diff --git a/src/api/MicCheck.Api/Features/FeatureUsageFlushBackgroundService.cs b/src/api/MicCheck.Api/Features/FeatureUsageFlushBackgroundService.cs
index 6562d40..f69a807 100644
--- a/src/api/MicCheck.Api/Features/FeatureUsageFlushBackgroundService.cs
+++ b/src/api/MicCheck.Api/Features/FeatureUsageFlushBackgroundService.cs
@@ -1,8 +1,10 @@
+using System.Diagnostics.CodeAnalysis;
 using MicCheck.Api.Data;
 using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
+[ExcludeFromCodeCoverage(Justification = "Timer-driven BackgroundService that issues raw SQL through a DI-scoped concrete MicCheckDbContext; exercising it cleanly requires a live DB, which CLAUDE.md disallows (no WebApplicationFactory/InMemory). DrainAccumulated's bucketing logic is covered by FeatureUsageMetricsTests.")]
 public class FeatureUsageFlushBackgroundService(
     FeatureUsageMetrics metrics,
     IServiceScopeFactory scopeFactory,
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/CreateFeatureRequestValidatorTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/CreateFeatureRequestValidatorTests.cs
new file mode 100644
index 0000000..16340d0
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/CreateFeatureRequestValidatorTests.cs
@@ -0,0 +1,61 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class CreateFeatureRequestValidatorTests
+{
+    private CreateFeatureRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateFeatureRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("dark_mode", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest(new string('a', 151), FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameContainsInvalidCharacters_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("dark mode!", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenInitialValueExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("flag", FeatureType.Standard, new string('a', 20_001), null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenInitialValueIsNull_ThenTheLengthRuleIsSkipped()
+    {
+        var result = _validator.Validate(new CreateFeatureRequest("flag", FeatureType.Standard, null, null));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/CreateTagRequestValidatorTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/CreateTagRequestValidatorTests.cs
new file mode 100644
index 0000000..d1d3323
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/CreateTagRequestValidatorTests.cs
@@ -0,0 +1,61 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class CreateTagRequestValidatorTests
+{
+    private CreateTagRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new CreateTagRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateTagRequest("Beta", "#FF0000"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenLabelIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateTagRequest("", "#FF0000"));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenLabelExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateTagRequest(new string('a', 101), "#FF0000"));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenColorIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateTagRequest("Beta", ""));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenColorIsNotAValidHexCode_ThenValidationFails()
+    {
+        var result = _validator.Validate(new CreateTagRequest("Beta", "red"));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenColorIsAShortHandHexCode_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new CreateTagRequest("Beta", "#F00"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureResponseTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureResponseTests.cs
new file mode 100644
index 0000000..4721fa1
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureResponseTests.cs
@@ -0,0 +1,88 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureResponseTests
+{
+    [Test]
+    public void WhenMappingAFeatureWithNoTags_ThenTheResponseHasAnEmptyTagsList()
+    {
+        var feature = new Feature { Id = 1, Name = "dark_mode", ProjectId = 5, Type = FeatureType.Standard, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Name, Is.EqualTo("dark_mode"));
+        Assert.That(response.ProjectId, Is.EqualTo(5));
+        Assert.That(response.Tags, Is.Empty);
+    }
+
+    [Test]
+    public void WhenMappingAFeature_ThenTypeIsUpperInvariant()
+    {
+        var feature = new Feature { Id = 1, Name = "flag", ProjectId = 1, Type = FeatureType.MultiVariate, CreatedAt = DateTimeOffset.UtcNow };
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Type, Is.EqualTo("MULTIVARIATE"));
+    }
+
+    [Test]
+    public void WhenMappingAFeatureWithTags_ThenEachTagIsMapped()
+    {
+        var feature = new Feature { Id = 1, Name = "flag", ProjectId = 1, CreatedAt = DateTimeOffset.UtcNow };
+        feature.Tags.Add(new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = 1 });
+
+        var response = FeatureResponse.From(feature);
+
+        Assert.That(response.Tags, Has.Count.EqualTo(1));
+        Assert.That(response.Tags[0].Label, Is.EqualTo("beta"));
+    }
+}
+
+[TestFixture]
+public class TagResponseTests
+{
+    [Test]
+    public void WhenMappingATag_ThenAllFieldsAreCopied()
