diff --git a/spec/factories/artifact_skills.rb b/spec/factories/artifact_skills.rb
new file mode 100644
index 0000000..a2f7137
--- /dev/null
+++ b/spec/factories/artifact_skills.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :artifact_skill do
+    skill_group { :group_i }
+    # Use high sequence numbers to avoid conflicts with seeded data (modifiers 1-29 used)
+    sequence(:modifier) { |n| 1000 + n }
+    sequence(:name_en) { |n| "Test Skill #{n}" }
+    name_jp { 'テストスキル' }
+    base_values { [1320, 1440, 1560, 1680, 1800] }
+    growth { 300.0 }
+    suffix_en { '' }
+    suffix_jp { '' }
+    polarity { :positive }
+
+    trait :group_i do
+      skill_group { :group_i }
+    end
+
+    trait :group_ii do
+      skill_group { :group_ii }
+    end
+
+    trait :group_iii do
+      skill_group { :group_iii }
+    end
+
+    trait :atk do
+      modifier { 1 }
+      name_en { 'ATK' }
+      name_jp { '攻撃力' }
+      base_values { [1320, 1440, 1560, 1680, 1800] }
+      growth { 300.0 }
+    end
+
+    trait :hp do
+      modifier { 2 }
+      name_en { 'HP' }
+      name_jp { 'HP' }
+      base_values { [660, 720, 780, 840, 900] }
+      growth { 150.0 }
+    end
+
+    trait :ca_dmg do
+      modifier { 3 }
+      name_en { 'C.A. DMG' }
+      name_jp { '奥義ダメ' }
+      base_values { [13.2, 14.4, 15.6, 16.8, 18.0] }
+      growth { 3.0 }
+      suffix_en { '%' }
+      suffix_jp { '%' }
+    end
+
+    trait :negative do
+      polarity { :negative }
+      growth { -6.0 }
+    end
+
+    trait :no_growth do
+      growth { nil }
+    end
+  end
+end
diff --git a/spec/factories/artifacts.rb b/spec/factories/artifacts.rb
new file mode 100644
index 0000000..8153c78
--- /dev/null
+++ b/spec/factories/artifacts.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :artifact do
+    # Use high sequence numbers to avoid conflicts with seeded data
+    sequence(:granblue_id) { |n| "39999#{n.to_s.rjust(4, '0')}" }
+    sequence(:name_en) { |n| "Test Artifact #{n}" }
+    name_jp { 'テストアーティファクト' }
+    proficiency { :sabre }
+    rarity { :standard }
+    release_date { Date.new(2025, 3, 10) }
+
+    trait :quirk do
+      rarity { :quirk }
+      proficiency { nil }
+      sequence(:granblue_id) { |n| "38888#{n.to_s.rjust(4, '0')}" }
+      sequence(:name_en) { |n| "Quirk Artifact #{n}" }
+      name_jp { 'クィルクアーティファクト' }
+    end
+
+    trait :dagger do
+      proficiency { :dagger }
+    end
+
+    trait :spear do
+      proficiency { :spear }
+    end
+
+    trait :staff do
+      proficiency { :staff }
+    end
+  end
+end
diff --git a/spec/factories/collection_artifacts.rb b/spec/factories/collection_artifacts.rb
new file mode 100644
index 0000000..0da50b4
--- /dev/null
+++ b/spec/factories/collection_artifacts.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :collection_artifact do
+    association :user
+    association :artifact
+    element { 'fire' }
+    level { 1 }
+    proficiency { nil }
+    nickname { nil }
+    skill1 { { 'modifier' => 1, 'strength' => 1800, 'level' => 1 } }
+    skill2 { { 'modifier' => 2, 'strength' => 900, 'level' => 1 } }
+    skill3 { { 'modifier' => 1, 'strength' => 18.0, 'level' => 1 } }
+    skill4 { { 'modifier' => 1, 'strength' => 10, 'level' => 1 } }
+
+    trait :max_level do
+      level { 5 }
+      skill1 { { 'modifier' => 1, 'strength' => 1800, 'level' => 2 } }
+      skill2 { { 'modifier' => 2, 'strength' => 900, 'level' => 2 } }
+      skill3 { { 'modifier' => 1, 'strength' => 18.0, 'level' => 2 } }
+      skill4 { { 'modifier' => 1, 'strength' => 10, 'level' => 2 } }
+    end
+
+    trait :quirk do
+      association :artifact, factory: [:artifact, :quirk]
+      proficiency { :sabre }
+      level { 1 }
+      skill1 { {} }
+      skill2 { {} }
+      skill3 { {} }
+      skill4 { {} }
+    end
+
+    trait :with_nickname do
+      nickname { 'My Favorite Artifact' }
+    end
+
+    trait :water do
+      element { 'water' }
+    end
+
+    trait :earth do
+      element { 'earth' }
+    end
+
+    trait :wind do
+      element { 'wind' }
+    end
+
+    trait :light do
+      element { 'light' }
+    end
+
+    trait :dark do
+      element { 'dark' }
+    end
+  end
+end
diff --git a/spec/factories/grid_artifacts.rb b/spec/factories/grid_artifacts.rb
new file mode 100644
index 0000000..4ec23d5
--- /dev/null
+++ b/spec/factories/grid_artifacts.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :grid_artifact do
+    association :grid_character
+    association :artifact
