diff --git a/GameCli/Program.cs b/GameCli/Program.cs
index 8e5680c..9ddd8b5 100644
--- a/GameCli/Program.cs
+++ b/GameCli/Program.cs
@@ -1,2 +1,98 @@
-// Placeholder. Rewritten in Task 8.
-System.Console.Error.WriteLine("GameCli placeholder");
+using System.Text.Json;
+using GameCli;
+
+// Deterministic PRNG for start-pose randomization on reset.
+// Seeded fresh on each reset if the caller supplies a seed.
+var rng = new Random();
+
+// Current world state and step counter. Reset on {"cmd":"reset"}.
+var ship = ShipState.AtCenter();
+int stepCount = 0;
+int lastAction = 0;
+
+// --- Startup handshake ---------------------------------------------------
+var init = new InitMessage
+{
+    Init = new InitPayload
+    {
+        World = new[] { 1f, 1f },
+        Dt = Physics.Dt,
+        ObsDim = Observation.Dim,
+        NActions = 4,
+    },
+};
+Console.WriteLine(JsonSerializer.Serialize(init, ProtocolJsonContext.Default.InitMessage));
+Console.Out.Flush();
+
+// --- Read-line loop ------------------------------------------------------
+string? line;
+while ((line = Console.In.ReadLine()) is not null)
+{
+    if (line.Length == 0) continue;
+
+    StepInput? input;
+    try
+    {
+        input = JsonSerializer.Deserialize(line, ProtocolJsonContext.Default.StepInput);
+    }
+    catch (JsonException ex)
+    {
+        Console.Error.WriteLine($"invalid json: {ex.Message}");
+        continue;
+    }
+    if (input is null) continue;
+
+    // Reset command.
+    if (input.Cmd == "reset")
+    {
+        if (input.Seed is int seed) rng = new Random(seed);
+        ship = RandomStartPose(rng);
+        stepCount = 0;
+        lastAction = 0;
+        WriteObservation((0.5f, 0.5f), reward: 0f, done: false);
+        continue;
+    }
+
+    // Step command.
+    int action = input.Action ?? 0;
+    var target = (input.Target is { Length: >= 2 })
+        ? (input.Target[0], input.Target[1])
+        : (0.5f, 0.5f);
+
+    ship = Physics.Step(ship, action);
+    lastAction = action;
+    stepCount++;
+
+    float reward = Reward.Compute(ship, target, action);
+    WriteObservation(target, reward, done: false);
+}
+
+// --- Helpers -------------------------------------------------------------
+void WriteObservation((float X, float Y) target, float reward, bool done)
+{
+    var output = new StepOutput
+    {
+        Obs = Observation.Build(ship, target),
+        State = new ShipStateDto
+        {
+            X = ship.X,
+            Y = ship.Y,
+            Angle = ship.Angle,
+            Engine = lastAction,
+        },
+        Reward = reward,
+        Done = done,
+        Step = stepCount,
+    };
+    Console.WriteLine(JsonSerializer.Serialize(output, ProtocolJsonContext.Default.StepOutput));
+    Console.Out.Flush();
+}
+
+static ShipState RandomStartPose(Random rng)
+{
+    // Random position in a comfortable box; zero velocity, near-upright.
+    float x = 0.2f + (float)rng.NextDouble() * 0.6f;
+    float y = 0.2f + (float)rng.NextDouble() * 0.4f;
+    float angle = ((float)rng.NextDouble() - 0.5f) * 0.4f;  // ±0.2 rad
+    return new ShipState(x, y, 0f, 0f, angle, 0f);
+}