+    {
+        var tag = new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = 5 };
+
+        var response = TagResponse.From(tag);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.Label, Is.EqualTo("beta"));
+        Assert.That(response.Color, Is.EqualTo("#FF0000"));
+        Assert.That(response.ProjectId, Is.EqualTo(5));
+    }
+}
+
+[TestFixture]
+public class FeatureStateResponseTests
+{
+    [Test]
+    public void WhenMappingAFeatureState_ThenAllFieldsAreCopied()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var state = new FeatureState
+        {
+            Id = 1,
+            FeatureId = 2,
+            EnvironmentId = 3,
+            Enabled = true,
+            Value = "on",
+            CreatedAt = now,
+            UpdatedAt = now
+        };
+
+        var response = FeatureStateResponse.From(state);
+
+        Assert.That(response.Id, Is.EqualTo(1));
+        Assert.That(response.FeatureId, Is.EqualTo(2));
+        Assert.That(response.EnvironmentId, Is.EqualTo(3));
+        Assert.That(response.Enabled, Is.True);
+        Assert.That(response.Value, Is.EqualTo("on"));
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureSegmentServiceTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureSegmentServiceTests.cs
new file mode 100644
index 0000000..45cf5d0
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureSegmentServiceTests.cs
@@ -0,0 +1,131 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Segments;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureSegmentServiceTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<Segment> _segments = null!;
+    private List<FeatureSegment> _featureSegments = null!;
+    private List<FeatureState> _featureStates = null!;
+    private FeatureSegmentService _service = null!;
+    private const int FeatureId = 1;
+    private const int EnvironmentId = 1;
+    private const int SegmentId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _segments = [new Segment { Id = SegmentId, Name = "Premium Users", ProjectId = 1, CreatedAt = DateTimeOffset.UtcNow }];
+        _db.SetupDbSetWithGeneratedIds(c => c.Segments, _segments);
+        _featureSegments = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.FeatureSegments, _featureSegments);
+        _featureStates = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.FeatureStates, _featureStates);
+
+        _service = new FeatureSegmentService(_db.Object);
+    }
+
+    [Test]
+    public async Task WhenCreatingAFeatureSegment_ThenItIsPersistedWithAMatchingFeatureState()
+    {
+        var response = await _service.CreateAsync(FeatureId, EnvironmentId, SegmentId, priority: 1, enabled: true, value: "on");
+
+        Assert.That(response.SegmentName, Is.EqualTo("Premium Users"));
+        Assert.That(response.Enabled, Is.True);
+        Assert.That(_featureSegments, Has.Count.EqualTo(1));
+        Assert.That(_featureStates.Single().FeatureSegmentId, Is.EqualTo(response.Id));
+    }
+
+    [Test]
+    public void WhenCreatingAFeatureSegmentForASegmentThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        Assert.ThrowsAsync<KeyNotFoundException>(() =>
+            _service.CreateAsync(FeatureId, EnvironmentId, 999, priority: 1, enabled: true, value: null));
+    }
+
+    [Test]
+    public async Task WhenListingFeatureSegments_ThenTheyAreOrderedByPriorityAndIncludeTheSegmentNameAndState()
+    {
+        await _service.CreateAsync(FeatureId, EnvironmentId, SegmentId, priority: 2, enabled: true, value: "second");
+        _segments.Add(new Segment { Id = 2, Name = "Beta Users", ProjectId = 1, CreatedAt = DateTimeOffset.UtcNow });
+        await _service.CreateAsync(FeatureId, EnvironmentId, 2, priority: 1, enabled: false, value: "first");
+
+        var results = await _service.ListByFeatureAsync(FeatureId, EnvironmentId);
+
+        Assert.That(results, Has.Count.EqualTo(2));
+        Assert.That(results[0].Priority, Is.EqualTo(1));
+        Assert.That(results[0].SegmentName, Is.EqualTo("Beta Users"));
+        Assert.That(results[1].Priority, Is.EqualTo(2));
+    }
+
+    [Test]
+    public async Task WhenListingFeatureSegmentsForADifferentEnvironment_ThenNoneAreReturned()
+    {
+        await _service.CreateAsync(FeatureId, EnvironmentId, SegmentId, priority: 1, enabled: true, value: null);
+
+        var results = await _service.ListByFeatureAsync(FeatureId, 999);
+
+        Assert.That(results, Is.Empty);
+    }
+
+    [Test]
+    public async Task WhenUpdatingAFeatureSegmentThatDoesNotExist_ThenNullIsReturned()
+    {
+        var result = await _service.UpdateAsync(999, priority: 1, enabled: true, value: null);
+
+        Assert.That(result, Is.Null);
+    }
+
+    [Test]
+    public async Task WhenUpdatingAFeatureSegment_ThenTheExistingStateIsUpdated()
+    {
+        var created = await _service.CreateAsync(FeatureId, EnvironmentId, SegmentId, priority: 1, enabled: false, value: "off");
+
+        var updated = await _service.UpdateAsync(created.Id, priority: 5, enabled: true, value: "on");
+
+        Assert.That(updated!.Priority, Is.EqualTo(5));
+        Assert.That(updated.Enabled, Is.True);
