diff --git a/tests/NSubstitute.Acceptance.Specs/AutoValuesForSubs.cs b/tests/NSubstitute.Acceptance.Specs/AutoValuesForSubs.cs
index 45f2c6b5..0384bd8d 100644
--- a/tests/NSubstitute.Acceptance.Specs/AutoValuesForSubs.cs
+++ b/tests/NSubstitute.Acceptance.Specs/AutoValuesForSubs.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -109,8 +110,8 @@ public void Should_auto_return_an_empty_string_from_a_func_that_returns_a_string
     public void Should_auto_return_for_iqueryable()
     {
         var sample = Substitute.For<ISample>();
-        Assert.IsEmpty(sample.Queryable().Select(x => x + 1).ToList());
-        Assert.NotNull(sample.Queryable().Expression);
+        ClassicAssert.IsEmpty(sample.Queryable().Select(x => x + 1).ToList());
+        ClassicAssert.NotNull(sample.Queryable().Expression);
     }
 
     [Test]
diff --git a/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs b/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
index 8868a470..e0915173 100644
--- a/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
+++ b/tests/NSubstitute.Acceptance.Specs/CallbackCalling.cs
@@ -1,6 +1,7 @@
 using NSubstitute.Acceptance.Specs.Infrastructure;
 using NSubstitute.Core;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -119,7 +120,7 @@ public void FirstIsRunOnce()
         Assert.That(callCount, Is.EqualTo(0), "Should not have been called yet");
         _something.Count();
         _something.Count();
-        Assert.AreEqual(new List<int> { 1 }, calls);
+        ClassicAssert.AreEqual(new List<int> { 1 }, calls);
     }
 
     [Test]
@@ -132,7 +133,7 @@ public void AlwaysIsCalledAlways()
         Assert.That(callCount, Is.EqualTo(0), "Should not have been called yet");
         _something.Count();
         _something.Count();
-        Assert.AreEqual(new List<int> { 1, 2 }, calls);
+        ClassicAssert.AreEqual(new List<int> { 1, 2 }, calls);
     }
 
     [Test]
@@ -149,7 +150,7 @@ public void ThenIsCalledAfterFirst()
         Assert.That(callCount, Is.EqualTo(0), "Should not have been called yet");
         _something.Count();
         _something.Count();
-        Assert.AreEqual(new List<int> { 1, 2 }, calls);
+        ClassicAssert.AreEqual(new List<int> { 1, 2 }, calls);
     }
 
     [Test]
@@ -166,11 +167,11 @@ public void TwoThensAreCalledInRightOrderAfterFirst()
         );
 
         _something.Count();
-        Assert.IsTrue(first);
+        ClassicAssert.IsTrue(first);
         _something.Count();
-        Assert.IsTrue(then);
+        ClassicAssert.IsTrue(then);
         _something.Count();
-        Assert.IsTrue(secondThen);
+        ClassicAssert.IsTrue(secondThen);
     }
 
     [Test]
@@ -187,11 +188,11 @@ public void AlwaysIsCalledEvenIfFirstAndThenIsDefined()
         );
 
         _something.Count();
-        Assert.IsTrue(first);
+        ClassicAssert.IsTrue(first);
         _something.Count();
-        Assert.IsTrue(then);
+        ClassicAssert.IsTrue(then);
         _something.Count();
-        Assert.AreEqual(3, callCount);
+        ClassicAssert.AreEqual(3, callCount);
     }
 
     [Test]
@@ -208,10 +209,10 @@ public void ThenIsCalledAfterFirstThrow()
 
         Assert.That(callCount, Is.EqualTo(0), "Should not have been called yet");
         var ex = Assert.Throws<Exception>(() => _something.Count());
-        Assert.AreSame(exception, ex);
+        ClassicAssert.AreSame(exception, ex);
 
         _something.Count();
-        Assert.AreEqual(new List<int> { 1 }, calls);
+        ClassicAssert.AreEqual(new List<int> { 1 }, calls);
     }
 
     [Test]
@@ -230,10 +231,10 @@ public void ThenIsCalledAfterThenThrow()
         Assert.That(callCount, Is.EqualTo(0), "Should not have been called yet");
         _something.Count();
         var ex = Assert.Throws<Exception>(() => _something.Count());
-        Assert.AreSame(exception, ex);
+        ClassicAssert.AreSame(exception, ex);
 
