Real Time Messaging API

The Real Time Messaging API is a WebSocket-based API that allows you to receive events from Slack in real time and send messages as users. It's sometimes referred to as simply the "RTM API".

It is the basis for all Slack clients. It's also commonly used with the bot user integration to create helper bots for your workspace.

If you prefer events to be pushed to you instead, we recommend using the HTTP-based Events API instead. Most event types supported by the RTM API are also available in the Events API.

Basics

To begin a RTM session make an authenticated call to the rtm.connect API method. This provides an initial set of workspace metadata and a message server WebSocket URL. Once you have connected to the message server it will provide a stream of events, including both messages and updates to the current state of the workspace. This allows a client to easily maintain a synchronized local copy of all workspace data and messages.

The Websocket URLs provided by rtm.connect are single-use and are only valid for 30 seconds, so make sure to connect quickly. If you connect successfully the first event received will be a hello:

{
    "type": "hello"
}

This will be followed by any events that occurred between the call to rtm.connect and the connection to the message server. If you're reconnecting after a network problem this initial set of events may include a response to the last message sent on a previous connection (with a reply_to) so a client can confirm that message was received.

If there was a problem connecting an error will be returned, including a descriptive error message:

{
    "type": "error",
    "error": {
        "code": 1,
        "msg": "Socket URL has expired"
    }
}

Events

Almost everything that happens in Slack will result in an event being sent to all connected clients. The simplest event is a message sent from a user:

{
    "type": "message",
    "ts": "1358878749.000002",
    "user": "U023BECGF",
    "text": "Hello"
}

Every event has a type property which describes the type of event. Our servers currently send the following event types:

