate a DM and then continue the conversation without switching to a different method.

Request the conversations.app_home:create scope during the typical OAuth or installation flow. Once that's granted, your workspace app can use the chat.postMessage method to start a conversation with a user from scratch in their App Home space. No separate call to im.open necessary. Simply call chat.postMessage and pass a user's ID (U0G9QF9C6) as the value of channel to post to that user's App Home channel.

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.

Errors

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.

Warnings

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.