diff --git a/GameCli.Tests/ProtocolTests.cs b/GameCli.Tests/ProtocolTests.cs
new file mode 100644
index 0000000..bc47a1e
--- /dev/null
+++ b/GameCli.Tests/ProtocolTests.cs
@@ -0,0 +1,81 @@
+using System.Text.Json;
+using GameCli;
+using Xunit;
+
+namespace GameCli.Tests;
+
+public class ProtocolTests
+{
+    [Fact]
+    public void StepInput_ParsesActionAndTarget()
+    {
+        var json = """{"action":2,"target":[0.35,0.60]}""";
+        var input = JsonSerializer.Deserialize(json, ProtocolJsonContext.Default.StepInput);
+        Assert.NotNull(input);
+        Assert.Equal(2, input!.Action);
+        Assert.NotNull(input.Target);
+        Assert.Equal(0.35f, input.Target![0]);
+        Assert.Equal(0.60f, input.Target![1]);
+        Assert.Null(input.Cmd);
+    }
+
+    [Fact]
+    public void StepInput_ParsesResetCommand()
+    {
+        var json = """{"cmd":"reset","seed":12345}""";
+        var input = JsonSerializer.Deserialize(json, ProtocolJsonContext.Default.StepInput);
+        Assert.NotNull(input);
+        Assert.Equal("reset", input!.Cmd);
+        Assert.Equal(12345, input.Seed);
+    }
+
+    [Fact]
+    public void StepInput_ParsesResetCommandWithoutSeed()
+    {
+        var json = """{"cmd":"reset"}""";
+        var input = JsonSerializer.Deserialize(json, ProtocolJsonContext.Default.StepInput);
+        Assert.NotNull(input);
+        Assert.Equal("reset", input!.Cmd);
+        Assert.Null(input.Seed);
+    }
+
+    [Fact]
+    public void StepOutput_SerializesAllFields()
+    {
+        var output = new StepOutput
+        {
+            Obs = new[] { 0.1f, 0.2f, 0.3f, 0.4f, 0.5f, 0.6f, 0.7f },
+            State = new ShipStateDto { X = 0.5f, Y = 0.6f, Angle = 0.1f, Engine = 2 },
+            Reward = -0.42f,
+            Done = false,
+            Step = 137,
+        };
+        var json = JsonSerializer.Serialize(output, ProtocolJsonContext.Default.StepOutput);
+        Assert.Contains("\"obs\":", json);
+        Assert.Contains("\"state\":", json);
+        Assert.Contains("\"reward\":", json);
+        Assert.Contains("\"done\":false", json);
+        Assert.Contains("\"step\":137", json);
+        Assert.Contains("\"engine\":2", json);
+    }
+
+    [Fact]
+    public void InitMessage_SerializesExpectedShape()
+    {
+        var init = new InitMessage
+        {
+            Init = new InitPayload
+            {
+                World = new[] { 1.0f, 1.0f },
+                Dt = 0.02f,
+                ObsDim = 7,
+                NActions = 4,
+            },
+        };
+        var json = JsonSerializer.Serialize(init, ProtocolJsonContext.Default.InitMessage);
+        Assert.Contains("\"world\":[1,1]", json);
+        Assert.Contains("\"dt\":0.02", json);
+        Assert.Contains("\"obs_dim\":7", json);
+        Assert.Contains("\"n_actions\":4", json);
+    }
+}
diff --git a/GameCli/GameCli.csproj b/GameCli/GameCli.csproj
index 206b89a..fb5fa1f 100644
--- a/GameCli/GameCli.csproj
+++ b/GameCli/GameCli.csproj
@@ -7,4 +7,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="GameCli.Tests" />
+  </ItemGroup>
+
 </Project>
diff --git a/GameCli/Protocol.cs b/GameCli/Protocol.cs
new file mode 100644
index 0000000..f3ede99
--- /dev/null
+++ b/GameCli/Protocol.cs
@@ -0,0 +1,56 @@
+using System.Text.Json.Serialization;
+
+namespace GameCli;
+
+/// <summary>Input line from the client (per step OR a reset command).</summary>
+public sealed class StepInput
+{
+    [JsonPropertyName("action")] public int? Action { get; set; }
+    [JsonPropertyName("target")] public float[]? Target { get; set; }
+    [JsonPropertyName("cmd")]    public string? Cmd { get; set; }
+    [JsonPropertyName("seed")]   public int? Seed { get; set; }
+}
+
+/// <summary>Output line from the CLI after each step or reset.</summary>
+public sealed class StepOutput
+{
+    [JsonPropertyName("obs")]    public float[] Obs { get; set; } = System.Array.Empty<float>();
+    [JsonPropertyName("state")]  public ShipStateDto State { get; set; } = new();
+    [JsonPropertyName("reward")] public float Reward { get; set; }
+    [JsonPropertyName("done")]   public bool Done { get; set; }
+    [JsonPropertyName("step")]   public int Step { get; set; }
+}
+
+/// <summary>Renderable ship pose (subset of ShipState that the UI actually draws).</summary>
+public sealed class ShipStateDto
+{
+    [JsonPropertyName("x")]      public float X { get; set; }
+    [JsonPropertyName("y")]      public float Y { get; set; }
+    [JsonPropertyName("angle")]  public float Angle { get; set; }
+    [JsonPropertyName("engine")] public int Engine { get; set; }
+}
+
+/// <summary>Startup handshake emitted as the very first stdout line.</summary>
+public sealed class InitMessage
+{
+    [JsonPropertyName("init")] public InitPayload Init { get; set; } = new();
+}
+
+public sealed class InitPayload
+{
+    [JsonPropertyName("world")]     public float[] World { get; set; } = new[] { 1f, 1f };
+    [JsonPropertyName("dt")]        public float Dt { get; set; }
+    [JsonPropertyName("obs_dim")]   public int ObsDim { get; set; }
+    [JsonPropertyName("n_actions")] public int NActions { get; set; }
+}
+
+/// <summary>Source-generated JSON contracts for AOT-friendly, allocation-lean (de)serialization.</summary>
+[JsonSourceGenerationOptions(WriteIndented = false)]
+[JsonSerializable(typeof(StepInput))]
+[JsonSerializable(typeof(StepOutput))]
+[JsonSerializable(typeof(ShipStateDto))]
+[JsonSerializable(typeof(InitMessage))]
+[JsonSerializable(typeof(InitPayload))]
+internal partial class ProtocolJsonContext : JsonSerializerContext
+{
+}