diff --git a/Gemfile b/Gemfile
index 138566780..1ad600224 100644
--- a/Gemfile
+++ b/Gemfile
@@ -159,3 +159,6 @@ end
 
 gem 'concurrent-ruby', require: false
 gem 'connection_pool', require: false
+
+gem 'xorcist', '~> 1.1'
+gem 'pluck_each', '~> 0.1.3'
diff --git a/Gemfile.lock b/Gemfile.lock
index aea592da1..915b45512 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -403,6 +403,9 @@ GEM
     pghero (2.7.2)
       activerecord (>= 5)
     pkg-config (1.4.4)
+    pluck_each (0.1.3)
+      activerecord (> 3.2.0)
+      activesupport (> 3.0.0)
     posix-spawn (0.3.15)
     premailer (1.14.2)
       addressable
@@ -662,6 +665,7 @@ GEM
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
     wisper (2.0.1)
+    xorcist (1.1.2)
     xpath (3.2.0)
       nokogiri (~> 1.8)
 
@@ -748,6 +752,7 @@ DEPENDENCIES
   pg (~> 1.2)
   pghero (~> 2.7)
   pkg-config (~> 1.4)
+  pluck_each (~> 0.1.3)
   posix-spawn
   premailer-rails
   private_address_check (~> 0.5)
@@ -796,3 +801,4 @@ DEPENDENCIES
   webmock (~> 3.9)
   webpacker (~> 5.2)
   webpush
+  xorcist (~> 1.1)
diff --git a/app/controllers/activitypub/followers_synchronizations_controller.rb b/app/controllers/activitypub/followers_synchronizations_controller.rb
new file mode 100644
index 000000000..525031105
--- /dev/null
+++ b/app/controllers/activitypub/followers_synchronizations_controller.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ActivityPub::FollowersSynchronizationsController < ActivityPub::BaseController
+  include SignatureVerification
+  include AccountOwnedConcern
+
+  before_action :require_signature!
+  before_action :set_items
+  before_action :set_cache_headers
+
+  def show
+    expires_in 0, public: false
+    render json: collection_presenter,
+           serializer: ActivityPub::CollectionSerializer,
+           adapter: ActivityPub::Adapter,
+           content_type: 'application/activity+json'
+  end
+
+  private
+
+  def uri_prefix
+    signed_request_account.uri[/http(s?):\/\/[^\/]+\//]
+  end
+
+  def set_items
+    @items = @account.followers.where(Account.arel_table[:uri].matches(uri_prefix + '%', false, true)).pluck(:uri)
+  end
+
+  def collection_presenter
+    ActivityPub::CollectionPresenter.new(
+      id: account_followers_synchronization_url(@account),
+      type: :ordered,
+      items: @items
+    )
+  end
+end
diff --git a/app/controllers/activitypub/inboxes_controller.rb b/app/controllers/activitypub/inboxes_controller.rb
index 0a561e7f0..fdb60d590 100644
--- a/app/controllers/activitypub/inboxes_controller.rb
+++ b/app/controllers/activitypub/inboxes_controller.rb
@@ -11,6 +11,7 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
 
   def create
     upgrade_account
+    process_collection_synchronization
     process_payload
     head 202
   end
@@ -52,6 +53,19 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
     DeliveryFailureTracker.reset!(signed_request_account.inbox_url)
   end
 
+  def process_collection_synchronization
+    raw_params = request.headers['Collection-Synchronization']
+    return if raw_params.blank? || ENV['DISABLE_FOLLOWERS_SYNCHRONIZATION'] == 'true'
+
+    # Re-using the syntax for signature parameters
+    tree   = SignatureParamsParser.new.parse(raw_params)
+    params = SignatureParamsTransformer.new.apply(tree)
+
+    ActivityPub::PrepareFollowersSynchronizationService.new.call(signed_request_account, params)
+  rescue Parslet::ParseFailed
+    Rails.logger.warn 'Error parsing Collection-Synchronization header'
+  end
+
   def process_payload
     ActivityPub::ProcessingWorker.perform_async(signed_request_account.id, body, @account&.id)
   end
diff --git a/app/lib/activitypub/tag_manager.rb b/app/lib/activitypub/tag_manager.rb
index 3f98dad2e..3f2ae1106 100644
--- a/app/lib/activitypub/tag_manager.rb
+++ b/app/lib/activitypub/tag_manager.rb
@@ -40,6 +40,10 @@ class ActivityPub::TagManager
     end
   end
 
