Slack–Mattermost Bridge

YEAR

2026

MY ROLE

Design, development and deployment

INDUSTRY

Collaboration between organisations

GitHub ↗

STACK USED

Python
Slack Socket Mode
Mattermost API
WebSocket
SQLite
Docker Compose

Project Overview

Two organisations, one on Slack and one on Mattermost, needed to work in a shared channel. The bridge connects one Slack channel to one Mattermost channel so it feels like a single room: messages, threads, files, edits, deletes and reactions cross in both directions, each under the author's own name and marked with the platform it came from.

It runs as a single container next to Mattermost, with no public endpoint. A setup check verifies tokens, permissions and server settings before a single message is copied.

Key Features

No inbound port

The bridge dials out to Slack over Socket Mode, so it needs no public hostname, certificate or firewall rule.

Only the channels you allow

An explicit allowlist decides which channels are connected.

If the app is invited anywhere else, it posts a notice once and leaves.

Tokens that expire

With rotation switched on, Slack tokens expire every 12 hours and are stored encrypted. Every secret can be read from a file, the way Docker secrets are mounted.

Project Gallery

Challenges Solved

Messages bouncing back forever

Each platform reports the bridge's own posts back to it as new messages. Every copied message is recorded as a pair of IDs, and an ID already in that table is proof the bridge posted it, which is more reliable than checking who the author is.

Threads, edits and reactions landing in the right place

The same ID pairs tell the bridge which message on the other side a reply, an edit or a reaction belongs to. A reply to a message the bridge never saw becomes a new message instead of being dropped.

Showing who actually wrote it

Mattermost ignores name overrides from bot accounts, so the bridge posts from an ordinary user account. If it is given a bot account anyway, it detects this at startup and writes the author's name into the message.

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