diff --git a/test/ApiFirstMediatR.Generator.Tests/ApiFirstMediatR.Generator.Tests.csproj b/test/ApiFirstMediatR.Generator.Tests/ApiFirstMediatR.Generator.Tests.csproj
index 789ecf5..a5092c7 100644
--- a/test/ApiFirstMediatR.Generator.Tests/ApiFirstMediatR.Generator.Tests.csproj
+++ b/test/ApiFirstMediatR.Generator.Tests/ApiFirstMediatR.Generator.Tests.csproj
@@ -16,7 +16,7 @@
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Analyzer.Testing.XUnit" Version="1.1.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" />
     <PackageReference Include="Microsoft.OpenApi" Version="1.6.3" />
-    <PackageReference Include="Moq" Version="4.18.4" />
+    <PackageReference Include="NSubstitute" Version="5.0.0" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
diff --git a/test/ApiFirstMediatR.Generator.Tests/GlobalUsings.cs b/test/ApiFirstMediatR.Generator.Tests/GlobalUsings.cs
index b7432eb..0f81663 100644
--- a/test/ApiFirstMediatR.Generator.Tests/GlobalUsings.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/GlobalUsings.cs
@@ -9,7 +9,6 @@
 global using ApiFirstMediatR.Generator.Models;
 global using ApiFirstMediatR.Generator.Models.Config;
 global using ApiFirstMediatR.Generator.Repositories;
-global using ApiFirstMediatR.Generator.Services;
 global using ApiFirstMediatR.Generator.Tests.Assertions;
 global using ApiFirstMediatR.Generator.Tests.Utils;
 global using FluentAssertions;
@@ -19,5 +18,5 @@
 global using Microsoft.OpenApi;
 global using Microsoft.OpenApi.Expressions;
 global using Microsoft.OpenApi.Models;
-global using Moq;
+global using NSubstitute;
 global using Xunit;
\ No newline at end of file
diff --git a/test/ApiFirstMediatR.Generator.Tests/Mappers/ControllerMapperTests.cs b/test/ApiFirstMediatR.Generator.Tests/Mappers/ControllerMapperTests.cs
index ef7cdc4..21cc4b8 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Mappers/ControllerMapperTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Mappers/ControllerMapperTests.cs
@@ -7,22 +7,22 @@ public class ControllerMapperTests
     public ControllerMapperTests()
     {
         var mockApiConfigRepo = MockApiConfig.Create();
-        var mockOperationNamingRepository = new Mock<IOperationNamingRepository>();
+        var mockOperationNamingRepository = Substitute.For<IOperationNamingRepository>();
         mockOperationNamingRepository
-            .Setup(mock => mock.GetControllerNameByPath("/test"))
+            .GetControllerNameByPath("/test")
             .Returns("Test");
 
-        var mockDiagnosticReporter = new Mock<IDiagnosticReporter>();
+        var mockDiagnosticReporter = Substitute.For<IDiagnosticReporter>();
         var typeMapper = new TypeMapper(mockApiConfigRepo);
         var parameterMapper = new ParameterMapper(typeMapper);
-        var responseMapper = new ResponseMapper(typeMapper, mockOperationNamingRepository.Object);
-        var securityMapper = new SecurityMapper(mockDiagnosticReporter.Object);
-        var mockApiConfigRepository = new Mock<IApiConfigRepository>();
+        var responseMapper = new ResponseMapper(typeMapper, mockOperationNamingRepository);
+        var securityMapper = new SecurityMapper(mockDiagnosticReporter);
+        var mockApiConfigRepository = Substitute.For<IApiConfigRepository>();
 
         var endpointMapper = new EndpointMapper(parameterMapper, responseMapper, typeMapper, securityMapper,
-            mockOperationNamingRepository.Object, mockApiConfigRepository.Object, mockDiagnosticReporter.Object);
+            mockOperationNamingRepository, mockApiConfigRepository, mockDiagnosticReporter);
 
-        _controllerMapper = new ControllerMapper(endpointMapper, mockOperationNamingRepository.Object);
+        _controllerMapper = new ControllerMapper(endpointMapper, mockOperationNamingRepository);
     }
 