+        Assert.That(updated.Value, Is.EqualTo("on"));
+        Assert.That(_featureStates, Has.Count.EqualTo(1));
+    }
+
+    [Test]
+    public async Task WhenUpdatingAFeatureSegmentThatHasNoExistingState_ThenANewStateIsCreated()
+    {
+        var featureSegment = new FeatureSegment { Id = 1, FeatureId = FeatureId, SegmentId = SegmentId, EnvironmentId = EnvironmentId, Priority = 1 };
+        _featureSegments.Add(featureSegment);
+
+        var updated = await _service.UpdateAsync(featureSegment.Id, priority: 3, enabled: true, value: "on");
+
+        Assert.That(updated!.Enabled, Is.True);
+        Assert.That(_featureStates, Has.Count.EqualTo(1));
+        Assert.That(_featureStates[0].FeatureSegmentId, Is.EqualTo(featureSegment.Id));
+    }
+
+    [Test]
+    public async Task WhenDeletingAFeatureSegmentThatDoesNotExist_ThenFalseIsReturned()
+    {
+        var result = await _service.DeleteAsync(999);
+
+        Assert.That(result, Is.False);
+    }
+
+    [Test]
+    public async Task WhenDeletingAFeatureSegment_ThenTrueIsReturnedAndItIsRemoved()
+    {
+        var created = await _service.CreateAsync(FeatureId, EnvironmentId, SegmentId, priority: 1, enabled: true, value: null);
+
+        var result = await _service.DeleteAsync(created.Id);
+
+        Assert.That(result, Is.True);
+        Assert.That(_featureSegments.Any(fsg => fsg.Id == created.Id), Is.False);
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureServiceTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureServiceTests.cs
index fc53292..45ec63c 100755
--- a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureServiceTests.cs
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureServiceTests.cs
@@ -179,4 +179,68 @@ public class FeatureServiceTests
 
         Assert.That(updated.Tags.Select(t => t.Id), Does.Not.Contain(tag.Id));
     }
+
+    [Test]
+    public async Task WhenFindingAFeatureByIdThatExists_ThenTheFeatureIsReturned()
+    {
+        var feature = await _service.CreateAsync(ProjectId, "flag", FeatureType.Standard, null, null);
+
+        var found = await _service.FindByIdAsync(feature.Id);
+
+        Assert.That(found, Is.Not.Null);
+        Assert.That(found!.Id, Is.EqualTo(feature.Id));
+    }
+
+    [Test]
+    public async Task WhenFindingAFeatureByIdThatDoesNotExist_ThenNullIsReturned()
+    {
+        var found = await _service.FindByIdAsync(999);
+
+        Assert.That(found, Is.Null);
+    }
+
+    [Test]
+    public void WhenUpdatingAFeatureThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.UpdateAsync(999, "renamed", null));
+    }
+
+    [Test]
+    public void WhenAssigningATagToAFeatureThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        var tag = new Tag { Label = "beta", Color = "#FF0000", ProjectId = ProjectId };
+        _db.Object.Tags.Add(tag);
+
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.AssignTagAsync(999, tag.Id));
+    }
+
+    [Test]
+    public async Task WhenAssigningATagThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        var feature = await _service.CreateAsync(ProjectId, "flag", FeatureType.Standard, null, null);
+
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.AssignTagAsync(feature.Id, 999));
+    }
+
+    [Test]
+    public void WhenRemovingATagFromAFeatureThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.RemoveTagAsync(999, 1));
+    }
+
+    [Test]
+    public async Task WhenRemovingATagThatIsNotAssigned_ThenTheFeatureIsUnchanged()
+    {
+        var feature = await _service.CreateAsync(ProjectId, "flag", FeatureType.Standard, null, null);
+
+        var updated = await _service.RemoveTagAsync(feature.Id, 999);
+
+        Assert.That(updated.Tags, Is.Empty);
+    }
+
+    [Test]
+    public void WhenDeletingAFeatureThatDoesNotExist_ThenNoExceptionIsThrown()
+    {
+        Assert.DoesNotThrowAsync(() => _service.DeleteAsync(999));
+    }
 }
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureStateServiceTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureStateServiceTests.cs
new file mode 100644
index 0000000..834991d
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureStateServiceTests.cs
@@ -0,0 +1,174 @@
+using MicCheck.Api.Audit;
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Organizations;
+using MicCheck.Api.Projects;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using MicCheck.Api.Webhooks;
+using Moq;
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureStateServiceTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<FeatureState> _featureStates = null!;
+    private List<Feature> _features = null!;
+    private List<AppEnvironment> _environments = null!;
+    private FeatureStateService _service = null!;
+    private const int OrganizationId = 1;
+    private const int ProjectId = 1;
+    private const int EnvironmentId = 1;
+    private const int FeatureId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+
+        _db.SetupDbSet(c => c.Organizations, [
+            new Organization { Id = OrganizationId, Name = "Test Org", CreatedAt = DateTimeOffset.UtcNow }
+        ]);
+        _db.SetupDbSet(c => c.Projects, [
