diff --git a/src/NATS.Server/ClientFlags.cs b/src/NATS.Server/ClientFlags.cs
new file mode 100644
index 0000000..8f9e915
--- /dev/null
+++ b/src/NATS.Server/ClientFlags.cs
@@ -0,0 +1,41 @@
+namespace NATS.Server;
+
+/// <summary>
+/// Connection state flags tracked per client.
+/// Corresponds to Go server/client.go clientFlag bitfield.
+/// Thread-safe via Interlocked operations on the backing int.
+/// </summary>
+[Flags]
+public enum ClientFlags
+{
+    ConnectReceived = 1 << 0,
+    FirstPongSent = 1 << 1,
+    HandshakeComplete = 1 << 2,
+    CloseConnection = 1 << 3,
+    WriteLoopStarted = 1 << 4,
+    IsSlowConsumer = 1 << 5,
+    ConnectProcessFinished = 1 << 6,
+}
+
+/// <summary>
+/// Thread-safe holder for client flags using Interlocked operations.
+/// </summary>
+public sealed class ClientFlagHolder
+{
+    private int _flags;
+
+    public void SetFlag(ClientFlags flag)
+    {
+        Interlocked.Or(ref _flags, (int)flag);
+    }
+
+    public void ClearFlag(ClientFlags flag)
+    {
+        Interlocked.And(ref _flags, ~(int)flag);
+    }
+
+    public bool HasFlag(ClientFlags flag)
+    {
+        return (Volatile.Read(ref _flags) & (int)flag) != 0;
+    }
+}
diff --git a/tests/NATS.Server.Tests/ClientFlagsTests.cs b/tests/NATS.Server.Tests/ClientFlagsTests.cs
new file mode 100644
index 0000000..9958c52
--- /dev/null
+++ b/tests/NATS.Server.Tests/ClientFlagsTests.cs
@@ -0,0 +1,53 @@
+namespace NATS.Server.Tests;
+
+public class ClientFlagsTests
+{
+    [Fact]
+    public void SetFlag_and_HasFlag_work()
+    {
+        var holder = new ClientFlagHolder();
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeFalse();
+
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void ClearFlag_removes_flag()
+    {
+        var holder = new ClientFlagHolder();
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.SetFlag(ClientFlags.IsSlowConsumer);
+
+        holder.ClearFlag(ClientFlags.ConnectReceived);
+
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeFalse();
+        holder.HasFlag(ClientFlags.IsSlowConsumer).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Multiple_flags_can_be_set_independently()
+    {
+        var holder = new ClientFlagHolder();
+        holder.SetFlag(ClientFlags.ConnectReceived);
+        holder.SetFlag(ClientFlags.WriteLoopStarted);
+        holder.SetFlag(ClientFlags.FirstPongSent);
+
+        holder.HasFlag(ClientFlags.ConnectReceived).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.WriteLoopStarted).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.FirstPongSent).ShouldBeTrue();
+        holder.HasFlag(ClientFlags.IsSlowConsumer).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void SetFlag_is_thread_safe()
+    {
+        var holder = new ClientFlagHolder();
+        var flags = Enum.GetValues<ClientFlags>();
+
+        Parallel.ForEach(flags, flag => holder.SetFlag(flag));
+
+        foreach (var flag in flags)
+            holder.HasFlag(flag).ShouldBeTrue();
+    }
+}