Kbin / Mbin — Complete Reference

Overview

Kbin (stylized /kbin) was a decentralized content aggregator and microblogging platform for the Fediverse — a Reddit + Mastodon hybrid. Created by Ernest Wiśniewski (Polish developer) with development becoming serious in early 2023, the flagship instance kbin.social launched in April 2023. The project received NLnet funding (NGI0 Entrust Fund, December 2022).

Kbin is now discontinued. Its active community fork Mbin (founded October 2023 by Melroy van den Berg) continues development with significant improvements to federation, performance, and governance. When this document says "Kbin/Mbin," the information applies to both; Mbin-specific features are noted.

Kbin's key differentiator from Lemmy: it combines two content models in one platform — Reddit-style link aggregation (threads organized into magazines) and Mastodon-style microblogging (short-form posts). This dual nature enables stronger integration with microblogging platforms like Mastodon.

Licensed AGPL-3.0. Built with PHP/Symfony/PostgreSQL.

1. Technology Stack

Language Breakdown

• PHP: 86.3%
• Twig: 9.4%
• SCSS: 2.5%
• JavaScript: 1.6%

Development Tools

• PHPUnit (testing), PHPStan (static analysis), Psalm (security scanning), Composer (dependencies), API Platform (API generation)

2. Content Model

Magazines

The primary organizational unit — equivalent to subreddits (Reddit), communities (Lemmy), or topics (PieFed). Any registered user can create a magazine and automatically becomes its owner with administrative tools including moderator appointment.

Content Types

Page

Page

Note

Note

inReplyTo

ChatMessage

The dual content model is what distinguishes Kbin/Mbin from Lemmy. Threads work like Reddit posts (link or self-text with a title, displayed in a list). Microblogs work like Mastodon posts (short-form, displayed in a stream). Both coexist within each magazine.

Voting

• Upvote — positive engagement, federated as

  Like

  activity
• Downvote ("Reduce") — negative engagement, local only — does NOT federate
• Boost — reshare/reblog, federated as

  Announce

  activity

The non-federation of downvotes is a critical interoperability detail. Lemmy federates downvotes via

Dislike

Reputation

Users accumulate reputation points from boosts and favourites on their content. Reputation is cosmetic only — visible on profile pages but has no functional effect (does not gate features or permissions).

Content Organization

• Follow magazines, users, or entire domains
• Block users, magazines, or domains
• Content categorized by tags and labels
• RSS feeds for threads, microblogs, or combined

3. ActivityPub Implementation

Actor Types

Application

/i/actor

Person

/u/username

Group

/m/name

JSON-LD Context

The

@context

toot:

Magazine (Group) Federation

Magazines use the same Group actor Announce relay pattern as Lemmy communities. When a user creates a thread in a magazine, the magazine's Group actor wraps the activity in an

Announce

User creates thread → Create{Page} sent to Magazine
Magazine wraps → Announce{Create{Page}} sent to all followers
Remote instances receive Announce → display thread in magazine

This means subscribing to a magazine is a

Follow

Announce

Activities Reference

Follow

Accept

Reject

Create

Update

Delete

Like

Dislike

Announce

Undo

Block

Flag

Add

Remove

Lock

Content Object Wire Format

Thread (Page):

{
  "type": "Page",
  "id": "https://instance.example/m/magazine/t/12345",
  "attributedTo": "https://instance.example/u/alice",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://instance.example/m/magazine/followers"],
  "name": "Thread title",
  "content": "<p>HTML body</p>",
  "mediaType": "text/html",
  "source": {
    "content": "Markdown body",
    "mediaType": "text/markdown"
  },
  "inReplyTo": null,
  "sensitive": false,
  "stickied": false,
  "commentsEnabled": true
}

Microblog (Note):

{
  "type": "Note",
  "id": "https://instance.example/m/magazine/p/67890",
  "attributedTo": "https://instance.example/u/alice",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "content": "<p>Short-form post content</p>",
  "source": {
    "content": "Short-form post content",
    "mediaType": "text/markdown"
  },
  "tag": [
    {"type": "Hashtag", "name": "#fediverse"}
  ]
}

Comment (Note with inReplyTo):

{
  "type": "Note",
  "id": "https://instance.example/m/magazine/t/12345/-/comment/99",
  "attributedTo": "https://instance.example/u/bob",
  "inReplyTo": "https://instance.example/m/magazine/t/12345",
  "content": "<p>Reply text</p>",
  "source": {
    "content": "Reply text",
    "mediaType": "text/markdown"
  }
}

Key Federation Details

• Downvotes do NOT federate — only

  Like

  (upvote) and

  Announce

  (boost) are sent. No

  Dislike

  activity.
• Reports (

  Flag

  ) are sent exclusively to instances housing the magazine's moderators.
• Bans (

  Block

  ) include optional expiration timestamps and specify scope (magazine-level vs instance-level).
• Content includes focal point metadata and blurhash via Mastodon extensions.
• Supports all ActivityPub Actor Types including

  Service

  for bot/robot accounts.
• Accepts

  application/json

  headers across all ActivityPub endpoints (not just

  application/activity+json

  ).
• All content preserves markdown source alongside HTML rendering.
• Multi-language content mapping supported.

