diff --git a/src/GameCult.Caching/CultCache.cs b/src/GameCult.Caching/CultCache.cs
index a50b3e03..dc0c8b99 100644
--- a/src/GameCult.Caching/CultCache.cs
+++ b/src/GameCult.Caching/CultCache.cs
@@ -2728,9 +2728,9 @@ namespace GameCult.Caching
                     return new CultUpdateResult<T>(CultUpdateOutcome.Updated, next);
                 if (outcome != CultCommitOutcome.Mismatch)
                     throw new InvalidOperationException($"Update of {key.Value} was refused: {outcome}.");
+                Held(() => { Home(next.GetType())?.PullAll(); return true; });
                 if (attempt >= maxAttempts)
                     return new CultUpdateResult<T>(CultUpdateOutcome.LostRace, Get<T>(key));
-                Held(() => { Home(next.GetType())?.PullAll(); return true; });
             }
         }
 
diff --git a/tests/GameCult.Caching.Tests/UpdateTests.cs b/tests/GameCult.Caching.Tests/UpdateTests.cs
new file mode 100644
index 00000000..837faddf
--- /dev/null
+++ b/tests/GameCult.Caching.Tests/UpdateTests.cs
@@ -0,0 +1,330 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using GameCult.Caching;
+using GameCult.Caching.MessagePack;
+using MessagePack;
+using NUnit.Framework;
+using R3;
+
+namespace GameCult.Caching.Tests
+{
+    // CultCache.Update: read the content version, apply a pure function to a private copy, commit conditionally, re-run on a lost
+    // race (variants:ruling:write-set-content-version-declared-ownership; variants:question:update-apply-copy).
+    public class UpdateTests
+    {
+        private static readonly CultRecordKey Key = new("counter");
+        private static readonly CultDocumentRegistry Registry =
+            CultDocumentRegistry.ForTypes(new[] { typeof(UpdateCounter), typeof(UpdateTally), typeof(UpdateGear) });
+
+        private string _directory = string.Empty;
+
+        [SetUp]
+        public void CreateDirectory()
+        {
+            _directory = Path.Combine(Path.GetTempPath(), $"cultlib-update-{Guid.NewGuid():N}");
+            Directory.CreateDirectory(_directory);
+        }
+
+        [TearDown]
+        public void DeleteDirectory()
+        {
+            if (Directory.Exists(_directory))
+                Directory.Delete(_directory, recursive: true);
+        }
+
+        private string PathOf(string name) => Path.Combine(_directory, name);
+
+        private static CultCache Open(string path, bool directory, bool codec = true)
+        {
+            var cache = codec ? new CultCache(Registry, CultCacheMessagePack.CreateCodec(Registry)) : new CultCache(Registry);
+            cache.AddBackingStore(directory ? new DirectoryMessagePackBackingStore(path) : new SingleFileMessagePackBackingStore(path));
+            return cache;
+        }
+
+        private static void Seed(string path, bool directory, int value)
+        {
+            using var cache = Open(path, directory);
+            Assert.That(cache.Commit(batch => batch.Upsert(new UpdateCounter { Name = "counter", Value = value }, new CultRecordHandle<UpdateCounter>(Key))), Is.True);
+        }
+
+        private static void Rival(CultCache cache, int value) =>
+            Assert.That(cache.Commit(batch => batch.Upsert(new UpdateCounter { Name = "counter", Value = value }, new CultRecordHandle<UpdateCounter>(Key))), Is.True);
+
+        private static int StoredValue(string path, bool directory)
+        {
+            using var reader = Open(path, directory);
+            return reader.Get<UpdateCounter>(Key)!.Value;
+        }
+
+        private static UpdateCounter Plus(UpdateCounter? counter, int by)
+        {
+            counter ??= new UpdateCounter { Name = "counter" };
+            counter.Value += by;
+            return counter;
+        }
+
+        [TestCase(false)]
+        [TestCase(true)]
+        public void TwoWritersUpdatingLoseNoIncrement(bool directory)
