src/api/MicCheck.Api/Common/Guard.cs

@@ -0,0 +1,40 @@
+namespace MicCheck.Api.Common;
+
+public static class Guard
+{
+    public static void Null<T>(T t, string parameterName) where T : class
+    {
+        if (t is null)
+            throw new ArgumentNullException(parameterName, $"{nameof(parameterName)} can not be null");
+    }
+
+    public static void Empty(string value, string parameterName)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+            throw new ArgumentException($"{parameterName} can not be empty", parameterName);
+    }
+
+    public static void Empty<T>(IEnumerable<T> collection, string parameterName)
+    {
+        if (collection == null || !collection.Any())
+            throw new ArgumentException($"{parameterName} can not be empty", parameterName);
+    }
+
+    public static void Negative(int value, string parameterName)
+    {
+        if (value < 0)
+            throw new ArgumentOutOfRangeException(parameterName, $"{parameterName} must be a positive number or zero");
+    }
+
+    public static void NegativeOrZero(int value, string parameterName)
+    {
+        if (value <= 0)
+            throw new ArgumentOutOfRangeException(parameterName, $"{nameof(parameterName)} must be a positive number greater then zero");
+    }
+
+    public static void Default<T>(T value, string parameterName)
+    {
+        if (EqualityComparer<T>.Default.Equals(value, default))
+            throw new ArgumentException($"{parameterName} can not be a default value", parameterName);
+    }
+}

src/api/MicCheck.Api/Common/Validation/IModelValidator.cs

@@ -0,0 +1,13 @@
+namespace MicCheck.Api.Common.Validation;
+
+public interface IModelValidator
+{
+    ValidationResult Validate(object model);
+}
+
+public interface IModelValidator<in T> : IModelValidator
+{
+    ValidationResult Validate(T model);
+
+    ValidationResult IModelValidator.Validate(object model) => Validate((T)model);
+}

src/api/MicCheck.Api/Common/Validation/ModelValidationActionFilter.cs

@@ -0,0 +1,28 @@
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace MicCheck.Api.Common.Validation;
+
+public class ModelValidationActionFilter : IActionFilter
+{
+    public void OnActionExecuting(ActionExecutingContext context)
+    {
+        foreach (var argument in context.ActionArguments.Values)
+        {
+            if (argument is null) continue;
+
+            var validatorType = typeof(IModelValidator<>).MakeGenericType(argument.GetType());
+            if (context.HttpContext.RequestServices.GetService(validatorType) is not IModelValidator validator) continue;
+
+            var result = validator.Validate(argument);
+            foreach (var error in result.Errors)
+                context.ModelState.AddModelError(error.PropertyName, error.Message);
+        }
+
+        if (!context.ModelState.IsValid)
+            context.Result = ValidationProblemResponseFactory.Create(context.ModelState);
+    }
+
+    public void OnActionExecuted(ActionExecutedContext context)
+    {
+    }
+}

src/api/MicCheck.Api/Common/Validation/ModelValidatorServiceCollectionExtensions.cs

@@ -0,0 +1,20 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace MicCheck.Api.Common.Validation;
+
+public static class ModelValidatorServiceCollectionExtensions
+{
+    public static IServiceCollection AddModelValidatorsFromAssemblyContaining<TMarker>(this IServiceCollection services)
+    {
+        var registrations = typeof(TMarker).Assembly.GetTypes()
+            .Where(type => !type.IsAbstract && !type.IsInterface)
+            .SelectMany(type => type.GetInterfaces()
+                .Where(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IModelValidator<>))
+                .Select(i => (Interface: i, Implementation: type)));
+
+        foreach (var (@interface, implementation) in registrations)
+            services.AddScoped(@interface, implementation);
+
+        return services;
+    }
+}

