Slack as_user: What It Does Now, Tested With Bot and User Tokens

as_user is a chat.postMessage argument from Slack's older bot model. With today's tokens it no longer decides who posts: the token does. On 2 October 2026 we sent messages with as_user=true, as_user=false and no as_user from a bot token and from a user token in our own test workspace. as_user=true was accepted and changed nothing. as_user=false was rejected with invalid_arguments. A bot token always posted as the bot, and a user token always posted as the person.

The test

We ran this with slack_sdk 3.45.0. The app had chat:write as a bot scope and as a user scope, so the install gave us both tokens:

import os
from slack_sdk import WebClient
from slack_sdk.errors import SlackApiError

CHANNEL = os.environ["CHANNEL_ID"]
tokens = {"bot": os.environ["SLACK_BOT_TOKEN"], "user": os.environ["SLACK_USER_TOKEN"]}

for name, token in tokens.items():
    client = WebClient(token=token)
    for as_user in (None, True, False):
        kwargs = {} if as_user is None else {"as_user": as_user}
        try:
            r = client.chat_postMessage(channel=CHANNEL, text=f"{name} token, as_user={as_user}", **kwargs)
            m = r["message"]
            print(f"{name:4} as_user={str(as_user):5} ok    user={m['user']} bot_id={m.get('bot_id')} ts={r['ts']}")
        except SlackApiError as e:
            d = e.response.data
            print(f"{name:4} as_user={str(as_user):5} {d['error']}  deprecated_argument={d.get('deprecated_argument')}")

Output:

bot  as_user=None  ok    user=U0C695L6YVA bot_id=B0C5Q3X48EB ts=1790905019.690349
bot  as_user=True  ok    user=U0C695L6YVA bot_id=B0C5Q3X48EB ts=1790905020.016449
bot  as_user=False invalid_arguments  deprecated_argument=as_user
user as_user=None  ok    user=U0B7L4YK420 bot_id=B0C695KUBJ8 ts=1790905020.654209
user as_user=True  ok    user=U0B7L4YK420 bot_id=B0C695KUBJ8 ts=1790905021.996629
user as_user=False invalid_arguments  deprecated_argument=as_user

The full response for as_user=false was:

{"ok": false, "error": "invalid_arguments", "deprecated_argument": "as_user"}

What each combination did

The other methods we tried with a bot token behaved the same way: chat.update and chat.postEphemeral accepted as_user=true and rejected as_user=false with the same error, chat.scheduleMessage accepted as_user=true, and chat.delete rejected as_user=false. If old code passes as_user=False to be explicit, delete the argument.

This is how Slack web showed the messages:

Both bot messages look the same, with the bot's display name and the APP label. The user-token message shows the person's name and avatar with no label. In the API it is not anonymous, though: the message carries bot_id B0C695KUBJ8 and the app's app_id, so anyone reading the raw message can tell an app sent it. The bot's own messages carry a different bot_id, B0C5Q3X48EB.

Posting as a user

To post as a person, use that person's user token:

• Add chat:write under User Token Scopes in the app's OAuth settings (or under oauth_config.scopes.user in the app manifest).
• The person installs or reinstalls the app. Each person who should post through the app has to authorize it themselves; the install response puts their token in authed_user.access_token. Our oauth.v2.access walkthrough shows that response.
• Call chat.postMessage with that xoxp- token and no as_user.

A bot token cannot post as a person, with or without as_user. The bot token page explains which token types exist.

About as_user_not_supported

Slack's docs list as_user_not_supported for chat.postMessage and describe it as the as_user parameter not working with workspace apps, an app type Slack retired. We did not get it with a bot token or a user token: every as_user=false returned invalid_arguments with deprecated_argument, and every as_user=true was ignored. If you see as_user_not_supported from an old integration, remove as_user from the call and check which kind of token it sends.

FAQ

Does as_user=true make a bot message look like it came from me?

No. In our test it posted under the bot's name with the APP label, exactly like a call without as_user.

Can an admin see that a message was sent through the API with a user token?

The message object shows bot_id and app_id, so anything that reads the raw message can. Slack web showed no label on it in our test.