4. API

REST API with Swagger/OpenAPI documentation at

/api/docs

Authentication

OAuth2-based:

1. Register client:

   POST /api/client

   → returns identifier and secret
2. Request token → returns Bearer token, expiration, refresh token
3. Include

   Authorization: Bearer <token>

   on requests

Scopes

read

write

5. Kbin vs Lemmy

Page

Note

Page

Dislike

Interoperability

Both use

Group

Page

Note

inReplyTo

Key friction points:

• Lemmy sends

  Dislike

  for downvotes; Kbin/Mbin ignores them (or had historical display issues)
• Kbin/Mbin microblog

  Note

  posts appear as comments in Lemmy (since Lemmy only understands

  Page

  for top-level posts)
• Mention federation works better from Mbin than from Lemmy

6. History and the Mbin Fork

Kbin Timeline

Mbin

Founded: October 12, 2023, by Melroy van den Berg.

Governance: Uses the Collective Code Construction Contract (C4) — no single-maintainer bottleneck. Multiple contributors have owner rights. Pull requests merged by any maintainer with community consensus on Matrix.

Key improvements over Kbin:

1. Federation compatibility — major fixes to interoperability issues
2. Performance and scaling — database optimizations, reduced background request overhead
3. Security — up-to-date dependencies, security-first patching
4. Documentation — hosted at docs.joinmbin.org
5. Thread/microblog locking
6. Discoverability and indexability settings
7. PeerTube channel following (since v1.7)
8. All ActivityPub Actor Types including

   Service

   for bots
9. Combined thread creation form (unified link/text/photo)

Current version: v1.9.1 (February 2026). 35 releases, 4,477 commits, 35+ contributors.

Around 8–10 months after forking, Mbin stopped porting Kbin code and focused on purely original development.

7. The Threadiverse Ecosystem

The "Threadiverse" is the link-aggregation / threaded-discussion portion of the Fediverse. All platforms interoperate via ActivityPub using

Group

Page

Note

8. Instances

Kbin Instances

8 tracked, mostly inactive or migrated to Mbin. kbin.social (flagship) is defunct.

Mbin Instances

23 tracked, almost all with >98% recent uptime:

• fedia.io — one of the largest, migrated from Kbin
• kbin.run — created due to kbin.social instability
• kbin.melroy.org — run by Mbin founder

9. Interoperability Notes for Implementers

1. Threads arrive as

   Page

   objects — same as Lemmy. If your software handles Lemmy's

   Page

   posts, Kbin/Mbin threads will work the same way. Title is in

   name

   , body in

   content

   /

   source

   .

2. Microblogs arrive as

   Note

   objects — these are short-form posts without titles, identical in structure to Mastodon posts. They originate from a magazine but look like standard microblog posts. Lemmy displays these as comments since it only understands

   Page

   for top-level content.

3. No

   Dislike

   activity — Kbin/Mbin never sends downvotes over federation. If you receive a

   Like

   from a Kbin/Mbin instance, it's an upvote. You will never receive a

   Dislike

   . If your platform sends

   Dislike

   to a Kbin/Mbin instance, it may be ignored or cause display issues.

4. Magazines are

   Group

   actors — follow a magazine by sending

   Follow

   to the Group actor at

   /m/name

   . The magazine relays all content via

   Announce

   , identical to Lemmy communities.

5. ChatMessage

   for DMs — Kbin/Mbin uses a custom

   ChatMessage

   type for private messages, not standard

   Note

   with direct addressing. Other platforms may not understand this type.

6. Accepts

   application/json

   — Kbin/Mbin accepts both

   application/activity+json

   and plain

   application/json

   on all ActivityPub endpoints. No need to worry about content-type strictness.

7. Report routing —

   Flag

   activities are sent only to instances that host the magazine's moderators, not broadcast widely.

8. Ban expiration —

   Block

   activities can include expiration timestamps and scope (magazine-level vs instance-level). Handle the expiration field if present.

9. Reputation is cosmetic — Kbin/Mbin exposes reputation scores on user profiles, but these have no functional effect and should not be used for trust decisions.

10. Mastodon content in magazines — Kbin/Mbin's microblog section displays Mastodon posts matching magazine hashtags. This means Mastodon

    Note

    objects may appear alongside native Kbin/Mbin content in magazine feeds.