+  def uri_for_username(username)
+    account_url(username: username)
+  end
+
   def generate_uri_for(_target)
     URI.join(root_url, 'payloads', SecureRandom.uuid)
   end
diff --git a/app/models/account.rb b/app/models/account.rb
index 5acc8d621..59d338f5a 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -352,6 +352,12 @@ class Account < ApplicationRecord
     shared_inbox_url.presence || inbox_url
   end
 
+  def synchronization_uri_prefix
+    return 'local' if local?
+
+    @synchronization_uri_prefix ||= uri[/http(s?):\/\/[^\/]+\//]
+  end
+
   class Field < ActiveModelSerializers::Model
     attributes :name, :value, :verified_at, :account, :errors
 
diff --git a/app/models/concerns/account_interactions.rb b/app/models/concerns/account_interactions.rb
index 6a0ad5aa9..e2c4b8acf 100644
--- a/app/models/concerns/account_interactions.rb
+++ b/app/models/concerns/account_interactions.rb
@@ -243,6 +243,26 @@ module AccountInteractions
          .where('users.current_sign_in_at > ?', User::ACTIVE_DURATION.ago)
   end
 
+  def remote_followers_hash(url_prefix)
+    Rails.cache.fetch("followers_hash:#{id}:#{url_prefix}") do
+      digest = "\x00" * 32
+      followers.where(Account.arel_table[:uri].matches(url_prefix + '%', false, true)).pluck_each(:uri) do |uri|
+        Xorcist.xor!(digest, Digest::SHA256.digest(uri))
+      end
+      digest.unpack('H*')[0]
+    end
+  end
+
+  def local_followers_hash
+    Rails.cache.fetch("followers_hash:#{id}:local") do
+      digest = "\x00" * 32
+      followers.where(domain: nil).pluck_each(:username) do |username|
+        Xorcist.xor!(digest, Digest::SHA256.digest(ActivityPub::TagManager.instance.uri_for_username(username)))
+      end
+      digest.unpack('H*')[0]
+    end
+  end
+
   private
 
   def remove_potential_friendship(other_account, mutual = false)
diff --git a/app/models/follow.rb b/app/models/follow.rb
index 0b4ddbf3f..55a9da792 100644
--- a/app/models/follow.rb
+++ b/app/models/follow.rb
@@ -41,8 +41,10 @@ class Follow < ApplicationRecord
 
   before_validation :set_uri, only: :create
   after_create :increment_cache_counters
+  after_create :invalidate_hash_cache
   after_destroy :remove_endorsements
   after_destroy :decrement_cache_counters
+  after_destroy :invalidate_hash_cache
 
