kb-mastodon

name	kb-mastodon
description	Background knowledge about Mastodon, the dominant federated microblogging platform and de facto reference implementation for ActivityPub social networking. Covers Mastodon's JSON-LD @context structure (three-element array: activitystreams, security/v1, inline object defining toot: http://joinmastodon.org/ns# with Emoji/discoverable/featured/featuredTags/ blurhash/votersCount/indexable/memorial/suspended/attributionDomains, schema: http://schema.org# with PropertyValue/value, as: with Hashtag/manuallyApprovesFollowers/movedTo/sensitive), content-type and Accept header requirements (application/activity+json or application/ ld+json with ActivityStreams profile, content negotiation for actor discovery), HTTP signature requirements (RSA-SHA256, Digest header mandatory for POST, signed headers include request-target/host/date/ digest), authorized fetch / secure mode (AUTHORIZED_FETCH env var, requires signatures on GET requests too, breaks unsigned federation), supported object types (Note and Question first-class, Article/Page/ Image/Audio/Video/Event converted with content or name + URL appended), poll implementation (Question wrapped in Create, oneOf for single-choice anyOf for multiple, options as Note with name and replies.totalItems, votes as Create of Note matching option name, endTime/closed/votersCount), edit/Update handling (since 3.5.0, requires updated timestamp, sends Update activity with full object, edit history not yet federated), Announce/boost behavior (object field is URI string, receiver fetches original, shares collection on statuses), direct message addressing (no as:Public, no follower collection, only actor URIs in to, all must be Mentioned in tag), visibility calculation (public=as:Public in to, unlisted=as:Public in cc, private=followers in to/cc without as:Public, limited=actors not all Mentioned, direct=all actors Mentioned), custom emoji (tag type Emoji with name shortcode and icon Image), HTML sanitization (allowed: a/p/br/span/b/i/em/strong/ blockquote/pre/code/ul/ol/li/div/audio/video/img/source, strips all other elements and attributes, allowed classes: mention/hashtag/ invisible/ellipsis/h-*/p-*/u-*/dt-*/e-*), character limits (500 default for composing but accepts and displays longer remote posts with truncation/Read More, hard limit 1MB JSON-LD serialization), thread fetching (up to 5 same-server replies fetched on discovery, inReplyTo recursive fetch, replies not forwarded upstream, 4.4+ fetch-all-replies feature, 4.5 enables by default with AsyncRefresh API), NodeInfo 2.0 at /.well-known/nodeinfo pointing to /nodeinfo/2.0, Mastodon REST API compatibility layer (implemented by Pleroma/Akkoma/GoToSocial/Firefish/ Sharkey/Pixelfed), recent version changes (4.3: attributionDomains/ fediverse:creator, like/boost counts in AP objects, summary as HTML; 4.4: quote post display via FEP-044f, experimental RFC9421 HTTP Message Signatures, FASP support; 4.5: quote post authoring, fetch- all-replies enabled by default, RFC9421 verification standard, AsyncRefresh API), size limits (username 2048, display name 2048 truncated, profile note 20kB, profile fields 50 max, field name/value 2047, poll options 500, emoji shortcode 2048, media descriptions 1500), and common federation gotchas (schema.org incorrect base URI, reply propagation gaps, counter inconsistencies across instances, Block activity sent server-to-server despite spec being client-only, Article type poorly supported, Linked-Data Signatures for public posts disabled in secure mode). Load when the user asks about Mastodon federation; implementing a Mastodon-compatible ActivityPub server; Mastodon's JSON-LD context or toot namespace; how Mastodon handles incoming ActivityPub objects; Mastodon content-type or Accept headers; Mastodon HTTP signatures or authorized fetch; how Mastodon polls work in ActivityPub; how Mastodon edits/Updates federate; how Mastodon boosts/Announces work; Mastodon direct messages in ActivityPub; Mastodon visibility levels; Mastodon HTML sanitization; Mastodon character limits; Mastodon thread fetching; Mastodon NodeInfo; the Mastodon client API compatibility; Mastodon custom emoji federation; Mastodon 4.3/4.4/4.5 changes; or common gotchas when federating with Mastodon.
user-invocable	false

Mastodon -- Complete Federation Reference

Overview

Mastodon is a free, open-source, decentralized microblogging platform -- the dominant software in the Fediverse and the de facto reference implementation for ActivityPub social networking. Created by Eugen Rochko (Gargron) in 2016. Built in Ruby on Rails with a PostgreSQL database, Redis for caching/queues, and a React.js frontend. Licensed AGPL-3.0.