+    element { 'fire' }
+    level { 1 }
+    proficiency { nil }
+    skill1 { { 'modifier' => 1, 'strength' => 1800, 'level' => 1 } }
+    skill2 { { 'modifier' => 2, 'strength' => 900, 'level' => 1 } }
+    skill3 { { 'modifier' => 1, 'strength' => 18.0, 'level' => 1 } }
+    skill4 { { 'modifier' => 1, 'strength' => 10, 'level' => 1 } }
+
+    trait :max_level do
+      level { 5 }
+      skill1 { { 'modifier' => 1, 'strength' => 1800, 'level' => 2 } }
+      skill2 { { 'modifier' => 2, 'strength' => 900, 'level' => 2 } }
+      skill3 { { 'modifier' => 1, 'strength' => 18.0, 'level' => 2 } }
+      skill4 { { 'modifier' => 1, 'strength' => 10, 'level' => 2 } }
+    end
+
+    trait :quirk do
+      association :artifact, factory: [:artifact, :quirk]
+      proficiency { :sabre }
+      level { 1 }
+      skill1 { {} }
+      skill2 { {} }
+      skill3 { {} }
+      skill4 { {} }
+    end
+  end
+end
diff --git a/spec/models/artifact_skill_spec.rb b/spec/models/artifact_skill_spec.rb
new file mode 100644
index 0000000..757c31f
--- /dev/null
+++ b/spec/models/artifact_skill_spec.rb
@@ -0,0 +1,181 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ArtifactSkill, type: :model do
+  describe 'validations' do
+    subject { build(:artifact_skill) }
+
+    it { is_expected.to validate_presence_of(:skill_group) }
+    it { is_expected.to validate_presence_of(:modifier) }
+    it { is_expected.to validate_presence_of(:name_en) }
+    it { is_expected.to validate_presence_of(:name_jp) }
+    it { is_expected.to validate_presence_of(:base_values) }
+    it { is_expected.to validate_presence_of(:polarity) }
+
+    it 'validates uniqueness of modifier within skill_group' do
+      # Create with unique modifier, then try to create duplicate
+      existing = create(:artifact_skill, skill_group: :group_i, modifier: 5000)
+      duplicate = build(:artifact_skill, skill_group: :group_i, modifier: 5000)
+      expect(duplicate).not_to be_valid
+      expect(duplicate.errors[:modifier]).to include('has already been taken')
+    end
+
+    it 'allows same modifier in different skill groups' do
+      create(:artifact_skill, skill_group: :group_i, modifier: 5001)
+      different_group = build(:artifact_skill, skill_group: :group_ii, modifier: 5001)
+      expect(different_group).to be_valid
+    end
+  end
+
+  describe 'enums' do
+    it 'defines skill_group enum' do
+      expect(ArtifactSkill.skill_groups).to eq(
+        'group_i' => 1,
+        'group_ii' => 2,
+        'group_iii' => 3
+      )
+    end
+
+    it 'defines polarity enum' do
+      expect(ArtifactSkill.polarities).to eq(
+        'positive' => 'positive',
+        'negative' => 'negative'
+      )
+    end
+  end
+
+  describe '.for_slot' do
+    before do
+      # Use unique modifiers that won't conflict with seeded data
+      @group_i_skill = create(:artifact_skill, :group_i, modifier: 6000)
+      @group_ii_skill = create(:artifact_skill, :group_ii, modifier: 6001)
+      @group_iii_skill = create(:artifact_skill, :group_iii, modifier: 6002)
+    end
+
+    it 'returns Group I skills for slot 1' do
+      expect(ArtifactSkill.for_slot(1)).to include(@group_i_skill)
+      expect(ArtifactSkill.for_slot(1)).not_to include(@group_ii_skill)
+    end
+
+    it 'returns Group I skills for slot 2' do
+      expect(ArtifactSkill.for_slot(2)).to include(@group_i_skill)
+      expect(ArtifactSkill.for_slot(2)).not_to include(@group_ii_skill)
+    end
+
+    it 'returns Group II skills for slot 3' do
+      expect(ArtifactSkill.for_slot(3)).to include(@group_ii_skill)
+      expect(ArtifactSkill.for_slot(3)).not_to include(@group_i_skill)
+    end
+
+    it 'returns Group III skills for slot 4' do
+      expect(ArtifactSkill.for_slot(4)).to include(@group_iii_skill)
+      expect(ArtifactSkill.for_slot(4)).not_to include(@group_i_skill)
+    end
+  end
+
+  describe '.find_skill' do
+    before do
+      ArtifactSkill.clear_cache!
+      @test_skill = create(:artifact_skill, skill_group: :group_i, modifier: 7000)
+    end
+
+    after do
+      ArtifactSkill.clear_cache!
+    end
+
+    it 'finds skill by group number and modifier' do
+      ArtifactSkill.clear_cache!
+      found = ArtifactSkill.find_skill(1, 7000)
+      expect(found).to eq(@test_skill)
+    end
+
+    it 'returns nil for non-existent skill' do
+      ArtifactSkill.clear_cache!
+      expect(ArtifactSkill.find_skill(1, 99999)).to be_nil
+    end
+
+    it 'caches skills for performance' do
+      ArtifactSkill.clear_cache!
