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chat.postMessage method

Facts​

DescriptionSends a message to a channel.
Method Access
POST https://slack.com/api/chat.postMessage
Scopes
Bot token:
User token:
Content types
application/x-www-form-urlencoded
application/json
Rate LimitsSpecial rate limits apply.

Arguments​

Required arguments

tokenstringRequired

Authentication token bearing required scopes. Tokens should be passed as an HTTP Authorization header or alternatively, as a POST parameter.

Example: xxxx-xxxxxxxxx-xxxx
channelstringRequired

An encoded ID or channel name that represents a channel, private group, or IM channel to send the message to. See below for more details.

Optional arguments

as_userbooleanOptional

(Legacy) Pass true to post the message as the authed user instead of as a bot. Defaults to false. Can only be used by classic apps. See legacy as_user parameter below.

Example: true
attachmentsOptional

A JSON-based array of structured attachments, presented as a URL-encoded string.

Example: [{"pretext": "pre-hello", "text": "text-world"}]
blocksOptional

A JSON-based array of structured blocks, presented as a URL-encoded string.

Example: [{"type": "section", "text": {"type": "plain_text", "text": "Hello world"}}]
current_draft_last_updated_tsstringOptional

This field represents the timestamp of the draft's last update at the time this API is called. If the current message is a draft, this field can be provided to ensure synchronization with the server.

Example: 1524523204.000192
icon_emojiOptional

Emoji to use as the icon for this message. Overrides icon_url.

Example: :chart_with_upwards_trend:
icon_urlOptional

URL to an image to use as the icon for this message.

Example: http://lorempixel.com/48/48
link_namesbooleanOptional

Find and link user groups. No longer supports linking individual users; use syntax shown in Mentioning Users instead.

Example: true
markdown_textstringOptional

Accepts message text formatted in markdown. This argument should not be used in conjunction with blocks or text. Limit this field to 12,000 characters.

Example: **This is bold text**
metadataOptional

JSON object with event_type and event_payload fields, presented as a URL-encoded string. You can also provide Work Object entity metadata using this parameter. Metadata you post to Slack is accessible to any app or user who is a member of that workspace.

Example: {"event_type": "task_created", "event_payload": { "id": "11223", "title": "Redesign Homepage"}}
mrkdwnbooleanOptional

Disable Slack markup parsing by setting to false. Enabled by default.

Default: true
Example: false
parseOptional

Change how messages are treated. See below.

Example: full
reply_broadcastbooleanOptional

Used in conjunction with thread_ts and indicates whether reply should be made visible to everyone in the channel or conversation. Defaults to false.

Example: true
textOptional

How this field works and whether it is required depends on other fields you use in your API call. See below for more detail.

Example: Hello world
thread_tsOptional

Provide another message's ts value to make this message a reply. Avoid using a reply's ts value; use its parent instead.

unfurl_linksbooleanOptional

Pass true to enable unfurling of primarily text-based content.

Example: true
unfurl_mediabooleanOptional

Pass false to disable unfurling of media content.

Example: false
usernameOptional

Set your bot's user name.

Example: My Bot

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.

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 conversations.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.

The chat.delete method has no accommodations for impersonation. If a message is sent impersonating another user, you will not be able to call chat.delete to delete that same message.

Response​

Response including 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.

{
  "ok": true,
  "channel": "C123ABC456",
  "ts": "1503435956.000247",
  "message": {
    "text": "Here's a message for you",
    "username": "ecto1",
    "bot_id": "B123ABC456",
    "attachments": [
      {
        "text": "This is an attachment",
        "id": 1,
        "fallback": "This is an attachment's fallback"
      }
    ],
    "type": "message",
    "subtype": "bot_message",
    "ts": "1503435956.000247"
  }
}

Typical error response if too many attachments are included

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

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 parameter in the response.