         _something.Count();
-        Assert.AreEqual(new List<int> { 1, 2 }, calls);
+        ClassicAssert.AreEqual(new List<int> { 1, 2 }, calls);
     }
 
     [Test]
@@ -250,7 +251,7 @@ public void AlwaysIsCalledEvenThrowIfIsDefined()
 
         Assert.Throws<Exception>(() => _something.Count());
         Assert.Throws<Exception>(() => _something.Count());
-        Assert.AreEqual(2, callCount);
+        ClassicAssert.AreEqual(2, callCount);
     }
 
     [Test]
diff --git a/tests/NSubstitute.Acceptance.Specs/ClearSubstitute.cs b/tests/NSubstitute.Acceptance.Specs/ClearSubstitute.cs
index d9eb20e6..00317247 100644
--- a/tests/NSubstitute.Acceptance.Specs/ClearSubstitute.cs
+++ b/tests/NSubstitute.Acceptance.Specs/ClearSubstitute.cs
@@ -1,6 +1,7 @@
 using NSubstitute.ClearExtensions;
 using NSubstitute.Exceptions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -40,7 +41,7 @@ public void Clear_results()
         var substitute = Substitute.For<ICalculator>();
         substitute.Add(1, 1).Returns(12);
         substitute.ClearSubstitute(ClearOptions.ReturnValues);
-        Assert.AreEqual(0, substitute.Add(1, 1));
+        ClassicAssert.AreEqual(0, substitute.Add(1, 1));
     }
 
     [Test]
@@ -53,6 +54,6 @@ public void Clear_callbacks()
         substitute.Add(1, 1);
         substitute.Add(1, 1);
         substitute.Add(1, 1);
-        Assert.AreEqual(0, count);
+        ClassicAssert.AreEqual(0, count);
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/CompatArgsTests.cs b/tests/NSubstitute.Acceptance.Specs/CompatArgsTests.cs
index 8e8c7dea..d3e99dfe 100644
--- a/tests/NSubstitute.Acceptance.Specs/CompatArgsTests.cs
+++ b/tests/NSubstitute.Acceptance.Specs/CompatArgsTests.cs
@@ -1,6 +1,7 @@
-using System.Reflection;
-using NSubstitute.Compatibility;
+using NSubstitute.Compatibility;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
+using System.Reflection;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -17,7 +18,7 @@ public void CompatAndCompatArgInstanceShouldBeInSync()
         var compatInstanceMembers =
             typeof(CompatArg).GetMethods(flags).Select(DescribeMethod).OrderBy(x => x);
 
-        Assert.AreEqual(
+        ClassicAssert.AreEqual(
             compatMembers.ToList(), compatInstanceMembers.ToList(),
             "Arg.Compat and CompatArg should have static/instance versions of the same methods"
             );
@@ -32,7 +33,7 @@ public void CompatAndArgShouldBeInSync()
         var compatMembers =
             typeof(Arg.Compat).GetMethods(flags).Select(DescribeMethod).OrderBy(x => x);
 
-        Assert.AreEqual(
+        ClassicAssert.AreEqual(
             argMembers.ToList(), compatMembers.ToList(),
             "Arg and Arg.Compat should have the same methods (just vary by out/ref)"
             );
diff --git a/tests/NSubstitute.Acceptance.Specs/EventChecking.cs b/tests/NSubstitute.Acceptance.Specs/EventChecking.cs
index 651a8f16..cbfb8aaf 100644
--- a/tests/NSubstitute.Acceptance.Specs/EventChecking.cs
+++ b/tests/NSubstitute.Acceptance.Specs/EventChecking.cs
@@ -1,5 +1,6 @@
 using NSubstitute.Exceptions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -25,7 +26,7 @@ public void Check_if_nullHandlers_are_ignored()
         source.Started += () => raised = true;
         source.Started += Raise.Event<Action>();
 
-        Assert.IsTrue(raised);
+        ClassicAssert.IsTrue(raised);
     }
 
     [Test]
@@ -40,9 +41,9 @@ public void Check_if_multiple_handlers_get_called()
         source.Started += () => raised3 = true;
         source.Started += Raise.Event<Action>();
 