Mastodon federates via ActivityPub (server-to-server). Because of its dominance, Mastodon's specific implementation choices have become de facto standards that other Fediverse software must accommodate, even when those choices deviate from or extend the ActivityPub specification.

---

1. JSON-LD @context Structure

Mastodon's @context is a three-element JSON array:

[
  "https://www.w3.org/ns/activitystreams",
  "https://w3id.org/security/v1",
  {
    "manuallyApprovesFollowers": "as:manuallyApprovesFollowers",
    "toot": "http://joinmastodon.org/ns#",
    "featured": { "@id": "toot:featured", "@type": "@id" },
    "featuredTags": { "@id": "toot:featuredTags", "@type": "@id" },
    "alsoKnownAs": { "@id": "as:alsoKnownAs", "@type": "@id" },
    "movedTo": { "@id": "as:movedTo", "@type": "@id" },
    "schema": "http://schema.org#",
    "PropertyValue": "schema:PropertyValue",
    "value": "schema:value",
    "discoverable": "toot:discoverable",
    "suspended": "toot:suspended",
    "memorial": "toot:memorial",
    "indexable": "toot:indexable",
    "attributionDomains": {
      "@id": "toot:attributionDomains",
      "@type": "@id"
    },
    "Hashtag": "as:Hashtag",
    "Emoji": "toot:Emoji",
    "sensitive": "as:sensitive",
    "votersCount": "toot:votersCount",
    "blurhash": "toot:blurhash",
    "focalPoint": { "@container": "@list", "@id": "toot:focalPoint" }
  }
]

Namespace Definitions

Prefix	URI	Notes
as:	https://www.w3.org/ns/activitystreams#	Standard ActivityStreams
toot:	http://joinmastodon.org/ns#	Mastodon-specific extensions
schema:	http://schema.org#	BUG: should be https://schema.org/ -- causes issues with strict JSON-LD processors
sec:	From https://w3id.org/security/v1	Public keys, signatures

Key toot: Namespace Terms

• toot:Emoji -- custom emoji type
• toot:featured -- pinned posts collection (@id, @type: @id)
• toot:featuredTags -- featured hashtags collection
• toot:discoverable -- boolean, whether to list in directory
• toot:indexable -- boolean, consent for search indexing (FEP-5feb)
• toot:suspended -- boolean, account suspension status
• toot:memorial -- boolean, memorial/deceased account
• toot:votersCount -- poll total voter count
• toot:blurhash -- BlurHash string for media placeholders
• toot:focalPoint -- [x, y] coordinates for image cropping
• toot:attributionDomains -- domains allowed to use fediverse:creator (since 4.3)

Key as: Extended Terms

• as:Hashtag -- hashtag tag type
• as:manuallyApprovesFollowers -- locked account flag
• as:movedTo -- account migration target
• as:sensitive -- content warning flag
• as:alsoKnownAs -- account aliases for migration

Key schema: Terms

• schema:PropertyValue -- profile metadata field type
• schema:value -- profile metadata field value

Gotcha: The schema: prefix maps to http://schema.org# (with hash, no HTTPS). This is technically incorrect per schema.org's own context (https://schema.org/). Strict JSON-LD processors may fail to resolve these terms. Implementations should accept both forms.

---

2. Content-Type and Accept Headers

For Fetching ActivityPub Resources (GET)

Send the Accept header as:

Accept: application/activity+json, application/ld+json

Mastodon uses HTTP content negotiation. If the Accept header does not include an ActivityPub media type, Mastodon serves the HTML profile page instead of the JSON-LD representation.

Both of these are valid and equivalent:

• application/activity+json
• application/ld+json; profile="https://www.w3.org/ns/activitystreams"

For Sending Activities (POST to inbox)

Set the Content-Type header to:

Content-Type: application/ld+json; profile="https://www.w3.org/ns/activitystreams"

Or equivalently:

Content-Type: application/activity+json

Gotcha: Some HTTP libraries default to application/json or application/x-www-form-urlencoded. Always set Content-Type explicitly. Send the request body as a raw JSON string, not form-encoded.

