diff --git a/web/backend/Shared/Shared.Application.Tests/Behaviors/ValidationBehaviorTests.cs b/web/backend/Shared/Shared.Application.Tests/Behaviors/ValidationBehaviorTests.cs
new file mode 100644
index 00000000..db91f2da
--- /dev/null
+++ b/web/backend/Shared/Shared.Application.Tests/Behaviors/ValidationBehaviorTests.cs
@@ -0,0 +1,189 @@
+using FluentAssertions;
+using FluentValidation;
+using FluentValidation.Results;
+using MediatR;
+using Moq;
+using Shared.Application.Behaviors;
+
+namespace Shared.Application.Tests.Behaviors;
+
+public class ValidationBehaviorTests
+{
+    private static ValidationBehavior<ValidationTestRequest, ValidationTestResponse> CreateBehavior(
+        params IValidator<ValidationTestRequest>[] validators
+    ) => new(validators);
+
+    [Fact]
+    public async Task Handle_WhenRequestPassesAllValidators_InvokesNextUnchanged()
+    {
+        Mock<IValidator<ValidationTestRequest>> validatorMock = new();
+        validatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult());
+        ValidationBehavior<ValidationTestRequest, ValidationTestResponse> behavior = CreateBehavior(
+            validatorMock.Object
+        );
+        ValidationTestRequest request = new("aaron");
+        ValidationTestResponse response = new("ok");
+        bool nextInvoked = false;
+        RequestHandlerDelegate<ValidationTestResponse> next = () =>
+        {
+            nextInvoked = true;
+            return Task.FromResult(response);
+        };
+
+        ValidationTestResponse result = await behavior.Handle(
+            request,
+            next,
+            CancellationToken.None
+        );
+
+        nextInvoked.Should().BeTrue();
+        result.Should().BeSameAs(response);
+    }
+
+    [Fact]
+    public async Task Handle_WhenValidatorReportsFailures_ThrowsValidationExceptionAndNeverInvokesNext()
+    {
+        ValidationFailure failure = new("Name", "is required");
+        Mock<IValidator<ValidationTestRequest>> validatorMock = new();
+        validatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult([failure]));
+        ValidationBehavior<ValidationTestRequest, ValidationTestResponse> behavior = CreateBehavior(
+            validatorMock.Object
+        );
+        ValidationTestRequest request = new("");
+        bool nextInvoked = false;
+        RequestHandlerDelegate<ValidationTestResponse> next = () =>
+        {
+            nextInvoked = true;
+            return Task.FromResult(new ValidationTestResponse("ok"));
+        };
+
+        Func<Task> act = async () => await behavior.Handle(request, next, CancellationToken.None);
+
+        (await act.Should().ThrowAsync<ValidationException>()).Which.Errors.Should()
+            .ContainSingle()
+            .Which.Should()
+            .BeSameAs(failure);
+        nextInvoked.Should().BeFalse();
+    }
+
+    [Fact]
+    public async Task Handle_WhenMultipleValidatorsAreRegistered_RunsAllOfThem()
+    {
+        Mock<IValidator<ValidationTestRequest>> firstValidatorMock = new();
+        firstValidatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult());
+        Mock<IValidator<ValidationTestRequest>> secondValidatorMock = new();
+        secondValidatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult());
+        ValidationBehavior<ValidationTestRequest, ValidationTestResponse> behavior = CreateBehavior(
+            firstValidatorMock.Object,
+            secondValidatorMock.Object
+        );
+        ValidationTestRequest request = new("aaron");
+        RequestHandlerDelegate<ValidationTestResponse> next = () =>
+            Task.FromResult(new ValidationTestResponse("ok"));
+
+        await behavior.Handle(request, next, CancellationToken.None);
+
+        firstValidatorMock.Verify(
+            v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                ),
+            Times.Once()
+        );
+        secondValidatorMock.Verify(
+            v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                ),
+            Times.Once()
+        );
+    }
+
+    [Fact]
+    public async Task Handle_WhenMultipleValidatorsReportFailures_AggregatesErrorsFromAll()
+    {
+        ValidationFailure firstFailure = new("Name", "is required");
+        ValidationFailure secondFailure = new("Name", "must be at least 3 characters");
+        Mock<IValidator<ValidationTestRequest>> firstValidatorMock = new();
+        firstValidatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult([firstFailure]));
+        Mock<IValidator<ValidationTestRequest>> secondValidatorMock = new();
+        secondValidatorMock
+            .Setup(v =>
+                v.ValidateAsync(
+                    It.IsAny<ValidationContext<ValidationTestRequest>>(),
+                    It.IsAny<CancellationToken>()
+                )
+            )
+            .ReturnsAsync(new ValidationResult([secondFailure]));
+        ValidationBehavior<ValidationTestRequest, ValidationTestResponse> behavior = CreateBehavior(
+            firstValidatorMock.Object,
+            secondValidatorMock.Object
+        );
+        ValidationTestRequest request = new("");
+        RequestHandlerDelegate<ValidationTestResponse> next = () =>
+            Task.FromResult(new ValidationTestResponse("ok"));
+
+        Func<Task> act = async () => await behavior.Handle(request, next, CancellationToken.None);
+
+        (await act.Should().ThrowAsync<ValidationException>()).Which.Errors.Should()
+            .BeEquivalentTo([firstFailure, secondFailure]);
+    }
+
+    [Fact]
+    public async Task Handle_WhenNoValidatorsAreRegistered_InvokesNext()
+    {
+        ValidationBehavior<ValidationTestRequest, ValidationTestResponse> behavior = CreateBehavior();
+        ValidationTestRequest request = new("aaron");
+        ValidationTestResponse response = new("ok");
+        RequestHandlerDelegate<ValidationTestResponse> next = () => Task.FromResult(response);
+
+        ValidationTestResponse result = await behavior.Handle(
+            request,
+            next,
+            CancellationToken.None
+        );
+
+        result.Should().BeSameAs(response);
+    }
+}
+
+public record ValidationTestRequest(string Name) : IRequest<ValidationTestResponse>;
+
+public record ValidationTestResponse(string Value);