Design: Matrix-Native MusicLink Routing

Goal

Make MusicLink a native Matrix bot that listens to multiple rooms and posts follow-up messages in the same room that originated the message. This removes Matterbridge from the routing path and eliminates cross-room fan-out.

Background

Legacy deployment used Matterbridge as an API gateway. When multiple Matrix rooms were configured in a single Matterbridge gateway, messages fanned out to other rooms, causing unintended cross-posting (including DM leakage). The system is now Matrix-native.

Objectives

Correct routing: Responses go back to the originating room (no cross-room fan-out).
Multi-room support: One MusicLink instance can monitor multiple Matrix rooms.
No fan-out bus: Remove Matterbridge dependency for routing.
Minimal operational complexity: Single service, single config, single token.

Non-Goals

Replacing mautrix-slack (Slack ↔ Matrix bridge remains).
Adding new link providers beyond current MusicLink behavior.

High-Level Architecture

Slack Room
  -> mautrix-slack (Matrix portal room)
  -> MusicLink (Matrix-native bot)
  -> same Matrix portal room (follow-up message)
  -> mautrix-slack -> Slack

Current Implementation

1) Matrix Client

Uses mautrix-go with a bot access token.
Syncs events from configured rooms.
Ignores messages from the bot itself.
Posts responses as standalone messages in the same room (no reply/thread relation).

2) Room Configuration

Matrix allowlist in config:

[matrix]
server = "https://clarun.xyz"
accessToken = "..."
userId = "@musiclink:clarun.xyz"
rooms = [
  "!DPQveBnfuDrbgOe6dm:clarun.xyz",
  "!dT40EUcemb8e6bPiig:clarun.xyz"
]

3) Message Handling

Parse message body for supported music links.
Call idonthavespotify (existing behavior).
Post formatted response in the same room.

4) Loop Prevention

Ignore events from @musiclink.
Ignore events without link matches.
Rate limit handling with retry/backoff.

Risks

Token handling and access permissions for the bot user.
Message deduplication and race conditions in sync processing.

Status

Matrix-native mode implemented.
Matterbridge routing removed.

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