   private
 
@@ -63,4 +65,10 @@ class Follow < ApplicationRecord
     account&.decrement_count!(:following_count)
     target_account&.decrement_count!(:followers_count)
   end
+
+  def invalidate_hash_cache
+    return if account.local? && target_account.local?
+
+    Rails.cache.delete("followers_hash:#{target_account_id}:#{account.synchronization_uri_prefix}")
+  end
 end
diff --git a/app/services/activitypub/prepare_followers_synchronization_service.rb b/app/services/activitypub/prepare_followers_synchronization_service.rb
new file mode 100644
index 000000000..2d22ed701
--- /dev/null
+++ b/app/services/activitypub/prepare_followers_synchronization_service.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class ActivityPub::PrepareFollowersSynchronizationService < BaseService
+  include JsonLdHelper
+
+  def call(account, params)
+    @account = account
+
+    return if params['collectionId'] != @account.followers_url || invalid_origin?(params['url']) || @account.local_followers_hash == params['digest']
+
+    ActivityPub::FollowersSynchronizationWorker.perform_async(@account.id, params['url'])
+  end
+end
diff --git a/app/services/activitypub/synchronize_followers_service.rb b/app/services/activitypub/synchronize_followers_service.rb
new file mode 100644
index 000000000..d83fcf55e
--- /dev/null
+++ b/app/services/activitypub/synchronize_followers_service.rb
@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+class ActivityPub::SynchronizeFollowersService < BaseService
+  include JsonLdHelper
+  include Payloadable
+
+  def call(account, partial_collection_url)
+    @account = account
+
+    items = collection_items(partial_collection_url)
+    return if items.nil?
+
+    # There could be unresolved accounts (hence the call to .compact) but this
+    # should never happen in practice, since in almost all cases we keep an
+    # Account record, and should we not do that, we should have sent a Delete.
+    # In any case there is not much we can do if that occurs.
+    @expected_followers = items.map { |uri| ActivityPub::TagManager.instance.uri_to_resource(uri, Account) }.compact
+
+    remove_unexpected_local_followers!
+    handle_unexpected_outgoing_follows!
+  end
+
+  private
+
+  def remove_unexpected_local_followers!
+    @account.followers.local.where.not(id: @expected_followers.map(&:id)).each do |unexpected_follower|
+      UnfollowService.new.call(unexpected_follower, @account)
+    end
+  end
+
+  def handle_unexpected_outgoing_follows!
+    @expected_followers.each do |expected_follower|
+      next if expected_follower.following?(@account)
+
+      if expected_follower.requested?(@account)
+        # For some reason the follow request went through but we missed it
+        expected_follower.follow_requests.find_by(target_account: @account)&.authorize!
+      else
+        # Since we were not aware of the follow from our side, we do not have an
+        # ID for it that we can include in the Undo activity. For this reason,
+        # the Undo may not work with software that relies exclusively on
+        # matching activity IDs and not the actor and target
+        follow = Follow.new(account: expected_follower, target_account: @account)
+        ActivityPub::DeliveryWorker.perform_async(build_undo_follow_json(follow), follow.account_id, follow.target_account.inbox_url)
+      end
+    end
+  end
+
+  def build_undo_follow_json(follow)
+    Oj.dump(serialize_payload(follow, ActivityPub::UndoFollowSerializer))
+  end
+
+  def collection_items(collection_or_uri)
+    collection = fetch_collection(collection_or_uri)
+    return unless collection.is_a?(Hash)
+
+    collection = fetch_collection(collection['first']) if collection['first'].present?
+    return unless collection.is_a?(Hash)
+
+    case collection['type']
+    when 'Collection', 'CollectionPage'
+      collection['items']
+    when 'OrderedCollection', 'OrderedCollectionPage'
+      collection['orderedItems']
+    end
+  end
+
+  def fetch_collection(collection_or_uri)
+    return collection_or_uri if collection_or_uri.is_a?(Hash)
+    return if invalid_origin?(collection_or_uri)
+
+    fetch_resource_without_id_validation(collection_or_uri, nil, true)
+  end
+end
diff --git a/app/workers/activitypub/delivery_worker.rb b/app/workers/activitypub/delivery_worker.rb
index 60775787a..6c5a576a7 100644
--- a/app/workers/activitypub/delivery_worker.rb
+++ b/app/workers/activitypub/delivery_worker.rb
@@ -2,6 +2,7 @@
 
 class ActivityPub::DeliveryWorker
   include Sidekiq::Worker
+  include RoutingHelper
   include JsonLdHelper
 
   STOPLIGHT_FAILURE_THRESHOLD = 10
@@ -38,9 +39,18 @@ class ActivityPub::DeliveryWorker
     Request.new(:post, @inbox_url, body: @json, http_client: http_client).tap do |request|
       request.on_behalf_of(@source_account, :uri, sign_with: @options[:sign_with])
       request.add_headers(HEADERS)
+      request.add_headers({ 'Collection-Synchronization' => synchronization_header }) if ENV['DISABLE_FOLLOWERS_SYNCHRONIZATION'] != 'true' && @options[:synchronize_followers]
     end
   end
 
+  def synchronization_header
+    "collectionId=\"#{account_followers_url(@source_account)}\", digest=\"#{@source_account.remote_followers_hash(inbox_url_prefix)}\", url=\"#{account_followers_synchronization_url(@source_account)}\""
+  end
+
+  def inbox_url_prefix
+    @inbox_url[/http(s?):\/\/[^\/]+\//]
+  end
+
   def perform_request
     light = Stoplight(@inbox_url) do
       request_pool.with(@host) do |http_client|
diff --git a/app/workers/activitypub/distribution_worker.rb b/app/workers/activitypub/distribution_worker.rb
index e4997ba0e..9b4814644 100644
--- a/app/workers/activitypub/distribution_worker.rb
+++ b/app/workers/activitypub/distribution_worker.rb
@@ -13,7 +13,7 @@ class ActivityPub::DistributionWorker
     return if skip_distribution?
 
