diff --git a/tests/GameCult.Caching.InteropPeer/GameCult.Caching.InteropPeer.csproj b/tests/GameCult.Caching.InteropPeer/GameCult.Caching.InteropPeer.csproj
new file mode 100644
index 00000000..dc982453
--- /dev/null
+++ b/tests/GameCult.Caching.InteropPeer/GameCult.Caching.InteropPeer.csproj
@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <RootNamespace>GameCult.Caching.InteropPeer</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\GameCult.Caching.MessagePack\GameCult.Caching.MessagePack.csproj" />
+    <ProjectReference Include="..\..\src\GameCult.Caching.MessagePack.Generator\GameCult.Caching.MessagePack.Generator.csproj"
+                      OutputItemType="Analyzer"
+                      ReferenceOutputAssembly="false" />
+    <PackageReference Include="MessagePack" Version="3.1.4" />
+  </ItemGroup>
+</Project>
diff --git a/tests/GameCult.Caching.InteropPeer/Program.cs b/tests/GameCult.Caching.InteropPeer/Program.cs
new file mode 100644
index 00000000..5de07335
--- /dev/null
+++ b/tests/GameCult.Caching.InteropPeer/Program.cs
@@ -0,0 +1,134 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text.Json;
+using System.Threading.Tasks;
+using GameCult.Caching;
+using GameCult.Caching.MessagePack;
+using MessagePack;
+
+return await ProgramMainAsync(args);
+
+static async Task<int> ProgramMainAsync(string[] args)
+{
+    try
+    {
+        if (args.Length == 0)
+        {
+            throw new InvalidOperationException("Expected mode: write | read");
+        }
+
+        var mode = args[0];
+        var options = ParseArgs(args.Skip(1).ToArray());
+        var file = RequireArg(options, "file");
+        switch (mode)
+        {
+            case "write":
+                await WriteAsync(file, RequireArg(options, "runtime-id"));
+                return 0;
+            case "read":
+                await ReadAsync(file);
+                return 0;
+            default:
+                throw new InvalidOperationException($"Unknown mode {mode}.");
+        }
+    }
+    catch (Exception error)
+    {
+        Console.Error.WriteLine(error);
+        return 1;
+    }
+}
+
+static async Task WriteAsync(string file, string runtimeId)
+{
+    var cache = BuildCache(file);
+    await cache.PullAllBackingStoresAsync();
+    var note = new CultCacheInteropNote
+    {
+        DocumentId = $"note:{runtimeId}",
+        AuthorRuntimeId = runtimeId,
+        Title = $"{runtimeId} wrote a CultCache note",
+        Body = "The v1 store format is the contract.",
+        Tags = [runtimeId, "csharp", "interop"]
+    };
+    await cache.AddAsync(note, new CultRecordHandle<CultCacheInteropNote>(new CultRecordKey(note.DocumentId)));
+    cache.FlushAllBackingStores();
+    WriteJsonLine(note);
+}
+
+static async Task ReadAsync(string file)
+{
+    var cache = BuildCache(file);
+    await cache.PullAllBackingStoresAsync();
+    var note = cache.AllEntries
+        .OfType<CultCacheInteropNote>()
+        .FirstOrDefault()
+        ?? throw new InvalidOperationException("No cultcache.interop-note records found.");
+    WriteJsonLine(note);
+}
+
+static CultCache BuildCache(string file)
+{
+    var cache = new CultCache();
+    cache.AddBackingStore(new SingleFileMessagePackBackingStore(file));
+    return cache;
+}
+
+static Dictionary<string, string> ParseArgs(string[] args)
+{
+    var parsed = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+    for (var index = 0; index < args.Length; index += 2)
+    {
+        var token = args[index];
+        if (!token.StartsWith("--", StringComparison.Ordinal))
+        {
+            continue;
+        }
+
+        if (index + 1 >= args.Length)
+        {
+            throw new InvalidOperationException($"Missing value for {token}.");
+        }
+
+        parsed[token[2..]] = args[index + 1];
+    }
+
+    return parsed;
+}
+
+static string RequireArg(Dictionary<string, string> options, string name)
+{
+    if (!options.TryGetValue(name, out var value) || string.IsNullOrWhiteSpace(value))
+    {
+        throw new InvalidOperationException($"Missing required argument --{name}.");
+    }
+
+    return value;
+}
+
+static void WriteJsonLine(CultCacheInteropNote note)
+{
+    Console.Out.WriteLine(JsonSerializer.Serialize(new
+    {
+        schemaVersion = note.SchemaVersion,
+        documentId = note.DocumentId,
+        authorRuntimeId = note.AuthorRuntimeId,
+        title = note.Title,
+        body = note.Body,
+        tags = note.Tags
+    }));
+}
+
+[CultDocument("cultcache.interop-note", "cultcache.interop_note.v1")]
+[MessagePackObject]
+public sealed class CultCacheInteropNote
+{
+    [Key(0)] public string SchemaVersion { get; set; } = "cultcache.interop_note.v1";
+    [Key(1)] [CultName] public string DocumentId { get; set; } = string.Empty;
+    [Key(2)] public string AuthorRuntimeId { get; set; } = string.Empty;
+    [Key(3)] public string Title { get; set; } = string.Empty;
+    [Key(4)] public string Body { get; set; } = string.Empty;
+    [Key(5)] public string[] Tags { get; set; } = Array.Empty<string>();
+}