-        Assert.IsTrue(raised1, "The first handler was not called");
-        Assert.IsTrue(raised2, "The second handler was not called");
-        Assert.IsTrue(raised3, "The third handler was not called");
+        ClassicAssert.IsTrue(raised1, "The first handler was not called");
+        ClassicAssert.IsTrue(raised2, "The second handler was not called");
+        ClassicAssert.IsTrue(raised3, "The third handler was not called");
     }
 
     public interface IEngine
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/EqualsBehaviourOnClassSubs.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/EqualsBehaviourOnClassSubs.cs
index 83e51993..3cea8969 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/EqualsBehaviourOnClassSubs.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/EqualsBehaviourOnClassSubs.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -10,8 +11,8 @@ public void Equals_should_work_as_expected_for_class_substitutes()
         var firstSub = Substitute.For<AClass>();
         var secondSub = Substitute.For<AClass>();
 
-        Assert.AreEqual(firstSub, firstSub);
-        Assert.AreNotEqual(firstSub, secondSub);
+        ClassicAssert.AreEqual(firstSub, firstSub);
+        ClassicAssert.AreNotEqual(firstSub, secondSub);
     }
 
     public class AClass { }
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue225_ConfiguredValueIsUsedInSubsequentSetups.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue225_ConfiguredValueIsUsedInSubsequentSetups.cs
index 271cf29a..ce6366ea 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue225_ConfiguredValueIsUsedInSubsequentSetups.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue225_ConfiguredValueIsUsedInSubsequentSetups.cs
@@ -1,5 +1,6 @@
 using NSubstitute.Acceptance.Specs.Infrastructure;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -16,11 +17,11 @@ public void ShouldNotUseTheConfiguredValueDuringSubsequentSetup()
         target.Echo(Arg.Is(1)).Returns("10", "11", "12");
 
         // Assert
-        Assert.AreEqual("00", target.Echo(0));
-        Assert.AreEqual("10", target.Echo(1));
-        Assert.AreEqual("01", target.Echo(0));
-        Assert.AreEqual("11", target.Echo(1));
-        Assert.AreEqual("02", target.Echo(0));
-        Assert.AreEqual("12", target.Echo(1));
+        ClassicAssert.AreEqual("00", target.Echo(0));
+        ClassicAssert.AreEqual("10", target.Echo(1));
+        ClassicAssert.AreEqual("01", target.Echo(0));
+        ClassicAssert.AreEqual("11", target.Echo(1));
+        ClassicAssert.AreEqual("02", target.Echo(0));
+        ClassicAssert.AreEqual("12", target.Echo(1));
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue271_DelegateOutArgument.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue271_DelegateOutArgument.cs
index 67333e7c..1d659ec6 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue271_DelegateOutArgument.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue271_DelegateOutArgument.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -15,6 +16,6 @@ public void DelegateReturnsOutParameter()
 
         foo(out bar);
 
-        Assert.AreEqual(42, bar);
+        ClassicAssert.AreEqual(42, bar);
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue282_MultipleReturnValuesParallelism.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue282_MultipleReturnValuesParallelism.cs
index 6ea29905..7f115f2f 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue282_MultipleReturnValuesParallelism.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue282_MultipleReturnValuesParallelism.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -24,7 +25,7 @@ public void ReturnsMultipleValuesInParallel()
 
         var results = Task.WhenAll(runningTask1, runningTask2).Result;
 
-        Assert.Contains(ret1, results);
-        Assert.Contains(ret2, results);
+        ClassicAssert.Contains(ret1, results);
+        ClassicAssert.Contains(ret2, results);
     }
 }
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue291_CannotReconfigureThrowingConfiguration.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue291_CannotReconfigureThrowingConfiguration.cs
index 9f7d5979..bd6880e2 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue291_CannotReconfigureThrowingConfiguration.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue291_CannotReconfigureThrowingConfiguration.cs
@@ -1,6 +1,7 @@
 using NSubstitute.Acceptance.Specs.Infrastructure;
 using NSubstitute.ExceptionExtensions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -29,7 +30,7 @@ public void ShouldBePossibleToReConfigureThrowingConfiguration()
 
         // Assert
         Assert.Throws<InvalidOperationException>(() => deliver.Send(new Message1()));
-        Assert.AreSame(response, deliver.Send(new Message2()));
+        ClassicAssert.AreSame(response, deliver.Send(new Message2()));
     }
 
     [Test]
@@ -44,6 +45,6 @@ public void ShouldBePossibleToConfigureConstantAfterThrowForAny()
 