Event	Description	Works with
accounts_changed	The list of accounts a user is signed into has changed	RTM
app_uninstalled	Your Slack app was uninstalled.		Events API
bot_added	An bot user was added	RTM
bot_changed	An bot user was changed	RTM
channel_archive	A channel was archived	RTM	Events API
channel_created	A channel was created	RTM	Events API
channel_deleted	A channel was deleted	RTM	Events API
channel_history_changed	Bulk updates were made to a channel's history	RTM	Events API
channel_joined	You joined a channel	RTM
channel_left	You left a channel	RTM
channel_marked	Your channel read marker was updated	RTM
channel_rename	A channel was renamed	RTM	Events API
channel_unarchive	A channel was unarchived	RTM	Events API
commands_changed	A slash command has been added or changed	RTM
dnd_updated	Do not Disturb settings changed for the current user	RTM	Events API
dnd_updated_user	Do not Disturb settings changed for a member	RTM	Events API
email_domain_changed	The workspace email domain has changed	RTM	Events API
emoji_changed	A custom emoji has been added or changed	RTM	Events API
file_change	A file was changed	RTM	Events API
file_comment_added	A file comment was added	RTM	Events API
file_comment_deleted	A file comment was deleted	RTM	Events API
file_comment_edited	A file comment was edited	RTM	Events API
file_created	A file was created	RTM	Events API
file_deleted	A file was deleted	RTM	Events API
file_public	A file was made public	RTM	Events API
file_shared	A file was shared	RTM	Events API
file_unshared	A file was unshared	RTM	Events API
goodbye	The server intends to close the connection soon.	RTM
grid_migration_finished	An enterprise grid migration has finished on this workspace.		Events API
grid_migration_started	An enterprise grid migration has started on this workspace.		Events API
group_archive	A private channel was archived	RTM	Events API
group_close	You closed a private channel	RTM	Events API
group_history_changed	Bulk updates were made to a private channel's history	RTM	Events API
group_joined	You joined a private channel	RTM
group_left	You left a private channel	RTM
group_marked	A private channel read marker was updated	RTM
group_open	You opened a private channel	RTM	Events API
group_rename	A private channel was renamed	RTM	Events API
group_unarchive	A private channel was unarchived	RTM	Events API
hello	The client has successfully connected to the server	RTM
im_close	You closed a DM	RTM	Events API
im_created	A DM was created	RTM	Events API
im_history_changed	Bulk updates were made to a DM's history	RTM	Events API
im_marked	A direct message read marker was updated	RTM
im_open	You opened a DM	RTM	Events API
link_shared	A message was posted containing one or more links relevant to your application		Events API
manual_presence_change	You manually updated your presence	RTM
member_joined_channel	A user joined a public or private channel	RTM	Events API
member_left_channel	A user left a public or private channel	RTM	Events API
message	A message was sent to a channel	RTM	Events API
message.channels	A message was posted to a channel		Events API
message.groups	A message was posted to a private channel		Events API
message.im	A message was posted in a direct message channel		Events API
message.mpim	A message was posted in a multiparty direct message channel		Events API
pin_added	A pin was added to a channel	RTM	Events API
pin_removed	A pin was removed from a channel	RTM	Events API
pref_change	You have updated your preferences	RTM
presence_change	A member's presence changed	RTM
presence_sub	Subscribe to presence events for the specified users	RTM
reaction_added	A member has added an emoji reaction to an item	RTM	Events API
reaction_removed	A member removed an emoji reaction	RTM	Events API
reconnect_url	Experimental	RTM
resources_added	Access to a set of resources was granted for your app		Events API
resources_removed	Access to a set of resources was removed for your app		Events API
scope_denied	OAuth scopes were denied to your app		Events API
scope_granted	OAuth scopes were granted to your app		Events API
star_added	A member has starred an item	RTM	Events API
star_removed	A member removed a star	RTM	Events API
subteam_created	A User Group has been added to the workspace	RTM	Events API
subteam_members_changed	The membership of an existing User Group has changed	RTM	Events API
subteam_self_added	You have been added to a User Group	RTM	Events API
subteam_self_removed	You have been removed from a User Group	RTM	Events API
subteam_updated	An existing User Group has been updated or its members changed	RTM	Events API
team_domain_change	The workspace domain has changed	RTM	Events API
team_join	A new member has joined	RTM	Events API
team_migration_started	The workspace is being migrated between servers	RTM
team_plan_change	The account billing plan has changed	RTM
team_pref_change	A preference has been updated	RTM
team_profile_change	The workspace profile fields have been updated	RTM
team_profile_delete	The workspace profile fields have been deleted	RTM
team_profile_reorder	The workspace profile fields have been reordered	RTM
team_rename	The workspace name has changed	RTM	Events API
tokens_revoked	API tokens for your app were revoked.		Events API
url_verification	Verifies ownership of an Events API Request URL		Events API
user_change	A member's data has changed	RTM	Events API
user_typing	A channel member is typing a message	RTM

New event types will be added in the future, clients should be able to handle unexpected event types.

Sending messages

You can send a message to Slack by sending JSON over the WebSocket connection.

Every event should have a unique (for that connection) positive integer ID. All replies to that message will include this ID allowing the client to correlate responses with the messages sent; replies may be "out of order" due to the asynchronous nature of the message servers.

Also, as with events sent from the server, each event sent by the client has a string type specifying what the message does — chat messages are of type message.

Channel selection

So to post the text "Hello world" to a channel, you can send this JSON:

{
    "id": 1,
    "type": "message",
    "channel": "C024BE91L",
    "text": "Hello world"
}

You can send a message to a private group or direct message channel in the same way, but using a Group ID (C024BE91L) or DM channel ID (D024BE91L).

To send a message both as and to the authenticating user, use the correct direct message channel ID for that user. The IM ID can be found as part of the response to rtm.connect, or by consulting im.list.

Formatting messages

The RTM API only supports posting simple messages formatted using our default message formatting mode. It does not support attachments or other message formatting modes. To post a more complex message as a user clients can call the chat.postMessage Web API method with as_user set to true.

