diff --git a/spec/serializers/activitypub/actor_serializer_spec.rb b/spec/serializers/activitypub/actor_serializer_spec.rb
new file mode 100644
index 0000000000..ad24455953
--- /dev/null
+++ b/spec/serializers/activitypub/actor_serializer_spec.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::ActorSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  describe '#type' do
+    context 'with the instance actor' do
+      let(:record) { Account.find(Account::INSTANCE_ACTOR_ID) }
+
+      it { is_expected.to include('type' => 'Application') }
+    end
+
+    context 'with an application actor' do
+      let(:record) { Fabricate :account, actor_type: 'Application' }
+
+      it { is_expected.to include('type' => 'Service') }
+    end
+
+    context 'with a service actor' do
+      let(:record) { Fabricate :account, actor_type: 'Service' }
+
+      it { is_expected.to include('type' => 'Service') }
+    end
+
+    context 'with a Group actor' do
+      let(:record) { Fabricate :account, actor_type: 'Group' }
+
+      it { is_expected.to include('type' => 'Group') }
+    end
+
+    context 'with a Person actor' do
+      let(:record) { Fabricate :account, actor_type: 'Person' }
+
+      it { is_expected.to include('type' => 'Person') }
+    end
+  end
+end
diff --git a/spec/serializers/activitypub/add_serializer_spec.rb b/spec/serializers/activitypub/add_serializer_spec.rb
new file mode 100644
index 0000000000..3b3eaeb1b0
--- /dev/null
+++ b/spec/serializers/activitypub/add_serializer_spec.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::AddSerializer do
+  describe '.serializer_for' do
+    subject { described_class.serializer_for(model, {}) }
+
+    context 'with a Status model' do
+      let(:model) { Status.new }
+
+      it { is_expected.to eq(described_class::UriSerializer) }
+    end
+
+    context 'with a FeaturedTag model' do
+      let(:model) { FeaturedTag.new }
+
+      it { is_expected.to eq(ActivityPub::HashtagSerializer) }
+    end
+
+    context 'with an Array' do
+      let(:model) { [] }
+
+      it { is_expected.to eq(ActiveModel::Serializer::CollectionSerializer) }
+    end
+  end
+end
diff --git a/spec/serializers/activitypub/collection_serializer_spec.rb b/spec/serializers/activitypub/collection_serializer_spec.rb
new file mode 100644
index 0000000000..7726df914f
--- /dev/null
+++ b/spec/serializers/activitypub/collection_serializer_spec.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::CollectionSerializer do
+  describe '.serializer_for' do
+    subject { described_class.serializer_for(model, {}) }
+
+    context 'with a Status model' do
+      let(:model) { Status.new }
+
+      it { is_expected.to eq(ActivityPub::NoteSerializer) }
+    end
+
+    context 'with a FeaturedTag model' do
+      let(:model) { FeaturedTag.new }
+
+      it { is_expected.to eq(ActivityPub::HashtagSerializer) }
+    end
+
+    context 'with an ActivityPub::CollectionPresenter' do
+      let(:model) { ActivityPub::CollectionPresenter.new }
+
+      it { is_expected.to eq(described_class) }
+    end
+
+    context 'with a String' do
+      let(:model) { '' }
+
+      it { is_expected.to eq(described_class::StringSerializer) }
+    end
+
+    context 'with an Array' do
+      let(:model) { [] }
+
+      it { is_expected.to eq(ActiveModel::Serializer::CollectionSerializer) }
+    end
+  end
+end
diff --git a/spec/serializers/activitypub/remove_serializer_spec.rb b/spec/serializers/activitypub/remove_serializer_spec.rb
new file mode 100644
index 0000000000..0e4b199838
--- /dev/null
+++ b/spec/serializers/activitypub/remove_serializer_spec.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::RemoveSerializer do
+  describe '.serializer_for' do
+    subject { described_class.serializer_for(model, {}) }
+
+    context 'with a Status model' do
+      let(:model) { Status.new }
+
+      it { is_expected.to eq(described_class::UriSerializer) }
+    end
+
+    context 'with a FeaturedTag model' do
+      let(:model) { FeaturedTag.new }
+
+      it { is_expected.to eq(ActivityPub::HashtagSerializer) }
+    end
+
+    context 'with an Array' do
+      let(:model) { [] }
+
+      it { is_expected.to eq(ActiveModel::Serializer::CollectionSerializer) }
+    end
+  end
+end
diff --git a/spec/serializers/rest/account_relationship_severance_event_serializer_spec.rb b/spec/serializers/rest/account_relationship_severance_event_serializer_spec.rb
new file mode 100644
index 0000000000..95b4d7dcd2
--- /dev/null
+++ b/spec/serializers/rest/account_relationship_severance_event_serializer_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::AccountRelationshipSeveranceEventSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Fabricate.build :account_relationship_severance_event, id: 123 }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'id' => be_a(String).and(eq('123'))
+        )
+    end
+  end
+end
diff --git a/spec/serializers/rest/account_warning_serializer_spec.rb b/spec/serializers/rest/account_warning_serializer_spec.rb
new file mode 100644
index 0000000000..0f335d1215
--- /dev/null
+++ b/spec/serializers/rest/account_warning_serializer_spec.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::AccountWarningSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Fabricate :account_warning, id: 123, status_ids: [456, 789] }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'id' => be_a(String).and(eq('123')),
+          'status_ids' => be_a(Array).and(eq(['456', '789']))
+        )
+    end
+  end
+end
diff --git a/spec/serializers/rest/admin/account_serializer_spec.rb b/spec/serializers/rest/admin/account_serializer_spec.rb
new file mode 100644
index 0000000000..58f58a997b
--- /dev/null