---

3. HTTP Signatures

Mastodon requires HTTP Signatures on all incoming POST requests and optionally on GET requests (when authorized fetch is enabled).

Algorithm

RSA-SHA256 (RSASSA-PKCS1-v1_5 with SHA-256) using the actor's RSA private key. The corresponding public key is published in the actor document under publicKey.

Signature Header Format

Signature: keyId="https://example.com/users/alice#main-key",
           headers="(request-target) host date digest",
           signature="<base64-encoded-signature>"

Required Signed Headers

For GET requests (when authorized fetch is enabled):

• (request-target) -- the HTTP method and path (e.g., get /users/bob)
• host -- target hostname
• date -- RFC 2616 date (must be within clock skew tolerance)

For POST requests (always required):

• (request-target)
• host
• date
• digest -- mandatory for POST; Mastodon will reject POSTs without a signed Digest header

Digest Header

For POST requests, compute the SHA-256 hash of the request body:

Digest: SHA-256=<base64-encoded-sha256-hash-of-body>

The digest header name must appear in the headers parameter of the Signature header.

Key ID Format

The keyId is typically {actor_uri}#main-key. Mastodon dereferences this to fetch the actor document, extracts publicKey.publicKeyPem, and verifies the signature.

---

4. Authorized Fetch (Secure Mode)

Enabled by setting AUTHORIZED_FETCH=true (or LIMITED_FEDERATION_MODE=true) in the Mastodon environment.

What Changes

• All GET requests to ActivityPub representations of public posts and profiles require HTTP signatures (normally these are anonymous)
• Linked-Data Signatures are no longer generated for public posts
• Suspended/blocked servers cannot fetch public content because they must identify themselves via signature
• Server-level and user-level blocks become more effective

Limitations

• Uses significantly more server resources (signature verification on every request)
• Causes compatibility problems with older software that does not sign GET requests
• Does not prevent determined adversaries -- a bad actor can set up a new instance to bypass blocks
• Authorized fetch is circumventable by design; it is a friction mechanism, not a security boundary

Limited Federation Mode

LIMITED_FEDERATION_MODE=true is a superset of authorized fetch. It additionally restricts federation to manually approved servers only. Administrators must explicitly allowlist each domain.

---

5. Supported Object Types

First-Class Types

Type	Mastodon Representation
Note	Regular status/toot
Question	Poll status

Converted Types (Best-Effort)

These types are accepted but not given full treatment:

Type	Handling
Article	Uses content (or name if no content) as status text; appends url; summary becomes CW; icon becomes thumbnail
Page	Same conversion logic as Article
Image	Same conversion logic
Audio	Same conversion logic
Video	Same conversion logic
Event	Same conversion logic

Critical gotcha: Article objects are not rendered with their full HTML content the way Note objects are. The content is used to generate status text, but formatting, structure, and richness are lost. Many platforms (WordPress, WriteFreely, Ghost) send Article objects that display poorly on Mastodon. Some platforms have switched to sending Note instead for better Mastodon compatibility.

---

6. Poll Implementation (Question/Answer)

Mastodon represents polls as Question objects (used as an Object type, not as an IntransitiveActivity). Polls are wrapped in Create activities like normal statuses.

Poll Structure

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://example.com/users/alice/statuses/12345",
  "type": "Question",
  "content": "<p>What is your favorite color?</p>",
  "published": "2024-01-01T00:00:00Z",
  "endTime": "2024-01-02T00:00:00Z",
  "oneOf": [
    {
      "type": "Note",
      "name": "Red",
      "replies": { "type": "Collection", "totalItems": 42 }
    },
    {
      "type": "Note",
      "name": "Blue",
      "replies": { "type": "Collection", "totalItems": 58 }
    }
  ],
  "toot:votersCount": 100
}

Key Properties

• oneOf -- array of options for single-choice polls
• anyOf -- array of options for multiple-choice polls
• Each option: type: "Note", name: "<option text>", replies.totalItems: <vote count>
• Options have no id property
• endTime -- ISO 8601 timestamp when voting closes
• closed -- ISO 8601 timestamp when voting actually closed (may differ from endTime)
• votersCount (toot:votersCount) -- total number of unique voters

Voting Mechanism

Votes are sent as Create activities where the object is a Note with:

• name exactly matching the poll option's name
• inReplyTo pointing to the Question's id
• to addressing the poll author

For multiple-choice polls, Mastodon sends separate Create activities for each selected option from the same voter.