+            new Project { Id = ProjectId, Name = "Test Project", OrganizationId = OrganizationId, CreatedAt = DateTimeOffset.UtcNow }
+        ]);
+        _environments = [new AppEnvironment { Id = EnvironmentId, Name = "Production", ApiKey = "test-key", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow }];
+        _db.SetupDbSet(c => c.Environments, _environments);
+        _features = [new Feature { Id = FeatureId, Name = "dark_mode", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow }];
+        _db.SetupDbSet(c => c.Features, _features);
+        _featureStates = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.FeatureStates, _featureStates);
+
+        var webhookQueue = new WebhookQueue();
+        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        auditService.Setup(a => a.RecordAsync(
+            It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
+            It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),
+            It.IsAny<object?>(), It.IsAny<object?>(), It.IsAny<CancellationToken>()))
+            .Returns(Task.CompletedTask);
+
+        _service = new FeatureStateService(_db.Object, webhookQueue, auditService.Object);
+    }
+
+    private FeatureState AddState(bool enabled = false, string? value = null)
+    {
+        var state = new FeatureState
+        {
+            Id = 1,
+            FeatureId = FeatureId,
+            EnvironmentId = EnvironmentId,
+            Enabled = enabled,
+            Value = value,
+            CreatedAt = DateTimeOffset.UtcNow,
+            UpdatedAt = DateTimeOffset.UtcNow
+        };
+        _featureStates.Add(state);
+        return state;
+    }
+
+    [Test]
+    public async Task WhenListingEnvironmentLevelStates_ThenIdentityAndSegmentOverridesAreExcluded()
+    {
+        AddState();
+        _featureStates.Add(new FeatureState { Id = 2, FeatureId = FeatureId, EnvironmentId = EnvironmentId, IdentityId = 5, CreatedAt = DateTimeOffset.UtcNow, UpdatedAt = DateTimeOffset.UtcNow });
+        _featureStates.Add(new FeatureState { Id = 3, FeatureId = FeatureId, EnvironmentId = EnvironmentId, FeatureSegmentId = 7, CreatedAt = DateTimeOffset.UtcNow, UpdatedAt = DateTimeOffset.UtcNow });
+
+        var results = await _service.ListByEnvironmentAsync(EnvironmentId);
+
+        Assert.That(results, Has.Count.EqualTo(1));
+        Assert.That(results[0].Id, Is.EqualTo(1));
+    }
+
+    [Test]
+    public async Task WhenFindingAStateByIdInTheCorrectEnvironment_ThenItIsReturned()
+    {
+        var state = AddState();
+
+        var found = await _service.FindByIdAsync(state.Id, EnvironmentId);
+
+        Assert.That(found, Is.Not.Null);
+    }
+
+    [Test]
+    public async Task WhenFindingAStateByIdInADifferentEnvironment_ThenNullIsReturned()
+    {
+        var state = AddState();
+
+        var found = await _service.FindByIdAsync(state.Id, 999);
+
+        Assert.That(found, Is.Null);
+    }
+
+    [Test]
+    public void WhenUpdatingAStateThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.UpdateAsync(999, true, "on"));
+    }
+
+    [Test]
+    public async Task WhenUpdatingAState_ThenEnabledAndValueAreChangedAndVersionIsIncremented()
+    {
+        var state = AddState(enabled: false, value: "old");
+
+        var updated = await _service.UpdateAsync(state.Id, true, "new");
+
+        Assert.That(updated.Enabled, Is.True);
+        Assert.That(updated.Value, Is.EqualTo("new"));
+        Assert.That(updated.Version, Is.EqualTo(1));
+    }
+
+    [Test]
+    public void WhenPatchingAStateThatDoesNotExist_ThenKeyNotFoundExceptionIsThrown()
+    {
+        Assert.ThrowsAsync<KeyNotFoundException>(() => _service.PatchAsync(999, true, null));
+    }
+
+    [Test]
+    public async Task WhenPatchingOnlyTheEnabledField_ThenValueIsUnchanged()
+    {
+        var state = AddState(enabled: false, value: "original");
+
+        var patched = await _service.PatchAsync(state.Id, true, null);
+
+        Assert.That(patched.Enabled, Is.True);
+        Assert.That(patched.Value, Is.EqualTo("original"));
+    }
+
+    [Test]
+    public async Task WhenPatchingOnlyTheValueField_ThenEnabledIsUnchanged()
+    {
+        var state = AddState(enabled: true, value: "original");
+
+        var patched = await _service.PatchAsync(state.Id, null, "updated");
+
+        Assert.That(patched.Enabled, Is.True);
+        Assert.That(patched.Value, Is.EqualTo("updated"));
+    }
+
+    [Test]
+    public async Task WhenUpdatingAState_ThenAWebhookEventIsEnqueued()
+    {
+        var state = AddState();
+        var queue = new WebhookQueue();
+
+        var auditService = new Mock<AuditService>(_db.Object, null!, queue);
+        auditService.Setup(a => a.RecordAsync(
+            It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
+            It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),
+            It.IsAny<object?>(), It.IsAny<object?>(), It.IsAny<CancellationToken>()))
+            .Returns(Task.CompletedTask);
+        var service = new FeatureStateService(_db.Object, queue, auditService.Object);
+
+        await service.UpdateAsync(state.Id, true, "on");
+
+        using var cts = new CancellationTokenSource();
