diff --git a/src/Fluxzy.Core/Clients/H2/H2ConnectionPool.cs b/src/Fluxzy.Core/Clients/H2/H2ConnectionPool.cs
index c5c9cb2e..861ca663 100644
--- a/src/Fluxzy.Core/Clients/H2/H2ConnectionPool.cs
+++ b/src/Fluxzy.Core/Clients/H2/H2ConnectionPool.cs
@@ -107,7 +107,7 @@ public void Init()
             if (_initDone)
                 return;
 
-            _initDone = false;
+            _initDone = true;
 
             //_baseStream.Write(Preface);
             SettingHelper.WriteWelcomeSettings(H2Constants.Preface, _baseStream, Setting, _logger);
diff --git a/src/Fluxzy.Core/Clients/H2/H2StreamSetting.cs b/src/Fluxzy.Core/Clients/H2/H2StreamSetting.cs
index a6bd83f4..e169a92b 100644
--- a/src/Fluxzy.Core/Clients/H2/H2StreamSetting.cs
+++ b/src/Fluxzy.Core/Clients/H2/H2StreamSetting.cs
@@ -13,7 +13,9 @@ public class H2StreamSetting
             SettingsMaxConcurrentStreams = 256
         };
 
-        public PeerSetting Remote { get; set; } = new();
+        public PeerSetting Remote { get; set; } = new() {
+            WindowSize = 65535 // HTTP/2 spec default (RFC 9113 §6.5.2)
+        };
 
         public int SettingsHeaderTableSize { get; set; } = 65536;
 
diff --git a/src/Fluxzy.Core/Clients/H2/StreamPool.cs b/src/Fluxzy.Core/Clients/H2/StreamPool.cs
index 91abe94c..79aef0b2 100644
--- a/src/Fluxzy.Core/Clients/H2/StreamPool.cs
+++ b/src/Fluxzy.Core/Clients/H2/StreamPool.cs
@@ -121,17 +121,18 @@ public void OnGoAway(Exception? ex)
 
         public int ShouldWindowUpdate(int dataLength)
         {
-            var windowIncrement = 0;
+            var newValue = Interlocked.Add(ref _overallWindowSize, dataLength);
+            var threshold = (int)(0.5 * Context.Setting.Local.WindowSize);
 
-            _overallWindowSize += dataLength;
+            if (newValue > threshold) {
+                // CAS to claim the accumulated value
+                var claimed = Interlocked.Exchange(ref _overallWindowSize, 0);
 
-            if (_overallWindowSize > 0.5 * Context.Setting.Local.WindowSize) {
-                windowIncrement = _overallWindowSize;
-
-                _overallWindowSize = 0;
+                if (claimed > 0)
+                    return claimed;
             }
 
-            return windowIncrement;
+            return 0;
         }
     }
 }
diff --git a/src/Fluxzy.Core/Clients/H2/StreamWorker.cs b/src/Fluxzy.Core/Clients/H2/StreamWorker.cs
index 22003515..5663615a 100644
--- a/src/Fluxzy.Core/Clients/H2/StreamWorker.cs
+++ b/src/Fluxzy.Core/Clients/H2/StreamWorker.cs
@@ -48,7 +48,7 @@ public StreamWorker(
             _resetTokenSource = resetTokenSource;
 
             RemoteWindowSize = new WindowSizeHolder(parent.Context.Logger,
-                parent.Context.Setting.OverallWindowSize,
+                parent.Context.Setting.Remote.WindowSize,
                 streamIdentifier);
 
             _logger = parent.Context.Logger;
diff --git a/src/Fluxzy.Core/Clients/H2/WindowSizeHolder.cs b/src/Fluxzy.Core/Clients/H2/WindowSizeHolder.cs
index 81914ff7..073a0599 100644
--- a/src/Fluxzy.Core/Clients/H2/WindowSizeHolder.cs
+++ b/src/Fluxzy.Core/Clients/H2/WindowSizeHolder.cs
@@ -81,8 +81,12 @@ public void UpdateWindowSize(int windowSizeIncrement)
                     break;
             }
 
-            // Signal ONE waiter if we crossed from ≤0 to >0.
-            if (before <= 0 && after > 0)
+            // Signal ONE waiter whenever window is positive — not just on the
+            // 0→positive transition.  When two concurrent UpdateWindowSize calls
+            // race, only one observes before≤0; the other sees before>0 and skips
+            // the wake even though a waiter may still be queued.  WakeOneWaiter is
+            // cheap (lock + empty-check) when no waiters exist.
+            if (after > 0)
             {
                 WakeOneWaiter();
             }
diff --git a/test/Fluxzy.Tests/UnitTests/Handlers/Http2ClientHandler.cs b/test/Fluxzy.Tests/UnitTests/Handlers/Http2ClientHandler.cs
index 49ac2b00..5e13c3ad 100644
--- a/test/Fluxzy.Tests/UnitTests/Handlers/Http2ClientHandler.cs
+++ b/test/Fluxzy.Tests/UnitTests/Handlers/Http2ClientHandler.cs
@@ -19,7 +19,7 @@ public static IEnumerable<object[]> GetHttpMethods
         {
             get
             {
-                int[] checkLength = { 0, 152, 12464, 150002 };
+                int[] checkLength = { 0, 152, 12464, 150002, 150002 * 16 };
 
                 foreach (var length in checkLength)
                 {