Poll Updates

Poll results are updated via Update activities on the Question object. Remote instances can re-fetch the poll to get current vote counts.

---

7. Edit / Update Activities

Mastodon has supported editing statuses since version 3.5.0 (March 2022).

How It Works

• An Update activity is sent with the full updated object as object
• The updated timestamp on the object signals that this is an edit (not a first-time creation)
• Receiving instances check: if a status with that id already exists locally, the Update triggers an edit; otherwise it is treated as a new status
• Mastodon's FetchRemoteStatusService automatically converts a Create to an Update when the status already exists in the local database

Edit History

• Edit history is stored locally and viewable on the originating instance
• Edit history is not federated in the ActivityPub representation (as of early 2026, there is an open issue requesting this: #23292)
• The StatusEdit entity in the REST API exposes edit history

Limitations

• Remote instances only see the latest version of a post
• There is no standard way to fetch previous versions via ActivityPub

---

8. Announce (Boost) Activities

Outgoing Announces

When a user boosts a post, Mastodon sends:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://example.com/users/alice/statuses/67890/activity",
  "type": "Announce",
  "actor": "https://example.com/users/alice",
  "published": "2024-01-01T12:00:00Z",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers",
          "https://remote.example/users/bob"],
  "object": "https://remote.example/users/bob/statuses/11111"
}

The object field is a URI string, not an inline object. The receiving instance must dereference (fetch) the URI to get the full status content.

Incoming Announces

Mastodon accepts both:

• object as a URI string (standard pattern) -- fetches the original
• object as an inline full object -- uses the embedded data

When processing an incoming Announce, Mastodon uses ActivityPub::FetchRemoteStatusService to retrieve the original status if needed.

Boost Counters

Statuses expose a shares collection:

• shares.totalItems -- number of boosts received
• Since Mastodon 4.3, like and boost counts are exposed in ActivityPub Note objects
• Since Mastodon 4.4, favorite and boost counts match remote server values when available

Gotcha: Boost/like counts are inherently inconsistent across instances. Only the originating instance knows the true total. Other instances only see counts from their own users plus what has been federated to them.

---

9. Direct Messages (Mentions-Only Visibility)

Mastodon does not have a dedicated direct message ActivityPub type. "Direct messages" are simply Note objects with restricted addressing.

Addressing Rules for Direct Messages

{
  "type": "Note",
  "to": ["https://remote.example/users/bob"],
  "cc": [],
  "tag": [
    {
      "type": "Mention",
      "href": "https://remote.example/users/bob",
      "name": "@[e-mail]"
    }
  ]
}

Requirements:

• No as:Public (https://www.w3.org/ns/activitystreams#Public) in to or cc
• No follower collection URI in to or cc
• Only specific actor URIs in to
• All actors in to/cc must have a corresponding Mention tag

Visibility Level Calculation

Mastodon calculates visibility from addressing, not from an explicit visibility field:

Visibility	to	cc	Mention tags
public	as:Public	followers collection	--
unlisted	followers collection	as:Public	--
private	followers collection	(no as:Public)	--
limited	specific actors	--	Not all actors are Mentioned
direct	specific actors	--	All actors are Mentioned

Gotcha: The distinction between limited and direct depends on whether all addressed actors have corresponding Mention tags. If you address actors in to without Mention tags, Mastodon treats it as limited (not direct).