+      ArtifactSkill.find_skill(1, 7000)
+      expect(ArtifactSkill.instance_variable_get(:@cached_skills)).not_to be_nil
+    end
+  end
+
+  describe '#calculate_value' do
+    let(:skill) { build(:artifact_skill, growth: 300.0) }
+
+    it 'returns base strength at level 1' do
+      expect(skill.calculate_value(1800, 1)).to eq(1800)
+    end
+
+    it 'adds growth for each level above 1' do
+      expect(skill.calculate_value(1800, 3)).to eq(2400) # 1800 + (300 * 2)
+    end
+
+    it 'handles level 5' do
+      expect(skill.calculate_value(1800, 5)).to eq(3000) # 1800 + (300 * 4)
+    end
+
+    context 'with nil growth' do
+      let(:skill) { build(:artifact_skill, :no_growth) }
+
+      it 'returns base strength regardless of level' do
+        expect(skill.calculate_value(10, 1)).to eq(10)
+        expect(skill.calculate_value(10, 5)).to eq(10)
+      end
+    end
+
+    context 'with negative growth' do
+      let(:skill) { build(:artifact_skill, :negative, growth: -6.0) }
+
+      it 'subtracts growth for each level' do
+        expect(skill.calculate_value(30, 3)).to eq(18) # 30 + (-6 * 2)
+      end
+    end
+  end
+
+  describe '#format_value' do
+    context 'with percentage suffix' do
+      let(:skill) { build(:artifact_skill, suffix_en: '%', suffix_jp: '%') }
+
+      it 'formats with English suffix' do
+        expect(skill.format_value(18.0, :en)).to eq('18.0%')
+      end
+
+      it 'formats with Japanese suffix' do
+        expect(skill.format_value(18.0, :jp)).to eq('18.0%')
+      end
+    end
+
+    context 'with no suffix' do
+      let(:skill) { build(:artifact_skill, suffix_en: '', suffix_jp: '') }
+
+      it 'returns value without suffix' do
+        expect(skill.format_value(1800, :en)).to eq('1800')
+      end
+    end
+  end
+
+  describe '#valid_strength?' do
+    let(:skill) { build(:artifact_skill, base_values: [1320, 1440, 1560, 1680, 1800]) }
+
+    it 'returns true for valid base values' do
+      expect(skill.valid_strength?(1800)).to be true
+      expect(skill.valid_strength?(1320)).to be true
+    end
+
+    it 'returns false for invalid values' do
+      expect(skill.valid_strength?(1500)).to be false
+      expect(skill.valid_strength?(9999)).to be false
+    end
+
+    context 'with nil in base_values (unknown values)' do
+      let(:skill) { build(:artifact_skill, base_values: [nil]) }
+
+      it 'returns true for any value' do
+        expect(skill.valid_strength?(9999)).to be true
+      end
+    end
+  end
+end
diff --git a/spec/models/artifact_spec.rb b/spec/models/artifact_spec.rb
new file mode 100644
index 0000000..c98efbf
--- /dev/null
+++ b/spec/models/artifact_spec.rb
@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Artifact, type: :model do
+  describe 'validations' do
+    subject { build(:artifact) }
+
+    it { is_expected.to validate_presence_of(:granblue_id) }
+    it 'validates uniqueness of granblue_id' do
+      create(:artifact)
+      duplicate = build(:artifact, granblue_id: Artifact.first.granblue_id)
+      expect(duplicate).not_to be_valid
+      expect(duplicate.errors[:granblue_id]).to include('has already been taken')
+    end
+    it { is_expected.to validate_presence_of(:name_en) }
+    it { is_expected.to validate_presence_of(:rarity) }
+
+    context 'when standard artifact' do
+      subject { build(:artifact, rarity: :standard) }
+
+      it { is_expected.to validate_presence_of(:proficiency) }
+    end
+
+    context 'when quirk artifact' do
+      subject { build(:artifact, :quirk) }
+
+      it 'requires proficiency to be nil' do
+        subject.proficiency = :sabre
+        expect(subject).not_to be_valid
+        expect(subject.errors[:proficiency]).to include('must be blank')
+      end
+
+      it 'is valid without proficiency' do
+        expect(subject).to be_valid
+      end
+    end
+  end
+
+  describe 'associations' do
+    it { is_expected.to have_many(:collection_artifacts).dependent(:restrict_with_error) }
+    it { is_expected.to have_many(:grid_artifacts).dependent(:restrict_with_error) }
+  end
+
+  describe 'enums' do
+    it 'defines proficiency enum' do
+      expect(Artifact.proficiencies).to include(
+        'sabre' => 1,
+        'dagger' => 2,
+        'spear' => 4
+      )
+    end
+
+    it 'defines rarity enum' do
+      expect(Artifact.rarities).to eq('standard' => 0, 'quirk' => 1)