     [Fact]
diff --git a/test/ApiFirstMediatR.Generator.Tests/Mappers/EndpointMapperTests.cs b/test/ApiFirstMediatR.Generator.Tests/Mappers/EndpointMapperTests.cs
index 421ff90..efdeece 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Mappers/EndpointMapperTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Mappers/EndpointMapperTests.cs
@@ -3,33 +3,33 @@ namespace ApiFirstMediatR.Generator.Tests.Mappers;
 public class EndpointMapperTests
 {
     private readonly IEndpointMapper _endpointMapper;
-    private readonly Mock<IDiagnosticReporter> _mockDiagnosticReporter;
+    private readonly IDiagnosticReporter _mockDiagnosticReporter;
 
     public EndpointMapperTests()
     {
         var mockApiConfigRepo = MockApiConfig.Create();
-        var mockOperationNamingRepository = new Mock<IOperationNamingRepository>();
+        var mockOperationNamingRepository = Substitute.For<IOperationNamingRepository>();
         mockOperationNamingRepository
-            .Setup(mock => mock.GetControllerNameByOperationId("TestOperation"))
+            .GetControllerNameByOperationId("TestOperation")
             .Returns("TestController");
 
         mockOperationNamingRepository
-            .Setup(mock => mock.GetOperationNameByOperationId("TestOperation"))
+            .GetOperationNameByOperationId("TestOperation")
             .Returns("TestOperation");
 
         mockOperationNamingRepository
-            .Setup(mock => mock.GetOperationNameByPathAndOperationType("/test", OperationType.Get))
+            .GetOperationNameByPathAndOperationType("/test", OperationType.Get)
             .Returns("TestOperation");
 
-        _mockDiagnosticReporter = new Mock<IDiagnosticReporter>();
+        _mockDiagnosticReporter = Substitute.For<IDiagnosticReporter>();
         var typeMapper = new TypeMapper(mockApiConfigRepo);
         var parameterMapper = new ParameterMapper(typeMapper);
-        var responseMapper = new ResponseMapper(typeMapper, mockOperationNamingRepository.Object);
-        var securityMapper = new SecurityMapper(_mockDiagnosticReporter.Object);
-        var mockApiConfigRepository = new Mock<IApiConfigRepository>();
+        var responseMapper = new ResponseMapper(typeMapper, mockOperationNamingRepository);
+        var securityMapper = new SecurityMapper(_mockDiagnosticReporter);
+        var mockApiConfigRepository = Substitute.For<IApiConfigRepository>();
 
         _endpointMapper = new EndpointMapper(parameterMapper, responseMapper, typeMapper, securityMapper,
-            mockOperationNamingRepository.Object, mockApiConfigRepository.Object, _mockDiagnosticReporter.Object);
+            mockOperationNamingRepository, mockApiConfigRepository, _mockDiagnosticReporter);
     }
 
     [Fact]
@@ -195,8 +195,7 @@ public void InvalidRequestBody_ThrowsDiagnostic()
         
         var endpoints = _endpointMapper.Map(paths);
         endpoints.Should().BeEmpty();
-        _mockDiagnosticReporter.Verify(m => m.ReportDiagnostic(It.IsAny<Diagnostic>()));
-        _mockDiagnosticReporter.VerifyNoOtherCalls();
+        _mockDiagnosticReporter.Received(1).ReportDiagnostic(Arg.Any<Diagnostic>());
     }
 
     [Fact]
diff --git a/test/ApiFirstMediatR.Generator.Tests/Mappers/ResponseMapperTests.cs b/test/ApiFirstMediatR.Generator.Tests/Mappers/ResponseMapperTests.cs
index 891d80e..693f74f 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Mappers/ResponseMapperTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Mappers/ResponseMapperTests.cs
@@ -7,17 +7,17 @@ public class ResponseMapperTests
     public ResponseMapperTests()
     {
         var mockApiConfigRepo = MockApiConfig.Create();
-        var mockOperationNamingRepository = new Mock<IOperationNamingRepository>();
+        var mockOperationNamingRepository = Substitute.For<IOperationNamingRepository>();
         mockOperationNamingRepository
-            .Setup(mock => mock.GetControllerNameByOperationId("TestOperation"))
+            .GetControllerNameByOperationId("TestOperation")
             .Returns("TestController");
 
         mockOperationNamingRepository
-            .Setup(mock => mock.GetOperationNameByOperationId("TestOperation"))
+            .GetOperationNameByOperationId("TestOperation")
             .Returns("TestOperation");
 
         var typeMapper = new TypeMapper(mockApiConfigRepo);
-        var operationNamingRepository = mockOperationNamingRepository.Object;
+        var operationNamingRepository = mockOperationNamingRepository;
         