Handling responses

Once the JSON has been sent to the server visual clients should immediately display the text in the channel, grayed out or otherwise marked to indicate that it is "pending". At some point after that, usually a few milliseconds later, the server will send a confirmation that the message was received:

{
    "ok": true,
    "reply_to": 1,
    "ts": "1355517523.000005",
    "text": "Hello world"
}

Replies to messages sent by clients will always contain two properties: a boolean ok indicating whether they succeeded and an integer reply_to indicating which message they are in response to.

In the case of a reply to a chat message, if successful, the reply will contain the canonical recorded timestamp of the message. All messages within a single channel are guaranteed to have a unique timestamp which is ASCII sortable. Given the precision of the timestamp, clients should treat these timestamps as strings, not floats/doubles. Once a successful reply has been returned, the message in the chat log should no longer be grayed out - it has now been delivered.

Chat message replies also contain the message text, which may vary from the sent message due to URL detection.

Errors

If there is an error processing an event the message server will reply with an error. For example:

{
    "ok": false,
    "reply_to": 1,
    "error": {
        "code": 2,
        "msg": "message text is missing"
    }
}

Typing indicators

Clients can send a typing indicator to indicate that the user is currently writing a message to send to a channel:

{
    "id": 1,
    "type": "typing",
    "channel": "C024BE91L"
}

This can be sent on every key press in the chat input unless one has been sent in the last three seconds. Unless there is an error the server will not send a reply, but it will send a "user_typing" event to all workspace members in the channel.

Presence

User and bot user presence on the RTM API is complicated enough to warrant an entire document. Learn all about presence subscriptions and batch presence events here.

Ping and Pong

Clients should try to quickly detect disconnections, even in idle periods, so that users can easily tell the difference between being disconnected and everyone being quiet. Not all web browsers support the WebSocket ping spec, so the RTM protocol also supports ping/pong messages. When there is no other activity clients should send a ping every few seconds. To send a ping, send the following JSON:

{
    "id": 1234, // ID, see "sending messages" above
    "type": "ping",
    ...
}

You can supply any number of extra "flat" arguments (that is: only scalar values, no arrays or objects). These will be included in the pong message that is sent back. For example, a client could include a local timestamp in the ping message so it can calculate round-trip latency:

{
    "id": 1234,
    "type": "ping",
    "time": 1403299273342
}

This will be included in the reply from the server:

{
    "reply_to": 1234,
    "type": "pong",
    "time": 1403299273342
}

Limits

The message server will disconnect any client that sends a message longer than 16 kilobytes. This includes all parts of the message, including JSON syntax, not just the message text. Clients should limit messages sent to channels to 4000 characters, which will always be under 16k bytes even with a message comprised solely of non-BMP Unicode characters at 4 bytes each. If the message is longer a client should prompt to split the message into multiple messages, create a snippet or create a post.

As with all Slack APIs, the RTM API is subject to rate limits. Clients should not send more than one message per second sustained. If you do you may receive an error message or be disconnected.

What's a WebSocket?

WebSockets are a standard way to open a long-lived bi-directional communication channel with a server over TCP. It's the protocol used when connecting to our RTM API. Many contributions from our community support the particulars of connecting to Slack via a WebSocket.

Connecting with rtm.connect vs. rtm.start

There are currently two ways to reserve websocket connections.

rtm.connect concerns itself only with getting your app connected to the RTM API, and only includes limited information about the connecting user and housing workspace.

rtm.start includes not only an entire kitchen sink but an entire kitchen filled with information about the user, its workspace, its channels, its current state in the universe. rtm.start is naturally more difficult to use with Enterprise Grid and other large workspaces.

We strongly recommend using rtm.connect to reserve your websocket connections and use the Web API in tandem to collect all the state information your app needs.

Events API vs. RTM API

Both APIs are useful for different reasons, despite them both serving the same kind of data. Find out why to choose one or the other (or both!) in the Events API FAQ.