+    end
+  end
+
+  describe 'scopes' do
+    let!(:standard_artifact) { create(:artifact, rarity: :standard) }
+    let!(:quirk_artifact) { create(:artifact, :quirk) }
+
+    it 'filters by rarity' do
+      expect(Artifact.standard).to include(standard_artifact)
+      expect(Artifact.standard).not_to include(quirk_artifact)
+      expect(Artifact.quirk).to include(quirk_artifact)
+      expect(Artifact.quirk).not_to include(standard_artifact)
+    end
+  end
+
+  describe '#quirk?' do
+    it 'returns true for quirk artifacts' do
+      artifact = build(:artifact, :quirk)
+      expect(artifact.quirk?).to be true
+    end
+
+    it 'returns false for standard artifacts' do
+      artifact = build(:artifact, rarity: :standard)
+      expect(artifact.quirk?).to be false
+    end
+  end
+end
diff --git a/spec/models/collection_artifact_spec.rb b/spec/models/collection_artifact_spec.rb
new file mode 100644
index 0000000..8645852
--- /dev/null
+++ b/spec/models/collection_artifact_spec.rb
@@ -0,0 +1,202 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe CollectionArtifact, type: :model do
+  describe 'associations' do
+    it { is_expected.to belong_to(:user) }
+    it { is_expected.to belong_to(:artifact) }
+  end
+
+  describe 'validations' do
+    subject { build(:collection_artifact) }
+
+    it { is_expected.to validate_presence_of(:element) }
+
+    it 'validates presence of level' do
+      subject.level = nil
+      expect(subject).not_to be_valid
+    end
+
+    it 'validates level is between 1 and 5' do
+      subject.level = 0
+      expect(subject).not_to be_valid
+
+      subject.level = 6
+      expect(subject).not_to be_valid
+
+      subject.level = 3
+      expect(subject).to be_valid
+    end
+  end
+
+  describe 'enums' do
+    it 'defines element enum' do
+      expect(CollectionArtifact.elements).to include(
+        'wind' => 1,
+        'fire' => 2,
+        'water' => 3,
+        'earth' => 4,
+        'dark' => 5,
+        'light' => 6
+      )
+    end
+
+    it 'defines proficiency enum' do
+      expect(CollectionArtifact.proficiencies).to include(
+        'sabre' => 1,
+        'dagger' => 2
+      )
+    end
+  end
+
+  describe '#effective_proficiency' do
+    context 'for standard artifact' do
+      let(:artifact) { create(:artifact, proficiency: :dagger) }
+
+      it 'returns proficiency from base artifact' do
+        collection_artifact = build(:collection_artifact,
+          artifact: artifact,
+          proficiency: nil,
+          skill1: {}, skill2: {}, skill3: {}, skill4: {}
+        )
+        expect(collection_artifact.effective_proficiency).to eq('dagger')
+      end
+    end
+
+    context 'for quirk artifact' do
+      let(:artifact) { create(:artifact, :quirk) }
+
+      it 'returns proficiency from instance' do
+        collection_artifact = build(:collection_artifact,
+          artifact: artifact,
+          proficiency: :staff,
+          level: 1,
+          skill1: {}, skill2: {}, skill3: {}, skill4: {}
+        )
+        expect(collection_artifact.effective_proficiency).to eq('staff')
+      end
+    end
+  end
+
+  describe 'skill validations' do
+    let(:artifact) { create(:artifact) }
+
+    before do
+      # Seed the required artifact skills for validation
+      ArtifactSkill.find_or_create_by!(skill_group: :group_i, modifier: 1) do |s|
+        s.name_en = 'ATK'
+        s.name_jp = '攻撃力'
+        s.base_values = [1320, 1440, 1560, 1680, 1800]
+        s.growth = 300.0
+        s.polarity = :positive
+      end
+      ArtifactSkill.find_or_create_by!(skill_group: :group_i, modifier: 2) do |s|
+        s.name_en = 'HP'
+        s.name_jp = 'HP'
+        s.base_values = [660, 720, 780, 840, 900]
+        s.growth = 150.0
+        s.polarity = :positive
+      end
+      ArtifactSkill.find_or_create_by!(skill_group: :group_ii, modifier: 1) do |s|
+        s.name_en = 'C.A. DMG'
+        s.name_jp = '奥義ダメ'
+        s.base_values = [13.2, 14.4, 15.6, 16.8, 18.0]
+        s.growth = 3.0
+        s.polarity = :positive
+      end
+      ArtifactSkill.find_or_create_by!(skill_group: :group_iii, modifier: 1) do |s|
+        s.name_en = 'Chain Burst DMG'
+        s.name_jp = 'チェインダメ'
+        s.base_values = [6, 7, 8, 9, 10]
+        s.growth = 2.5
+        s.polarity = :positive
+      end
+      ArtifactSkill.clear_cache!
