diff --git a/tests/DeterministicSim.Engine.Tests.csproj b/tests/DeterministicSim.Engine.Tests.csproj
index ca14eb3..10e4f42 100644
--- a/tests/DeterministicSim.Engine.Tests.csproj
+++ b/tests/DeterministicSim.Engine.Tests.csproj
@@ -34,6 +34,7 @@
 
   <!-- Engine unit tests -->
   <ItemGroup>
+    <Compile Include="unit/core/EntityAgnosticTests.cs" />
     <Compile Include="unit/engine/FixedMathTests.cs" />
     <Compile Include="unit/determinism/StateHashTests.cs" />
     <Compile Include="unit/determinism/SimulationDeterminismTests.cs" />
diff --git a/tests/unit/core/EntityAgnosticTests.cs b/tests/unit/core/EntityAgnosticTests.cs
new file mode 100644
index 0000000..4ecf879
--- /dev/null
+++ b/tests/unit/core/EntityAgnosticTests.cs
@@ -0,0 +1,130 @@
+using DeterministicSim.Engine.Core;
+using DeterministicSim.Engine.Net;
+using Xunit;
+using System.Runtime.CompilerServices;
+
+namespace DeterministicSim.Engine.Tests.Core;
+
+public class EntityAgnosticTests
+{
+    [Fact]
+    public void GameState_1Entity_InitializesCorrectly()
+    {
+        var state = new GameState(1);
+        Assert.Equal(1, state.EntityCount);
+
+        state.SetPosX(0, 100);
+        state.SetPosY(0, 200);
+
+        Assert.Equal(100, state.GetPosX(0));
+        Assert.Equal(200, state.GetPosY(0));
+        Assert.True(state.IsActive(0)); // Should be active by default
+    }
+
+    [Fact]
+    public void GameState_100Entities_InitializesCorrectly()
+    {
+        var state = new GameState(100);
+        Assert.Equal(100, state.EntityCount);
+
+        for (int i = 0; i < 100; i++)
+        {
+            state.SetPosX(i, i * 10);
+            state.SetPosY(i, i * 20);
+            state.SetVelX(i, i * 2);
+            state.SetVelY(i, i * 3);
+            state.SetActive(i, i % 2 == 0);
+        }
+
+        for (int i = 0; i < 100; i++)
+        {
+            Assert.Equal(i * 10, state.GetPosX(i));
+            Assert.Equal(i * 20, state.GetPosY(i));
+            Assert.Equal(i * 2, state.GetVelX(i));
+            Assert.Equal(i * 3, state.GetVelY(i));
+            Assert.Equal(i % 2 == 0, state.IsActive(i));
+        }
+    }
+
+    [Fact]
+    public void GameState_CopyFrom_MismatchEntityCount_HandlesGracefully()
+    {
+        var src = new GameState(10);
+        for(int i = 0; i < 10; i++)
+        {
+            src.SetPosX(i, i * 100);
+        }
+
+        var dst = new GameState(5);
+        dst.CopyFrom(src);
+
+        // Should only copy up to Math.Min(src.EntityCount, dst.EntityCount)
+        for(int i = 0; i < 5; i++)
+        {
+            Assert.Equal(i * 100, dst.GetPosX(i));
+        }
+
+        var dst2 = new GameState(15);
+        dst2.CopyFrom(src);
+        for(int i = 0; i < 10; i++)
+        {
+            Assert.Equal(i * 100, dst2.GetPosX(i));
+        }
+        for(int i = 10; i < 15; i++)
+        {
+            Assert.Equal(0, dst2.GetPosX(i));
+        }
+    }
+
+    [Fact]
+    public void RollbackController_1Player_WorksCorrectly()
+    {
+        var config = RollbackConfig.Default();
+        config.PlayerCount = 1;
+
+        var controller = new RollbackController(config);
+
+        var inputFrame = new InputFrame(1, 1);
+        inputFrame.SetPlayerInputs(0, 42);
+
+        controller.SaveInputs(1, inputFrame);
+
+        var storedFrame = controller.GetInputs(1);
+        Assert.Equal(1, storedFrame.PlayerCount);
+        Assert.Equal(42, storedFrame.GetPlayerInputs(0));
+
+        controller.SaveState(1);
+        var state = controller.GetState(1);
+        Assert.NotNull(state);
+        Assert.Equal(1, state!.EntityCount);
+    }
+
+    [Fact]
+    public void RollbackController_4Players_WorksCorrectly()
+    {
+        var config = RollbackConfig.Default();
+        config.PlayerCount = 4;
+
+        var controller = new RollbackController(config);
+
+        var inputFrame = new InputFrame(1, 4);
+        inputFrame.SetPlayerInputs(0, 10);
+        inputFrame.SetPlayerInputs(1, 20);
+        inputFrame.SetPlayerInputs(2, 30);
+        inputFrame.SetPlayerInputs(3, 40);
+
+        controller.SaveInputs(1, inputFrame);
+
+        var storedFrame = controller.GetInputs(1);
+        Assert.Equal(4, storedFrame.PlayerCount);
+        Assert.Equal(10, storedFrame.GetPlayerInputs(0));
+        Assert.Equal(20, storedFrame.GetPlayerInputs(1));
+        Assert.Equal(30, storedFrame.GetPlayerInputs(2));
+        Assert.Equal(40, storedFrame.GetPlayerInputs(3));
+
+        controller.SaveState(1);
+        var state = controller.GetState(1);
+        Assert.NotNull(state);
+        Assert.Equal(4, state!.EntityCount);
+    }
+}