Vibefed kb-lemmy

install

source · Clone the upstream repo

git clone https://github.com/reiver/vibefed

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/reiver/vibefed "$T" && mkdir -p ~/.claude/skills && cp -r "$T/skills/kb-lemmy" ~/.claude/skills/reiver-vibefed-kb-lemmy && rm -rf "$T"

manifest: skills/kb-lemmy/SKILL.md

source content

Lemmy — Complete Reference

Overview

Lemmy is a self-hosted, federated social link aggregation and discussion forum — the Fediverse equivalent of Reddit. Created by Dessalines in February 2019, with Nutomic as co-creator and primary federation implementer. Built in Rust (Actix-web, Diesel ORM, PostgreSQL) with a TypeScript/Inferno.js frontend. Licensed AGPL-3.0.

Lemmy federates via ActivityPub. Its fundamental design choice is representing communities as Group actors that automatically relay all content via

Announce

activities, ensuring cross-instance consistency. It is the largest platform in the "Threadiverse" — the threaded discussion subset of the Fediverse.

1. Technology Stack

Component	Technology
Backend language	Rust
Web framework	Actix-web (actor-model based)
ORM	Diesel (type-safe)
Database	PostgreSQL (R2D2 connection pooling)
Frontend language	TypeScript + CSS
Frontend framework	Inferno.js (React-compatible)
API client	lemmy-js-client
Image service	pictrs
Deployment	Docker, Ansible, ARM64/RPi supported
License	AGPL-3.0
Federation library	activitypub-federation-rust (standalone reusable crate)

2. ActivityPub Object Type Mapping

Lemmy Concept	ActivityPub Type	Notes
Community	`Group`	Automated relay actor; Announces all content to followers
User	`Person`	Creates content, moderates, sends PMs
Instance	`Application`	Instance-level actor at root path
Post	`Page`	Mandatory title ( `name` ), optional URL and body
Comment	`Note`	Tree structure via `inReplyTo`
Private Message	`ChatMessage`	Direct user-to-user, no threading

Why Page for Posts?

Lemmy posts have a mandatory title (

name

), distinguishing them from

Note

(typically untitled) and

Article

(long-form). The

Page

type aligns with the concept of a web page with a URL, title, and optional body.

3. Communities as Group Actors

This is the fundamental architectural decision. Communities are

Group

actors with their own inboxes. Every post, comment, vote, and moderation action is received by the community and then Announced (wrapped and forwarded) to all followers across all instances.

Community Object

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://join-lemmy.org/context.json"
  ],
  "id": "https://enterprise.lemmy.ml/c/main",
  "type": "Group",
  "preferredUsername": "main",
  "name": "The Main Community",
  "summary": "<p>Welcome to the main community!</p>",
  "inbox": "https://enterprise.lemmy.ml/c/main/inbox",
  "outbox": "https://enterprise.lemmy.ml/c/main/outbox",
  "followers": "https://enterprise.lemmy.ml/c/main/followers",
  "featured": "https://enterprise.lemmy.ml/c/main/featured",
  "attributedTo": ["https://enterprise.lemmy.ml/u/picard"],
  "publicKey": {
    "id": "https://enterprise.lemmy.ml/c/main#main-key",
    "owner": "https://enterprise.lemmy.ml/c/main",
    "publicKeyPem": "..."
  }
}

Key Fields

Field	Description
`preferredUsername`	Machine-readable community name
`name`	Human-readable display name
`summary`	Description / rules (HTML)
`icon` / `image`	Avatar / banner
`outbox`	Limited to 20 latest posts
`followers`	Count only (no individual references — privacy)
`attributedTo`	Creators and moderators
`featured`	Pinned posts collection
`postingRestrictedToMods`	Whether only mods can post

The Announce Relay Pattern

User on Instance A              Community on Instance B         Instance C subscriber
       |                               |                              |
       |--- Create{Page} -----------> |                              |
       |                               |--- Announce{Create{Page}} ->|
       |                               |                              |

All content activities (Create, Update, Delete, Like, Dislike, Remove, Lock) flow through the community Group actor, which wraps each in an

Announce

and broadcasts to all followers. This is how cross-instance consistency is maintained.