+    end
+
+    it 'is valid with correct skills' do
+      collection_artifact = build(:collection_artifact,
+        artifact: artifact,
+        level: 1,
+        skill1: { 'modifier' => 1, 'strength' => 1800, 'level' => 1 },
+        skill2: { 'modifier' => 2, 'strength' => 900, 'level' => 1 },
+        skill3: { 'modifier' => 1, 'strength' => 18.0, 'level' => 1 },
+        skill4: { 'modifier' => 1, 'strength' => 10, 'level' => 1 }
+      )
+      expect(collection_artifact).to be_valid
+    end
+
+    it 'is invalid when skill1 and skill2 have the same modifier' do
+      collection_artifact = build(:collection_artifact,
+        artifact: artifact,
+        level: 1,
+        skill1: { 'modifier' => 1, 'strength' => 1800, 'level' => 1 },
+        skill2: { 'modifier' => 1, 'strength' => 1800, 'level' => 1 }, # Same modifier
+        skill3: { 'modifier' => 1, 'strength' => 18.0, 'level' => 1 },
+        skill4: { 'modifier' => 1, 'strength' => 10, 'level' => 1 }
+      )
+      expect(collection_artifact).not_to be_valid
+      expect(collection_artifact.errors[:base]).to include('Skill 1 and Skill 2 cannot have the same modifier')
+    end
+
+    it 'validates skill levels sum correctly' do
+      # At level 1, skill levels must sum to 4 (1 + 3)
+      collection_artifact = build(:collection_artifact,
+        artifact: artifact,
+        level: 1,
+        skill1: { 'modifier' => 1, 'strength' => 1800, 'level' => 2 },
+        skill2: { 'modifier' => 2, 'strength' => 900, 'level' => 2 },
+        skill3: { 'modifier' => 1, 'strength' => 18.0, 'level' => 2 },
+        skill4: { 'modifier' => 1, 'strength' => 10, 'level' => 2 }
+      )
+      expect(collection_artifact).not_to be_valid
+      expect(collection_artifact.errors[:base].first).to include('Skill levels must sum to')
+    end
+  end
+
+  describe 'quirk artifact constraints' do
+    let(:quirk_artifact) { create(:artifact, :quirk) }
+
+    it 'requires level 1 for quirk artifacts' do
+      collection_artifact = build(:collection_artifact,
+        artifact: quirk_artifact,
+        proficiency: :sabre,
+        level: 3,
+        skill1: {},
+        skill2: {},
+        skill3: {},
+        skill4: {}
+      )
+      expect(collection_artifact).not_to be_valid
+      expect(collection_artifact.errors[:level]).to include('must be 1 for quirk artifacts')
+    end
+
+    it 'requires empty skills for quirk artifacts' do
+      collection_artifact = build(:collection_artifact,
+        artifact: quirk_artifact,
+        proficiency: :sabre,
+        level: 1,
+        skill1: { 'modifier' => 1, 'strength' => 1800, 'level' => 1 },
+        skill2: {},
+        skill3: {},
+        skill4: {}
+      )
+      expect(collection_artifact).not_to be_valid
+      expect(collection_artifact.errors[:skill1]).to include('must be empty for quirk artifacts')
+    end
+
+    it 'is valid with empty skills and level 1' do
+      collection_artifact = build(:collection_artifact,
+        artifact: quirk_artifact,
+        proficiency: :sabre,
+        level: 1,
+        skill1: {},
+        skill2: {},
+        skill3: {},
+        skill4: {}
+      )
+      expect(collection_artifact).to be_valid
+    end
+  end
+end
diff --git a/spec/models/grid_artifact_spec.rb b/spec/models/grid_artifact_spec.rb
new file mode 100644
index 0000000..40fc5f8
--- /dev/null
+++ b/spec/models/grid_artifact_spec.rb
@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe GridArtifact, type: :model do
+  describe 'associations' do
+    it { is_expected.to belong_to(:grid_character) }
+    it { is_expected.to belong_to(:artifact) }
+  end
+
+  describe 'validations' do
+    subject { build(:grid_artifact) }
+
+    it { is_expected.to validate_presence_of(:element) }
+
+    it 'validates presence of level' do
+      subject.level = nil
+      expect(subject).not_to be_valid
+    end
+
+    it 'validates level is between 1 and 5' do
+      subject.level = 0
+      expect(subject).not_to be_valid
+
+      subject.level = 6
+      expect(subject).not_to be_valid
+
+      subject.level = 3
+      expect(subject).to be_valid
+    end
+  end
+
+  describe 'enums' do
+    it 'defines element enum' do
+      expect(GridArtifact.elements).to include(
+        'wind' => 1,
+        'fire' => 2,
+        'water' => 3,
+        'earth' => 4
+      )
+    end
+  end
+
+  describe '#effective_proficiency' do
+    context 'for standard artifact' do
+      let(:artifact) { create(:artifact, proficiency: :spear) }
+
+      it 'returns proficiency from base artifact' do
+        grid_artifact = build(:grid_artifact,
+          artifact: artifact,
+          proficiency: nil,
+          skill1: {}, skill2: {}, skill3: {}, skill4: {}
+        )
+        expect(grid_artifact.effective_proficiency).to eq('spear')
+      end
+    end
+
+    context 'for quirk artifact' do
+      let(:artifact) { create(:artifact, :quirk) }
+
+      it 'returns proficiency from instance' do
+        grid_artifact = build(:grid_artifact,
+          artifact: artifact,
+          proficiency: :melee,
+          level: 1,
+          skill1: {}, skill2: {}, skill3: {}, skill4: {}
+        )
+        expect(grid_artifact.effective_proficiency).to eq('melee')
+      end
+    end
+  end
+
+  describe 'unique constraint on grid_character' do
+    it 'does not allow duplicate grid_artifacts for same grid_character' do
+      grid_character = create(:grid_character)
+      create(:grid_artifact,
+        grid_character: grid_character,
+        skill1: {}, skill2: {}, skill3: {}, skill4: {}
+      )
+      duplicate = build(:grid_artifact,
+        grid_character: grid_character,
+        skill1: {}, skill2: {}, skill3: {}, skill4: {}
+      )
+      expect(duplicate).not_to be_valid
+    end
+  end
+
+  describe 'amoeba duplication' do
+    let(:grid_character) { create(:grid_character) }
+    let(:grid_artifact) { create(:grid_artifact, grid_character: grid_character) }
+
+    it 'can be duplicated via amoeba' do
+      expect(GridArtifact).to respond_to(:amoeba_block)
+    end
+  end
+end
diff --git a/spec/requests/artifact_skills_controller_spec.rb b/spec/requests/artifact_skills_controller_spec.rb
new file mode 100644