         // Assert
         Assert.Throws<InvalidOperationException>(() => something.Echo(100));
-        Assert.AreEqual("42", something.Echo(42));
+        ClassicAssert.AreEqual("42", something.Echo(42));
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue372_InterfaceSameNameOfMethods.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue372_InterfaceSameNameOfMethods.cs
index 1425d064..740cd677 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue372_InterfaceSameNameOfMethods.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue372_InterfaceSameNameOfMethods.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -22,6 +23,6 @@ public interface X
     public void Should_create_substitute()
     {
         var sut = Substitute.For<X>();
-        Assert.NotNull(sut);
+        ClassicAssert.NotNull(sut);
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue59_ArgDoWithReturns.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue59_ArgDoWithReturns.cs
index c5c84ddd..abc54dfe 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue59_ArgDoWithReturns.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue59_ArgDoWithReturns.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -21,9 +22,9 @@ public void Returns_and_arg_do_for_method()
         sub.Say("hello").Returns("world");
         var resultOfHowdy = sub.Say("howdy");
 
-        Assert.AreEqual("", resultOfHowdy);
-        Assert.AreEqual("howdy", lastArgUsedInCallToSayMethod);
-        Assert.AreEqual("world", sub.Say("hello"));
+        ClassicAssert.AreEqual("", resultOfHowdy);
+        ClassicAssert.AreEqual("howdy", lastArgUsedInCallToSayMethod);
+        ClassicAssert.AreEqual("world", sub.Say("hello"));
     }
 
     [Test]
@@ -36,8 +37,8 @@ public void Returns_and_arg_do_for_property()
         sub.Name = "Jane";
         sub.Name.Returns("Bob");
 
-        Assert.AreEqual("Jane", name);
-        Assert.AreEqual("Bob", sub.Name);
+        ClassicAssert.AreEqual("Jane", name);
+        ClassicAssert.AreEqual("Bob", sub.Name);
     }
 
     [Test]
@@ -51,7 +52,7 @@ public void Returns_and_when_do()
 
         sub.Name.Returns("Bob");
 
-        Assert.AreEqual("Jane", name);
-        Assert.AreEqual("Bob", sub.Name);
+        ClassicAssert.AreEqual("Jane", name);
+        ClassicAssert.AreEqual("Bob", sub.Name);
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue61_ArgAnyStringRegression.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue61_ArgAnyStringRegression.cs
index 412bfc3e..ff16d04c 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue61_ArgAnyStringRegression.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue61_ArgAnyStringRegression.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports;
 
@@ -12,6 +13,6 @@ public void Stub_any_string_and_call_with_null()
         var foo = Substitute.For<IFoo>();
         foo.Bar(Arg.Any<string>(), Arg.Any<double>()).ReturnsForAnyArgs("hello");
 
-        Assert.AreEqual("hello", foo.Bar(null, 0));
+        ClassicAssert.AreEqual("hello", foo.Bar(null, 0));
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/NSubstitute.Acceptance.Specs.csproj b/tests/NSubstitute.Acceptance.Specs/NSubstitute.Acceptance.Specs.csproj
index 45fa9f3c..6cbe3007 100644
--- a/tests/NSubstitute.Acceptance.Specs/NSubstitute.Acceptance.Specs.csproj
+++ b/tests/NSubstitute.Acceptance.Specs/NSubstitute.Acceptance.Specs.csproj
@@ -6,8 +6,8 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="6.0.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
+    <PackageReference Include="NUnit" Version="4.1.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
   </ItemGroup>
 
diff --git a/tests/NSubstitute.Acceptance.Specs/OutAndRefParameters.cs b/tests/NSubstitute.Acceptance.Specs/OutAndRefParameters.cs
index 2ded49ac..d440db4b 100644
--- a/tests/NSubstitute.Acceptance.Specs/OutAndRefParameters.cs
+++ b/tests/NSubstitute.Acceptance.Specs/OutAndRefParameters.cs
@@ -1,5 +1,6 @@
 using NSubstitute.Exceptions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
diff --git a/tests/NSubstitute.Acceptance.Specs/PartialSubs.cs b/tests/NSubstitute.Acceptance.Specs/PartialSubs.cs
index 153a38dd..f299e29a 100644
--- a/tests/NSubstitute.Acceptance.Specs/PartialSubs.cs
+++ b/tests/NSubstitute.Acceptance.Specs/PartialSubs.cs
@@ -2,6 +2,7 @@
 using NSubstitute.Exceptions;
 using NSubstitute.Extensions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -31,7 +32,7 @@ public void CallAbstractMethod()
         var testAbstractClass = Substitute.ForPartsOf<TestAbstractClass>();
         testAbstractClass.VoidAbstractMethod();
         var result = testAbstractClass.AbstractMethodReturnsSameInt(123);
-        Assert.AreEqual(0, result);
+        ClassicAssert.AreEqual(0, result);
     }
 
