diff --git a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
index df7302de..b66ed716 100644
--- a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
+++ b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
@@ -548,6 +548,12 @@ public class SingleFileMessagePackBackingStore : SingleFileBackingStore
         return snapshot;
     }
 
+    protected override string? ReadFormatVersion(byte[] data)
+    {
+        var reader = new MessagePackReader(data);
+        return reader.ReadArrayHeader() > 0 ? reader.ReadString() : null;
+    }
+
     protected override byte[] SerializePayload(object document)
     {
         return CultDocumentMessagePackSerialization.SerializeUntyped(document, document.GetType(), Registry);
diff --git a/src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs b/src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs
index 61e65208..436ec4f4 100644
--- a/src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs
+++ b/src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs
@@ -449,6 +449,14 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
         }
 
         var snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(manifestBytes);
+        if (CultPersistedStoreSnapshot.FormatNumber(snapshot.FormatVersion) is int number && number > 4)
+        {
+            // Every write path parses the manifest under its commit lease before it writes a page or the manifest, so a
+            // store another process has since stamped newer is refused here, untouched.
+            throw new NotSupportedException(
+                $"Directory store {_manifestFile.FullName} is {snapshot.FormatVersion}, a newer format than this runtime reads ({IndexedFormatVersion}).");
+        }
+
         if (!string.Equals(snapshot.FormatVersion, IndexedFormatVersion, StringComparison.Ordinal))
         {
             throw new InvalidOperationException(
diff --git a/src/GameCult.Caching/CultCache.cs b/src/GameCult.Caching/CultCache.cs
index 124bea21..f4ebe368 100644
--- a/src/GameCult.Caching/CultCache.cs
+++ b/src/GameCult.Caching/CultCache.cs
@@ -66,6 +66,24 @@ namespace GameCult.Caching
         public const string FormatV2 = "cultcache.store.v2";
 
         public string FormatVersion { get; set; } = FormatV1;
+
+        /// <summary>
+        /// The N of a header shaped exactly <c>cultcache.store.v&lt;N&gt;</c> or <c>cultcache.store.v&lt;N&gt;.&lt;suffix&gt;</c>;
+        /// null for any other header. A header whose N exceeds what a runtime reads is a newer format, which that runtime
+        /// refuses and must never rewrite.
+        /// </summary>
+        internal static int? FormatNumber(string? header)
+        {
+            const string prefix = "cultcache.store.v";
+            if (header == null || !header.StartsWith(prefix, StringComparison.Ordinal))
+                return null;
+            var end = prefix.Length;
+            while (end < header.Length && header[end] >= '0' && header[end] <= '9')
+                end++;
+            if (end == prefix.Length || end - prefix.Length > 6 || (end < header.Length && header[end] != '.'))
+                return null;
+            return int.Parse(header.Substring(prefix.Length, end - prefix.Length), System.Globalization.CultureInfo.InvariantCulture);
+        }
         public CultSchemaCatalogEntry[] SchemaCatalog { get; set; } = Array.Empty<CultSchemaCatalogEntry>();
         public CultPersistedRecord[] Records { get; set; } = Array.Empty<CultPersistedRecord>();
     }
@@ -3354,6 +3372,14 @@ namespace GameCult.Caching
         protected FileInfo FileInfo { get; }
         protected abstract byte[] SerializeSnapshot(CultPersistedStoreSnapshot snapshot);
         protected abstract CultPersistedStoreSnapshot DeserializeSnapshot(byte[] data);
+
+        /// <summary>
+        /// Reads only the format header of a stored file, or null when this store cannot tell. A plain flush rewrites the
+        /// whole file from this cache's view, so it first reads this header and refuses a file stamped with a newer
+        /// format than the store reads; a store that returns null here is not guarded.
+        /// </summary>
+        protected virtual string? ReadFormatVersion(byte[] data) => null;
+
         protected abstract byte[] SerializePayload(object document);
         protected abstract object DeserializePayload(Type documentType, byte[] payload);
 
@@ -3429,6 +3455,7 @@ namespace GameCult.Caching
             {
                 using (AcquireLock(wait: true))
                 {
+                    RefuseToRewriteANewerFormat();
                     WriteSnapshot(
                         Entries.Values.Select(entry => ToPersistedRecord(entry, SerializePayload)),
                         Entries.Values.Select(entry => entry.Descriptor.ToCatalogEntry()));
@@ -3520,6 +3547,30 @@ namespace GameCult.Caching
             Judging!(arriving, departing);
         }
 
+        // A plain flush replaces the file without merging it, so under the lock it first reads the stamp: a file another
+        // process has since written in a newer format is refused and left as it is. An unreadable stamp (missing, empty,
+        // corrupt) is not newer, and the flush proceeds as it always has, so it can still heal a damaged store.
+        private void RefuseToRewriteANewerFormat()
+        {
+            FileInfo.Refresh();
+            if (!FileInfo.Exists)
+                return;
+            string? header;
+            try
+            {
+                header = ReadFormatVersion(ReadAllBytesShared(FileInfo.FullName));
+            }
+            catch (Exception)
+            {
+                return;
+            }
+
+            if (FormatNumber(header) is int number && number > 2)
+                throw new NotSupportedException(
+                    $"Store {FileInfo.FullName} is {header}, a newer format than this runtime reads " +
+                    $"({CultPersistedStoreSnapshot.FormatV1} and {CultPersistedStoreSnapshot.FormatV2}); a flush would downgrade it, so none was written.");
+        }
+
         private CultPersistedStoreSnapshot? ReadSnapshot()
         {
             FileInfo.Refresh();
diff --git a/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs b/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
index 0f64f3ff..393d1300 100644
--- a/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
+++ b/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
@@ -156,6 +156,100 @@ namespace GameCult.Caching.Tests
             }
         }
 