src/api/MicCheck.Api/Common/Validation/ValidationProblemResponseFactory.cs

@@ -0,0 +1,18 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace MicCheck.Api.Common.Validation;
+
+public static class ValidationProblemResponseFactory
+{
+    public static IActionResult Create(ModelStateDictionary modelState)
+    {
+        var errors = modelState
+            .Where(e => e.Value?.Errors.Count > 0)
+            .ToDictionary(
+                kvp => kvp.Key,
+                kvp => kvp.Value!.Errors.Select(e => e.ErrorMessage).ToArray());
+
+        return new UnprocessableEntityObjectResult(new { errors });
+    }
+}

src/api/MicCheck.Api/Common/Validation/ValidationResult.cs

@@ -0,0 +1,14 @@
+namespace MicCheck.Api.Common.Validation;
+
+public record ValidationError(string PropertyName, string Message);
+
+public class ValidationResult
+{
+    private readonly List<ValidationError> _errors = [];
+
+    public IReadOnlyList<ValidationError> Errors => _errors;
+    public bool IsValid => _errors.Count == 0;
+    public bool IsInvalid => _errors.Count > 0;
+
+    public void AddError(string propertyName, string message) => _errors.Add(new ValidationError(propertyName, message));
+}

src/api/MicCheck.Api/Environments/CloneEnvironmentRequest.cs

@@ -1,13 +1,20 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Environments;
 
 public record CloneEnvironmentRequest(string Name);
 
-public class CloneEnvironmentRequestValidator : AbstractValidator<CloneEnvironmentRequest>
+public class CloneEnvironmentRequestValidator : IModelValidator<CloneEnvironmentRequest>
 {
-    public CloneEnvironmentRequestValidator()
+    public ValidationResult Validate(CloneEnvironmentRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Environments/CreateEnvironmentRequest.cs

@@ -1,14 +1,23 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Environments;
 
 public record CreateEnvironmentRequest(string Name, int ProjectId);
 
-public class CreateEnvironmentRequestValidator : AbstractValidator<CreateEnvironmentRequest>
+public class CreateEnvironmentRequestValidator : IModelValidator<CreateEnvironmentRequest>
 {
-    public CreateEnvironmentRequestValidator()
+    public ValidationResult Validate(CreateEnvironmentRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
-        RuleFor(x => x.ProjectId).GreaterThan(0);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        if (model.ProjectId <= 0)
+            result.AddError(nameof(model.ProjectId), "'Project Id' must be greater than 0.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Environments/UpdateEnvironmentRequest.cs

@@ -1,13 +1,20 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Environments;
 
 public record UpdateEnvironmentRequest(string Name);
 
-public class UpdateEnvironmentRequestValidator : AbstractValidator<UpdateEnvironmentRequest>
+public class UpdateEnvironmentRequestValidator : IModelValidator<UpdateEnvironmentRequest>
 {
-    public UpdateEnvironmentRequestValidator()
+    public ValidationResult Validate(UpdateEnvironmentRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Features/CreateFeatureRequest.cs

@@ -1,4 +1,5 @@
-using FluentValidation;
+using System.Text.RegularExpressions;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Features;
 
@@ -9,18 +10,24 @@ public record CreateFeatureRequest(
     string? Description
 );
 
-public class CreateFeatureRequestValidator : AbstractValidator<CreateFeatureRequest>
+public class CreateFeatureRequestValidator : IModelValidator<CreateFeatureRequest>
 {
-    public CreateFeatureRequestValidator()
-    {
-        RuleFor(x => x.Name)
-            .NotEmpty()
-            .MaximumLength(150)
-            .Matches("^[a-zA-Z0-9_-]+$")
-            .WithMessage("Name may only contain letters, digits, underscores, and hyphens.");
+    private static readonly Regex NamePattern = new("^[a-zA-Z0-9_-]+$");
 
-        RuleFor(x => x.InitialValue)
-            .MaximumLength(20_000)
-            .When(x => x.InitialValue is not null);
+    public ValidationResult Validate(CreateFeatureRequest model)
+    {
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 150)
+            result.AddError(nameof(model.Name), "'Name' must be 150 characters or fewer.");
+        else if (!NamePattern.IsMatch(model.Name))
+            result.AddError(nameof(model.Name), "Name may only contain letters, digits, underscores, and hyphens.");
+
+        if (model.InitialValue is not null && model.InitialValue.Length > 20_000)
+            result.AddError(nameof(model.InitialValue), "'Initial Value' must be 20000 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Features/CreateTagRequest.cs