+        await using var enumerator = queue.ReadAllAsync(cts.Token).GetAsyncEnumerator(cts.Token);
+        Assert.That(await enumerator.MoveNextAsync(), Is.True);
+        Assert.That(enumerator.Current.EventType, Is.EqualTo(WebhookEventTypes.FlagUpdated));
+        Assert.That(enumerator.Current.EnvironmentId, Is.EqualTo(EnvironmentId));
+        Assert.That(enumerator.Current.OrganizationId, Is.EqualTo(OrganizationId));
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureUsageControllerTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureUsageControllerTests.cs
new file mode 100644
index 0000000..051f809
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeatureUsageControllerTests.cs
@@ -0,0 +1,93 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Caching.Memory;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeatureUsageControllerTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<FeatureUsageDaily> _usage = null!;
+    private IMemoryCache _cache = null!;
+    private FeatureUsageController _controller = null!;
+    private const int EnvironmentId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _usage = [];
+        _db.SetupDbSet(c => c.FeatureUsageDaily, _usage);
+        _cache = new MemoryCache(new MemoryCacheOptions());
+
+        _controller = new FeatureUsageController(new FeatureUsageQueryService(_db.Object), _cache);
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageWithNoData_ThenEmptyResultsAreReturned()
+    {
+        var result = await _controller.GetDashboardUsage(EnvironmentId);
+
+        var ok = result.Result as OkObjectResult;
+        var response = (DashboardUsageResponse)ok!.Value!;
+        Assert.That(response.TopFeaturesLastDay, Is.Empty);
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageASecondTime_ThenTheCachedResultIsReturnedWithoutQueryingAgain()
+    {
+        var first = await _controller.GetDashboardUsage(EnvironmentId);
+        _usage.Add(new FeatureUsageDaily
+        {
+            EnvironmentId = EnvironmentId,
+            FeatureId = 1,
+            FeatureName = "flag",
+            UsageDate = DateOnly.FromDateTime(DateTime.UtcNow),
+            Count = 100,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var second = await _controller.GetDashboardUsage(EnvironmentId);
+
+        var firstOk = (DashboardUsageResponse)((OkObjectResult)first.Result!).Value!;
+        var secondOk = (DashboardUsageResponse)((OkObjectResult)second.Result!).Value!;
+        Assert.That(secondOk, Is.SameAs(firstOk));
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageForDifferentDayRanges_ThenEachRangeIsCachedSeparately()
+    {
+        var sevenDays = await _controller.GetDashboardUsage(EnvironmentId, days: 7);
+        var fourteenDays = await _controller.GetDashboardUsage(EnvironmentId, days: 14);
+
+        var sevenOk = (DashboardUsageResponse)((OkObjectResult)sevenDays.Result!).Value!;
+        var fourteenOk = (DashboardUsageResponse)((OkObjectResult)fourteenDays.Result!).Value!;
+        Assert.That(sevenOk, Is.Not.SameAs(fourteenOk));
+    }
+
+    [Test]
+    public async Task WhenDaysExceedsTheMaximum_ThenItIsClampedTo90()
+    {
+        _usage.Add(new FeatureUsageDaily
+        {
+            EnvironmentId = EnvironmentId,
+            FeatureId = 1,
+            FeatureName = "flag",
+            UsageDate = DateOnly.FromDateTime(DateTime.UtcNow),
+            Count = 5,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var clamped = await _controller.GetDashboardUsage(EnvironmentId, days: 1000);
+        var explicitNinety = await _controller.GetDashboardUsage(EnvironmentId, days: 90);
+
+        var clampedOk = (DashboardUsageResponse)((OkObjectResult)clamped.Result!).Value!;
+        var explicitOk = (DashboardUsageResponse)((OkObjectResult)explicitNinety.Result!).Value!;
+        Assert.That(clampedOk.DailyUsage.Count, Is.EqualTo(explicitOk.DailyUsage.Count));
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/FeaturesControllerTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/FeaturesControllerTests.cs
new file mode 100644
index 0000000..36456da
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/FeaturesControllerTests.cs
@@ -0,0 +1,282 @@
+using MicCheck.Api.Audit;
+using MicCheck.Api.Common;
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Organizations;
+using MicCheck.Api.Projects;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Microsoft.AspNetCore.Mvc;
+using Moq;
+using NUnit.Framework;
+using AppEnvironment = MicCheck.Api.Environments.Environment;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class FeaturesControllerTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<Feature> _features = null!;
+    private List<Tag> _tags = null!;
+    private FeaturesController _controller = null!;
+    private const int OrganizationId = 1;
+    private const int ProjectId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+
+        _db.SetupDbSet(c => c.Organizations, [
+            new Organization { Id = OrganizationId, Name = "Test Org", CreatedAt = DateTimeOffset.UtcNow }
+        ]);
+        _db.SetupDbSet(c => c.Projects, [