+        {
+            const int n = 100;
+            var path = PathOf("race.cc");
+            Seed(path, directory, 0);
+            using var a = Open(path, directory);
+            using var b = Open(path, directory);
+            var applied = 0;
+
+            var racers = new[] { a, b }.Select(cache => Task.Run(() =>
+            {
+                var results = new List<CultUpdateOutcome>();
+                for (var i = 0; i < n; i++)
+                    results.Add(cache.Update<UpdateCounter>(Key, counter =>
+                    {
+                        Interlocked.Increment(ref applied);
+                        return Plus(counter, 1);
+                    }, maxAttempts: 200).Outcome);
+                return results;
+            })).ToArray();
+            Assert.That(Task.WaitAll(racers, TimeSpan.FromMinutes(2)), Is.True, "the racers did not finish");
+
+            Assert.That(racers.SelectMany(racer => racer.Result), Is.All.EqualTo(CultUpdateOutcome.Updated));
+            Assert.That(StoredValue(path, directory), Is.EqualTo(2 * n));
+            Assert.That(applied, Is.GreaterThan(2 * n), "the writers never actually raced");
+        }
+
+        [Test]
+        public void LostAttemptRerunsApplyOnTheFreshValue()
+        {
+            var path = PathOf("rerun.cc");
+            Seed(path, directory: false, 1);
+            using var cache = Open(path, false);
+            using var rival = Open(path, false);
+            var seen = new List<int>();
+
+            var result = cache.Update<UpdateCounter>(Key, counter =>
+            {
+                seen.Add(counter!.Value);
+                if (seen.Count == 1)
+                    Rival(rival, 100);
+                return Plus(counter, 1);
+            });
+
+            Assert.That(seen, Is.EqualTo(new[] { 1, 100 }));
+            Assert.That(result.Outcome, Is.EqualTo(CultUpdateOutcome.Updated));
+            Assert.That(result.Document!.Value, Is.EqualTo(101));
+            Assert.That(StoredValue(path, false), Is.EqualTo(101));
+        }
+
+        [Test]
+        public void ExhaustedAttemptsReturnLostRace()
+        {
+            var path = PathOf("lost.cc");
+            Seed(path, false, 1);
+            using var cache = Open(path, false);
+            using var rival = Open(path, false);
+            var calls = 0;
+
+            var result = cache.Update<UpdateCounter>(Key, counter =>
+            {
+                Rival(rival, 100 * ++calls);
+                return Plus(counter, 1);
+            }, maxAttempts: 3);
+
+            Assert.That(calls, Is.EqualTo(3));
+            Assert.That(result.Outcome, Is.EqualTo(CultUpdateOutcome.LostRace));
+            Assert.That(result.Document!.Value, Is.EqualTo(300));
+            Assert.That(StoredValue(path, false), Is.EqualTo(300), "the store holds the rival's last write");
+        }
+
+        [Test]
+        public void ApplyGetsAPrivateCopy()
+        {
+            var path = PathOf("copy.cc");
+            Seed(path, false, 1);
+            using var cache = Open(path, false);
+            using var rival = Open(path, false);
+            var published = new List<int>();
+            using var subscription = cache.Watch<UpdateCounter>().Subscribe(change => published.Add(change.Document!.Value));
+            var arguments = new List<int>();
+            var calls = 0;
+
+            var result = cache.Update<UpdateCounter>(Key, counter =>
+            {
+                calls++;
+                arguments.Add(counter!.Value);
+                Assert.That(counter, Is.Not.SameAs(cache.Get<UpdateCounter>(Key)), "apply was handed the held instance");
+                counter.Value += 1000;
+                Assert.That(cache.Get<UpdateCounter>(Key)!.Value, Is.Not.EqualTo(counter.Value), "the change reached the cache before any commit");
