diff --git a/tests/GameCult.Caching.Tests/StoreReplaceTests.cs b/tests/GameCult.Caching.Tests/StoreReplaceTests.cs
new file mode 100644
index 00000000..e464187b
--- /dev/null
+++ b/tests/GameCult.Caching.Tests/StoreReplaceTests.cs
@@ -0,0 +1,203 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Threading;
+using NUnit.Framework;
+
+namespace GameCult.Caching.Tests
+{
+    public class StoreReplaceTests
+    {
+        private sealed class FakeKernel : IReplaceKernel
+        {
+            private readonly Queue<int> _posix;
+            private readonly Queue<int> _moveEx;
+            public readonly List<string> Log = new List<string>();
+            public readonly List<int> Sleeps = new List<int>();
+
+            public FakeKernel(int[] posix, int[] moveEx)
+            {
+                _posix = new Queue<int>(posix);
+                _moveEx = new Queue<int>(moveEx);
+            }
+
+            public int RenamePosix(string staged, string destination)
+            {
+                Log.Add("posix");
+                return _posix.Count > 0 ? _posix.Dequeue() : 0;
+            }
+
+            public int MoveFileEx(string staged, string destination, uint flags)
+            {
+                Log.Add("moveex:" + flags);
+                return _moveEx.Count > 0 ? _moveEx.Dequeue() : 0;
+            }
+
+            public void Sleep(int milliseconds) => Sleeps.Add(milliseconds);
+        }
+
+        private static IOException Fails(FakeKernel kernel)
+            => Assert.Throws<IOException>(() => StoreReplace.ReplaceWindows("a", "b", kernel))!;
+
+        [Test]
+        public void APosixRenameThatWorksIsTheWholeReplace()
+        {
+            var kernel = new FakeKernel(new[] { 0 }, new int[0]);
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix" }));
+            Assert.That(kernel.Sleeps, Is.Empty);
+        }
+
+        [TestCase(1)]
+        [TestCase(50)]
+        [TestCase(87)]
+        public void WhereAPosixRenameIsUnsupportedTheReplaceFallsBackToWriteThroughMoveFileEx(int unsupported)
+        {
+            var kernel = new FakeKernel(new[] { unsupported }, new[] { 0 });
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+        }
+
+        [Test]
+        public void OnceThePosixRenameIsUnsupportedTheRestOfTheCallStaysOnMoveFileEx()
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { 32, 0 });
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9", "moveex:9" }));
+            Assert.That(kernel.Sleeps, Is.EqualTo(new[] { 20 }));
+        }
+
+        [TestCase(5)]
+        [TestCase(32)]
+        [TestCase(33)]
+        public void ATransientErrorRetriesTheSamePrimitiveAfterTwentyMilliseconds(int transient)
+        {
+            var kernel = new FakeKernel(new[] { transient, transient, 0 }, new int[0]);
+            StoreReplace.ReplaceWindows("a", "b", kernel);
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "posix", "posix" }));
+            Assert.That(kernel.Sleeps, Is.EqualTo(new[] { 20, 20 }));
+        }
+
+        [TestCase(1)]
+        [TestCase(50)]
+        [TestCase(87)]
+        public void AnUnsupportedCodeAfterTheFallbackIsAnErrorNotAnotherFallback(int unsupported)
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { unsupported });
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("Win32 error " + unsupported));
+            Assert.That(kernel.Log, Is.EqualTo(new[] { "posix", "moveex:9" }));
+        }
+
+        [Test]
+        public void ARetryThatNeverSucceedsGivesUpAfterTwentyFiveAttemptsNamingTheCode()
+        {
+            var kernel = new FakeKernel(new int[30].Fill(32), new int[0]);
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("25 attempts").And.Contain("32"));
+            Assert.That(kernel.Log.Count, Is.EqualTo(25));
+            Assert.That(kernel.Sleeps.Count, Is.EqualTo(25));
+        }
+
+        [Test]
+        public void AnyOtherCodeThrowsAtOnceNamingTheCode()
+        {
+            var kernel = new FakeKernel(new[] { 2 }, new int[0]);
+            var error = Fails(kernel);
