diff --git a/source/Halibut.Tests/Support/Streams/FlushBufferedStream.cs b/source/Halibut.Tests/Support/Streams/FlushBufferedStream.cs
new file mode 100644
index 000000000..0aa5b9be1
--- /dev/null
+++ b/source/Halibut.Tests/Support/Streams/FlushBufferedStream.cs
@@ -0,0 +1,81 @@
+#nullable enable
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Halibut.Tests.Support.Streams
+{
+    /// <summary>
+    /// A stream that buffers all writes in memory and only forwards them to the underlying
+    /// stream when Flush or FlushAsync is called.
+    /// </summary>
+    public class TestOnlySendDataWhenFlushedStream : DelegateStreamBase
+    {
+        readonly Stream inner;
+        MemoryStream writeBuffer = new MemoryStream();
+
+        public TestOnlySendDataWhenFlushedStream(Stream inner)
+        {
+            this.inner = inner;
+        }
+
+        public override Stream Inner => inner;
+
+        public override void Write(byte[] buffer, int offset, int count)
+            => writeBuffer.Write(buffer, offset, count);
+
+        public override void WriteByte(byte value)
+            => writeBuffer.WriteByte(value);
+
+        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            => writeBuffer.WriteAsync(buffer, offset, count, cancellationToken);
+
+        public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback? callback, object? state)
+            => writeBuffer.BeginWrite(buffer, offset, count, callback, state);
+
+        public override void EndWrite(IAsyncResult asyncResult)
+            => writeBuffer.EndWrite(asyncResult);
+
+#if !NETFRAMEWORK
+        public override async ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
+            => await writeBuffer.WriteAsync(buffer, cancellationToken);
+
+        public override void Write(ReadOnlySpan<byte> buffer)
+            => writeBuffer.Write(buffer);
+#endif
+
+        public override void Flush()
+        {
+            writeBuffer.Position = 0;
+            writeBuffer.CopyTo(inner);
+            writeBuffer = new MemoryStream();
+            inner.Flush();
+        }
+
+        public override async Task FlushAsync(CancellationToken cancellationToken)
+        {
+            writeBuffer.Position = 0;
+            await writeBuffer.CopyToAsync(inner, 8192, cancellationToken);
+            writeBuffer = new MemoryStream();
+            await inner.FlushAsync(cancellationToken);
+        }
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                writeBuffer.Dispose();
+            }
+            base.Dispose(disposing);
+        }
+
+#if !NETFRAMEWORK
+        public override async ValueTask DisposeAsync()
+        {
+            await writeBuffer.DisposeAsync();
+            await inner.DisposeAsync();
+        }
+#endif
+    }
+}
diff --git a/source/Halibut.Tests/Transport/WhenTransportUsesFlushBufferedStreamFixture.cs b/source/Halibut.Tests/Transport/WhenTransportUsesFlushBufferedStreamFixture.cs
new file mode 100644
index 000000000..d22f7c1c0
--- /dev/null
+++ b/source/Halibut.Tests/Transport/WhenTransportUsesFlushBufferedStreamFixture.cs
@@ -0,0 +1,52 @@
+using System.Threading.Tasks;
+using FluentAssertions;
+using Halibut.Tests.Support;
+using Halibut.Tests.Support.Streams;
+using Halibut.Tests.Support.TestAttributes;
+using Halibut.Tests.Support.TestCases;
+using Halibut.Tests.TestServices.Async;
+using Halibut.TestUtils.Contracts;
+using NUnit.Framework;
+
+namespace Halibut.Tests.Transport
+{
+    public class WhenTransportUsesFlushBufferedStreamFixture : BaseTest
+    {
+#if !NETFRAMEWORK
+        // On net48, SslStream uses APM (BeginWrite/EndWrite) internally during the TLS handshake rather than
+        // WriteAsync/FlushAsync. TestOnlySendDataWhenFlushedStream.BeginWrite only writes to an in-memory buffer
+        // and never flushes, so handshake data from both sides sits buffered and neither end receives anything —
+        // the client times out waiting for the ServerHello. On modern .NET, SslStream calls FlushAsync after each
+        // TLS record, so the handshake works correctly. There is no hook to inject a flush between SslStream's
+        // internal BeginWrite/EndWrite calls on net48, so this test cannot run there.
+        //
+        // Why net48's SslStream never calls Flush: it was designed assuming the underlying stream sends data
+        // immediately on Write/BeginWrite — a valid assumption for NetworkStream over TCP, where bytes go out
+        // without needing an explicit Flush. The net48 implementation treated Flush as a no-op concern and never
+        // added the call. When SslStream was rewritten for modern .NET it was made transport-agnostic, explicitly
+        // calling FlushAsync after each TLS record so it works correctly with any stream implementation.
+        // TestOnlySendDataWhenFlushedStream violates the net48 assumption by holding writes in a MemoryStream
+        // until Flush is called, which net48's SslStream never does.
+        
+        [Test]
+        [LatestClientAndLatestServiceTestCases(testNetworkConditions: false)]
+        public async Task RequestsSucceed_WhenStreamsOnlyForwardDataOnFlush(ClientAndServiceTestCase clientAndServiceTestCase)
+        {
+            await using var clientAndService = await clientAndServiceTestCase.CreateTestCaseBuilder()
+                .AsLatestClientAndLatestServiceBuilder()
+                .WithClientStreamFactory(new StreamWrappingStreamFactory { WrapStreamWith = s => new TestOnlySendDataWhenFlushedStream(s) })
+                .WithServiceStreamFactory(new StreamWrappingStreamFactory { WrapStreamWith = s => new TestOnlySendDataWhenFlushedStream(s) })
+                .WithEchoService()
+                .Build(CancellationToken);
+
+            var echo = clientAndService.CreateAsyncClient<IEchoService, IAsyncClientEchoService>();
+
+            for (var i = 0; i < clientAndServiceTestCase.RecommendedIterations; i++)
+            {
+                var result = await echo.SayHelloAsync("hello");
+                result.Should().Be("hello...");
+            }
+        }
+#endif
+    }
+}
diff --git a/source/Halibut/Transport/Protocol/MessageExchangeStream.cs b/source/Halibut/Transport/Protocol/MessageExchangeStream.cs
index 334462a4c..1f7050104 100644
--- a/source/Halibut/Transport/Protocol/MessageExchangeStream.cs
+++ b/source/Halibut/Transport/Protocol/MessageExchangeStream.cs
@@ -191,6 +191,7 @@ public async Task SendAsync<T>(T message, CancellationToken cancellationToken)
             var serializedStreams = await serializer.WriteMessageAsync(stream, message, cancellationToken);
             await WriteEachStreamAsync(serializedStreams, cancellationToken);
             
+            await stream.FlushAsync(cancellationToken);
             log.Write(EventType.Diagnostic, "Sent message");
         }