@@ -1,16 +1,30 @@
-using FluentValidation;
+using System.Text.RegularExpressions;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Features;
 
 public record CreateTagRequest(string Label, string Color);
 
-public class CreateTagRequestValidator : AbstractValidator<CreateTagRequest>
+public class CreateTagRequestValidator : IModelValidator<CreateTagRequest>
 {
-    public CreateTagRequestValidator()
+    private static readonly Regex ColorPattern = new("^#[0-9A-Fa-f]{3,6}$");
+
+    public ValidationResult Validate(CreateTagRequest model)
     {
-        RuleFor(x => x.Label).NotEmpty().MaximumLength(100);
-        RuleFor(x => x.Color).NotEmpty().MaximumLength(20)
-            .Matches("^#[0-9A-Fa-f]{3,6}$")
-            .WithMessage("Color must be a valid hex color (e.g. #FF0000).");
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Label))
+            result.AddError(nameof(model.Label), "'Label' must not be empty.");
+        else if (model.Label.Length > 100)
+            result.AddError(nameof(model.Label), "'Label' must be 100 characters or fewer.");
+
+        if (string.IsNullOrEmpty(model.Color))
+            result.AddError(nameof(model.Color), "'Color' must not be empty.");
+        else if (model.Color.Length > 20)
+            result.AddError(nameof(model.Color), "'Color' must be 20 characters or fewer.");
+        else if (!ColorPattern.IsMatch(model.Color))
+            result.AddError(nameof(model.Color), "Color must be a valid hex color (e.g. #FF0000).");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Features/FeatureService.cs

@@ -23,13 +23,7 @@ public class FeatureService(IMicCheckDbContext db, AuditService auditService, We
         return await db.Features.Include(f => f.Tags).FirstOrDefaultAsync(f => f.Id == id, ct);
     }
 
-    public async Task<Feature> CreateAsync(
-        int projectId,
-        string name,
-        FeatureType type,
-        string? initialValue,
-        string? description,
-        CancellationToken ct = default)
+    public async Task<Feature> CreateAsync(int projectId, string name, FeatureType type, string? initialValue, string? description, CancellationToken ct = default)
     {
         var count = await db.Features.CountAsync(f => f.ProjectId == projectId, ct);
         if (count >= MaxFeaturesPerProject)
@@ -78,8 +72,7 @@ public class FeatureService(IMicCheckDbContext db, AuditService auditService, We
         return feature;
     }
 
-    public async Task<Feature> UpdateAsync(
-        int id, string name, string? description, CancellationToken ct = default)
+    public async Task<Feature> UpdateAsync(int id, string name, string? description, CancellationToken ct = default)
     {
         var feature = await db.Features.Include(f => f.Tags).FirstOrDefaultAsync(f => f.Id == id, ct)
             ?? throw new KeyNotFoundException($"Feature {id} not found.");
@@ -162,11 +155,7 @@ public class FeatureService(IMicCheckDbContext db, AuditService auditService, We
                     EventType = WebhookEventTypes.FlagDeleted,
                     EnvironmentId = env.Id,
                     OrganizationId = project.OrganizationId,
-                    Data = new FlagDeletedData(
-                        null,
-                        DateTimeOffset.UtcNow,
-                        new FeatureSummary(feature.Id, feature.Name))
-                });
+                    Data = new FlagDeletedData(null, DateTimeOffset.UtcNow, new FeatureSummary(feature.Id, feature.Name)) });
             }
         }
     }

src/api/MicCheck.Api/Features/UpdateFeatureRequest.cs

@@ -1,4 +1,5 @@
-using FluentValidation;
+using System.Text.RegularExpressions;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Features;
 
@@ -7,14 +8,21 @@ public record UpdateFeatureRequest(
     string? Description
 );
 