+        // Two processes: this one attached a v1 store, then a later build rewrote it as v3. A plain flush, a staged write
+        // and a staged delete must each refuse to downgrade it, leaving the file as the other process wrote it.
+        [Test]
+        public async Task SingleFileFlushRefusesAStoreAnotherProcessStampedNewerSinceAttach()
+        {
+            var root = CopyFixture("single-file-v1");
+            try
+            {
+                var file = Path.Combine(root, "store.msgpack");
+                using var cache = new CultCache();
+                cache.AddBackingStore(new SingleFileMessagePackBackingStore(file));
+                await cache.PullAllBackingStoresAsync();
+
+                var snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(File.ReadAllBytes(file));
+                snapshot.FormatVersion = "cultcache.store.v3";
+                File.WriteAllBytes(file, CultDocumentMessagePackSerialization.SerializeSnapshot(snapshot));
+                var before = File.ReadAllBytes(file);
+
+                await cache.AddAsync(new PreCut2FixtureNote { Text = "staged after the other process wrote v3" });
+                Assert.Throws<NotSupportedException>(() => cache.FlushAllBackingStores());
+                Assert.That(File.ReadAllBytes(file), Is.EqualTo(before));
+
+                cache.Remove(new CultRecordKey("item:anvil"));
+                Assert.Throws<NotSupportedException>(() => cache.BackingStores.Single().PushAll());
+                Assert.That(File.ReadAllBytes(file), Is.EqualTo(before));
+            }
+            finally
+            {
+                if (Directory.Exists(root)) Directory.Delete(root, recursive: true);
+            }
+        }
+
+        // An unreadable stamp is not a newer format: a flush still heals a damaged store.
+        [Test]
+        public async Task SingleFileFlushStillHealsAStoreWhoseStampIsUnreadable()
+        {
+            var root = CopyFixture("single-file-v1");
+            try
+            {
+                var file = Path.Combine(root, "store.msgpack");
+                using var cache = new CultCache();
+                cache.AddBackingStore(new SingleFileMessagePackBackingStore(file));
+                await cache.PullAllBackingStoresAsync();
+                File.WriteAllBytes(file, new byte[] { 0xc1, 0xc1 });
+
+                await cache.AddAsync(new PreCut2FixtureNote { Text = "heals" });
+                cache.FlushAllBackingStores();
+
+                Assert.That(CultDocumentMessagePackSerialization.DeserializeSnapshot(File.ReadAllBytes(file)).FormatVersion,
+                    Is.EqualTo("cultcache.store.v1"));
+            }
+            finally
+            {
+                if (Directory.Exists(root)) Directory.Delete(root, recursive: true);
+            }
+        }
+
+        [TestCase("cultcache.store.v5.directory-content-addressed-pages")]
+        [TestCase("cultcache.store.v9")]
+        public async Task DirectoryWritesRefuseAStoreAnotherProcessStampedNewerSinceAttach(string newer)
+        {
+            var root = CopyFixture("directory-v4");
+            try
+            {
+                var manifest = Path.Combine(root, "store.cc");
+                using var cache = new CultCache();
+                cache.AddBackingStore(new DirectoryMessagePackBackingStore(manifest));
+                await cache.PullAllBackingStoresAsync();
+
+                var snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(File.ReadAllBytes(manifest));
+                snapshot.FormatVersion = newer;
+                File.WriteAllBytes(manifest, CultDocumentMessagePackSerialization.SerializeSnapshot(snapshot));
+                Func<System.Collections.Generic.Dictionary<string, byte[]>> files = () => Directory
+                    .GetFiles(root, "*", SearchOption.AllDirectories)
+                    .Where(path => !path.EndsWith(".lock", StringComparison.Ordinal))
+                    .ToDictionary(path => path, File.ReadAllBytes);
+                var before = files();
+
+                await cache.AddAsync(new PreCut2FixtureNote { Text = "staged after the other process wrote the newer format" });
+                Assert.Throws<NotSupportedException>(() => cache.FlushAllBackingStores());
+                Assert.That(files().Keys, Is.EquivalentTo(before.Keys));
+                foreach (var pair in before) Assert.That(File.ReadAllBytes(pair.Key), Is.EqualTo(pair.Value), pair.Key);
+
+                cache.Remove(new CultRecordKey("item:anvil"));
+                Assert.Throws<NotSupportedException>(() => cache.FlushAllBackingStores());
+                Assert.That(files().Keys, Is.EquivalentTo(before.Keys));
+                foreach (var pair in before) Assert.That(File.ReadAllBytes(pair.Key), Is.EqualTo(pair.Value), pair.Key);
+            }
+            finally
+            {
+                if (Directory.Exists(root)) Directory.Delete(root, recursive: true);
+            }
+        }
+
         [Test]
         public void SingleFileRefusesExtraRecordSlotNamingTheRecord()
         {