ActivityPub Federation#

Misskey implements the ActivityPub protocol to federate with Mastodon, Pleroma, Akkoma, Fedibird, and other Fediverse servers. All AP logic lives under packages/backend/src/core/activitypub/ 1.

The five primary services are:

A models/ subdirectory contains ApNoteService and ApPersonService, which materialize remote AP objects into the local database.

Outbound: ApRendererService#

ApRendererService converts internal Misskey entities into AP-compliant JSON-LD objects 2. Rendered types include:

• Objects: Note, Person, Document, Image, Emoji
• Activities: Create, Announce, Follow, Like, Block, Delete, Update, Undo, Accept, Reject

Outgoing notes include a _misskey_content field carrying the original MFM source text; ApNoteService uses a typeof === 'string' guard on this field to avoid shadowing the standard content field when _misskey_content is null 3.

Emoji reactions are federated as Like activities with a content property. Custom emoji reactions additionally include an Emoji object in the tag array 4.

Activities are signed via attachLdSignature() using RSA-SHA256 (RsaSignature2017). Ed25519 key support is in progress under PR #16250; it adds additionalPublicKeys to the AP Person object (following the Mastodon publicKey spec) and selects the signing algorithm per remote instance based on their nodeinfo httpMessageSignaturesImplementationLevel 5.

Inbound: ApInboxService#

ApInboxService handles the full range of AP activity types: Create, Delete, Update, Follow, Accept, Reject, Add, Remove, Announce, Like, Undo, Block, Flag, Move 6.

Actor validation flow:

1. Actor suspension status is checked before any processing.
2. All uses of activity.actor are wrapped with getApId() to handle both embedded actor objects and plain URI strings — improving compatibility with non-Mastodon AP implementations 7.
3. isFederationAllowedUri enforces domain-level allow/block lists.
4. A distributed lock (acquireApObjectLock) prevents race conditions on the same AP object.

Security hardening applied:

• Reject{Follow} validates the rejecting actor's URI matches the Follow's target; mismatches are skipped to block forced-unfollow attacks 3.
• Remote actor name fields are HTML-stripped before storage to prevent stored XSS 3.
• Activities missing an actor field return HTTP 400 rather than crashing the worker queue 7.
• Delete activities for actors that don't exist locally are silently skipped 8.

Relay handling:
RelayService.isRelayActor() identifies relay actors by matching their inbox/sharedInbox against accepted relay entries. Relay-delivered Announce activities publish the inner note directly to the stream instead of creating a renote — a distinction corrected in PR #17308 9.

Delivery: ApDeliverManagerService#

ApDeliverManagerService builds a delivery list from followers and direct recipients, deduplicates by preferring shared inboxes, then enqueues jobs via QueueService 10. Retry logic and concurrency are managed at the queue layer.

Default queue concurrency values (updated in PR #16250 to reduce server load when Ed25519 becomes widespread):

Clock skew tolerance is ±300s in both directions (previously only 2000ms forward); HTTP 401 responses from remote inboxes are retried to accommodate clock-skew scenarios 5.

Image Format Compatibility#

Federation constrains which image formats Misskey can serve and publish. Two constants and one service govern this:

FILE_TYPE_BROWSERSAFE (const.ts) is the whitelist of MIME types served with their real Content-Type. Everything else is returned as application/octet-stream. SVG is explicitly excluded to prevent XSS 11.

DriveService.generateAlts() creates two derivatives for every locally-uploaded image 12:

• webpublic — a web-optimized version referenced in AP Document attachments. Rules for when it is generated:
  • Not generated for animated images (compatibility).
  • Not generated for remote-origin files (federation direction).
  • AVIF originals are converted to WebP, because AVIF is "not supported by Mastodon and MS Edge" 13.
  • Images with EXIF/IPTC/XMP metadata embedded are always re-encoded (strips private metadata).
  • Images exceeding 2048×2048 are downscaled.
  • PNG/BMP/SVG originals produce a PNG webpublic; JPEG/WebP/AVIF produce a WebP webpublic.
• thumbnail — always WebP, always generated (498×422 for stills, 374×317 for animated).

WebP encoding defaults: quality 77, alphaQuality 95, smartSubsample: true, mixed: true, effort 2 14.

AVIF history: AVIF support was added in 2022 then immediately rolled back due to incompatibility with Mastodon and Microsoft Edge 15. AVIF remains in FILE_TYPE_BROWSERSAFE for direct display of AVIF originals, but is never used as the output format for webpublic derivatives.

HEIF/HEIC: Not in FILE_TYPE_BROWSERSAFE and not auto-converted. Server-side conversion to JPEG/WebP on upload is the recommended implementation path to avoid federation compatibility problems 15.

Cross-Platform Compatibility Notes#

• Emoji reactions: Misskey federates emoji reactions as Like with content (and an Emoji tag for custom emoji). FEP-c0e0 is largely implemented; the remaining gap is exposing an emojiReactions collection endpoint. Fedibird uses a per-reaction array format, while Misskey stores aggregated JSON counts in its DB — requiring mapping on each side 16 4.

• Image attachments: PR #17563 adds width and height metadata to AP Document objects, enabling remote clients to pre-allocate layout space before fetching media 17.

• Actor embedding: Some AP implementations embed full actor objects inside activities rather than providing a URI. getApId() normalizes both forms throughout ApInboxService, preventing silent activity rejection 7.

• Relay LD signatures: As of Misskey 2026.6.0, LD Signature verification fails for posts relayed through some relay servers (Fedibird's relay, activitypub-relay) 18. This is an active bug.

• Secure Mode / Private Mode: Administrators can require HTTP Signatures for all remote requests (Secure Mode) or restrict federation to an explicit allowlist (Private Mode) via instance settings 19.