Error
Description
access_denied

Access to a resource specified in the request is denied.

accesslimited

Access to this method is limited on the current network

account_inactive

Authentication token is for a deleted user or workspace when using a bot token.

as_user_not_supported

The as_user parameter does not function with workspace apps.

attachment_payload_limit_exceeded

Attachment payload size is too long.

cannot_reply_to_message

This message type cannot have thread replies.

channel_not_found

Value passed for channel was invalid.

deprecated_endpoint

The endpoint has been deprecated.

draft_already_deleted

The draft has already been deleted.

draft_already_sent

The draft has already been sent.

draft_has_conflict

The client draft version is out of sync with the server draft version.

draft_not_found

The draft was not found.

duplicate_channel_not_found

Channel associated with client_msg_id was invalid.

duplicate_message_not_found

No duplicate message exists associated with client_msg_id.

ekm_access_denied

Your message couldn’t be sent because your admins have disabled sending messages to this channel.

ekm_access_denied

Administrators have suspended the ability to post a message.

enterprise_is_restricted

The method cannot be called from an Enterprise.

fatal_error

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.

internal_error

The server could not complete your operation(s) without encountering an error, likely due to a transient issue on our end. It's possible some aspect of the operation succeeded before the error was raised.

invalid_arg_name

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.

invalid_arguments

The method was called with invalid arguments.

invalid_array_arg

The method was passed an array as an argument. Please only input valid strings.

invalid_auth

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.

invalid_blocks

Blocks submitted with this message are not valid.

invalid_blocks_format

The blocks is not a valid JSON object or doesn't match the Block Kit syntax.

invalid_charset

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.

invalid_form_data

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.

invalid_metadata_format

Invalid metadata format provided.

invalid_metadata_schema

Invalid metadata schema provided.

invalid_post_type

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

is_archived

Channel has been archived.

markdown_text_conflict

Markdown text cannot be used in conjunction with blocks or text argument.

message_limit_exceeded

Members on this team are sending too many messages. For more details, see https://slack.com/help/articles/115002422943-Usage-limits-for-free-workspaces.

messages_tab_disabled

Messages tab for the app is disabled.

metadata_must_be_sent_from_app

Message metadata can only be posted or updated using an app-level token.

metadata_too_large

Metadata exceeds size limit.

method_deprecated

The method has been deprecated.

missing_file_data

Attempted to share a file but some required data was missing.

missing_post_type

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

missing_scope

The token used is not granted the specific scope permissions required to complete this request.

msg_blocks_too_long

Blocks submitted with this message are too long.

no_permission

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.

no_text

No message text provided.

not_allowed_token_type

The token type used in this request is not allowed.

not_authed

No authentication token provided.

not_in_channel

Cannot post user messages to a channel they are not in.

org_login_required

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

rate_limited

Application has posted too many messages, read the Rate Limit documentation for more information.

ratelimited

The request has been ratelimited. Refer to the Retry-After header for when to retry the request.

request_timeout

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

restricted_action

A workspace preference prevents the authenticated user from posting.

restricted_action_non_threadable_channel

Cannot post thread replies into a non_threadable channel.

restricted_action_read_only_channel

Cannot post any message into a read-only channel.

restricted_action_thread_locked

Cannot post replies to a thread that has been locked by admins.

restricted_action_thread_only_channel

Cannot post top-level messages into a thread-only channel.

service_unavailable

The service is temporarily unavailable

slack_connect_canvas_sharing_blocked

Admin has disabled Canvas File sharing in all Slack Connect communications.

slack_connect_file_link_sharing_blocked

Admin has disabled Slack File sharing in all Slack Connect communications.

slack_connect_lists_sharing_blocked

Admin has disabled Lists sharing in all Slack Connect communications.

team_access_not_granted

The token used is not granted the specific workspace access required to complete this request.

team_added_to_org

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.

team_not_found

This error occurs if, when using an org-wide token, the channel_name is passed instead of the channel_id.

token_expired

Authentication token has expired

token_revoked

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

too_many_attachments

Too many attachments were provided with this message. A maximum of 100 attachments are allowed on a message.

too_many_contact_cards

Too many contact_cards were provided with this message. A maximum of 10 contact cards are allowed on a message.

two_factor_setup_required

Two factor setup is required.

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.