+            new Project { Id = ProjectId, Name = "Test Project", OrganizationId = OrganizationId, CreatedAt = DateTimeOffset.UtcNow }
+        ]);
+        _db.SetupDbSet(c => c.Environments, new List<AppEnvironment>());
+        _features = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.Features, _features);
+        _db.SetupDbSet(c => c.FeatureStates, new List<FeatureState>());
+        _tags = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.Tags, _tags);
+
+        var webhookQueue = new MicCheck.Api.Webhooks.WebhookQueue();
+        var auditService = new Mock<AuditService>(_db.Object, null!, webhookQueue);
+        auditService.Setup(a => a.RecordAsync(
+            It.IsAny<string>(), It.IsAny<string>(), It.IsAny<string>(),
+            It.IsAny<int>(), It.IsAny<int?>(), It.IsAny<int?>(),
+            It.IsAny<object?>(), It.IsAny<object?>(), It.IsAny<CancellationToken>()))
+            .Returns(Task.CompletedTask);
+
+        var service = new FeatureService(_db.Object, auditService.Object, webhookQueue);
+        _controller = new FeaturesController(service);
+    }
+
+    private static CreateFeatureRequest ValidRequest(string name = "dark_mode") =>
+        new(name, FeatureType.Standard, null, null);
+
+    [Test]
+    public async Task WhenListingFeaturesForAProject_ThenAllFeaturesAreReturnedInAPage()
+    {
+        await _controller.Create(ProjectId, ValidRequest("feature_a"), CancellationToken.None);
+        await _controller.Create(ProjectId, ValidRequest("feature_b"), CancellationToken.None);
+
+        var result = await _controller.List(ProjectId);
+
+        var ok = result.Result as OkObjectResult;
+        var page = (PaginatedResponse<FeatureResponse>)ok!.Value!;
+        Assert.That(page.Count, Is.EqualTo(2));
+        Assert.That(page.Results, Has.Count.EqualTo(2));
+    }
+
+    [Test]
+    public async Task WhenListingFeaturesWithAPageSizeAboveTheMaximum_ThenThePageSizeIsClampedTo100()
+    {
+        for (var i = 0; i < 3; i++)
+            await _controller.Create(ProjectId, ValidRequest($"feature_{i}"), CancellationToken.None);
+
+        var result = await _controller.List(ProjectId, page: 1, pageSize: 1000);
+
+        var ok = result.Result as OkObjectResult;
+        var page = (PaginatedResponse<FeatureResponse>)ok!.Value!;
+        Assert.That(page.Results, Has.Count.EqualTo(3));
+    }
+
+    [Test]
+    public async Task WhenCreatingAFeatureWithValidData_ThenACreatedResultWithTheFeatureIsReturned()
+    {
+        var result = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+
+        var created = result.Result as CreatedAtActionResult;
+        Assert.That(created, Is.Not.Null);
+        Assert.That(((FeatureResponse)created!.Value!).Name, Is.EqualTo("dark_mode"));
+    }
+
+    [Test]
+    public async Task WhenCreatingAFeatureThatExceedsTheProjectLimit_ThenBadRequestIsReturned()
+    {
+        for (var i = 0; i < 400; i++)
+            _features.Add(new Feature { Id = i + 1, Name = $"feature_{i}", ProjectId = ProjectId, CreatedAt = DateTimeOffset.UtcNow });
+
+        var result = await _controller.Create(ProjectId, ValidRequest("overflow"), CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<BadRequestObjectResult>());
+    }
+
+    [Test]
+    public async Task WhenGettingAFeatureByIdInTheCorrectProject_ThenTheFeatureIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.GetById(ProjectId, featureId, CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(ok, Is.Not.Null);
+        Assert.That(((FeatureResponse)ok!.Value!).Id, Is.EqualTo(featureId));
+    }
+
+    [Test]
+    public async Task WhenGettingAFeatureThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.GetById(ProjectId, 999, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenGettingAFeatureThatBelongsToADifferentProject_ThenNotFoundIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.GetById(999, featureId, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenUpdatingAFeatureThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.Update(ProjectId, 999, new UpdateFeatureRequest("renamed", null), CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenUpdatingAFeatureInTheCorrectProject_ThenTheUpdatedFeatureIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Update(ProjectId, featureId, new UpdateFeatureRequest("renamed", "new desc"), CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(ok, Is.Not.Null);
+        Assert.That(((FeatureResponse)ok!.Value!).Name, Is.EqualTo("renamed"));
+    }
+
+    [Test]
+    public async Task WhenPatchingAFeatureThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.Patch(ProjectId, 999, new PatchFeatureRequest("renamed", null, null), CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenPatchingOnlyTheNameField_ThenDescriptionAndDefaultEnabledAreUnchanged()
+    {
+        var created = await _controller.Create(ProjectId, new CreateFeatureRequest("original", FeatureType.Standard, null, "original desc"), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Patch(ProjectId, featureId, new PatchFeatureRequest("renamed", null, null), CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        var response = (FeatureResponse)ok!.Value!;
