Mattermost "Reply in Channel" Plugin

Disclaimer: This project was built with the guidance of a human developer and implemented primarily by Claude Code (Anthropic's AI coding assistant). I didn't have time to dev all of this by hand myself, but did have enough to guide Claude to do so! If LLM-assisted development is something you actively avoid, consider this your fair warning.

What it does

Adds a /ric slash command to Mattermost that lets you reply to a thread inline in the channel — similar to Slack's "Also send to channel" feature. Works on all platforms including web, desktop, iOS, and Android.

When you run /ric your message from within a thread, the plugin:

1. Posts a formatted reply in the channel with a quote of the thread's root message and a clickable link back to the thread
2. Posts your plain reply in the thread with a link to the channel post

What it looks like

In the channel

Replying to @erik's thread:
> Kinda cool to see rust sorta taking off in its own niche. It's a pretty cool language

its slowly taking over parts of the linux stack too isn't it?

The "Replying to @erik's thread" text is a clickable link that opens the thread.

In the thread

its slowly taking over parts of the linux stack too isn't it?

> Also sent to ~town-square

The "Also sent to ~town-square" is a clickable link to the channel post.

Usage

From within any thread, type:

/ric your reply message here

• Must be used from within a thread (not the main channel)
• The message cannot be empty
• Supports full Mattermost markdown, emoji, etc. in your reply
• File attachments are forwarded on a best-effort basis (see Limitations below)

Features

• Cross-platform: Works on web, desktop, and mobile (it's a server-side slash command)
• URL defanging: URLs in the quoted original message are wrapped in backticks to prevent duplicate link previews
• Blockquote stripping: Existing blockquotes in the original message are stripped to avoid nested quotes
• Long message truncation: Original messages over 200 characters are truncated in the quote
• Permalink support: Both the channel post and thread reply include clickable links to each other
• Live "view newer replies" link: When new replies are added to a thread that was shared via /ric, the channel post automatically updates with a "view newer replies" link pointing to the latest reply
• Bidirectional edit sync: Editing either the channel post or the thread post automatically syncs the change to the other — they behave as one post
• DM/GM support: Works in direct messages and group messages (picks the user's first team for permalink construction)

Building

make dist

The plugin bundle will be at dist/reply-in-channel-*.tar.gz.

Other useful targets:

• make test — run Go tests
• make server — compile server only
• make deploy — deploy to a Mattermost instance (requires MM_SERVICESETTINGS_SITEURL and MM_ADMIN_TOKEN env vars)

Installation

1. Build the plugin or download a release
2. Go to System Console > Plugins > Plugin Management
3. Upload the .tar.gz file
4. Enable the plugin

Architecture

This is a server-only plugin (no webapp component). All functionality is implemented via:

• Slash command (/ric) registered in OnActivate
• ExecuteCommand hook handles the command, creates posts via p.API.CreatePost(), and builds permalinks
• MessageHasBeenPosted hook listens for new thread replies and updates /ric channel posts with a "view newer replies" link
• MessageHasBeenUpdated hook syncs edits bidirectionally between paired channel/thread posts
• KV store maps thread root IDs to /ric channel post IDs, and stores bidirectional post pair mappings for edit sync
• formatQuotedReply handles message formatting (quoting, truncation, URL defanging, blockquote stripping)

How the permalink dance works

Since the channel post needs a link to the thread reply (and vice versa), and neither post ID exists before creation:

1. Create channel post without a permalink
2. Create thread reply with a link to the channel post
3. Update the channel post with a link to the thread reply

Limitations

• File attachments: Mattermost's slash command API does not pass file attachment IDs to plugins. The plugin attempts to recover recently uploaded orphaned files, but this is best-effort and may not work in all cases. If attachments don't appear, try posting the file separately.

Security

• Acting user is identified by the Mattermost-User-ID header (set by the Mattermost server)
• The command only works in threads the user has access to
• Posts are created as the acting user (no BOT tag)
• Rate limiting is handled by Mattermost's built-in post creation limits