🚨 Slack Error: 'as_user_not_supported' — Cause, Fix, and Troubleshooting Guide

Overview

as_user_not_supported means you passed as_user=true on a call like chat.postMessage with a bot token. Slack retired the ability for bot tokens to post “as the authorising user”; a bot always posts as itself.

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

It surfaces on chat.postMessage, chat.update, and chat.meMessage whenever the as_user argument is present with a xoxb- token.

Symptoms

• A call that worked on a legacy token now fails the moment you send as_user=true.
• Removing as_user makes the identical payload succeed.
• Only affects bot (xoxb-) tokens; nothing to do with scopes or channel membership.

Common Root Causes

1. Legacy code still sends as_user

Older SDK samples and copy-pasted snippets set as_user: true to impersonate the installing user. Bot tokens no longer honour it.

2. Migrated from a legacy custom-integration token

Legacy tokens allowed impersonation. After moving to a real Slack app with a bot token, the same argument is now rejected.

3. Trying to override display name/icon the old way

Teams used as_user together with username/icon_url to control appearance. That override now requires an explicit scope instead.

How to diagnose

Reproduce with and without the argument:

# Fails
curl -s -X POST https://slack.com/api/chat.postMessage \
  -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"channel":"C0123456789","text":"deploy done","as_user":true}'

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

# Succeeds
curl -s -X POST https://slack.com/api/chat.postMessage \
  -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"channel":"C0123456789","text":"deploy done"}'

Fixes

• Delete the as_user argument. A bot token always posts as the app’s bot user; nothing else is needed for a normal message.
• To customise name/icon per message, request the chat:write.customize scope and pass username and icon_url (or icon_emoji) instead of as_user.
• To genuinely post as a human, use a user token (xoxp-) with chat:write — but prefer the bot identity for ops automation so messages are clearly machine-generated.

What to watch out for

• chat:write.customize only changes the visible name/avatar; the author is still the app. Do not use it to fake a specific human.
• Grep your codebase for as_user before shipping a token migration — it is the single most common breakage when moving off legacy integrations.
• The bot’s default name/icon come from the app’s configuration; set them there so most messages need no per-call override.

Related

• Slack Error: ‘invalid_blocks’
• Slack Error: ‘missing_scope’
• Slack Block Kit message design for ops