+                if (calls == 1)
+                    Rival(rival, 5);
+                return counter;
+            });
+
+            Assert.That(arguments, Is.EqualTo(new[] { 1, 5 }), "the second call saw the first call's change");
+            Assert.That(result.Outcome, Is.EqualTo(CultUpdateOutcome.Updated));
+            Assert.That(published, Does.Not.Contain(1001), "an uncommitted value was published");
+            Assert.That(published.Count(value => value == 1005), Is.EqualTo(1));
+            Assert.That(StoredValue(path, false), Is.EqualTo(1005));
+        }
+
+        [Test]
+        public void NullFromApplyDeclines()
+        {
+            var path = PathOf("decline.cc");
+            Seed(path, false, 7);
+            var before = File.ReadAllBytes(path);
+            using var cache = Open(path, false);
+            var changes = 0;
+            using var subscription = cache.Watch<UpdateCounter>().Subscribe(_ => changes++);
+
+            var result = cache.Update<UpdateCounter>(Key, _ => null);
+
+            Assert.That(result.Outcome, Is.EqualTo(CultUpdateOutcome.Declined));
+            Assert.That(result.Document!.Value, Is.EqualTo(7));
+            Assert.That(File.ReadAllBytes(path), Is.EqualTo(before));
+            Assert.That(changes, Is.EqualTo(0));
+        }
+
+        [TestCase(false)]
+        [TestCase(true)]
+        public void AbsentKeyCreatesOnceUnderRace(bool directory)
+        {
+            var path = PathOf("create.cc");
+            using var a = Open(path, directory);
+            using var b = Open(path, directory);
+            var started = new Barrier(2);
+
+            var racers = new[] { a, b }.Select(cache => Task.Run(() =>
+            {
+                started.SignalAndWait();
+                return cache.Update<UpdateCounter>(Key, counter => Plus(counter, 1)).Outcome;
+            })).ToArray();
+            Assert.That(Task.WaitAll(racers, TimeSpan.FromMinutes(1)), Is.True);
+
+            Assert.That(racers.Select(racer => racer.Result), Is.All.EqualTo(CultUpdateOutcome.Updated));
+            Assert.That(StoredValue(path, directory), Is.EqualTo(2));
+        }
+
+        [Test]
+        public void RefusalsAreNotRetried()
+        {
+            var path = PathOf("refuse.cc");
+            Seed(path, false, 1);
+            var before = File.ReadAllBytes(path);
+            var calls = 0;
+            Func<UpdateCounter?, UpdateCounter?> apply = counter => { calls++; return Plus(counter, 1); };
+
+            using (var codecLess = Open(path, false, codec: false))
+                Assert.That(() => codecLess.Update(Key, apply), Throws.InvalidOperationException);
+            using (var cache = Open(path, false))
+            {
+                Assert.That(() => cache.Update(Key, apply, maxAttempts: 0), Throws.TypeOf<ArgumentOutOfRangeException>());
+                Assert.That(() => cache.Update<UpdateCounter>(Key, null!), Throws.TypeOf<ArgumentNullException>());
+                Assert.That(() => cache.Update<UpdateTally>(Key, _ => new UpdateTally { Name = "counter" }), Throws.InvalidOperationException);
+            }
+
+            using (var cache = Open(path, false))
+            {
+                cache.Commit(batch =>
+                {
+                    batch.Upsert(new UpdateGear { Name = "gear", Power = 1 }, new CultRecordHandle<UpdateGear>(new CultRecordKey("gear")));
+                });
+                cache.Commit(batch => batch.UpsertVariant(new CultRecordKey("gear-big"), new CultRecordKey("gear"),
+                    new[] { cache.Override<UpdateGear>(nameof(UpdateGear.Name), "gear big"), cache.Override<UpdateGear>(nameof(UpdateGear.Power), 9) }));
+            }
+
+            before = File.ReadAllBytes(path);
+            using (var cache = Open(path, false))
