diff --git a/tests/GameCult.Caching.Tests/SlotNilVectorTests.cs b/tests/GameCult.Caching.Tests/SlotNilVectorTests.cs
new file mode 100644
index 00000000..f7e113d9
--- /dev/null
+++ b/tests/GameCult.Caching.Tests/SlotNilVectorTests.cs
@@ -0,0 +1,123 @@
+#nullable enable
+using System;
+using System.Buffers;
+using System.Collections.Generic;
+using System.IO;
+using GameCult.Caching.MessagePack;
+using MessagePack;
+using NUnit.Framework;
+
+namespace GameCult.Caching.Tests
+{
+    // The slot golden in tests/vectors/slot-nil: what the reference reads and rewrites for a nil or a short record in each
+    // kind of slot. The C# reference is the only writer (CULTCACHE_WRITE_VECTORS=1); every other run reads the committed
+    // bytes and compares them byte for byte. Other runtimes are held to these files.
+    public class SlotNilVectorTests
+    {
+        private const string WriteVariable = "CULTCACHE_WRITE_VECTORS";
+
+        [MessagePackObject] public sealed class Inner { [Key(0)] public string X = string.Empty; }
+
+        // One slot of each kind: a string, a value type, a byte[], a record and a list.
+        [MessagePackObject]
+        public sealed class Probe
+        {
+            [Key(0)] public string S = string.Empty;
+            [Key(1)] public int I;
+            [Key(2)] public byte[] B = Array.Empty<byte>();
+            [Key(3)] public Inner? R;
+            [Key(4)] public List<string> L = new();
+        }
+
+        private static bool Writing => Environment.GetEnvironmentVariable(WriteVariable) == "1";
+
+        private static readonly MessagePackSerializerOptions Options = CultDocumentMessagePackSerialization.OptionsFor(typeof(SlotNilVectorTests).Assembly);
+
+        // [name, payload]: the whole case table, in file order.
+        private static readonly (string Name, string Hex)[] Cases =
+        {
+            ("full", "95a16107c402616291a17890"),
+            ("nil-string", "95c007c4026162c090"),
+            ("nil-bytes", "95a16107c0c090"),
+            ("nil-record", "95a16107c4026162c090"),
+            ("nil-list", "95a16107c4026162c0c0"),
+            ("nil-int", "95a161c0c4026162c090"),
+            ("bytes-str", "95a16107a3616263c090"),
+            ("bytes-bin", "95a16107c403616263c090"),
+            ("short-0", "90"),
+            ("short-1", "91a161"),
+            ("short-2", "92a16107"),
+            ("short-3", "93a16107c4026162"),
+            ("short-3-nil-string", "93c007c4026162"),
+            ("short-3-nil-int", "93a161c0c4026162"),
+        };
+
+        private static string VectorPath(string name)
+        {
+            for (var dir = new DirectoryInfo(TestContext.CurrentContext.TestDirectory); dir != null; dir = dir.Parent)
+            {
+                var vectors = Path.Combine(dir.FullName, "tests", "vectors");
+                if (Directory.Exists(vectors)) return Path.Combine(vectors, "slot-nil", name);
+            }
+
+            throw new DirectoryNotFoundException($"tests/vectors not found above {TestContext.CurrentContext.TestDirectory}.");
+        }
+
+        private static void Check(string name, byte[] reference)
+        {
+            var path = VectorPath(name);
+            if (Writing)
+            {
+                Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+                File.WriteAllBytes(path, reference);
+                return;
+            }
+
+            Assert.That(File.Exists(path), Is.True, $"vector {name} is committed");
+            Assert.That(Convert.ToHexString(reference), Is.EqualTo(Convert.ToHexString(File.ReadAllBytes(path))), $"vector {name}");
+        }
+
+        private static (bool Accepted, byte[] Rewritten) Read(byte[] payload)
+        {
+            try
+            {
+                return (true, MessagePackSerializer.Serialize(MessagePackSerializer.Deserialize<Probe>(payload, Options), Options));
+            }
+            catch (MessagePackSerializationException)
+            {
+                return (false, Array.Empty<byte>());
+            }
+        }
+
+        // reads.msgpack: an array of [name, payload (bin), accepted, rewritten (bin: what the reference writes after reading it; empty when refused)].
+        private static byte[] ReadsVector()
+        {
+            var buffer = new ArrayBufferWriter<byte>();
+            var writer = new MessagePackWriter(buffer);
+            writer.WriteArrayHeader(Cases.Length);
+            foreach (var (name, hex) in Cases)
+            {
+                var payload = Convert.FromHexString(hex);
+                var (accepted, rewritten) = Read(payload);
+                writer.WriteArrayHeader(4);
+                writer.Write(name);
+                writer.Write(payload);
+                writer.Write(accepted);
+                writer.Write(rewritten);
+            }
+
+            writer.Flush();
+            return buffer.WrittenSpan.ToArray();
+        }
+
+        [Test]
+        public void ReadsVectorMatches() => Check("reads.msgpack", ReadsVector());
+
+        [Test]
+        public void NilIsAcceptedInAReferenceSlotAndRefusedInAValueSlot()
+        {
+            Assert.That(Read(Convert.FromHexString("95c007c4026162c090")).Accepted, Is.True);
+            Assert.That(Read(Convert.FromHexString("95a161c0c4026162c090")).Accepted, Is.False);
+        }
+    }
+}
diff --git a/tests/vectors/slot-nil/README.md b/tests/vectors/slot-nil/README.md
new file mode 100644
index 00000000..f7ac1a2b
--- /dev/null
+++ b/tests/vectors/slot-nil/README.md
@@ -0,0 +1,12 @@
+# slot-nil vectors
+
+Written only by the C# reference, `tests/GameCult.Caching.Tests/SlotNilVectorTests.cs`, with
+`CULTCACHE_WRITE_VECTORS=1`. Every other run reads this file and compares it byte for byte; a changed vector is a
+reviewed commit. Other runtimes decode it and are held to the same answers.
+
+A record of five slots: `S` string, `I` int, `B` byte[], `R` a record of one string, `L` List of string.
+
+- `reads.msgpack`: an array of `[name, payload, accepted, rewritten]`. `payload` (bin) is the record fed to the
+  reference; `accepted` is whether it read; `rewritten` (bin) is what the reference writes after reading it, and empty
+  when refused. It covers a nil in each kind of slot, a MessagePack str in the byte[] slot, and records short of their
+  last slots.