4. Post (Page) Object

{
  "type": "Page",
  "id": "https://enterprise.lemmy.ml/post/123",
  "attributedTo": "https://enterprise.lemmy.ml/u/riker",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public",
    "https://ds9.lemmy.ml/c/main"
  ],
  "audience": "https://ds9.lemmy.ml/c/main",
  "name": "An interesting link",
  "content": "<p>Check this out!</p>",
  "source": {
    "content": "Check this out!",
    "mediaType": "text/markdown"
  },
  "attachment": [],
  "image": {},
  "commentsEnabled": true,
  "sensitive": false,
  "stickied": false,
  "published": "2024-01-15T12:00:00.000Z"
}

Field	Description
`name`	Title (mandatory — distinguishes Post from Comment)
`content`	Body text (HTML-rendered)
`source`	Original markdown with `mediaType: "text/markdown"`
`audience`	Target community (for routing)
`attachment`	Links/images array
`image`	Thumbnail
`commentsEnabled`	Post lock status (custom extension)
`sensitive`	NSFW flag
`stickied`	Pinned status (custom extension)
`language`	Language metadata ( `identifier` + `name` )

5. Comment (Note) Object

{
  "type": "Note",
  "id": "https://enterprise.lemmy.ml/comment/95",
  "attributedTo": "https://enterprise.lemmy.ml/u/picard",
  "to": "https://enterprise.lemmy.ml/c/main",
  "content": "I agree with this take.",
  "source": {
    "content": "I agree with this take.",
    "mediaType": "text/markdown"
  },
  "inReplyTo": [
    "https://enterprise.lemmy.ml/post/38",
    "https://voyager.lemmy.ml/comment/73"
  ],
  "tag": [
    {
      "type": "Mention",
      "href": "https://voyager.lemmy.ml/u/janeway",
      "name": "@janeway@voyager.lemmy.ml"
    }
  ],
  "distinguished": false,
  "published": "2024-01-15T13:00:00.000Z"
}

Field	Description
`inReplyTo`	Parent post and/or parent comment (builds tree)
`tag`	`Mention` objects for user references
`distinguished`	Mod-highlighted comment (custom extension)

6. Voting — Like and Dislike

Lemmy uses both

Like

(upvote) and

Dislike

(downvote) as first-class ActivityPub activities. This is unique among major Fediverse platforms — most only support

Like

Upvote

{
  "type": "Like",
  "actor": "https://enterprise.lemmy.ml/u/picard",
  "object": "https://enterprise.lemmy.ml/post/123",
  "audience": "https://ds9.lemmy.ml/c/main"
}

Downvote

{
  "type": "Dislike",
  "actor": "https://enterprise.lemmy.ml/u/worf",
  "object": "https://enterprise.lemmy.ml/post/123",
  "audience": "https://ds9.lemmy.ml/c/main"
}

Undo Vote

{
  "type": "Undo",
  "actor": "https://enterprise.lemmy.ml/u/picard",
  "object": {
    "type": "Like",
    "actor": "https://enterprise.lemmy.ml/u/picard",
    "object": "https://enterprise.lemmy.ml/post/123"
  }
}

All votes include

audience

for community routing and are wrapped in

Announce

by the community actor.

Privacy note: Every instance that subscribes to the community receives all vote activities — instance admins can see who voted on what.

Ranking Algorithms

Algorithm	Description
Active (default)	Score + latest comment time with decay
Hot	Score + post publication time with decay
Scaled	Hot with boost for less active communities
New / Old	Chronological
Most Comments	Comment count
New Comments	Forum-style bump on new reply
Top (Day/Week/Month/Year/All)	Score within time window

Score = upvotes minus downvotes.

7. Full Activity Reference

User → Community

Activity	Purpose
`Follow`	Subscribe to community
`Undo{Follow}`	Unsubscribe
`Create{Page}`	New post
`Create{Note}`	New comment
`Update{Page}`	Edit post
`Update{Note}`	Edit comment
`Like`	Upvote
`Dislike`	Downvote
`Undo{Like}` / `Undo{Dislike}`	Remove vote
`Delete`	Delete own content
`Undo{Delete}`	Restore deleted content
`Flag`	Report content (with `summary` reason)

Community → Followers

Activity	Purpose
`Accept{Follow}`	Automatic response to Follow
`Announce{*}`	Rebroadcast any received activity to all followers

Moderation

Activity	Purpose
`Remove`	Remove content (with `summary` reason)
`Block`	Ban user (with `target` scope, optional `removeData` , `expires` , `summary` )
`Lock`	Prevent new comments on post
`Add` to `/moderators`	Add moderator
`Remove` from `/moderators`	Remove moderator
`Add` to `/featured`	Pin post
`Remove` from `/featured`	Unpin post

User → User

Activity	Purpose
`Create{ChatMessage}`	Send private message
`Update{ChatMessage}`	Edit private message
`Delete{ChatMessage}`	Delete private message

Instance-Level

Instance actor (type
```
Application
```
) at root path
Admins can remove any content instance-wide
```
Delete
```
on user actor with
```
removeData: true
```
for account deletion

All activities are signed with RSA keys (key ID:

{actor-url}#main-key

8. JSON-LD Context and Custom Extensions

Required Context

[
  "https://join-lemmy.org/context.json",
  "https://www.w3.org/ns/activitystreams"
]

Namespace Prefixes

{
  "lemmy": "https://join-lemmy.org/ns#",
  "litepub": "http://litepub.social/ns#",
  "pt": "https://joinpeertube.org/ns#",
  "sc": "http://schema.org/"
}

Custom Properties

Property	Namespace	Used on	Description
`stickied`	lemmy	Page	Pinned post
`distinguished`	lemmy	Note	Mod-highlighted comment
`commentsEnabled`	lemmy	Page	Post lock status
`postingRestrictedToMods`	lemmy	Group	Restricted posting
`moderators`	lemmy	Group	Moderator collection
`language`	lemmy	Page, Note	Language metadata
`sensitive`	as	Page	NSFW flag
`matrixUserId`	lemmy	Person	Matrix chat ID

Important: Lemmy parses ActivityPub objects as plain JSON, not as JSON-LD. The

@context

is included for other software's benefit but Lemmy does not perform JSON-LD expansion or compaction.