+                Assert.That(() => cache.Update<UpdateGear>(new CultRecordKey("gear-big"), gear => gear), Throws.InvalidOperationException);
+            Assert.That(calls, Is.EqualTo(0), "apply ran for a refused call");
+            Assert.That(File.ReadAllBytes(path), Is.EqualTo(before));
+
+            using (var cache = Open(path, false))
+            {
+                cache.UpsertAsync(new UpdateTally { Name = "staged" }).GetAwaiter().GetResult();
+                Assert.That(() => cache.Update(Key, apply), Throws.InvalidOperationException.With.Message.Contains("flush before a conditional commit"));
+                Assert.That(calls, Is.LessThanOrEqualTo(1));
+            }
+
+            Assert.That(File.ReadAllBytes(path), Is.EqualTo(before), "a refusal wrote");
+        }
+
+        [Test]
+        public void VariantStoreConflictIsALostAttempt()
+        {
+            var path = PathOf("variant.cc");
+            Seed(path, false, 1);
+            using (var seed = Open(path, false))
+            {
+                seed.Commit(batch => batch.Upsert(new UpdateGear { Name = "gear", Power = 1 }, new CultRecordHandle<UpdateGear>(new CultRecordKey("gear"))));
+                seed.Commit(batch => batch.UpsertVariant(new CultRecordKey("gear-big"), new CultRecordKey("gear"),
+                    new[] { seed.Override<UpdateGear>(nameof(UpdateGear.Name), "gear big"), seed.Override<UpdateGear>(nameof(UpdateGear.Power), 9) }));
+            }
+
+            using var cache = Open(path, false);
+            using (var rival = Open(path, false))
+                Assert.That(rival.Commit(batch => batch.Upsert(new UpdateTally { Name = "other" }, new CultRecordHandle<UpdateTally>(new CultRecordKey("other")))), Is.True);
+            var calls = 0;
+
+            var result = cache.Update<UpdateCounter>(Key, counter => { calls++; return Plus(counter, 1); });
+
+            Assert.That(result.Outcome, Is.EqualTo(CultUpdateOutcome.Updated));
+            Assert.That(calls, Is.EqualTo(2), "the conflict was not a lost attempt that re-ran apply");
+            Assert.That(StoredValue(path, false), Is.EqualTo(2));
+        }
+
+        [Test]
+        public void InMemoryCacheUpdatesUnderThreadRace()
+        {
+            using var cache = new CultCache(Registry, CultCacheMessagePack.CreateCodec(Registry));
+            cache.Commit(batch => batch.Upsert(new UpdateCounter { Name = "counter" }, new CultRecordHandle<UpdateCounter>(Key)));
+
+            var threads = Enumerable.Range(0, 2).Select(_ => Task.Run(() =>
+            {
+                for (var i = 0; i < 100; i++)
+                    Assert.That(cache.Update<UpdateCounter>(Key, counter => Plus(counter, 1), maxAttempts: 200).Outcome, Is.EqualTo(CultUpdateOutcome.Updated));
+            })).ToArray();
+            Assert.That(Task.WaitAll(threads, TimeSpan.FromMinutes(1)), Is.True);
+
+            Assert.That(cache.Get<UpdateCounter>(Key)!.Value, Is.EqualTo(200));
+        }
+
+        [CultDocument("tests.update_counter", "tests.update_counter.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        internal sealed class UpdateCounter
+        {
+            [Key(0)]
+            [CultName]
+            public string Name = string.Empty;
+
+            [Key(1)]
+            public int Value;
+        }
+
+        [CultDocument("tests.update_tally", "tests.update_tally.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        internal sealed class UpdateTally
+        {
+            [Key(0)]
+            [CultName]
+            public string Name = string.Empty;
+        }
+
+        [CultDocument("tests.update_gear", "tests.update_gear.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        internal sealed class UpdateGear
+        {
+            [Key(0)]
+            [CultName]
+            public string Name = string.Empty;
+
+            [Key(1)]
+            public int Power;
+        }
+    }
+}