How can a Slack bot detect a direct message vs a message in a channel?

FYI as of July 2017, for "message.im" events (via your app's Event Subscriptions), the event payload seems to now return additional fields to detect if the message is coming from your own bot (pasted in here from my logs):

    INFO[0012] got Slack message: (bot.SlackMessage) {
    SlackEvent: (bot.SlackEvent) {
        Type: (string) (len=7) "message",
        EventTs: (string) (len=17) "1501076832.063834",
        User: (string) ""
    },
    SubType: (string) (len=11) "bot_message",
    Channel: (string) (len=9) "D6CJWD132",
    Text: (string) (len=20) "this is my bot reply",
    Username: (string) (len=15) "Myapp Local",
    BotID: (string) (len=9) "B6DAZKTGG",
    Ts: (string) (len=17) "1501076832.063834"
}

Slack have added Conversations API some time ago. You should use it to differentiate between PM/channel instead of relying on prefix.

From Conversations API documentation:

Each channel has a unique-to-the-team ID that begins with a single letter prefix, either C, G, or D. When a channel is shared across teams (see Developing for Shared Channels), the prefix of the channel ID may be changed, e.g. a private channel with ID G0987654321 may become ID C0987654321.

This is one reason you should use the conversations methods instead of the previous API methods! You cannot rely on a private shared channel's unique ID remaining constant during its entire lifetime.

Get conversation info using conversations.info method and check is_im flag. is_im == true means that the conversation is a direct message between two distinguished individuals or a user and a bot.

I talked to James at Slack and he gave me a simply way to determine if a message is a DM or not; if a channel ID begins with a:

• C, it's a public channel
• D, it's a DM with the user
• G, it's either a private channel or multi-person DM

However, these values aren't set in stone and could change at some point, or be added to.

So if that syntax goes away, another way to detect a DM to use both channels.info and groups.info. If they both return “false” for the “ok” field, then you know it’s a DM.

Note:

• channels.info is for public channels only
• groups.info is for private channels and multi-person DMs only

Bonus info: Once you detect a that a message is a DM, use either the user ID or channel ID and search for it in the results of im.list; if you find it, then you’ll know it’s a DM to the bot.

• “id” from im.list is the channel ID
• “user” from im.list is the user ID from the person DM’ing with the bot
• You don’t pass in the bot’s user ID, because it’s extracted from the token