---

10. Custom Emoji Federation

Custom emoji are represented as Emoji tags on objects:

{
  "type": "Note",
  "content": "<p>Hello :coolcat:</p>",
  "tag": [
    {
      "id": "https://example.com/emojis/coolcat",
      "type": "Emoji",
      "name": ":coolcat:",
      "icon": {
        "type": "Image",
        "mediaType": "image/png",
        "url": "https://example.com/system/custom_emojis/images/coolcat.png"
      }
    }
  ]
}

Properties

• type: "Emoji" (from toot:Emoji)
• name: shortcode including colons (e.g., :coolcat:)
• icon.type: "Image"
• icon.mediaType: MIME type (e.g., image/png)
• icon.url: direct URL to the emoji image

Behavior

• Mastodon scans name, summary, and content for shortcode substrings and links them to the emoji image
• Custom emoji from remote instances are visible in federated posts
• Administrators can see and copy emoji from federated instances
• Emoji updates and deletions do not reliably federate
• Custom emoji may not render in display names and bios in all contexts

---

11. HTML Sanitization

Mastodon sanitizes all incoming HTML to protect API client developers from unexpected markup.

Allowed Elements

a, abbr, audio, b, blockquote, br, code, div, em, i, img, li, ol, p,
pre, source, span, strong, ul, video

Allowed CSS Classes

mention, hashtag, ellipsis, invisible, h-*, p-*, u-*, dt-*, e-*

(Microformats2 classes are preserved.)

Behavior

• Unsupported elements are stripped (content preserved, tags removed)
• Unsupported attributes are stripped
• Links (<a>) are kept only if the protocol is supported (http, https); other protocols are converted to plain text
• Mentions and hashtags are represented as <a> tags with class="mention" or class="hashtag"

Gotcha: If your platform sends rich HTML (tables, headings, definition lists, details/summary, etc.), Mastodon will strip all of it. Only the plain text content inside those elements survives. Design your content to degrade gracefully.

---

12. Character Limits

Composing Limit

Mastodon's default composing limit is 500 characters. This is enforced client-side and in the REST API. Instance administrators can modify this by changing source code, but it is not a simple configuration option.

Receiving Limit

Mastodon accepts and displays posts longer than 500 characters from remote instances. The ActivityPub protocol has no character limit.

• Long posts are displayed with automatic truncation and a "Read More" link in the web UI
• Most Mastodon-compatible mobile clients also truncate with "Read More"
• The post content is stored in full -- no data is lost

Hard Size Limits (FEDERATION.md)

Property	Limit	Behavior if exceeded
JSON-LD serialization	1 MB	Activity rejected
Username	2048 chars	Actor rejected
Display name	2048 chars	Truncated
Profile note (bio)	20 kB	Truncated
Profile fields	50 fields	List truncated
Field name/value	2047 chars	Truncated
Poll options	500	List truncated
Custom emoji shortcode	2048 chars	Rejected
Media descriptions (alt)	1500 chars	Truncated
Account aliases	256	List truncated
Attribution domains	256	List truncated

---

13. Thread Fetching and Reply Dereferencing

inReplyTo Resolution

When Mastodon encounters a status with an inReplyTo pointing to an unknown URI, it fetches the parent status from the remote server. This works recursively up the reply chain to build the thread context.

Same-Server Reply Fetching

Upon discovering a remote status, Mastodon fetches up to 5 replies from the same server as the original post. This helps fill in thread context but is limited in scope.

Reply Distribution Problem

Replies are sent to the parent post's instance but are not forwarded through follower networks automatically. This creates a fundamental gap:

• If A posts, B replies, and C replies to B: A's instance may never see C's reply unless C explicitly mentions A
• Each instance has an incomplete view of any conversation
• Reply counts, like counts, and boost counts differ across instances

Fetch All Replies (4.4+)

Mastodon 4.4 introduced a "fetch all replies" feature (disabled by default). When triggered:

• Paginates through the first layer of replies collections from the source server
• Collects reply URIs
• Launches background workers to fetch them

Mastodon 4.5 Improvements

• Fetch all replies is enabled by default
• Automatically checks for missing replies on page load and every 15 min
• New AsyncRefresh API: the /api/v1/statuses/:id/context endpoint returns a Mastodon-Async-Refresh HTTP header when background fetch jobs are pending, allowing clients to poll for completion

---

14. NodeInfo Implementation

Discovery Endpoint

GET /.well-known/nodeinfo

Response:

{
  "links": [
    {
      "rel": "http://nodeinfo.diaspora.software/ns/schema/2.0",
      "href": "https://mastodon.example/nodeinfo/2.0"
    }
  ]
}

NodeInfo 2.0 Response

GET /nodeinfo/2.0

{
  "version": "2.0",
  "software": {
    "name": "mastodon",
    "version": "4.5.0"
  },
  "protocols": ["activitypub"],
  "services": {
    "outbound": [],
    "inbound": []
  },
  "usage": {
    "users": {
      "total": 12345,
      "activeMonth": 5678,
      "activeHalfyear": 9012
    },
    "localPosts": 678901
  },
  "openRegistrations": true,
  "metadata": {
    "nodeName": "Example Mastodon",
    "nodeDescription": "A Mastodon instance"
  }
}

Note: software.name is always lowercase "mastodon". Use this to detect Mastodon instances programmatically.

---

15. Mastodon REST API Compatibility Layer

Mastodon's REST API has become a de facto standard. Many Fediverse platforms implement it (partially or fully) to allow Mastodon client apps to work with them:

Platform	Mastodon API Support
Pleroma	Near-complete, with Pleroma-specific extensions
Akkoma	Near-complete (Pleroma fork), with additional extensions
GoToSocial	Well-implemented, works with major Mastodon apps
Firefish	Partial support (now discontinued)
Sharkey	Partial support (Misskey fork)
Pixelfed	Partial support for photo-related endpoints
Friendica	Partial support

API Discovery Challenge

Different platforms support different subsets of the Mastodon API and add their own extensions. There is no standardized way to discover which endpoints a server supports. Client libraries like Megalodon abstract over platform differences.

---

16. Recent Version Changes (4.x Series)

Mastodon 4.3 (October 2024)

Federation-relevant changes:

• attributionDomains: new toot: property for fediverse:creator author attribution on external articles
• Like and boost counts now exposed in ActivityPub Note objects
• summary field in non-Note objects treated as HTML (was plain text)
• Grouped notifications in REST API
• Numeric actor IDs: new users get numeric-based ActivityPub URIs (preparation for future account renaming)

Mastodon 4.4 (July 2025)

Federation-relevant changes:

• Quote post display: incoming quote posts per FEP-044f rendered in web UI; new quote attribute on Status entity
• RFC 9421 HTTP Message Signatures: experimental support for verification of incoming signatures (behind feature flag)
• FASP (Fediverse Auxiliary Service Providers): initial support for shared decentralized services (behind feature flag)
• Fetch all replies: new backend feature to fetch missing remote replies (disabled by default)
• Remote counts: favorite/boost counts match remote server values

Mastodon 4.5 (November 2025)

Federation-relevant changes:

• Quote post authoring: users can compose quote posts; quoted_status_id API parameter; new quote states (blocked_account, blocked_domain, muted_account)
• Fetch all replies enabled by default: automatic background fetching of missing conversation participants
• AsyncRefresh API: Mastodon-Async-Refresh HTTP header for async background job status on context endpoint
• RFC 9421 standard: experimental flag removed; incoming HTTP Message Signatures now verified by default

Supported FEPs

• FEP-67ff: FEDERATION.md
• FEP-f1d5: NodeInfo in Fediverse Software
• FEP-8fcf: Followers collection synchronization
• FEP-5feb: Search indexing consent (toot:indexable)
• FEP-044f: Consent-respecting quote posts
• FEP-3b86: Activity Intents (Follow, Announce, Like, Object intents)

---

17. Common Federation Gotchas

JSON-LD Issues

1. schema.org base URI bug: Mastodon maps schema: to http://schema.org# instead of https://schema.org/. Strict JSON-LD processors will fail. Workaround: accept both forms.

2. Context order matters: Some implementations are sensitive to the order of items in the @context array. Match Mastodon's order when possible.

3. Unknown properties ignored: Mastodon silently ignores JSON-LD properties it does not understand. Your custom extensions will not cause errors but will not be processed.

Reply Propagation

4. Incomplete threads: Mastodon instances have fundamentally incomplete views of conversations. Replies are not forwarded upstream. Only the instance where a post originated has the full picture.

5. Counter inconsistency: Like counts, boost counts, and reply counts differ across instances. Never assume these numbers are authoritative on any instance except the originating one.

