diff --git a/src/NATS.Server/JetStream/Api/JetStreamApiError.cs b/src/NATS.Server/JetStream/Api/JetStreamApiError.cs
new file mode 100644
index 0000000..4aaf120
--- /dev/null
+++ b/src/NATS.Server/JetStream/Api/JetStreamApiError.cs
@@ -0,0 +1,7 @@
+namespace NATS.Server.JetStream.Api;
+
+public sealed class JetStreamApiError
+{
+    public int Code { get; init; }
+    public string Description { get; init; } = string.Empty;
+}
diff --git a/src/NATS.Server/JetStream/Api/JetStreamApiResponse.cs b/src/NATS.Server/JetStream/Api/JetStreamApiResponse.cs
new file mode 100644
index 0000000..aba69c7
--- /dev/null
+++ b/src/NATS.Server/JetStream/Api/JetStreamApiResponse.cs
@@ -0,0 +1,46 @@
+namespace NATS.Server.JetStream.Api;
+
+public sealed class JetStreamApiResponse
+{
+    public JetStreamApiError? Error { get; init; }
+    public JetStreamStreamInfo? StreamInfo { get; init; }
+    public JetStreamConsumerInfo? ConsumerInfo { get; init; }
+
+    public static JetStreamApiResponse NotFound(string subject) => new()
+    {
+        Error = new JetStreamApiError
+        {
+            Code = 404,
+            Description = $"unknown api subject '{subject}'",
+        },
+    };
+
+    public static JetStreamApiResponse Ok() => new();
+}
+
+public sealed class JetStreamStreamInfo
+{
+    public required JetStreamStreamConfig Config { get; init; }
+    public required JetStreamStreamState State { get; init; }
+}
+
+public sealed class JetStreamConsumerInfo
+{
+    public required JetStreamConsumerConfig Config { get; init; }
+}
+
+public sealed class JetStreamStreamConfig
+{
+    public string Name { get; init; } = string.Empty;
+}
+
+public sealed class JetStreamStreamState
+{
+    public ulong Messages { get; init; }
+    public ulong FirstSeq { get; init; }
+}
+
+public sealed class JetStreamConsumerConfig
+{
+    public string DurableName { get; init; } = string.Empty;
+}
diff --git a/src/NATS.Server/JetStream/Api/JetStreamApiRouter.cs b/src/NATS.Server/JetStream/Api/JetStreamApiRouter.cs
new file mode 100644
index 0000000..f5445a8
--- /dev/null
+++ b/src/NATS.Server/JetStream/Api/JetStreamApiRouter.cs
@@ -0,0 +1,7 @@
+namespace NATS.Server.JetStream.Api;
+
+public sealed class JetStreamApiRouter
+{
+    public JetStreamApiResponse Route(string subject, ReadOnlySpan<byte> payload)
+        => JetStreamApiResponse.NotFound(subject);
+}
diff --git a/src/NATS.Server/NatsServer.cs b/src/NATS.Server/NatsServer.cs
index 1a394b7..537acb6 100644
--- a/src/NATS.Server/NatsServer.cs
+++ b/src/NATS.Server/NatsServer.cs
@@ -11,6 +11,7 @@ using NATS.Server.Auth;
 using NATS.Server.Configuration;
 using NATS.Server.Gateways;
 using NATS.Server.JetStream;
+using NATS.Server.JetStream.Api;
 using NATS.Server.LeafNodes;
 using NATS.Server.Monitoring;
 using NATS.Server.Protocol;
@@ -47,6 +48,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly GatewayManager? _gatewayManager;
     private readonly LeafNodeManager? _leafNodeManager;
     private readonly JetStreamService? _jetStreamService;
+    private readonly JetStreamApiRouter? _jetStreamApiRouter;
     private Socket? _listener;
     private Socket? _wsListener;
     private readonly TaskCompletionSource _wsAcceptLoopExited = new(TaskCreationOptions.RunContinuationsAsynchronously);
@@ -84,6 +86,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public bool IsShuttingDown => Volatile.Read(ref _shutdown) != 0;
     public bool IsLameDuckMode => Volatile.Read(ref _lameDuck) != 0;
     public string? ClusterListen => _routeManager?.ListenEndpoint;
+    public JetStreamApiRouter? JetStreamApiRouter => _jetStreamApiRouter;
     public Action? ReOpenLogFile { get; set; }
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 
@@ -327,6 +330,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         if (options.JetStream != null)
         {
             _jetStreamService = new JetStreamService(options.JetStream);
+            _jetStreamApiRouter = new JetStreamApiRouter();
         }
 
         if (options.HasTls)
diff --git a/tests/NATS.Server.Tests/JetStreamApiFixture.cs b/tests/NATS.Server.Tests/JetStreamApiFixture.cs
new file mode 100644
index 0000000..3547905
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStreamApiFixture.cs
@@ -0,0 +1,14 @@
+using System.Text;
+using NATS.Server.JetStream.Api;
+
+namespace NATS.Server.Tests;
+
+internal static class JetStreamApiFixture
+{
+    private static readonly JetStreamApiRouter Router = new();
+
+    public static Task<JetStreamApiResponse> RequestAsync(string subject, string payload)
+    {
+        return Task.FromResult(Router.Route(subject, Encoding.UTF8.GetBytes(payload)));
+    }
+}
diff --git a/tests/NATS.Server.Tests/JetStreamApiRouterTests.cs b/tests/NATS.Server.Tests/JetStreamApiRouterTests.cs
new file mode 100644
index 0000000..b0a9cf5
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStreamApiRouterTests.cs
@@ -0,0 +1,12 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamApiRouterTests
+{
+    [Fact]
+    public async Task Unknown_js_api_subject_returns_structured_error()
+    {
+        var response = await JetStreamApiFixture.RequestAsync("$JS.API.BAD", "{}");
+        response.Error.ShouldNotBeNull();
+        response.Error!.Code.ShouldBe(404);
+    }
+}