diff --git a/src/NATS.Server/JetStream/Models/StreamState.cs b/src/NATS.Server/JetStream/Models/StreamState.cs
new file mode 100644
index 0000000..9fe197f
--- /dev/null
+++ b/src/NATS.Server/JetStream/Models/StreamState.cs
@@ -0,0 +1,8 @@
+namespace NATS.Server.JetStream.Models;
+
+public sealed class StreamState
+{
+    public ulong Messages { get; set; }
+    public ulong FirstSeq { get; set; }
+    public ulong LastSeq { get; set; }
+}
diff --git a/src/NATS.Server/JetStream/Storage/IStreamStore.cs b/src/NATS.Server/JetStream/Storage/IStreamStore.cs
new file mode 100644
index 0000000..8073199
--- /dev/null
+++ b/src/NATS.Server/JetStream/Storage/IStreamStore.cs
@@ -0,0 +1,9 @@
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Storage;
+
+public interface IStreamStore
+{
+    ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct);
+    ValueTask<StreamState> GetStateAsync(CancellationToken ct);
+}
diff --git a/src/NATS.Server/JetStream/Storage/StoredMessage.cs b/src/NATS.Server/JetStream/Storage/StoredMessage.cs
new file mode 100644
index 0000000..b702613
--- /dev/null
+++ b/src/NATS.Server/JetStream/Storage/StoredMessage.cs
@@ -0,0 +1,9 @@
+namespace NATS.Server.JetStream.Storage;
+
+public sealed class StoredMessage
+{
+    public ulong Sequence { get; init; }
+    public string Subject { get; init; } = string.Empty;
+    public ReadOnlyMemory<byte> Payload { get; init; }
+    public bool Redelivered { get; init; }
+}
diff --git a/tests/NATS.Server.Tests/StreamStoreContractTests.cs b/tests/NATS.Server.Tests/StreamStoreContractTests.cs
new file mode 100644
index 0000000..f5243f8
--- /dev/null
+++ b/tests/NATS.Server.Tests/StreamStoreContractTests.cs
@@ -0,0 +1,31 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests;
+
+public class StreamStoreContractTests
+{
+    [Fact]
+    public async Task Append_increments_sequence_and_updates_state()
+    {
+        var store = new FakeStreamStore();
+        var seq = await store.AppendAsync("foo", "bar"u8.ToArray(), default);
+
+        seq.ShouldBe((ulong)1);
+        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)1);
+    }
+
+    private sealed class FakeStreamStore : IStreamStore
+    {
+        private ulong _last;
+
+        public ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct)
+        {
+            _last++;
+            return ValueTask.FromResult(_last);
+        }
+
+        public ValueTask<StreamState> GetStateAsync(CancellationToken ct)
+            => ValueTask.FromResult(new StreamState { Messages = _last });
+    }
+}