index 0000000..4878943
--- /dev/null
+++ b/spec/requests/artifact_skills_controller_spec.rb
@@ -0,0 +1,90 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Artifact Skills API', type: :request do
+  before do
+    # Create test skills in different groups
+    create(:artifact_skill, :group_i, :atk, modifier: 1)
+    create(:artifact_skill, :group_i, :hp, modifier: 2)
+    create(:artifact_skill, :group_ii, modifier: 1, name_en: 'C.A. DMG', name_jp: '奥義ダメ')
+    create(:artifact_skill, :group_iii, modifier: 1, name_en: 'Chain Burst DMG', name_jp: 'チェインダメ')
+  end
+
+  describe 'GET /api/v1/artifact_skills' do
+    it 'returns all artifact skills' do
+      get '/api/v1/artifact_skills'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].length).to eq(4)
+    end
+
+    it 'filters by skill group' do
+      get '/api/v1/artifact_skills', params: { group: 'group_i' }
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].length).to eq(2)
+      expect(json['artifact_skills'].all? { |s| s['skill_group'] == 'group_i' }).to be true
+    end
+
+    it 'filters by polarity' do
+      create(:artifact_skill, :group_i, :negative, modifier: 99)
+
+      get '/api/v1/artifact_skills', params: { polarity: 'negative' }
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].all? { |s| s['polarity'] == 'negative' }).to be true
+    end
+  end
+
+  describe 'GET /api/v1/artifact_skills/for_slot/:slot' do
+    it 'returns Group I skills for slot 1' do
+      get '/api/v1/artifact_skills/for_slot/1'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].all? { |s| s['skill_group'] == 'group_i' }).to be true
+    end
+
+    it 'returns Group I skills for slot 2' do
+      get '/api/v1/artifact_skills/for_slot/2'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].all? { |s| s['skill_group'] == 'group_i' }).to be true
+    end
+
+    it 'returns Group II skills for slot 3' do
+      get '/api/v1/artifact_skills/for_slot/3'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].all? { |s| s['skill_group'] == 'group_ii' }).to be true
+    end
+
+    it 'returns Group III skills for slot 4' do
+      get '/api/v1/artifact_skills/for_slot/4'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifact_skills'].all? { |s| s['skill_group'] == 'group_iii' }).to be true
+    end
+
+    it 'returns error for invalid slot' do
+      get '/api/v1/artifact_skills/for_slot/5'
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      json = JSON.parse(response.body)
+      expect(json['error']).to include('Slot must be between 1 and 4')
+    end
+
+    it 'returns error for slot 0' do
+      get '/api/v1/artifact_skills/for_slot/0'
+
+      expect(response).to have_http_status(:unprocessable_entity)
+    end
+  end
+end
diff --git a/spec/requests/artifacts_controller_spec.rb b/spec/requests/artifacts_controller_spec.rb
new file mode 100644
index 0000000..7d5f442
--- /dev/null
+++ b/spec/requests/artifacts_controller_spec.rb
@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Artifacts API', type: :request do
+  describe 'GET /api/v1/artifacts' do
+    let!(:standard_artifact) { create(:artifact, proficiency: :sabre) }
+    let!(:quirk_artifact) { create(:artifact, :quirk) }
+
+    it 'returns all artifacts' do
+      get '/api/v1/artifacts'
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].length).to eq(2)
+    end
+
+    it 'filters by rarity' do
+      get '/api/v1/artifacts', params: { rarity: 'standard' }
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].length).to eq(1)
+      expect(json['artifacts'].first['rarity']).to eq('standard')
+    end
+
+    it 'filters by proficiency' do
+      create(:artifact, proficiency: :dagger)
+
+      get '/api/v1/artifacts', params: { proficiency: 'sabre' }
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].all? { |a| a['proficiency'] == 'sabre' }).to be true
+    end
+  end
+
+  describe 'GET /api/v1/artifacts/:id' do
+    let!(:artifact) { create(:artifact) }
+
+    it 'returns the artifact' do
+      get "/api/v1/artifacts/#{artifact.id}"
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['id']).to eq(artifact.id)
+      expect(json['name']['en']).to eq(artifact.name_en)
+    end
+
+    it 'returns not found for non-existent artifact' do
+      get "/api/v1/artifacts/#{SecureRandom.uuid}"
+
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+end
diff --git a/spec/requests/collection_artifacts_controller_spec.rb b/spec/requests/collection_artifacts_controller_spec.rb
new file mode 100644
index 0000000..a2e89b2
--- /dev/null
+++ b/spec/requests/collection_artifacts_controller_spec.rb
@@ -0,0 +1,318 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Collection Artifacts API', type: :request do
+  let(:user) { create(:user) }
+  let(:other_user) { create(:user) }
+  let(:access_token) do
+    Doorkeeper::AccessToken.create!(resource_owner_id: user.id, expires_in: 30.days, scopes: 'public')
+  end
+  let(:headers) do
+    { 'Authorization' => "Bearer #{access_token.token}", 'Content-Type' => 'application/json' }
+  end
+
+  let(:artifact) { create(:artifact) }
+  let(:quirk_artifact) { create(:artifact, :quirk) }
+
+  before do
+    # Seed required artifact skills for validation
+    ArtifactSkill.find_or_create_by!(skill_group: :group_i, modifier: 1) do |s|
+      s.name_en = 'ATK'
+      s.name_jp = '攻撃力'
+      s.base_values = [1320, 1440, 1560, 1680, 1800]
+      s.growth = 300.0
+      s.polarity = :positive
+    end
+    ArtifactSkill.find_or_create_by!(skill_group: :group_i, modifier: 2) do |s|
+      s.name_en = 'HP'
+      s.name_jp = 'HP'
+      s.base_values = [660, 720, 780, 840, 900]
+      s.growth = 150.0
+      s.polarity = :positive
+    end
+    ArtifactSkill.find_or_create_by!(skill_group: :group_ii, modifier: 1) do |s|
+      s.name_en = 'C.A. DMG'
+      s.name_jp = '奥義ダメ'
+      s.base_values = [13.2, 14.4, 15.6, 16.8, 18.0]
+      s.growth = 3.0
+      s.polarity = :positive
+    end
+    ArtifactSkill.find_or_create_by!(skill_group: :group_iii, modifier: 1) do |s|
+      s.name_en = 'Chain Burst DMG'
+      s.name_jp = 'チェインダメ'
+      s.base_values = [6, 7, 8, 9, 10]
+      s.growth = 2.5
+      s.polarity = :positive
+    end
+    ArtifactSkill.clear_cache!