+        Assert.That(response.Name, Is.EqualTo("renamed"));
+        Assert.That(response.Description, Is.EqualTo("original desc"));
+        Assert.That(response.DefaultEnabled, Is.False);
+    }
+
+    [Test]
+    public async Task WhenPatchingTheDefaultEnabledField_ThenItIsUpdated()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Patch(ProjectId, featureId, new PatchFeatureRequest(null, null, true), CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(((FeatureResponse)ok!.Value!).DefaultEnabled, Is.True);
+    }
+
+    [Test]
+    public async Task WhenDeletingAFeatureThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.Delete(ProjectId, 999, CancellationToken.None);
+
+        Assert.That(result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenDeletingAFeatureInTheCorrectProject_ThenNoContentIsReturnedAndTheFeatureIsRemoved()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Delete(ProjectId, featureId, CancellationToken.None);
+
+        Assert.That(result, Is.InstanceOf<NoContentResult>());
+        Assert.That(_features.Any(f => f.Id == featureId), Is.False);
+    }
+
+    [Test]
+    public async Task WhenAssigningATagThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.AssignTag(ProjectId, featureId, 999, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenAssigningATagFromAnotherProject_ThenBadRequestIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+        var foreignTag = new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = 999 };
+        _tags.Add(foreignTag);
+
+        var result = await _controller.AssignTag(ProjectId, featureId, foreignTag.Id, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<BadRequestObjectResult>());
+    }
+
+    [Test]
+    public async Task WhenAssigningAValidTag_ThenTheFeatureResponseIncludesTheTag()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+        var tag = new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = ProjectId };
+        _tags.Add(tag);
+
+        var result = await _controller.AssignTag(ProjectId, featureId, tag.Id, CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(((FeatureResponse)ok!.Value!).Tags.Select(t => t.Id), Does.Contain(tag.Id));
+    }
+
+    [Test]
+    public async Task WhenFeatureForAssignTagDoesNotExistInTheProject_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.AssignTag(ProjectId, 999, 1, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenRemovingTagFromFeatureThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.RemoveTag(ProjectId, 999, 1, CancellationToken.None);
+
+        Assert.That(result.Result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenRemovingAnAssignedTag_ThenTheFeatureResponseNoLongerIncludesIt()
+    {
+        var created = await _controller.Create(ProjectId, ValidRequest(), CancellationToken.None);
+        var featureId = ((FeatureResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+        var tag = new Tag { Id = 1, Label = "beta", Color = "#FF0000", ProjectId = ProjectId };
+        _tags.Add(tag);
+        await _controller.AssignTag(ProjectId, featureId, tag.Id, CancellationToken.None);
+
+        var result = await _controller.RemoveTag(ProjectId, featureId, tag.Id, CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        Assert.That(((FeatureResponse)ok!.Value!).Tags.Select(t => t.Id), Does.Not.Contain(tag.Id));
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/TagServiceTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/TagServiceTests.cs
new file mode 100644
index 0000000..1d2cf08
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/TagServiceTests.cs
@@ -0,0 +1,84 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class TagServiceTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<Tag> _tags = null!;
+    private TagService _service = null!;
+    private const int ProjectId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _tags = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.Tags, _tags);
+
+        _service = new TagService(_db.Object);
+    }
+
+    [Test]
+    public async Task WhenCreatingATag_ThenItIsPersistedWithTheGivenProject()
+    {
+        var tag = await _service.CreateAsync(ProjectId, "Beta", "#FF0000");
+
+        Assert.That(tag.Label, Is.EqualTo("Beta"));
+        Assert.That(tag.Color, Is.EqualTo("#FF0000"));
+        Assert.That(tag.ProjectId, Is.EqualTo(ProjectId));
+        Assert.That(_tags, Has.Count.EqualTo(1));
+    }
+
+    [Test]
+    public async Task WhenListingTagsForAProject_ThenOnlyThatProjectsTagsAreReturned()
+    {
+        await _service.CreateAsync(ProjectId, "Beta", "#FF0000");
+        await _service.CreateAsync(999, "Other", "#00FF00");
+
+        var tags = await _service.ListByProjectAsync(ProjectId);
+
+        Assert.That(tags, Has.Count.EqualTo(1));
+        Assert.That(tags[0].Label, Is.EqualTo("Beta"));
+    }
+
+    [Test]
+    public async Task WhenFindingATagByIdThatExists_ThenTheTagIsReturned()
+    {
+        var tag = await _service.CreateAsync(ProjectId, "Beta", "#FF0000");
+
+        var found = await _service.FindByIdAsync(tag.Id);
+
+        Assert.That(found, Is.Not.Null);
+        Assert.That(found!.Id, Is.EqualTo(tag.Id));
+    }
+
