chat.postMessage

Sends a message to a channel.

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 as_user parameter may not be used by new Slack apps. Read more about Authorship to understand how as_user works for classic Slack apps.

Using text with blocks or attachments

The usage of the text field changes depending on whether you're using blocks. If you are 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.

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.

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

Pass the channel's ID (C024BE91L) 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 multi-person direct message

As long as the authenticated user is a member of the multi-person direct message (AKA "private group" or MPIM), you can pass the group's ID (G012AC86C), 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 are meaning to accomplish.

If you simply want your app's bot user to start a 1:1 conversation with another user in a workspace, provide the user's user ID as the channel value and a direct message conversation will be opened, if it hasn't 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 can't 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 might 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.

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 (U0G9QF9C6) 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 im.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.

With all that in mind, the act of authoring a message as a user is relatively simple. With the aforementioned scope, you'll be able to make calls to chat.postMessage and provide:

• 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.

Legacy concerns

Legacy authorship

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

For classic Slack apps, the best way to control the authorship of a message is to be explicit with the as_user parameter.

If you don't use the as_user parameter, chat.postMessage will 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
  • 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

Legacy identity rules in DMs

If you're using icon_url, icon_emoji, or username with chat.postMessage and a direct message, some special rules apply to make sure the receiver is crystal clear on just who is sending the message:

• If as_user is false:
  • Pass the DM channel's ID (D023BB3L2) 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 as_user is true:
  • Pass the DM channel's ID (D023BB3L2) or a user's ID (U0G9QF9C6) 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.

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.