diff --git a/Directory.Packages.props b/Directory.Packages.props
index 6beb132e85..a84164d03e 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -9,7 +9,6 @@
     <PackageVersion Include="DataGridExtensions" Version="2.6.0" />
     <PackageVersion Include="DiffLib" Version="2025.0.0" />
     <PackageVersion Include="Dirkster.AvalonDock.Themes.VS2013" Version="4.72.1" />
-    <PackageVersion Include="FluentAssertions" Version="7.0.0" /><!-- License change from 7 to 8, we need to stay on 7 -->
     <PackageVersion Include="ILCompiler.Reflection.ReadyToRun.Experimental" Version="9.0.1-rtm.24557.9" />
     <PackageVersion Include="Iced" Version="1.21.0" />
     <PackageVersion Include="JunitXml.TestLogger" Version="5.0.0" />
@@ -39,6 +38,7 @@
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="NuGet.Protocol" Version="6.12.1" />
     <PackageVersion Include="PowerShellStandard.Library" Version="5.1.1" />
+    <PackageVersion Include="Shouldly" Version="4.2.1" />
     <PackageVersion Include="System.Composition.AttributedModel" Version="9.0.1" />
     <PackageVersion Include="System.Collections.Immutable" Version="9.0.1" />
     <PackageVersion Include="System.Memory" Version="4.6.0" />
diff --git a/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj b/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
index 3f60f45482..305d22d969 100644
--- a/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
+++ b/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
@@ -67,7 +67,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="NUnit" />
-    <PackageReference Include="FluentAssertions" />
+    <PackageReference Include="Shouldly" />
     <PackageReference Include="JunitXml.TestLogger" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="System.Memory" />
diff --git a/ILSpy.BamlDecompiler.Tests/ILSpy.BamlDecompiler.Tests.csproj b/ILSpy.BamlDecompiler.Tests/ILSpy.BamlDecompiler.Tests.csproj
index d92e79c75d..849406f392 100644
--- a/ILSpy.BamlDecompiler.Tests/ILSpy.BamlDecompiler.Tests.csproj
+++ b/ILSpy.BamlDecompiler.Tests/ILSpy.BamlDecompiler.Tests.csproj
@@ -39,7 +39,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="NUnit" />
     <PackageReference Include="NUnit3TestAdapter" />
-    <PackageReference Include="FluentAssertions" />
+    <PackageReference Include="Shouldly" />
     <PackageReference Include="JunitXml.TestLogger" />
     <PackageReference Include="coverlet.collector">
       <PrivateAssets>all</PrivateAssets>
diff --git a/ILSpy.Tests/CommandLineArgumentsTests.cs b/ILSpy.Tests/CommandLineArgumentsTests.cs
index 20704250ce..eb99f59fe2 100644
--- a/ILSpy.Tests/CommandLineArgumentsTests.cs
+++ b/ILSpy.Tests/CommandLineArgumentsTests.cs
@@ -1,6 +1,6 @@
 using System;
 
-using FluentAssertions;
+using Shouldly;
 
 using ICSharpCode.ILSpy.AppEnv;
 
@@ -16,27 +16,28 @@ public void VerifyEmptyArgumentsArray()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { });
 
-			cmdLineArgs.AssembliesToLoad.Should().BeEmpty();
-			cmdLineArgs.SingleInstance.Should().BeNull();
-			cmdLineArgs.NavigateTo.Should().BeNull();
-			cmdLineArgs.Search.Should().BeNull();
-			cmdLineArgs.Language.Should().BeNull();
-			cmdLineArgs.NoActivate.Should().BeFalse();
-			cmdLineArgs.ConfigFile.Should().BeNull();
+			cmdLineArgs.AssembliesToLoad.ShouldBeEmpty();
+			cmdLineArgs.SingleInstance.ShouldBeNull();
+			cmdLineArgs.NavigateTo.ShouldBeNull();
+			cmdLineArgs.Search.ShouldBeNull();
+			cmdLineArgs.Language.ShouldBeNull();
+			cmdLineArgs.NoActivate.ShouldBeFalse();
+			cmdLineArgs.ConfigFile.ShouldBeNull();
 		}
 
 		[Test]
 		public void VerifyHelpOption()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--help" });
-			cmdLineArgs.ArgumentsParser.IsShowingInformation.Should().BeTrue();
+			cmdLineArgs.ArgumentsParser.IsShowingInformation.ShouldBeTrue();
 		}
 
