diff --git a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
index f591d764..51193806 100644
--- a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
+++ b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
@@ -224,7 +224,7 @@ public static class CultDocumentMessagePackSerialization
             !string.Equals(version, CultPersistedStoreSnapshot.FormatV2, StringComparison.Ordinal))
         {
             throw new NotSupportedException(
-                $"Store format {version} is not readable; this runtime reads {CultPersistedStoreSnapshot.FormatV1} and {CultPersistedStoreSnapshot.FormatV2}.");
+                $"Store format {DescribeHeader(version)} is not readable; this runtime reads {CultPersistedStoreSnapshot.FormatV1} and {CultPersistedStoreSnapshot.FormatV2}.");
         }
 
         // Only a v2 store may hold a variant: a v1 header over a variant record is a store no reader could trust.
@@ -239,6 +239,17 @@ public static class CultDocumentMessagePackSerialization
         }
     }
 
+    // A store header as a refusal may show it: echoed in the shape cultcache.store.v<digits>, and otherwise described
+    // only by its length, since the bytes are the store's.
+    private static string DescribeHeader(string header)
+    {
+        const string prefix = "cultcache.store.v";
+        var version = header.StartsWith(prefix, StringComparison.Ordinal) ? header.Substring(prefix.Length) : "";
+        return version.Length > 0 && version.All(character => character >= '0' && character <= '9')
+            ? header
+            : $"an unrecognised cultcache.store.* header of {System.Text.Encoding.UTF8.GetByteCount(header)} bytes";
+    }
+
     private static void WritePersistedRecord(ref MessagePackWriter writer, CultPersistedRecord record)
     {
         writer.WriteArrayHeader(record.Variant == null ? PersistedRecordFieldCount : VariantRecordFieldCount);
diff --git a/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs b/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
index 0bba017e..e9d8797d 100644
--- a/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
+++ b/tests/GameCult.Caching.Tests/PreCut2StoreFormatTests.cs
@@ -202,6 +202,26 @@ namespace GameCult.Caching.Tests
             }
         }
 
+        // A refusal echoes a header only in the shape cultcache.store.v<digits>; any other is described by its length.
+        [TestCase("cultcache.store.v12", true)]
+        [TestCase("cultcache.store.SECRET-HEADER", false)]
+        [TestCase("cultcache.store.v12SECRET", false)]
+        [TestCase("cultcache.store.v", false)]
+        public void SingleFileRefusalEchoesAHeaderOnlyInTheKnownShape(string header, bool echoed)
+        {
+            var snapshot = new CultPersistedStoreSnapshot { FormatVersion = header };
+            var error = Assert.Throws<NotSupportedException>(() => CultDocumentMessagePackSerialization.RequireSingleFileFormat(snapshot))!;
+            if (echoed)
+            {
+                Assert.That(error.Message, Does.Contain("format " + header + " is not"));
+            }
+            else
+            {
+                Assert.That(error.Message, Does.Not.Contain("SECRET").And.Not.Contain("format " + header + " is not"));
+                Assert.That(error.Message, Does.Contain($"of {header.Length} bytes"));
+            }
+        }
+
         [Test]
         public void V1StoreWrittenAtTheBaseCommitStillDecodesByteForByte()
         {