     ActivityPub::DeliveryWorker.push_bulk(inboxes) do |inbox_url|
-      [payload, @account.id, inbox_url]
+      [payload, @account.id, inbox_url, { synchronize_followers: !@status.distributable? }]
     end
 
     relay! if relayable?
diff --git a/app/workers/activitypub/followers_synchronization_worker.rb b/app/workers/activitypub/followers_synchronization_worker.rb
new file mode 100644
index 000000000..35a3ef0b9
--- /dev/null
+++ b/app/workers/activitypub/followers_synchronization_worker.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class ActivityPub::FollowersSynchronizationWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'push', lock: :until_executed
+
+  def perform(account_id, url)
+    @account = Account.find_by(id: account_id)
+    return true if @account.nil?
+
+    ActivityPub::SynchronizeFollowersService.new.call(@account, url)
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index a21dbd45e..e0ef23723 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -83,6 +83,7 @@ Rails.application.routes.draw do
     resource :inbox, only: [:create], module: :activitypub
     resource :claim, only: [:create], module: :activitypub
     resources :collections, only: [:show], module: :activitypub
+    resource :followers_synchronization, only: [:show], module: :activitypub
   end
 
   resource :inbox, only: [:create], module: :activitypub
diff --git a/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb b/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb
new file mode 100644
index 000000000..a24d3f8e0
--- /dev/null
+++ b/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb
@@ -0,0 +1,58 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::FollowersSynchronizationsController, type: :controller do
+  let!(:account)    { Fabricate(:account) }
+  let!(:follower_1) { Fabricate(:account, domain: 'example.com', uri: 'https://example.com/users/a') }
+  let!(:follower_2) { Fabricate(:account, domain: 'example.com', uri: 'https://example.com/users/b') }
+  let!(:follower_3) { Fabricate(:account, domain: 'foo.com', uri: 'https://foo.com/users/a') }
+
+  before do
+    follower_1.follow!(account)
+    follower_2.follow!(account)
+    follower_3.follow!(account)
+  end
+
+  before do
+    allow(controller).to receive(:signed_request_account).and_return(remote_account)
+  end
+
+  describe 'GET #show' do
+    context 'without signature' do
+      let(:remote_account) { nil }
+
+      before do
+        get :show, params: { account_username: account.username }
+      end
+
+      it 'returns http not authorized' do
+        expect(response).to have_http_status(401)
+      end
+    end
+
+    context 'with signature from example.com' do
+      let(:remote_account) { Fabricate(:account, domain: 'example.com', uri: 'https://example.com/instance') }
+
+      before do
+        get :show, params: { account_username: account.username }
+      end
+
+      it 'returns http success' do
+        expect(response).to have_http_status(200)
+      end
+
+      it 'returns application/activity+json' do
+        expect(response.content_type).to eq 'application/activity+json'
+      end
+
+      it 'returns orderedItems with followers from example.com' do
+        json = body_as_json
+        expect(json[:orderedItems]).to be_an Array
+        expect(json[:orderedItems].sort).to eq [follower_1.uri, follower_2.uri]
+      end
+
+      it 'returns private Cache-Control header' do
+        expect(response.headers['Cache-Control']).to eq 'max-age=0, private'
+      end
+    end
+  end
+end
diff --git a/spec/controllers/activitypub/inboxes_controller_spec.rb b/spec/controllers/activitypub/inboxes_controller_spec.rb
index f3bc23953..e5c004611 100644
--- a/spec/controllers/activitypub/inboxes_controller_spec.rb
+++ b/spec/controllers/activitypub/inboxes_controller_spec.rb
@@ -22,6 +22,56 @@ RSpec.describe ActivityPub::InboxesController, type: :controller do
       end
     end
 
