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

Overview

not_enough_users is returned by conversations.open when you didn’t supply enough valid users to open a conversation. Opening a DM needs a real user; opening a multi-person DM (MPIM) needs enough distinct users.

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

Symptoms

• conversations.open fails with an empty or malformed users value.
• A group-DM open fails when only one user (or none) resolves.
• The error appears before any message is sent.

Common Root Causes

1. Empty users parameter

A code path builds the users string from a list that turned out empty (all filtered/invalid), sending nothing.

2. Only invalid/duplicate ids

After deduping or validation, no real user remains — e.g. the only id was the bot itself or a bad value.

3. MPIM with too few distinct users

A multi-person DM needs at least two distinct human users; supplying one isn’t enough.

How to diagnose

Reproduce with an empty list:

curl -s -X POST https://slack.com/api/conversations.open \
  -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"users":""}'

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

A valid single user opens a DM fine:

curl -s -X POST https://slack.com/api/conversations.open \
  -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"users":"U0USER0001"}'

{"ok": true, "channel": {"id": "D0DM000001"}}

Fixes

• Validate the user list before calling: ensure at least one real U… id remains after filtering.
• Resolve emails/names to ids first (users.lookupByEmail) and drop anything that fails to resolve.
• For MPIMs, pass a comma-separated list of at least two distinct human users.
• Guard the call: short-circuit if the resolved list is empty rather than calling the API with nothing.

What to watch out for

• not_enough_users (too few valid users) differs from too_many_users (over the MPIM cap) and user_not_found (a specific id didn’t resolve).
• Don’t include the bot’s own id to pad the list — it won’t count as another participant.
• Bad email lookups silently drop users; log which ids failed to resolve so an empty list is obvious.

Related

• Slack Error: ‘user_not_found’
• Slack Error: ‘cannot_dm_bot’
• Ephemeral Slack messages for quieter ops bots