-public class UpdateFeatureRequestValidator : AbstractValidator<UpdateFeatureRequest>
+public class UpdateFeatureRequestValidator : IModelValidator<UpdateFeatureRequest>
 {
-    public UpdateFeatureRequestValidator()
+    private static readonly Regex NamePattern = new("^[a-zA-Z0-9_-]+$");
+
+    public ValidationResult Validate(UpdateFeatureRequest model)
     {
-        RuleFor(x => x.Name)
-            .NotEmpty()
-            .MaximumLength(150)
-            .Matches("^[a-zA-Z0-9_-]+$")
-            .WithMessage("Name may only contain letters, digits, underscores, and hyphens.");
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 150)
+            result.AddError(nameof(model.Name), "'Name' must be 150 characters or fewer.");
+        else if (!NamePattern.IsMatch(model.Name))
+            result.AddError(nameof(model.Name), "Name may only contain letters, digits, underscores, and hyphens.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/MicCheck.Api.csproj

@@ -9,7 +9,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="FluentValidation.AspNetCore" Version="11.3.0" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.5" />
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.5" />
     <PackageReference Include="Microsoft.OpenApi" Version="2.9.0" />

src/api/MicCheck.Api/Organizations/CreateOrganizationRequest.cs

@@ -1,13 +1,20 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Organizations;
 
 public record CreateOrganizationRequest(string Name);
 
-public class CreateOrganizationRequestValidator : AbstractValidator<CreateOrganizationRequest>
+public class CreateOrganizationRequestValidator : IModelValidator<CreateOrganizationRequest>
 {
-    public CreateOrganizationRequestValidator()
+    public ValidationResult Validate(CreateOrganizationRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Organizations/InviteUserRequest.cs

@@ -1,15 +1,23 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Organizations;
 
 public record InviteUserRequest(int UserId, string Role);
 
-public class InviteUserRequestValidator : AbstractValidator<InviteUserRequest>
+public class InviteUserRequestValidator : IModelValidator<InviteUserRequest>
 {
-    public InviteUserRequestValidator()
+    public ValidationResult Validate(InviteUserRequest model)
     {
-        RuleFor(x => x.UserId).GreaterThan(0);
-        RuleFor(x => x.Role).NotEmpty().Must(r => Enum.TryParse<OrganizationRole>(r, true, out _))
-            .WithMessage("Role must be 'User' or 'Admin'.");
+        var result = new ValidationResult();
+
+        if (model.UserId <= 0)
+            result.AddError(nameof(model.UserId), "'User Id' must be greater than 0.");
+
+        if (string.IsNullOrEmpty(model.Role))
+            result.AddError(nameof(model.Role), "'Role' must not be empty.");
+        else if (!Enum.TryParse<OrganizationRole>(model.Role, true, out _))
+            result.AddError(nameof(model.Role), "Role must be 'User' or 'Admin'.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Organizations/UpdateOrganizationRequest.cs

@@ -1,13 +1,20 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Organizations;
 
 public record UpdateOrganizationRequest(string Name);
 
-public class UpdateOrganizationRequestValidator : AbstractValidator<UpdateOrganizationRequest>
+public class UpdateOrganizationRequestValidator : IModelValidator<UpdateOrganizationRequest>
 {
-    public UpdateOrganizationRequestValidator()
+    public ValidationResult Validate(UpdateOrganizationRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Program.cs

@@ -1,11 +1,10 @@
 using System.Text;
 using System.Threading.RateLimiting;
-using FluentValidation;
-using FluentValidation.AspNetCore;
 using MicCheck.Api.Common.Security.ApiKeys;
 using MicCheck.Api.Audit;
 using MicCheck.Api.Common.Security.Authentication;
 using MicCheck.Api.Common.Security.Authorization;
+using MicCheck.Api.Common.Validation;
 using MicCheck.Api.Data;
 using MicCheck.Api.Environments;
 using MicCheck.Api.Features;
@@ -42,7 +41,7 @@ try
               .WriteTo.Console());
 
     builder.Services.AddOpenApi();
-    builder.Services.AddControllers()
+    builder.Services.AddControllers(options => options.Filters.Add<ModelValidationActionFilter>())
         .AddJsonOptions(options =>
             options.JsonSerializerOptions.Converters.Add(
                 new System.Text.Json.Serialization.JsonStringEnumConverter()));
@@ -92,20 +91,11 @@ try
             limiter.QueueLimit = 0;
         }));
 