+    [Test]
+    public async Task WhenFindingATagByIdThatDoesNotExist_ThenNullIsReturned()
+    {
+        var found = await _service.FindByIdAsync(999);
+
+        Assert.That(found, Is.Null);
+    }
+
+    [Test]
+    public async Task WhenDeletingATag_ThenItIsRemovedFromTheDatabase()
+    {
+        var tag = await _service.CreateAsync(ProjectId, "Beta", "#FF0000");
+
+        await _service.DeleteAsync(tag.Id);
+
+        Assert.That(_tags.Any(t => t.Id == tag.Id), Is.False);
+    }
+
+    [Test]
+    public void WhenDeletingATagThatDoesNotExist_ThenNoExceptionIsThrown()
+    {
+        Assert.DoesNotThrowAsync(() => _service.DeleteAsync(999));
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/TagsControllerTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/TagsControllerTests.cs
new file mode 100644
index 0000000..ae5323a
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/TagsControllerTests.cs
@@ -0,0 +1,82 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Microsoft.AspNetCore.Mvc;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class TagsControllerTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<Tag> _tags = null!;
+    private TagsController _controller = null!;
+    private const int ProjectId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _tags = [];
+        _db.SetupDbSetWithGeneratedIds(c => c.Tags, _tags);
+
+        _controller = new TagsController(new TagService(_db.Object));
+    }
+
+    [Test]
+    public async Task WhenListingTagsForAProject_ThenAllOfThatProjectsTagsAreReturned()
+    {
+        await _controller.Create(ProjectId, new CreateTagRequest("Beta", "#FF0000"), CancellationToken.None);
+        await _controller.Create(999, new CreateTagRequest("Other", "#00FF00"), CancellationToken.None);
+
+        var result = await _controller.List(ProjectId, CancellationToken.None);
+
+        var ok = result.Result as OkObjectResult;
+        var tags = (IReadOnlyList<TagResponse>)ok!.Value!;
+        Assert.That(tags, Has.Count.EqualTo(1));
+        Assert.That(tags[0].Label, Is.EqualTo("Beta"));
+    }
+
+    [Test]
+    public async Task WhenCreatingATag_ThenACreatedResultWithTheTagIsReturned()
+    {
+        var result = await _controller.Create(ProjectId, new CreateTagRequest("Beta", "#FF0000"), CancellationToken.None);
+
+        var created = result.Result as CreatedAtActionResult;
+        Assert.That(created, Is.Not.Null);
+        Assert.That(((TagResponse)created!.Value!).Label, Is.EqualTo("Beta"));
+    }
+
+    [Test]
+    public async Task WhenDeletingATagThatDoesNotExist_ThenNotFoundIsReturned()
+    {
+        var result = await _controller.Delete(ProjectId, 999, CancellationToken.None);
+
+        Assert.That(result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenDeletingATagFromADifferentProject_ThenNotFoundIsReturned()
+    {
+        var created = await _controller.Create(ProjectId, new CreateTagRequest("Beta", "#FF0000"), CancellationToken.None);
+        var tagId = ((TagResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Delete(999, tagId, CancellationToken.None);
+
+        Assert.That(result, Is.InstanceOf<NotFoundResult>());
+    }
+
+    [Test]
+    public async Task WhenDeletingATagInTheCorrectProject_ThenNoContentIsReturnedAndTheTagIsRemoved()
+    {
+        var created = await _controller.Create(ProjectId, new CreateTagRequest("Beta", "#FF0000"), CancellationToken.None);
+        var tagId = ((TagResponse)((CreatedAtActionResult)created.Result!).Value!).Id;
+
+        var result = await _controller.Delete(ProjectId, tagId, CancellationToken.None);
+
+        Assert.That(result, Is.InstanceOf<NoContentResult>());
+        Assert.That(_tags.Any(t => t.Id == tagId), Is.False);
+    }
+}
diff --git a/tests/api/MicCheck.Api.Tests.Unit/Features/UpdateFeatureRequestValidatorTests.cs b/tests/api/MicCheck.Api.Tests.Unit/Features/UpdateFeatureRequestValidatorTests.cs
new file mode 100644
index 0000000..29d1fca
--- /dev/null
+++ b/tests/api/MicCheck.Api.Tests.Unit/Features/UpdateFeatureRequestValidatorTests.cs
@@ -0,0 +1,45 @@
+using MicCheck.Api.Features;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features;
+
+[TestFixture]
+public class UpdateFeatureRequestValidatorTests
+{
+    private UpdateFeatureRequestValidator _validator = null!;
+
+    [SetUp]
+    public void SetUp() => _validator = new UpdateFeatureRequestValidator();
+
+    [Test]
+    public void WhenTheRequestIsValid_ThenValidationSucceeds()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("dark_mode", "A description"));
+
+        Assert.That(result.IsValid, Is.True);
+    }
+
+    [Test]
+    public void WhenNameIsEmpty_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("", null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameExceedsMaximumLength_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest(new string('a', 151), null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+
+    [Test]
+    public void WhenNameContainsInvalidCharacters_ThenValidationFails()
+    {
+        var result = _validator.Validate(new UpdateFeatureRequest("dark mode!", null));
+
+        Assert.That(result.IsValid, Is.False);
+    }
+}