+    context 'with Collection-Synchronization header' do
+      let(:remote_account)             { Fabricate(:account, followers_url: 'https://example.com/followers', domain: 'example.com', uri: 'https://example.com/actor', protocol: :activitypub) }
+      let(:synchronization_collection) { remote_account.followers_url }
+      let(:synchronization_url)        { 'https://example.com/followers-for-domain' }
+      let(:synchronization_hash)       { 'somehash' }
+      let(:synchronization_header)     { "collectionId=\"#{synchronization_collection}\", digest=\"#{synchronization_hash}\", url=\"#{synchronization_url}\"" }
+
+      before do
+        allow(ActivityPub::FollowersSynchronizationWorker).to receive(:perform_async).and_return(nil)
+        allow_any_instance_of(Account).to receive(:local_followers_hash).and_return('somehash')
+
+        request.headers['Collection-Synchronization'] = synchronization_header
+        post :create, body: '{}'
+      end
+
+      context 'with mismatching target collection' do
+        let(:synchronization_collection) { 'https://example.com/followers2' }
+
+        it 'does not start a synchronization job' do
+          expect(ActivityPub::FollowersSynchronizationWorker).not_to have_received(:perform_async)
+        end
+      end
+
+      context 'with mismatching domain in partial collection attribute' do
+        let(:synchronization_url) { 'https://example.org/followers' }
+
+        it 'does not start a synchronization job' do
+          expect(ActivityPub::FollowersSynchronizationWorker).not_to have_received(:perform_async)
+        end
+      end
+
+      context 'with matching digest' do
+        it 'does not start a synchronization job' do
+          expect(ActivityPub::FollowersSynchronizationWorker).not_to have_received(:perform_async)
+        end
+      end
+
+      context 'with mismatching digest' do
+        let(:synchronization_hash) { 'wronghash' }
+
+        it 'starts a synchronization job' do
+          expect(ActivityPub::FollowersSynchronizationWorker).to have_received(:perform_async)
+        end
+      end
+
+      it 'returns http accepted' do
+        expect(response).to have_http_status(202)
+      end
+    end
+
     context 'without signature' do
       before do
         post :create, body: '{}'
diff --git a/spec/models/concerns/account_interactions_spec.rb b/spec/models/concerns/account_interactions_spec.rb
index f0380179c..85fbf7e79 100644
--- a/spec/models/concerns/account_interactions_spec.rb
+++ b/spec/models/concerns/account_interactions_spec.rb
@@ -539,6 +539,49 @@ describe AccountInteractions do
     end
   end
 