+            Assert.That(error.Message, Does.Contain("Win32 error 2"));
+            Assert.That(kernel.Log.Count, Is.EqualTo(1));
+            Assert.That(kernel.Sleeps, Is.Empty);
+        }
+
+        [Test]
+        public void AFallbackThatFailsOtherwiseThrowsItsCode()
+        {
+            var kernel = new FakeKernel(new[] { 87 }, new[] { 3 });
+            Assert.That(Fails(kernel).Message, Does.Contain("Win32 error 3"));
+        }
+
+        [Test]
+        public void ReplaceOverAnExistingAndThenAnAbsentDestinationLeavesTheStagedBytes()
+        {
+            var dir = Path.Combine(TestContext.CurrentContext.WorkDirectory, "storereplace-" + Guid.NewGuid().ToString("N"));
+            Directory.CreateDirectory(dir);
+            try
+            {
+                var destination = Path.Combine(dir, "store.cc");
+                var staged = Path.Combine(dir, "staged");
+                File.WriteAllBytes(destination, new byte[] { 1 });
+                File.WriteAllBytes(staged, new byte[] { 2 });
+                StoreReplace.Replace(staged, destination);
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 2 }));
+                Assert.That(File.Exists(staged), Is.False);
+
+                File.Delete(destination);
+                File.WriteAllBytes(staged, new byte[] { 3 });
+                StoreReplace.Replace(staged, destination);
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 3 }));
+            }
+            finally { Directory.Delete(dir, true); }
+        }
+
+        [Test]
+        public void OnWindowsAReplaceOverADestinationAReaderHoldsOpenSucceedsOnTheFirstAttempt()
+        {
+            if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                Assert.Ignore("The real kernel runs on Windows only.");
+
+            var dir = Path.Combine(TestContext.CurrentContext.WorkDirectory, "storereplace-" + Guid.NewGuid().ToString("N"));
+            Directory.CreateDirectory(dir);
+            try
+            {
+                var destination = Path.Combine(dir, "store.cc");
+                var staged = Path.Combine(dir, "staged");
+                File.WriteAllBytes(destination, new byte[] { 1 });
+                File.WriteAllBytes(staged, new byte[] { 2 });
+                using (new FileStream(destination, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
+                {
+                    var kernel = new CountingKernel(StoreReplace.WindowsKernel);
+                    StoreReplace.ReplaceWindows(staged, destination, kernel);
+                    Assert.That(kernel.PosixCalls, Is.EqualTo(1));
+                    Assert.That(kernel.MoveExCalls, Is.EqualTo(0));
+                    Assert.That(kernel.Sleeps, Is.EqualTo(0));
+                    Assert.That(kernel.LastCode, Is.EqualTo(0));
+                }
+
+                Assert.That(File.ReadAllBytes(destination), Is.EqualTo(new byte[] { 2 }));
+            }
+            finally { Directory.Delete(dir, true); }
+        }
+
+        private sealed class CountingKernel : IReplaceKernel
+        {
+            private readonly IReplaceKernel _inner;
+            public int PosixCalls, MoveExCalls, Sleeps, LastCode;
+            public CountingKernel(IReplaceKernel inner) { _inner = inner; }
+
+            public int RenamePosix(string staged, string destination)
+            {
+                PosixCalls++;
+                return LastCode = _inner.RenamePosix(staged, destination);
+            }
+
+            public int MoveFileEx(string staged, string destination, uint flags)
+            {
+                MoveExCalls++;
+                return LastCode = _inner.MoveFileEx(staged, destination, flags);
+            }
+
+            public void Sleep(int milliseconds) { Sleeps++; _inner.Sleep(milliseconds); }
+        }
+    }
+
+    internal static class ArrayFill
+    {
+        public static int[] Fill(this int[] array, int value)
+        {
+            for (var i = 0; i < array.Length; i++) array[i] = value;
+            return array;
+        }
+    }
+}