         _responseMapper = new ResponseMapper(typeMapper, operationNamingRepository);
     }
diff --git a/test/ApiFirstMediatR.Generator.Tests/Mappers/SecurityMapperTests.cs b/test/ApiFirstMediatR.Generator.Tests/Mappers/SecurityMapperTests.cs
index f3d0351..0ee4b44 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Mappers/SecurityMapperTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Mappers/SecurityMapperTests.cs
@@ -3,13 +3,13 @@
 public class SecurityMapperTests
 {
     private readonly ISecurityMapper _securityMapper;
-    private readonly Mock<IDiagnosticReporter> _mockDiagnosticReporter;
+    private readonly IDiagnosticReporter _mockDiagnosticReporter;
     
     public SecurityMapperTests()
     {
-        _mockDiagnosticReporter = new Mock<IDiagnosticReporter>();
+        _mockDiagnosticReporter = Substitute.For<IDiagnosticReporter>();
 
-        _securityMapper = new SecurityMapper(_mockDiagnosticReporter.Object);
+        _securityMapper = new SecurityMapper(_mockDiagnosticReporter);
     }
 
     [Fact]
@@ -75,6 +75,6 @@ public void Unsupported_ThrowsDiagnostic()
 
         security.Policies.Should().NotBeNull();
         security.Policies.Should().BeEmpty();
-        _mockDiagnosticReporter.Verify(m => m.ReportDiagnostic(It.IsAny<Diagnostic>()));
+        _mockDiagnosticReporter.Received(1).ReportDiagnostic(Arg.Any<Diagnostic>());
     }
 }
\ No newline at end of file
diff --git a/test/ApiFirstMediatR.Generator.Tests/Repositories/ApiConfigRepositoryTests.cs b/test/ApiFirstMediatR.Generator.Tests/Repositories/ApiConfigRepositoryTests.cs
index c2b14a4..916bd01 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Repositories/ApiConfigRepositoryTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Repositories/ApiConfigRepositoryTests.cs
@@ -5,27 +5,27 @@ namespace ApiFirstMediatR.Generator.Tests.Repositories;
 public class ApiConfigRepositoryTests
 {
     private readonly ApiConfigRepository _apiConfigRepository;
-    private readonly Mock<AnalyzerConfigOptions> _mockAnalyzerConfigOptions;
-    private readonly Mock<ICompilation> _mockICompilation;
-    private readonly Mock<IDiagnosticReporter> _mockDiagnosticReporter;
+    private readonly AnalyzerConfigOptions _mockAnalyzerConfigOptions;
+    private readonly ICompilation _mockICompilation;
+    private readonly IDiagnosticReporter _mockDiagnosticReporter;
     
     public ApiConfigRepositoryTests()
     {
-        _mockAnalyzerConfigOptions = new Mock<AnalyzerConfigOptions>();
+        _mockAnalyzerConfigOptions = Substitute.For<AnalyzerConfigOptions>();
 
-        var mockAnalyzerConfigOptionsProvider = new Mock<AnalyzerConfigOptionsProvider>();
-        mockAnalyzerConfigOptionsProvider
-            .Setup(m => m.GlobalOptions)
-            .Returns(_mockAnalyzerConfigOptions.Object);
+        var analyzerConfigOptionsProvider = Substitute.For<AnalyzerConfigOptionsProvider>();
+        analyzerConfigOptionsProvider
+            .GlobalOptions
+            .Returns(_mockAnalyzerConfigOptions);
 
-        _mockICompilation = new Mock<ICompilation>();
+        _mockICompilation = Substitute.For<ICompilation>();
         _mockICompilation
-            .Setup(m => m.AnalyzerConfigOptions)
-            .Returns(mockAnalyzerConfigOptionsProvider.Object);
+            .AnalyzerConfigOptions
+            .Returns(analyzerConfigOptionsProvider);
         
-        _mockDiagnosticReporter = new Mock<IDiagnosticReporter>();
+        _mockDiagnosticReporter = Substitute.For<IDiagnosticReporter>();
         
-        _apiConfigRepository = new ApiConfigRepository(_mockICompilation.Object, _mockDiagnosticReporter.Object);
+        _apiConfigRepository = new ApiConfigRepository(_mockICompilation, _mockDiagnosticReporter);
     }
     