+++ b/spec/serializers/rest/admin/account_serializer_spec.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::Admin::AccountSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  describe 'created_by_application_id' do
+    context 'when account is application-created' do
+      let(:record) { Fabricate :account, user: Fabricate(:user, created_by_application: application) }
+      let(:application) { Fabricate :application }
+
+      it { is_expected.to include('created_by_application_id' => application.id.to_s) }
+    end
+  end
+
+  describe 'invited_by_account_id' do
+    context 'when account was invited' do
+      let(:record) { Fabricate :account, user: Fabricate(:user, invite: invite) }
+      let(:invite) { Fabricate :invite }
+
+      it { is_expected.to include('invited_by_account_id' => invite.user.account.id.to_s) }
+    end
+  end
+end
diff --git a/spec/serializers/rest/admin/cohort_serializer_spec.rb b/spec/serializers/rest/admin/cohort_serializer_spec.rb
new file mode 100644
index 0000000000..ed6067c0d2
--- /dev/null
+++ b/spec/serializers/rest/admin/cohort_serializer_spec.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::Admin::CohortSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Admin::Metrics::Retention.new('2024-01-01', '2024-01-02', 'day').cohorts.first }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'data' => be_a(Array),
+          'period' => /2024-01-01/
+        )
+    end
+  end
+end
diff --git a/spec/serializers/rest/admin/webhook_event_serializer_spec.rb b/spec/serializers/rest/admin/webhook_event_serializer_spec.rb
new file mode 100644
index 0000000000..3cbfbd92a3
--- /dev/null
+++ b/spec/serializers/rest/admin/webhook_event_serializer_spec.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::Admin::WebhookEventSerializer do
+  describe '.serializer_for' do
+    subject { described_class.serializer_for(model, {}) }
+
+    context 'with an Account model' do
+      let(:model) { Account.new }
+
+      it { is_expected.to eq(REST::Admin::AccountSerializer) }
+    end
+
+    context 'with a Report model' do
+      let(:model) { Report.new }
+
+      it { is_expected.to eq(REST::Admin::ReportSerializer) }
+    end
+
+    context 'with a Status model' do
+      let(:model) { Status.new }
+
+      it { is_expected.to eq(REST::StatusSerializer) }
+    end
+
+    context 'with an Array' do
+      let(:model) { [] }
+
+      it { is_expected.to eq(ActiveModel::Serializer::CollectionSerializer) }
+    end
+  end
+end
diff --git a/spec/serializers/rest/appeal_serializer_spec.rb b/spec/serializers/rest/appeal_serializer_spec.rb
new file mode 100644
index 0000000000..1ae6617de5
--- /dev/null
+++ b/spec/serializers/rest/appeal_serializer_spec.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::AppealSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  describe 'state' do
+    context 'when appeal is approved' do
+      let(:record) { Fabricate.build :appeal, approved_at: 2.days.ago }
+
+      it { is_expected.to include('state' => 'approved') }
+    end
+
+    context 'when appeal is rejected' do
+      let(:record) { Fabricate.build :appeal, rejected_at: 2.days.ago }
+
+      it { is_expected.to include('state' => 'rejected') }
+    end
+
+    context 'when appeal is not approved or rejected' do
+      let(:record) { Fabricate.build :appeal, approved_at: nil, rejected_at: nil }
+
+      it { is_expected.to include('state' => 'pending') }
+    end
+  end
+end
diff --git a/spec/serializers/rest/custom_emoji_serializer_spec.rb b/spec/serializers/rest/custom_emoji_serializer_spec.rb
new file mode 100644
index 0000000000..a694ca515a
--- /dev/null
+++ b/spec/serializers/rest/custom_emoji_serializer_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::CustomEmojiSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Fabricate.build :custom_emoji, id: 123, category: Fabricate(:custom_emoji_category, name: 'Category Name') }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'category' => be_a(String).and(eq('Category Name'))
+        )
+    end
+  end
+end
diff --git a/spec/serializers/rest/extended_description_serializer_spec.rb b/spec/serializers/rest/extended_description_serializer_spec.rb
new file mode 100644
index 0000000000..d7bd078e1d
--- /dev/null
+++ b/spec/serializers/rest/extended_description_serializer_spec.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::ExtendedDescriptionSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  describe 'serialization' do
+    context 'with text present' do
+      let(:record) { ExtendedDescription.new text: 'Hello world', updated_at: Date.new(2024, 1, 1) }
+
+      it 'returns expected values' do
+        expect(subject)
+          .to include(
+            'content' => eq(<<~HTML),
+              <p>Hello world</p>
+            HTML
+            'updated_at' => eq('2024-01-01')
+          )
+      end
+    end
+
+    context 'with text missing' do
+      let(:record) { ExtendedDescription.new text: nil, updated_at: Date.new(2024, 1, 1) }
+
+      it 'returns expected values' do
+        expect(subject)
+          .to include(
+            'content' => eq(''),
+            'updated_at' => eq('2024-01-01')
+          )
+      end
+    end
+  end
+end
diff --git a/spec/serializers/rest/rule_serializer_spec.rb b/spec/serializers/rest/rule_serializer_spec.rb
new file mode 100644
index 0000000000..4d801e77d3
--- /dev/null
+++ b/spec/serializers/rest/rule_serializer_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::RuleSerializer do
+  subject { serialized_record_json(record, described_class) }
+
+  let(:record) { Fabricate.build :rule, id: 123 }
+
+  describe 'serialization' do
+    it 'returns expected values' do
+      expect(subject)
+        .to include(
+          'id' => be_a(String).and(eq('123'))
+        )
+    end
+  end
+end