Cannot post a message as a user via an application/bot

[jpventura]

(Filling out the following with as much detail as you can provide will help us solve your issue sooner.)

Reproducible in:

 "@slack/bolt": "^3.17.1"

The Slack SDK version

(Paste the output of)

Not relevant

or refer to your package.json

Node.js runtime version

node v20.11.1

OS info

Ubuntu	macOS
Ubuntu 22.04.4 LTS	v13.6.4

(Paste the output of sw_vers && uname -v on macOS/Linux or ver on Windows OS)

Steps to reproduce:

Same issue was reported at bolt-python when trying to use Bolt to post a channel message an specific user:

const result = await app.client.chat.postMessage({
      token: process.env.SLACK_BOT_TOKEN,
      channel: channel_id,
      text: "All your bases are belong to us", 
      as_user: true,
      username: 'username',
      icon_url: "https://i.pinimg.com/736x/b5/13/5f/b5135f5280b8ccb7b48c0bb9eaedc158.jpg"
});

Expected result:

Bolt Slack bot should post a message by the specified username and with a different profile picture,

Actual result

As reported at bolt-python, this feature is not working at their framework, neither bolt-js.

What the chat:write.customize scope enables developers is to customize the message appearance (icon, name) while the message is still by a bot user.

If your situation here is that the username argument for a customized bot message does not work as you expect, please let us know. We'll try to reproduce the situation on our end.

I also does not work at chat.postMessage test page.

I would recommend modify the API to use the user_id instead of username, considering the last one may be changed.

[seratch]

Hi @jpventura, thanks for asking the question! What "chat:write.customize" scope gives to your app might be confusing, but as mentioned at slackapi/bolt-python#611, this bot scope allows your app to send a "bot" message with a personalized icon and username. This feature is supposed to be used for visualizing bot and various user interactions within a Slack thread conversation.

For your use case, your app needs to serve the OAuth flow for app installation and ask each your end user to grant your app to post a message on behalf of them. More specifically, chat:write in user_scope in the OAuth flow is necessary. Behaving as a human user is so critical that your app needs to receive approval from each user to do so.

I hope this helps.

[jpventura]

@seratch I tried exactly this call and the icon and name change does not happen. The permission was added to the bot, but it does not seems to work on new apps.

[seratch]

Just in case, I verified there is no such issue with a newly created app. I create a new app and confirmed the app works for me without any issues. To narrow your issue down, I'd suggest making sure the following:

• Your app needs to have not only chat:write but also chat:write.customize scope
• If you add chat:write.customize after the app installation, you need to re-install the app to grant the addiional scope
• If you're using a user token (xoxp- prefix), it's not possible to customize name and icon for it

I hope this helps.

[jpventura]

@seratch Thank you for your attention.

We discovered the problem. According to the documentation, as_user parameter is required only for legacy apps. Including it into new apps causes the unexpected behavior.

The following code works as expected:

const result = await app.client.chat.postMessage({
      channel: channel_id,
      icon_url: 'https://i.pinimg.com/736x/b5/13/5f/b5135f5280b8ccb7b48c0bb9eaedc158.jpg',
      text: 'All your bases are belong to us', 
      token: process.env.SLACK_BOT_TOKEN,
      username: 'CATS',
});

However it does not seem to work with app.client.files.upload function. Is it an API restriction or should it work as well?

[jpventura]

I just checked the code and compared the app.client.files.upload arguments:

export interface FilesUploadArguments extends FileUpload, WebAPICallOptions, TokenOverridable {
}

interface FileUpload {
    channels?: string;
    content?: string;
    file?: Buffer | Stream | string;
    filename?: string;
    filetype?: string;
    initial_comment?: string;
    thread_ts?: string;
    title?: string;
}

and app.client.chat.postMessage

export interface ChatPostMessageArguments extends WebAPICallOptions, TokenOverridable {
      channel: string;
      text?: string;
      as_user?: boolean;
      attachments?: MessageAttachment[];
      blocks?: (KnownBlock | Block)[];
      icon_emoji?: string;
      icon_url?: string;
      metadata?: MessageMetadata;
      link_names?: boolean;
      mrkdwn?: boolean;
      parse?: 'full' | 'none';
      reply_broadcast?: boolean;
      thread_ts?: string;
      unfurl_links?: boolean;
      unfurl_media?: boolean;
      username?: string;
}

So I would guess that this feature is unsupported and it would require a patch into the library.

[seratch]

Hi @jpventura, I overlooked you were trying to use as_user option, but you're right that the option should not be used along with username and icon for the latest permission. As for files.upload API, it does not support the parameters. However, you can do the following instead:

• Upload files using files.uploadV2 without channel_id and initial_comment parameters (at this point, the files are uploaded but not yet shared)
• Post a message with uploaded file permalink URLs in a channel along with username and icon using chat.postMessage API

I hope this helps.

[github-actions]

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