     [Fact]
@@ -33,7 +33,7 @@ public void NoConfigValuesSet_HappyPath()
     {
         var compilation = CSharpCompilation.Create(null);
         _mockICompilation
-            .Setup(m => m.Compilation)
+            .Compilation
             .Returns(compilation);
 
         var apiConfig = _apiConfigRepository.Get();
@@ -50,12 +50,16 @@ public void ConfigValuesSet_HappyPath()
     {
         var jsonLibrary = "Newtonsoft.Json";
         _mockAnalyzerConfigOptions
-            .Setup(m => m.TryGetValue("build_property.ApiFirstMediatR_SerializationLibrary", out jsonLibrary))
-            .Returns(true);
+            .TryGetValue("build_property.ApiFirstMediatR_SerializationLibrary", out Arg.Any<string>()!)
+            .Returns(x =>
+            {
+                x[1] = jsonLibrary; // This is the out value
+                return true;
+            });
         
         var compilation = CSharpCompilation.Create("ApiFirstChanged");
         _mockICompilation
-            .Setup(m => m.Compilation)
+            .Compilation
             .Returns(compilation);
 
         var apiConfig = _apiConfigRepository.Get();
@@ -72,12 +76,16 @@ public void InvalidSerializationLibrary_ThrowsDiagnostic()
     {
         var jsonLibrary = "BadJsonSerializer";
         _mockAnalyzerConfigOptions
-            .Setup(m => m.TryGetValue("build_property.ApiFirstMediatR_SerializationLibrary", out jsonLibrary))
-            .Returns(true);
+            .TryGetValue("build_property.ApiFirstMediatR_SerializationLibrary", out Arg.Any<string>()!)
+            .Returns(x =>
+            {
+                x[1] = jsonLibrary; // This is the out value
+                return true;
+            });
         
         var compilation = CSharpCompilation.Create(null);
         _mockICompilation
-            .Setup(m => m.Compilation)
+            .Compilation
             .Returns(compilation);
 
         var apiConfig = _apiConfigRepository.Get();
@@ -87,8 +95,7 @@ public void InvalidSerializationLibrary_ThrowsDiagnostic()
         
         apiConfig.SerializationLibrary
             .Should().Be(SerializationLibrary.SystemTextJson);
-        
-        _mockDiagnosticReporter.Verify(m => m.ReportDiagnostic(It.IsAny<Diagnostic>()));
-        _mockDiagnosticReporter.VerifyNoOtherCalls();
+
+        _mockDiagnosticReporter.Received(1).ReportDiagnostic(Arg.Any<Diagnostic>());
     }
 }
\ No newline at end of file
diff --git a/test/ApiFirstMediatR.Generator.Tests/Repositories/OperationNamingRepositoryTests.cs b/test/ApiFirstMediatR.Generator.Tests/Repositories/OperationNamingRepositoryTests.cs
index d985f80..286750f 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Repositories/OperationNamingRepositoryTests.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Repositories/OperationNamingRepositoryTests.cs
@@ -5,9 +5,9 @@ public class OperationNamingRepositoryTests
     [Fact]
     public void ValidAPISpec_OperationId_HappyPath()
     {
-        var mockApiSpecRepository = new Mock<IApiSpecRepository>();
+        var mockApiSpecRepository = Substitute.For<IApiSpecRepository>();
         mockApiSpecRepository
-            .Setup(mock => mock.Get())
+            .Get()
             .Returns(new OpenApiDocument
             {
                 Paths = new OpenApiPaths
@@ -33,7 +33,7 @@ public void ValidAPISpec_OperationId_HappyPath()
                 }
             });
 
-        var operationNamingRepository = new OperationNamingRepository(mockApiSpecRepository.Object);
+        var operationNamingRepository = new OperationNamingRepository(mockApiSpecRepository);
         
         const string expectedControllerName = "Api";
         const string expectedOperationName = "GetHelloWorld";
diff --git a/test/ApiFirstMediatR.Generator.Tests/Utils/MockApiConfig.cs b/test/ApiFirstMediatR.Generator.Tests/Utils/MockApiConfig.cs
index d6a5e57..aab8de8 100644
--- a/test/ApiFirstMediatR.Generator.Tests/Utils/MockApiConfig.cs
+++ b/test/ApiFirstMediatR.Generator.Tests/Utils/MockApiConfig.cs
@@ -4,9 +4,9 @@ internal static class MockApiConfig
 {
     public static IApiConfigRepository Create(string namespaceName = "Test")
     {
-        var mockApiConfigRepo = new Mock<IApiConfigRepository>();
+        var mockApiConfigRepo = Substitute.For<IApiConfigRepository>();
         mockApiConfigRepo
-            .Setup(mock => mock.Get())
+            .Get()
             .Returns(new ApiConfig
             {
                 Namespace = namespaceName,
@@ -14,6 +14,6 @@ public static IApiConfigRepository Create(string namespaceName = "Test")
                 RequestBodyName = "Body"
             });
 
-        return mockApiConfigRepo.Object;
+        return mockApiConfigRepo;
     }
 }
\ No newline at end of file