+  describe '#followers_hash' do
+    let(:me) { Fabricate(:account, username: 'Me') }
+    let(:remote_1) { Fabricate(:account, username: 'alice', domain: 'example.org', uri: 'https://example.org/users/alice') }
+    let(:remote_2) { Fabricate(:account, username: 'bob', domain: 'example.org', uri: 'https://example.org/users/bob') }
+    let(:remote_3) { Fabricate(:account, username: 'eve', domain: 'foo.org', uri: 'https://foo.org/users/eve') }
+
+    before do
+      remote_1.follow!(me)
+      remote_2.follow!(me)
+      remote_3.follow!(me)
+      me.follow!(remote_1)
+    end
+
+    context 'on a local user' do
+      it 'returns correct hash for remote domains' do
+        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
+        expect(me.remote_followers_hash('https://foo.org/')).to eq 'ccb9c18a67134cfff9d62c7f7e7eb88e6b803446c244b84265565f4eba29df0e'
+      end
+
+      it 'invalidates cache as needed when removing or adding followers' do
+        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
+        remote_1.unfollow!(me)
+        expect(me.remote_followers_hash('https://example.org/')).to eq '241b00794ce9b46aa864f3220afadef128318da2659782985bac5ed5bd436bff'
+        remote_1.follow!(me)
+        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
+      end
+    end
+
+    context 'on a remote user' do
+      it 'returns correct hash for remote domains' do
+        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
+      end
+
+      it 'invalidates cache as needed when removing or adding followers' do
+        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
+        me.unfollow!(remote_1)
+        expect(remote_1.local_followers_hash).to eq '0000000000000000000000000000000000000000000000000000000000000000'
+        me.follow!(remote_1)
+        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
+      end
+    end
+  end
+
   describe 'muting an account' do
     let(:me) { Fabricate(:account, username: 'Me') }
     let(:you) { Fabricate(:account, username: 'You') }
diff --git a/spec/services/activitypub/synchronize_followers_service_spec.rb b/spec/services/activitypub/synchronize_followers_service_spec.rb
new file mode 100644
index 000000000..75dcf204b
--- /dev/null
+++ b/spec/services/activitypub/synchronize_followers_service_spec.rb
@@ -0,0 +1,105 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::SynchronizeFollowersService, type: :service do
+  let(:actor)          { Fabricate(:account, domain: 'example.com', uri: 'http://example.com/account', inbox_url: 'http://example.com/inbox') }
+  let(:alice)          { Fabricate(:account, username: 'alice') }
+  let(:bob)            { Fabricate(:account, username: 'bob') }
+  let(:eve)            { Fabricate(:account, username: 'eve') }
+  let(:mallory)        { Fabricate(:account, username: 'mallory') }
+  let(:collection_uri) { 'http://example.com/partial-followers' }
+
+  let(:items) do
+    [
+      ActivityPub::TagManager.instance.uri_for(alice),
+      ActivityPub::TagManager.instance.uri_for(eve),
+      ActivityPub::TagManager.instance.uri_for(mallory),
+    ]
+  end
+
+  let(:payload) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      type: 'Collection',
+      id: collection_uri,
+      items: items,
+    }.with_indifferent_access
+  end
+
+  subject { described_class.new }
+
+  shared_examples 'synchronizes followers' do
+    before do
+      alice.follow!(actor)
+      bob.follow!(actor)
+      mallory.request_follow!(actor)
+
+      allow(ActivityPub::DeliveryWorker).to receive(:perform_async)
+
+      subject.call(actor, collection_uri)
+    end
+
+    it 'keeps expected followers' do
+      expect(alice.following?(actor)).to be true
+    end
+
+    it 'removes local followers not in the remote list' do
+      expect(bob.following?(actor)).to be false
+    end
+
+    it 'converts follow requests to follow relationships when they have been accepted' do
+      expect(mallory.following?(actor)).to be true
+    end
+
+    it 'sends an Undo Follow to the actor' do
+      expect(ActivityPub::DeliveryWorker).to have_received(:perform_async).with(anything, eve.id, actor.inbox_url)
+    end
+  end
+
+  describe '#call' do
+    context 'when the endpoint is a Collection of actor URIs' do
+      before do
+        stub_request(:get, collection_uri).to_return(status: 200, body: Oj.dump(payload))
+      end
+
+      it_behaves_like 'synchronizes followers'
+    end
+
+    context 'when the endpoint is an OrderedCollection of actor URIs' do
+      let(:payload) do
+        {
+          '@context': 'https://www.w3.org/ns/activitystreams',
+          type: 'OrderedCollection',
+          id: collection_uri,
+          orderedItems: items,
+        }.with_indifferent_access
+      end
+
+      before do
+        stub_request(:get, collection_uri).to_return(status: 200, body: Oj.dump(payload))
+      end
+
+      it_behaves_like 'synchronizes followers'
+    end
+
+    context 'when the endpoint is a paginated Collection of actor URIs' do
+      let(:payload) do
+        {
+          '@context': 'https://www.w3.org/ns/activitystreams',
+          type: 'Collection',
+          id: collection_uri,
+          first: {
+            type: 'CollectionPage',
+            partOf: collection_uri,
+            items: items,
+          }
+        }.with_indifferent_access
+      end
+
+      before do
+        stub_request(:get, collection_uri).to_return(status: 200, body: Oj.dump(payload))
+      end
+
+      it_behaves_like 'synchronizes followers'
+    end
+  end
+end
diff --git a/spec/workers/activitypub/delivery_worker_spec.rb b/spec/workers/activitypub/delivery_worker_spec.rb
index 351be185c..f4633731e 100644
--- a/spec/workers/activitypub/delivery_worker_spec.rb
+++ b/spec/workers/activitypub/delivery_worker_spec.rb
@@ -3,16 +3,22 @@
 require 'rails_helper'
 
 describe ActivityPub::DeliveryWorker do
+  include RoutingHelper
+
   subject { described_class.new }
 
   let(:sender)  { Fabricate(:account) }
   let(:payload) { 'test' }
 
+  before do
+    allow_any_instance_of(Account).to receive(:remote_followers_hash).with('https://example.com/').and_return('somehash')
+  end
+
   describe 'perform' do
     it 'performs a request' do
       stub_request(:post, 'https://example.com/api').to_return(status: 200)
-      subject.perform(payload, sender.id, 'https://example.com/api')
-      expect(a_request(:post, 'https://example.com/api')).to have_been_made.once
+      subject.perform(payload, sender.id, 'https://example.com/api', { synchronize_followers: true })
+      expect(a_request(:post, 'https://example.com/api').with(headers: { 'Collection-Synchronization' => "collectionId=\"#{account_followers_url(sender)}\", digest=\"somehash\", url=\"#{account_followers_synchronization_url(sender)}\"" })).to have_been_made.once
     end
 
     it 'raises when request fails' do