Content Handling

6. Article objects degraded: Mastodon does not render Article objects with full fidelity. Long-form content platforms should consider sending Note for Mastodon compatibility or accept that their content will be reduced to plain text + URL.

7. HTML stripped aggressively: Tables, headings, definition lists, details/summary, horizontal rules, and other block-level elements are stripped. Only a minimal subset of HTML survives.

8. Hashtag/mention encoding: When a mention is present, hashtags in the Note activity can sometimes be replaced with mentions during federation delivery (the object itself retains correct tags when fetched directly).

Protocol Behavior

9. Block activity sent S2S: Mastodon sends Block activities server-to-server when a user blocks a remote account, even though the ActivityPub spec defines Block for client-to-server only. Your implementation should handle incoming Block activities gracefully.

10. Linked-Data Signatures disabled in secure mode: When AUTHORIZED_FETCH=true, Mastodon stops generating LD-Signatures on public posts. Implementations that rely on LD-Signatures for relay forwarding will break.

11. Clock skew sensitivity: HTTP signature verification checks the Date header. If your server's clock is significantly off, signatures will be rejected.

Performance

12. Link preview DDoS effect: Every instance that receives a post with a URL will independently fetch the link preview (OpenGraph data). A post that goes viral can cause hundreds of concurrent requests to the linked website.

13. Inbox forwarding: Mastodon does not forward activities through inboxes the way the AP spec describes. Replies to your posts from third-party servers may never reach followers who are not on the replier's server.

---

18. WebFinger

Mastodon requires WebFinger for user discovery. The endpoint:

GET https://example.com/.well-known/webfinger?resource=acct:[e-mail]

Must return a JRD document with at minimum:

• subject: the acct: URI
• links: array including a link with rel: "self", type: "application/activity+json", and href pointing to the actor URI

Mastodon uses username@domain as the canonical user identifier. The WebFinger lookup is fundamental to Mastodon's database design: every remote actor is stored with a username and domain pair resolved via WebFinger.

---

19. Actor Structure

Mastodon actors are Person type (or Application for the instance actor, Service for bot/automated accounts). Key properties:

• preferredUsername -- the local username (before the @)
• name -- display name
• summary -- bio (HTML)
• url -- profile page URL (HTML)
• inbox -- ActivityPub inbox endpoint
• outbox -- ActivityPub outbox endpoint
• followers -- followers collection URI
• following -- following collection URI
• featured (toot:featured) -- pinned posts collection
• featuredTags (toot:featuredTags) -- featured hashtags
• publicKey -- RSA public key for HTTP signature verification
• endpoints.sharedInbox -- shared inbox for efficient delivery
• icon -- avatar image
• image -- header/banner image
• attachment -- array of schema:PropertyValue for profile fields
• discoverable (toot:discoverable) -- directory listing consent
• indexable (toot:indexable) -- search indexing consent
• memorial (toot:memorial) -- deceased account flag
• suspended (toot:suspended) -- suspension flag
• manuallyApprovesFollowers -- locked account
• movedTo -- migration target actor URI
• alsoKnownAs -- array of aliases (for migration verification)
• attributionDomains (toot:attributionDomains) -- fediverse:creator domains

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20. Supported Activities (Inbox Processing)

Activity	Object Type	Mastodon Behavior
Create	Note	Creates a new status
Create	Question	Creates a poll status
Create	Other	Best-effort conversion to status
Update	Note/Question	Edits status (if updated present, since 3.5)
Update	Person	Updates remote actor profile
Delete	Note	Deletes remote status locally
Delete	Person	Removes remote actor and content
Announce	Status URI	Creates a boost
Like	Status URI	Creates a favorite
Follow	Actor URI	Sends follow request
Accept	Follow	Confirms follow relationship
Reject	Follow	Rejects follow request
Undo	Follow	Unfollows
Undo	Like	Removes favorite
Undo	Announce	Removes boost
Undo	Block	Removes block
Block	Actor URI	Records block (non-standard S2S use)
Flag	Actor + objects	Reports content/user to moderators
Move	Actor URI	Triggers account migration
Add	Status to featured	Pins a status
Remove	Status from featured	Unpins a status