+  end
+
+  describe 'GET /api/v1/users/:user_id/collection/artifacts' do
+    let!(:collection_artifact1) { create(:collection_artifact, user: user, artifact: artifact) }
+    let!(:collection_artifact2) { create(:collection_artifact, user: user, element: :water) }
+    let!(:other_user_artifact) { create(:collection_artifact, user: other_user) }
+
+    it "returns the user's collection artifacts" do
+      get "/api/v1/users/#{user.id}/collection/artifacts", headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].length).to eq(2)
+    end
+
+    it 'supports pagination' do
+      get "/api/v1/users/#{user.id}/collection/artifacts", params: { page: 1, limit: 1 }, headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].length).to eq(1)
+      expect(json['meta']['total_pages']).to be >= 2
+    end
+
+    it 'filters by artifact_id' do
+      get "/api/v1/users/#{user.id}/collection/artifacts", params: { artifact_id: artifact.id }, headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].length).to eq(1)
+    end
+
+    it 'filters by element' do
+      get "/api/v1/users/#{user.id}/collection/artifacts", params: { element: 'water' }, headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['artifacts'].all? { |a| a['element'] == 'water' }).to be true
+    end
+
+    it 'returns unauthorized without authentication' do
+      other_user.update!(collection_visibility: 'private')
+      get "/api/v1/users/#{other_user.id}/collection/artifacts"
+
+      expect(response).to have_http_status(:unauthorized)
+    end
+  end
+
+  describe 'GET /api/v1/users/:user_id/collection/artifacts/:id' do
+    let!(:collection_artifact) { create(:collection_artifact, user: user, artifact: artifact) }
+
+    it 'returns the collection artifact' do
+      get "/api/v1/users/#{user.id}/collection/artifacts/#{collection_artifact.id}", headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['id']).to eq(collection_artifact.id)
+      expect(json['artifact']['id']).to eq(artifact.id)
+    end
+  end
+
+  describe 'POST /api/v1/collection/artifacts' do
+    let(:valid_attributes) do
+      {
+        collection_artifact: {
+          artifact_id: artifact.id,
+          element: 'fire',
+          level: 1,
+          skill1: { modifier: 1, strength: 1800, level: 1 },
+          skill2: { modifier: 2, strength: 900, level: 1 },
+          skill3: { modifier: 1, strength: 18.0, level: 1 },
+          skill4: { modifier: 1, strength: 10, level: 1 }
+        }
+      }
+    end
+
+    it 'creates a new collection artifact' do
+      expect do
+        post '/api/v1/collection/artifacts', params: valid_attributes.to_json, headers: headers
+      end.to change(CollectionArtifact, :count).by(1)
+
+      expect(response).to have_http_status(:created)
+      json = JSON.parse(response.body)
+      expect(json['artifact']['id']).to eq(artifact.id)
+      expect(json['element']).to eq('fire')
+    end
+
+    it 'allows multiple copies of the same artifact' do
+      create(:collection_artifact, user: user, artifact: artifact)
+
+      expect do
+        post '/api/v1/collection/artifacts', params: valid_attributes.to_json, headers: headers
+      end.to change(CollectionArtifact, :count).by(1)
+
+      expect(response).to have_http_status(:created)
+    end
+
+    it 'creates artifact with nickname' do
+      attributes_with_nickname = valid_attributes.deep_merge(
+        collection_artifact: { nickname: 'My Best Artifact' }
+      )
+
+      post '/api/v1/collection/artifacts', params: attributes_with_nickname.to_json, headers: headers
+
+      expect(response).to have_http_status(:created)
+      json = JSON.parse(response.body)
+      expect(json['nickname']).to eq('My Best Artifact')
+    end
+
+    it 'creates quirk artifact with proficiency' do
+      quirk_attributes = {
+        collection_artifact: {
+          artifact_id: quirk_artifact.id,
+          element: 'dark',
+          proficiency: 'staff',
+          level: 1,
+          skill1: {},
+          skill2: {},
+          skill3: {},
+          skill4: {}
+        }
+      }
+
+      post '/api/v1/collection/artifacts', params: quirk_attributes.to_json, headers: headers
+
+      expect(response).to have_http_status(:created)
+      json = JSON.parse(response.body)
+      expect(json['proficiency']).to eq('staff')
+    end
+
+    it 'returns error when skill1 and skill2 have same modifier' do
+      invalid_attributes = valid_attributes.deep_merge(
+        collection_artifact: {
+          skill1: { modifier: 1, strength: 1800, level: 1 },
+          skill2: { modifier: 1, strength: 1800, level: 1 }