-    builder.Services.AddFluentValidationAutoValidation();
-    builder.Services.AddValidatorsFromAssemblyContaining<Program>();
+    builder.Services.AddModelValidatorsFromAssemblyContaining<Program>();
 
     builder.Services.Configure<ApiBehaviorOptions>(options =>
     {
-        options.InvalidModelStateResponseFactory = context =>
-        {
-            var errors = context.ModelState
-                .Where(e => e.Value?.Errors.Count > 0)
-                .ToDictionary(
-                    kvp => kvp.Key,
-                    kvp => kvp.Value!.Errors.Select(e => e.ErrorMessage).ToArray());
-            return new UnprocessableEntityObjectResult(new { errors });
-        };
+        options.InvalidModelStateResponseFactory = context => ValidationProblemResponseFactory.Create(context.ModelState);
     });
 
     builder.Services.AddMemoryCache();

src/api/MicCheck.Api/Projects/CreateProjectRequest.cs

@@ -1,14 +1,23 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Projects;
 
 public record CreateProjectRequest(string Name, int OrganizationId);
 
-public class CreateProjectRequestValidator : AbstractValidator<CreateProjectRequest>
+public class CreateProjectRequestValidator : IModelValidator<CreateProjectRequest>
 {
-    public CreateProjectRequestValidator()
+    public ValidationResult Validate(CreateProjectRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
-        RuleFor(x => x.OrganizationId).GreaterThan(0);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        if (model.OrganizationId <= 0)
+            result.AddError(nameof(model.OrganizationId), "'Organization Id' must be greater than 0.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Projects/SetUserPermissionsRequest.cs

@@ -1,17 +1,25 @@
-using FluentValidation;
-using MicCheck.Api.Common.Security.Authorization;
-
-namespace MicCheck.Api.Projects;
-
-public record SetUserPermissionsRequest(int UserId, bool IsAdmin, List<string> Permissions);
-
-public class SetUserPermissionsRequestValidator : AbstractValidator<SetUserPermissionsRequest>
-{
-    public SetUserPermissionsRequestValidator()
-    {
-        RuleFor(x => x.UserId).GreaterThan(0);
-        RuleForEach(x => x.Permissions)
-            .Must(p => Enum.TryParse<ProjectPermission>(p, true, out _))
-            .WithMessage("Invalid permission value.");
-    }
-}
+using MicCheck.Api.Common.Security.Authorization;
+using MicCheck.Api.Common.Validation;
+
+namespace MicCheck.Api.Projects;
+
+public record SetUserPermissionsRequest(int UserId, bool IsAdmin, List<string> Permissions);
+
+public class SetUserPermissionsRequestValidator : IModelValidator<SetUserPermissionsRequest>
+{
+    public ValidationResult Validate(SetUserPermissionsRequest model)
+    {
+        var result = new ValidationResult();
+
+        if (model.UserId <= 0)
+            result.AddError(nameof(model.UserId), "'User Id' must be greater than 0.");
+
+        foreach (var permission in model.Permissions)
+        {
+            if (!Enum.TryParse<ProjectPermission>(permission, true, out _))
+                result.AddError(nameof(model.Permissions), "Invalid permission value.");
+        }
+
+        return result;
+    }
+}

src/api/MicCheck.Api/Projects/UpdateProjectRequest.cs

@@ -1,13 +1,20 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Projects;
 
 public record UpdateProjectRequest(string Name, bool HideDisabledFlags);
 
