Lemmy

Lemmy is a link aggregator for the Fediverse, similar to Reddit.

Overview

Property	Value
Language	Rust (backend), TypeScript (frontend)
Database	PostgreSQL
License	AGPL-3.0
Repository	github.com/LemmyNet/lemmy
Latest stable	0.19.20 (2026-07-21); 1.0.0-beta.2 (2026-09-11)
Documentation	join-lemmy.org/docs

ActivityPub Implementation

The examples below are trimmed from Lemmy's own federation test fixtures (crates/apub/assets/lemmy, 0.19.20). For full interop details see the Lemmy compatibility guide.

Actor Types

Person (User) at /u/{name}, and Group (Community) at /c/{name} (group.json):

{
  "id": "https://enterprise.lemmy.ml/c/tenforward",
  "type": "Group",
  "preferredUsername": "tenforward",
  "name": "Ten Forward",
  "inbox": "https://enterprise.lemmy.ml/c/tenforward/inbox",
  "followers": "https://enterprise.lemmy.ml/c/tenforward/followers",
  "attributedTo": "https://enterprise.lemmy.ml/c/tenforward/moderators",
  "featured": "https://enterprise.lemmy.ml/c/tenforward//featured",
  "postingRestrictedToMods": false,
  "endpoints": { "sharedInbox": "https://enterprise.lemmy.ml/inbox" }
}

name is the display title and preferredUsername the slug. Moderators are listed in the collection attributedTo points to; pinned posts are in featured.

Content Types

Page (Post) (page.json) — the link of a link post is an attachment of type Link, not url:

{
  "id": "https://enterprise.lemmy.ml/post/55143",
  "type": "Page",
  "attributedTo": "https://enterprise.lemmy.ml/u/picard",
  "to": [
    "https://enterprise.lemmy.ml/c/tenforward",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "audience": "https://enterprise.lemmy.ml/c/tenforward",
  "name": "Post title",
  "content": "<p>This is a post in the /c/tenforward community</p>\n",
  "mediaType": "text/html",
  "source": { "content": "This is a post in the /c/tenforward community", "mediaType": "text/markdown" },
  "attachment": [{ "type": "Link", "href": "https://enterprise.lemmy.ml/pictrs/image/eOtYb9iEiB.png" }],
  "sensitive": false,
  "language": { "identifier": "fr", "name": "Français" }
}

Note (Comment) — a Note whose inReplyTo is the post or the parent comment.

Supported Activities

Activity	Used for
Create / Update	Posts (Page) and comments (Note)
Delete / Undo(Delete)	Deleting and restoring content
Like / Dislike, Undo	Up- and downvotes (votes carry audience)
Follow / Accept, Undo(Follow)	Subscribing to a community
Announce	The community re-broadcasts every activity in it to its followers
Lock / Undo(Lock)	Locking a post
Block / Undo(Block)	Moderator bans (with target)
Add / Remove	Moderators and featured (pinned) posts
Flag	Reports

Sources: activities/ fixtures, e.g. announce_create_page.json, lock_page.json, like_note.json.

Community Federation

A remote user follows the Group; the community then wraps every activity posted to it in an Announce and sends it to its followers:

┌──────────────────┐      Follow       ┌──────────────┐
│ lemmy.ml         │◀──────────────────│ lemmy.world  │
│ /c/programming   │                   │ (subscriber) │
└──────────────────┘                   └──────────────┘
        │                                     ▲
        │  Announce(Create(Page))             │
        └─────────────────────────────────────┘

Custom Extensions

Property	On	Description
audience	Page, Note, activities	The community the content belongs to
postingRestrictedToMods	Group	Only moderators can post
featured	Group	Pinned posts collection
language	Page, Note, Group	{identifier, name} objects
source	Page, Note, Group	Original Markdown (text/markdown)

Lemmy 0.19.20's Page type does not read commentsEnabled or stickied (older fixtures still show them); locking uses Lock and pinning uses the featured collection.

Release Status

As of October 2026 the latest stable release is the 0.19.x series; Lemmy 1.0 is in beta (1.0.0-beta.2, September 2026) with no release date announced. 1.0 introduces API v4 (with a v3 compatibility layer for existing clients) and renames the actor_id database column to ap_id. No major federation behaviour changes have been announced; check the releases page before relying on 1.0 behaviour.

API Endpoints

Actor Endpoints

GET /u/{username}          # Person
GET /c/{community}         # Group
GET /post/{id}            # Page
GET /comment/{id}         # Note

Collections

GET /c/{community}/followers
GET /c/{community}/outbox
GET /c/{community}/moderators
GET /c/{community}/featured

Voting System

Lemmy supports both upvotes (Like) and downvotes (Dislike), each with the community as audience (like_note.json):

{
  "type": "Like",
  "id": "http://ds9.lemmy.ml/activities/like/fd61d070-7382-46a9-b2b7-6bb253732877",
  "actor": "http://ds9.lemmy.ml/u/lemmy_alpha",
  "object": "http://ds9.lemmy.ml/comment/1",
  "audience": "https://enterprise.lemmy.ml/c/tenforward"
}

Mastodon only understands Like, so downvotes are invisible there.

See Also

• Lemmy Compatibility Guide - Interop details and code
• Lemmy Documentation
• Rust Libraries
• Server Software Overview