9. Federation Modes and Fetching

Federation Modes

Mode	Description
Allowlist	Only connect to specified instances
Blocklist	Block specific instances, open to all others
Open (default)	Connect to any instance

Cascading Fetch Behavior

Community fetch: Also fetches most recent posts (not comments/votes)
Post fetch: Also fetches community and author
Comment fetch: Also fetches all parent comments, the post, authors, and the community

This ensures referential integrity across the federated network.

10. REST API

Aspect	Details
Version	v3 ( `/api/v3/...` )
Auth	JWT (cookies or `Authorization` header since 0.19.0)
Image service	pictrs — upload, resize, format conversion
Feeds	RSS/Atom at `/feeds/` paths
Rate limiting	IP-based (Docker: requires `X-Forwarded-For` header)
Client library	lemmy-js-client (official TypeScript)
OpenAPI spec	Unofficial: `mv-gh.github.io/lemmy_openapi_spec/`

11. Interoperability Issues

name vs preferredUsername (Historical)

Lemmy initially reversed these:

name

for usernames,

preferredUsername

for display names. This is the opposite of the ActivityPub spec and Mastodon. Nutomic acknowledged it as "completely counterintuitive."

JSON-LD Context Errors

Lemmy's

@context

had incorrect declarations:

```
"stickied": "as:stickied"
```
— falsely implied
```
stickied
```
is in the ActivityStreams namespace
```
"pt"
```
prefix expanded to Lemmy's domain instead of PeerTube's

Nutomic admitted: "the truth is that I don't really understand how [@context] works."

Mastodon Reply Propagation

Replies from Mastodon can fail to reach all Lemmy instances because Mastodon doesn't include the community inbox in its recipient lists. The community Group actor must be in the recipient list to Announce the reply.

Mastodon Vote Federation

Mastodon's

Like

targets personal inboxes, not shared inboxes. Lemmy processes the vote locally but does not Announce it, causing vote count inconsistencies across instances.

Strict Parsing

Lemmy is "really inflexible when it comes to incoming activities." Arrays vs single values, or object IDs vs full inlined objects, can cause parsing errors. The Lemmy protocol is described as "a strict subset of the ActivityPub Protocol."

12. The Threadiverse Ecosystem

"Threadiverse" refers to Reddit-style threaded discussion platforms in the Fediverse. All federate with each other via ActivityPub.

Platform	Language	Terminology	Notes
Lemmy	Rust	Communities	Largest, most mature
Mbin	PHP (fork of Kbin)	Magazines	Also supports microblogs (Mastodon-style)
PieFed	Python	Topics	Rapid development, approaching feature-parity
Sublinks	Java	—	Drop-in Lemmy replacement (alpha)

Users on any Threadiverse platform can see and interact with content from any other, including subscribing to Lemmy communities from Mbin or PieFed.

13. Known Quirks for Implementers

Posts use
```
Page
```
, not
Note
— if your server sends
```
Note
```
objects to a Lemmy community, they will be treated as comments, not posts. Posts require
```
Page
```
type with a
```
name
```
(title).
Community must be in recipients — activities targeting a Lemmy community must include the community in
```
to
```
or
```
cc
```
. Sending only to the post author's inbox will not distribute the activity.
```
audience
```
field — Lemmy uses the
```
audience
```
property to route activities to the correct community. Include it for reliable delivery.
```
Dislike
```
is real — unlike most Fediverse platforms, Lemmy uses
```
Dislike
```
as a distinct activity type for downvotes, not
```
Undo{Like}
```
.
```
source
```
with markdown — Lemmy preserves original markdown in the
```
source
```
field with
```
mediaType: "text/markdown"
```
. The
```
content
```
field contains the HTML-rendered version.
Outbox is limited — community outbox only contains the 20 most recent posts. Comments and votes are not included.
Followers collection is opaque — returns only a total count, not individual follower references (for privacy).
No JSON-LD processing — Lemmy parses JSON directly. Do not rely on JSON-LD compaction/expansion when interoperating with Lemmy.
```
ChatMessage
```
for PMs — Lemmy uses the non-standard
```
ChatMessage
```
type for private messages rather than
```
Note
```
with direct addressing.