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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.

PropertyValue
Websitefed.brid.gy
Repositorygithub.com/snarfed/bridgy-fed
LicensePublic Domain

Supported protocols:

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

How it works:​

  1. User opts in via their profile
  2. Bridge creates proxy accounts
  3. Posts and interactions sync bidirectionally

Matrix-ActivityPub​

Bridges Matrix chat to ActivityPub.

PropertyValue
StatusExperimental
Use CaseChat ↔ Social

Protocol Bridges​

Bluesky (AT Protocol)​

Bridgy Fed provides the primary bridge:

@user@bsky.brid.gy ← Bluesky user on Fediverse
@user@ap.brid.gy   ← Fediverse user on Bluesky

Nostr​

Two main bridges connect Nostr to the Fediverse:

Mostr​

Native Nostr-Fediverse bridge by Soapbox.

PropertyValue
Websitemostr.pub
Documentationsoapbox.pub/blog/mostr-fediverse-nostr-bridge
StatusActive

How it works:

  • Nostr users appear as npub...@mostr.pub on the Fediverse
  • Fediverse users can follow Nostr accounts directly
  • Posts sync bidirectionally
  • Uses Nostr relays for message transport
Nostr user:     npub1abc...@mostr.pub
Fediverse user: @alice@mastodon.social (follow npub via mostr.pub)

Bridgy Fed (Nostr)​

Bridgy Fed also supports Nostr alongside Bluesky and IndieWeb:

npub...@nostr.brid.gy ← Nostr user on Fediverse

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 official bridge exists. Historical options:

  • BirdsiteLive (discontinued)
  • Self-hosted instances (legal concerns)

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​

ApproachProsCons
Proxy accountsSeamless UXAccount pollution
Explicit opt-inUser consentFriction
Domain-basedClear ownershipComplex setup

Content Mapping​

ChallengeSolution
Different character limitsTruncation/threading
Media format differencesTranscoding
Missing featuresGraceful degradation
Reactions/likesMap 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​