-public class UpdateProjectRequestValidator : AbstractValidator<UpdateProjectRequest>
+public class UpdateProjectRequestValidator : IModelValidator<UpdateProjectRequest>
 {
-    public UpdateProjectRequestValidator()
+    public ValidationResult Validate(UpdateProjectRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        return result;
     }
 }

src/api/MicCheck.Api/Segments/CreateSegmentRequest.cs

@@ -1,4 +1,4 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Segments;
 
@@ -16,24 +16,38 @@ public record CreateSegmentRequest(
     string Name,
     IReadOnlyList<CreateSegmentRuleRequest> Rules);
 
-public class CreateSegmentRequestValidator : AbstractValidator<CreateSegmentRequest>
+public class CreateSegmentRequestValidator : IModelValidator<CreateSegmentRequest>
 {
-    public CreateSegmentRequestValidator()
+    public ValidationResult Validate(CreateSegmentRequest model)
     {
-        RuleFor(x => x.Name).NotEmpty().MaximumLength(200);
-        RuleFor(x => x.Rules).NotNull();
-        RuleForEach(x => x.Rules).ChildRules(rule =>
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Name))
+            result.AddError(nameof(model.Name), "'Name' must not be empty.");
+        else if (model.Name.Length > 200)
+            result.AddError(nameof(model.Name), "'Name' must be 200 characters or fewer.");
+
+        if (model.Rules is null)
         {
-            rule.RuleFor(r => r.Type)
-                .Must(t => Enum.TryParse<SegmentRuleType>(t, true, out _))
-                .WithMessage("Rule type must be 'All', 'Any', or 'None'.");
-            rule.RuleForEach(r => r.Conditions).ChildRules(cond =>
+            result.AddError(nameof(model.Rules), "'Rules' must not be empty.");
+            return result;
+        }
+
+        foreach (var rule in model.Rules)
+        {
+            if (!Enum.TryParse<SegmentRuleType>(rule.Type, true, out _))
+                result.AddError(nameof(model.Rules), "Rule type must be 'All', 'Any', or 'None'.");
+
+            foreach (var condition in rule.Conditions)
             {
-                cond.RuleFor(c => c.Property).NotEmpty();
-                cond.RuleFor(c => c.Operator)
-                    .Must(o => Enum.TryParse<SegmentConditionOperator>(o, true, out _))
-                    .WithMessage("Invalid operator.");
-            });
-        });
+                if (string.IsNullOrEmpty(condition.Property))
+                    result.AddError(nameof(model.Rules), "'Property' must not be empty.");
+
+                if (!Enum.TryParse<SegmentConditionOperator>(condition.Operator, true, out _))
+                    result.AddError(nameof(model.Rules), "Invalid operator.");
+            }
+        }
+
+        return result;
     }
 }

src/api/MicCheck.Api/Webhooks/CreateWebhookRequest.cs

@@ -1,14 +1,22 @@
-using FluentValidation;
+using MicCheck.Api.Common.Validation;
 
 namespace MicCheck.Api.Webhooks;
 
 public record CreateWebhookRequest(string Url, string? Secret, bool Enabled);
 
-public class CreateWebhookRequestValidator : AbstractValidator<CreateWebhookRequest>
+public class CreateWebhookRequestValidator : IModelValidator<CreateWebhookRequest>
 {
-    public CreateWebhookRequestValidator()
+    public ValidationResult Validate(CreateWebhookRequest model)
     {
-        RuleFor(x => x.Url).NotEmpty().MaximumLength(500).Must(u => Uri.TryCreate(u, UriKind.Absolute, out _))
-            .WithMessage("Url must be a valid absolute URL.");
+        var result = new ValidationResult();
+
+        if (string.IsNullOrEmpty(model.Url))
+            result.AddError(nameof(model.Url), "'Url' must not be empty.");
+        else if (model.Url.Length > 500)
+            result.AddError(nameof(model.Url), "'Url' must be 500 characters or fewer.");
+        else if (!Uri.TryCreate(model.Url, UriKind.Absolute, out _))
+            result.AddError(nameof(model.Url), "Url must be a valid absolute URL.");
+
+        return result;
     }
 }