Mattermost — Open-Source Slack

Think of Mattermost as the self-hosted, open-source cousin of Slack—same chat channels, same team workflows, but the keys to the server stay in your pocket.

Why Mattermost, Not Slack?

Slack is fantastic, but it lives in someone else’s cloud. Mattermost gives you the same channel-based messaging experience on your own hardware. That means total control over data, custom integrations, and no per-user licensing fees. For teams with strict compliance rules (healthcare, finance, government), this is a game-changer.

Hermes Agent connects to Mattermost just as easily as it connects to Slack—but without the public-URL hassle. Since Mattermost supports WebSocket connections, your Hermes bot can live behind a firewall, on a laptop, or in a private data center.

The Key Difference: No Public Endpoints

With Slack’s Socket Mode, you need a bot token and an app-level token. Mattermost simplifies this: you just need a bot account and a WebSocket endpoint.

Here’s the mental model:

Slack	Mattermost
App-Level Token (xapp-)	Personal Access Token
Socket Mode (WebSocket)	WebSocket built-in
Slack API dashboard	Your own server admin panel

Step 1: Create a Bot Account

In Mattermost, log in as an admin, go to Integrations → Bot Accounts, and create a bot. Give it a username like hermes-agent. You’ll get a bot token that starts with xoxb- (similar to Slack).

Step 2: Connect Hermes

Hermes has a built-in Mattermost connector. Run:

hermes mattermost connect --token xoxb-your-token --server https://chat.yourcompany.com

That’s it. Hermes will open a WebSocket connection to your Mattermost server and start listening for mentions.

Step 3: Test Your Bot

In any Mattermost channel, mention your bot:

@hermes-agent what's the weather today?

Hermes will respond directly in the channel. No public IP, no ngrok, no firewall changes.

Practical Tip: Use Channels for Separation

Create a dedicated channel like #hermes-ops for bot commands. This keeps your main team channels clean and makes it easy to audit what the bot is doing.

The Bottom Line

Mattermost + Hermes Agent = a fully private, open-source AI assistant for your team. You get Slack-like convenience without the cloud dependency. If you’re already running Mattermost, this is a five-minute setup. If you’re evaluating options, this combo might just be the reason to switch.

Try this today: Spin up a Mattermost server on a spare VM (their Docker image is one command), create a bot, and connect Hermes. You’ll have a private AI teammate by lunch.