 		[Test]
 		public void VerifyForceNewInstanceOption()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--newinstance" });
-			cmdLineArgs.SingleInstance.Should().BeFalse();
+			cmdLineArgs.SingleInstance.ShouldNotBeNull();
+			cmdLineArgs.SingleInstance.Value.ShouldBeFalse();
 		}
 
 		[Test]
@@ -44,14 +45,14 @@ public void VerifyNavigateToOption()
 		{
 			const string navigateTo = "MyNamespace.MyClass";
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--navigateto", navigateTo });
-			cmdLineArgs.NavigateTo.Should().BeEquivalentTo(navigateTo);
+			cmdLineArgs.NavigateTo.ShouldBe(navigateTo);
 		}
 
 		[Test]
 		public void VerifyNavigateToOption_NoneTest_Matching_VSAddin()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--navigateto:none" });
-			cmdLineArgs.NavigateTo.Should().BeEquivalentTo("none");
+			cmdLineArgs.NavigateTo.ShouldBe("none");
 		}
 
 		[Test]
@@ -59,7 +60,7 @@ public void VerifyCaseSensitivityOfOptionsDoesntThrow()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--navigateTo:none" });
 
-			cmdLineArgs.ArgumentsParser.RemainingArguments.Should().HaveCount(1);
+			cmdLineArgs.ArgumentsParser.RemainingArguments.Count.ShouldBe(1);
 		}
 
 		[Test]
@@ -67,7 +68,7 @@ public void VerifySearchOption()
 		{
 			const string searchWord = "TestContainers";
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--search", searchWord });
-			cmdLineArgs.Search.Should().BeEquivalentTo(searchWord);
+			cmdLineArgs.Search.ShouldBe(searchWord);
 		}
 
 		[Test]
@@ -75,7 +76,7 @@ public void VerifyLanguageOption()
 		{
 			const string language = "csharp";
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--language", language });
-			cmdLineArgs.Language.Should().BeEquivalentTo(language);
+			cmdLineArgs.Language.ShouldBe(language);
 		}
 
 		[Test]
@@ -83,21 +84,21 @@ public void VerifyConfigOption()
 		{
 			const string configFile = "myilspyoptions.xml";
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--config", configFile });
-			cmdLineArgs.ConfigFile.Should().BeEquivalentTo(configFile);
+			cmdLineArgs.ConfigFile.ShouldBe(configFile);
 		}
 
 		[Test]
 		public void VerifyNoActivateOption()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "--noactivate" });
-			cmdLineArgs.NoActivate.Should().BeTrue();
+			cmdLineArgs.NoActivate.ShouldBeTrue();
 		}
 
 		[Test]
 		public void MultipleAssembliesAsArguments()
 		{
 			var cmdLineArgs = CommandLineArguments.Create(new string[] { "assembly1", "assembly2", "assembly3" });
-			cmdLineArgs.AssembliesToLoad.Should().HaveCount(3);
+			cmdLineArgs.AssembliesToLoad.Count.ShouldBe(3);
 		}
 
 		[Test]
@@ -117,9 +118,10 @@ public void PassAtFileArguments()
 			{
 			}
 
-			cmdLineArgs.SingleInstance.Should().BeFalse();
-			cmdLineArgs.NoActivate.Should().BeTrue();
-			cmdLineArgs.AssembliesToLoad.Should().HaveCount(3);
+			cmdLineArgs.SingleInstance.ShouldNotBeNull();
+			cmdLineArgs.SingleInstance.Value.ShouldBeFalse();
+			cmdLineArgs.NoActivate.ShouldBeTrue();
+			cmdLineArgs.AssembliesToLoad.Count.ShouldBe(3);
 		}
 	}
 }
diff --git a/ILSpy.Tests/ILSpy.Tests.csproj b/ILSpy.Tests/ILSpy.Tests.csproj
index 8340c2b46a..7fff3f2eb1 100644
--- a/ILSpy.Tests/ILSpy.Tests.csproj
+++ b/ILSpy.Tests/ILSpy.Tests.csproj
@@ -74,7 +74,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="NUnit" />
-    <PackageReference Include="FluentAssertions" />
+    <PackageReference Include="Shouldly" />
     <PackageReference Include="JunitXml.TestLogger" />
   </ItemGroup>