     [Test]
@@ -90,7 +91,7 @@ public void UseImplementedVirtualMethod()
     public void ReturnDefaultForUnimplementedAbstractMethod()
     {
         var testAbstractClass = Substitute.ForPartsOf<TestAbstractClass>();
-        Assert.AreEqual(default(int), testAbstractClass.AbstractMethodReturnsSameInt(1));
+        ClassicAssert.AreEqual(default(int), testAbstractClass.AbstractMethodReturnsSameInt(1));
     }
 
     [Test]
diff --git a/tests/NSubstitute.Acceptance.Specs/ReceivedCalls.cs b/tests/NSubstitute.Acceptance.Specs/ReceivedCalls.cs
index d7de58b4..cbc74635 100644
--- a/tests/NSubstitute.Acceptance.Specs/ReceivedCalls.cs
+++ b/tests/NSubstitute.Acceptance.Specs/ReceivedCalls.cs
@@ -1,6 +1,7 @@
 using NSubstitute.Exceptions;
 using NSubstitute.ReceivedExtensions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
diff --git a/tests/NSubstitute.Acceptance.Specs/ReturnsAndDoes.cs b/tests/NSubstitute.Acceptance.Specs/ReturnsAndDoes.cs
index 25460561..43102278 100644
--- a/tests/NSubstitute.Acceptance.Specs/ReturnsAndDoes.cs
+++ b/tests/NSubstitute.Acceptance.Specs/ReturnsAndDoes.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -44,8 +45,8 @@ public void Add_multiple_callbacks_and_ignore_args()
         var result2 = sub.GetBar(9999);
         Assert.That(wasCalled, Is.True);
         Assert.That(argsUsed, Is.EquivalentTo(new[] { 42, 9999 }));
-        Assert.AreSame(result1, bar);
-        Assert.AreSame(result2, bar);
+        ClassicAssert.AreSame(result1, bar);
+        ClassicAssert.AreSame(result2, bar);
     }
 
     [Test]
@@ -59,7 +60,7 @@ public void Interaction_with_other_callback_methods()
         sub.When(x => x.GetBar(2)).Do(x => calledViaWhenDo = true);
         sub.GetBar(Arg.Do<int>(x => calledViaArgDo = true));
 
-        Assert.False(calledViaAndDoes && calledViaArgDo && calledViaWhenDo);
+        ClassicAssert.False(calledViaAndDoes && calledViaArgDo && calledViaWhenDo);
 
         sub.GetBar(2);
 
diff --git a/tests/NSubstitute.Acceptance.Specs/SubstitutingForDelegates.cs b/tests/NSubstitute.Acceptance.Specs/SubstitutingForDelegates.cs
index a48a92e6..259d8e1d 100644
--- a/tests/NSubstitute.Acceptance.Specs/SubstitutingForDelegates.cs
+++ b/tests/NSubstitute.Acceptance.Specs/SubstitutingForDelegates.cs
@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -36,7 +37,7 @@ public void SubForFuncThatReturnsValueType()
     {
         var func = Substitute.For<Func<int>>();
         func().Returns(10);
-        Assert.AreEqual(10, func());
+        ClassicAssert.AreEqual(10, func());
     }
 
     [Test]
diff --git a/tests/NSubstitute.Acceptance.Specs/ThrowingAsyncExceptions.cs b/tests/NSubstitute.Acceptance.Specs/ThrowingAsyncExceptions.cs
index 5f59b0b5..29fe4e11 100644
--- a/tests/NSubstitute.Acceptance.Specs/ThrowingAsyncExceptions.cs
+++ b/tests/NSubstitute.Acceptance.Specs/ThrowingAsyncExceptions.cs
@@ -1,6 +1,7 @@
 using NSubstitute.Acceptance.Specs.Infrastructure;
 using NSubstitute.ExceptionExtensions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -36,7 +37,7 @@ public void ThrowExceptionWithMessage()
             _something.Async().ThrowsAsync(new Exception(exceptionMessage));
 
