Bridges

Bridges connect the Fediverse to other social networks and protocols.

What Are Bridges?

Bridges translate between ActivityPub and other protocols, allowing users on different networks to interact.

┌─────────────┐     ┌──────────┐     ┌─────────────┐
│  Mastodon   │◄───▶│  Bridge  │◄───▶│   Bluesky   │
│ (ActivityPub)│     │          │     │  (AT Proto) │
└─────────────┘     └──────────┘     └─────────────┘

Active Bridges

Bridgy Fed

Connects ActivityPub to multiple protocols.

Property	Value
Website	fed.brid.gy
Repository	github.com/snarfed/bridgy-fed
License	CC0-1.0 (public domain)

Supported protocols:

• Bluesky (AT Protocol)
• Web (IndieWeb/Webmention)

Nostr is not supported; the docs say only that Farcaster and Nostr are being considered.

How it works:

1. Users opt in on each side: fediverse users follow @bsky.brid.gy@bsky.brid.gy, Bluesky users follow @ap.brid.gy (blocking that account opts out)
2. Bridge creates proxy accounts
3. Only fully public posts are bridged — unlisted, followers-only posts and DMs never are

Handle formats and other interop details: Big Platforms → Bridgy Fed.

Protocol Bridges

Bluesky (AT Protocol)

Bridgy Fed provides the primary bridge:

@handle@bsky.brid.gy        ← Bluesky user on the Fediverse (e.g. @snarfed.bsky.social@bsky.brid.gy)
user.instance.ap.brid.gy    ← Fediverse user on Bluesky (e.g. snarfed.indieweb.social.ap.brid.gy)

Nostr

The main bridge between Nostr and the Fediverse is Mostr (Bridgy Fed does not support Nostr):

Mostr

Nostr–Fediverse bridge by Soapbox.

Property	Value
Website	mostr.pub
Repository	gitlab.com/soapbox-pub/mostr (active, October 2026)
Documentation	Soapbox announcement

How to find people, per the announcement:

Fediverse user on Nostr:  alex_at_gleasonator.com@mostr.pub   (NIP-05, via the wss://relay.mostr.pub relay)
Nostr user on Fediverse:  https://mostr.pub/users/<hex pubkey>  (search for this URL)

Identity Linking (Alternative to Bridging)

Instead of bridging content, you can link identities across protocols using alsoKnownAs:

{
  "type": "Person",
  "id": "https://example.com/alice#me",
  "alsoKnownAs": ["did:nostr:abc123..."]
}

This approach (used by Fedbox) declares "I am the same person on both networks" without duplicating content. Users can then choose which protocol to interact with you on.

See the did:nostr spec for the DID format.

Twitter/X

No bridge exists. BirdsiteLive, which mirrored Twitter accounts into the fediverse, has had no commits since March 2023.

Building a Bridge

Architecture

┌─────────────────────────────────────────────────┐
│                    Bridge                       │
├─────────────────────────────────────────────────┤
│  Protocol A Adapter  │  Protocol B Adapter     │
│  - Receive activities│  - Receive messages     │
│  - Convert format    │  - Convert format       │
│  - Send to other side│  - Send to other side   │
└─────────────────────────────────────────────────┘

Key Components

1. Adapter per protocol - Handle protocol specifics
2. Mapping layer - Convert between formats
3. Identity management - Map users across systems
4. Queue - Handle async delivery

Example: Activity Mapping

// ActivityPub Note → Other format
function mapNoteToOther(apNote) {
  return {
    id: generateId(),
    text: stripHtml(apNote.content),
    author: mapActor(apNote.attributedTo),
    timestamp: apNote.published,
    // Protocol-specific fields
  };
}

// Other format → ActivityPub Note
function mapOtherToNote(otherPost) {
  return {
    "@context": "https://www.w3.org/ns/activitystreams",
    "type": "Note",
    "content": escapeHtml(otherPost.text),
    "attributedTo": mapToActor(otherPost.author),
    "published": otherPost.timestamp
  };
}

Considerations

Identity

Approach	Pros	Cons
Proxy accounts	Seamless UX	Account pollution
Explicit opt-in	User consent	Friction
Domain-based	Clear ownership	Complex setup

Content Mapping

Challenge	Solution
Different character limits	Truncation/threading
Media format differences	Transcoding
Missing features	Graceful degradation
Reactions/likes	Map to closest equivalent

Privacy

• Users should explicitly opt in
• Respect block/mute across bridges
• Handle deletions appropriately
• Consider data residency

Moderation

• How do reports work cross-protocol?
• Who handles abuse?
• Content policy alignment

Self-Hosting

Bridgy Fed

git clone https://github.com/snarfed/bridgy-fed
cd bridgy-fed
# Follow setup instructions

Considerations

• Domain and SSL required
• Protocol-specific API access
• Storage for mapping data
• Rate limiting

See Also

• Bridgy Fed
• Other Servers (Relays section)
• SocialHub Forum