+        }
+      )
+
+      post '/api/v1/collection/artifacts', params: invalid_attributes.to_json, headers: headers
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      json = JSON.parse(response.body)
+      expect(json['errors'].to_s).to include('cannot have the same modifier')
+    end
+
+    it 'returns unauthorized without authentication' do
+      post '/api/v1/collection/artifacts', params: valid_attributes.to_json
+
+      expect(response).to have_http_status(:unauthorized)
+    end
+  end
+
+  describe 'PUT /api/v1/collection/artifacts/:id' do
+    let!(:collection_artifact) { create(:collection_artifact, user: user, artifact: artifact, level: 1) }
+
+    it 'updates the collection artifact' do
+      update_attributes = {
+        collection_artifact: {
+          nickname: 'Updated Name',
+          element: 'water'
+        }
+      }
+
+      put "/api/v1/collection/artifacts/#{collection_artifact.id}",
+          params: update_attributes.to_json, headers: headers
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json['nickname']).to eq('Updated Name')
+      expect(json['element']).to eq('water')
+    end
+
+    it 'returns not found for other user\'s artifact' do
+      other_collection = create(:collection_artifact, user: other_user)
+
+      put "/api/v1/collection/artifacts/#{other_collection.id}",
+          params: { collection_artifact: { nickname: 'Hack' } }.to_json, headers: headers
+
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+
+  describe 'DELETE /api/v1/collection/artifacts/:id' do
+    let!(:collection_artifact) { create(:collection_artifact, user: user, artifact: artifact) }
+
+    it 'deletes the collection artifact' do
+      expect do
+        delete "/api/v1/collection/artifacts/#{collection_artifact.id}", headers: headers
+      end.to change(CollectionArtifact, :count).by(-1)
+
+      expect(response).to have_http_status(:no_content)
+    end
+
+    it 'returns not found for other user\'s artifact' do
+      other_collection = create(:collection_artifact, user: other_user)
+
+      expect do
+        delete "/api/v1/collection/artifacts/#{other_collection.id}", headers: headers
+      end.not_to change(CollectionArtifact, :count)
+
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+
+  describe 'POST /api/v1/collection/artifacts/batch' do
+    let(:artifact2) { create(:artifact, :dagger) }
+
+    it 'creates multiple collection artifacts' do
+      batch_attributes = {
+        collection_artifacts: [
+          {
+            artifact_id: artifact.id,
+            element: 'fire',
+            level: 1,
+            skill1: { modifier: 1, strength: 1800, level: 1 },
+            skill2: { modifier: 2, strength: 900, level: 1 },
+            skill3: { modifier: 1, strength: 18.0, level: 1 },
+            skill4: { modifier: 1, strength: 10, level: 1 }
+          },
+          {
+            artifact_id: artifact2.id,
+            element: 'water',
+            level: 1,
+            skill1: { modifier: 1, strength: 1800, level: 1 },
+            skill2: { modifier: 2, strength: 900, level: 1 },
+            skill3: { modifier: 1, strength: 18.0, level: 1 },
+            skill4: { modifier: 1, strength: 10, level: 1 }
+          }
+        ]
+      }
+
+      expect do
+        post '/api/v1/collection/artifacts/batch', params: batch_attributes.to_json, headers: headers
+      end.to change(CollectionArtifact, :count).by(2)
+
+      expect(response).to have_http_status(:created)
+      json = JSON.parse(response.body)
+      expect(json['meta']['created']).to eq(2)
+      expect(json['meta']['errors']).to be_empty
+    end
+
+    it 'returns multi_status when some items fail' do
+      batch_attributes = {
+        collection_artifacts: [
+          {
+            artifact_id: artifact.id,
+            element: 'fire',
+            level: 1,
+            skill1: { modifier: 1, strength: 1800, level: 1 },
+            skill2: { modifier: 2, strength: 900, level: 1 },
+            skill3: { modifier: 1, strength: 18.0, level: 1 },
+            skill4: { modifier: 1, strength: 10, level: 1 }
+          },
+          {
+            artifact_id: artifact.id,
+            element: 'invalid_element', # Invalid
+            level: 1
+          }
+        ]
+      }
+
+      post '/api/v1/collection/artifacts/batch', params: batch_attributes.to_json, headers: headers
+
+      expect(response).to have_http_status(:multi_status)
+      json = JSON.parse(response.body)
+      expect(json['meta']['created']).to eq(1)
+      expect(json['meta']['errors'].length).to eq(1)
+    end
+  end
+end