             Exception exceptionThrown = AssertFaultedTaskException<Exception>(() => _something.Async());
-            Assert.AreEqual(exceptionMessage, exceptionThrown.Message);
+            ClassicAssert.AreEqual(exceptionMessage, exceptionThrown.Message);
         }
 
         [Test]
@@ -47,8 +48,8 @@ public void ThrowExceptionWithInnerException()
 
             Exception exceptionThrown = AssertFaultedTaskException<Exception>(() => _something.Async());
 
-            Assert.IsNotNull(exceptionThrown.InnerException);
-            Assert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
+            ClassicAssert.IsNotNull(exceptionThrown.InnerException);
+            ClassicAssert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
         }
 
         [Test]
@@ -136,7 +137,7 @@ public void ThrowExceptionWithMessage()
             _something.CountAsync().ThrowsAsync(new Exception(exceptionMessage));
 
             Exception exceptionThrown = AssertFaultedTaskException<Exception>(() => _something.CountAsync());
-            Assert.AreEqual(exceptionMessage, exceptionThrown.Message);
+            ClassicAssert.AreEqual(exceptionMessage, exceptionThrown.Message);
         }
 
         [Test]
@@ -147,8 +148,8 @@ public void ThrowExceptionWithInnerException()
 
             Exception exceptionThrown = AssertFaultedTaskException<Exception>(() => _something.CountAsync());
 
-            Assert.IsNotNull(exceptionThrown.InnerException);
-            Assert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
+            ClassicAssert.IsNotNull(exceptionThrown.InnerException);
+            ClassicAssert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
         }
 
         [Test]
@@ -237,7 +238,7 @@ public void ThrowExceptionWithMessage()
             _something.CountValueTaskAsync().ThrowsAsync(new Exception(exceptionMessage));
 
             Exception exceptionThrown = AssertFaultedTaskException<int, Exception>(() => _something.CountValueTaskAsync());
-            Assert.AreEqual(exceptionMessage, exceptionThrown.Message);
+            ClassicAssert.AreEqual(exceptionMessage, exceptionThrown.Message);
         }
 
         [Test]
@@ -248,8 +249,8 @@ public void ThrowExceptionWithInnerException()
 
             Exception exceptionThrown = AssertFaultedTaskException<int, Exception>(() => _something.CountValueTaskAsync());
 
-            Assert.IsNotNull(exceptionThrown.InnerException);
-            Assert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
+            ClassicAssert.IsNotNull(exceptionThrown.InnerException);
+            ClassicAssert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
         }
 
         [Test]
diff --git a/tests/NSubstitute.Acceptance.Specs/ThrowingExceptions.cs b/tests/NSubstitute.Acceptance.Specs/ThrowingExceptions.cs
index 4fac6381..4ac6e18c 100644
--- a/tests/NSubstitute.Acceptance.Specs/ThrowingExceptions.cs
+++ b/tests/NSubstitute.Acceptance.Specs/ThrowingExceptions.cs
@@ -1,6 +1,7 @@
 using NSubstitute.Acceptance.Specs.Infrastructure;
 using NSubstitute.ExceptionExtensions;
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -41,7 +42,7 @@ public void ThrowExceptionWithMessage()
         _something.Count().Throws(new Exception(exceptionMessage));
 
         Exception exceptionThrown = Assert.Catch<Exception>(() => _something.Count());
-        Assert.AreEqual(exceptionMessage, exceptionThrown.Message);
+        ClassicAssert.AreEqual(exceptionMessage, exceptionThrown.Message);
     }
 
     [Test]
@@ -52,8 +53,8 @@ public void ThrowExceptionWithInnerException()
 
         Exception exceptionThrown = Assert.Catch<Exception>(() => _something.Count());
 
-        Assert.IsNotNull(exceptionThrown.InnerException);
-        Assert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
+        ClassicAssert.IsNotNull(exceptionThrown.InnerException);
+        ClassicAssert.IsInstanceOf<ArgumentException>(exceptionThrown.InnerException);
     }
 
     [Test]
diff --git a/tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj b/tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj
index 9d9508c2..c5d32130 100644
--- a/tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj
+++ b/tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj
@@ -6,7 +6,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.13.10" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.13.12" />
   </ItemGroup>
 
   <ItemGroup>