diff --git a/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.cs b/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.cs
index b16548c7b4c374..45a5b9f0372696 100644
--- a/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.cs
+++ b/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.cs
@@ -45,6 +45,8 @@ public partial class Thread
     {
         [ThreadStatic]
         public static bool ThrowOnBlockingWaitOnJSInteropThread;
+        [ThreadStatic]
+        public static bool WarnOnBlockingWaitOnJSInteropThread;
 
         public static void AssureBlockingPossible() { throw null; }
         public static void ForceBlockingWait(Action<object?> action, object? state) { throw null; }
diff --git a/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.txt b/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.txt
index 3b80cb0de6753b..2a6434973ff1e3 100644
--- a/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.txt
+++ b/src/libraries/System.Private.CoreLib/ref/System.Private.CoreLib.ExtraApis.txt
@@ -7,4 +7,5 @@ T:System.Diagnostics.DebugProvider
 M:System.Diagnostics.Debug.SetProvider(System.Diagnostics.DebugProvider)
 M:System.Threading.Thread.AssureBlockingPossible
 F:System.Threading.Thread.ThrowOnBlockingWaitOnJSInteropThread
+F:System.Threading.Thread.WarnOnBlockingWaitOnJSInteropThread
 F:System.Threading.Thread.ForceBlockingWait
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs
index 7f7bddf24737b3..39233c87c15c96 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs
@@ -41,6 +41,10 @@ public bool Wait(int timeoutMs, bool spinWait)
         {
             Debug.Assert(timeoutMs >= -1);
 
+#if FEATURE_WASM_MANAGED_THREADS
+            Thread.AssureBlockingPossible();
+#endif
+
             int spinCount = spinWait ? _spinCount : 0;
 
             // Try to acquire the semaphore or
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/ManualResetEventSlim.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/ManualResetEventSlim.cs
index a385543f9174ab..516fb42bf0a52e 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/ManualResetEventSlim.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/ManualResetEventSlim.cs
@@ -485,6 +485,10 @@ public bool Wait(int millisecondsTimeout, CancellationToken cancellationToken)
 
             ArgumentOutOfRangeException.ThrowIfLessThan(millisecondsTimeout, -1);
 
+#if FEATURE_WASM_MANAGED_THREADS
+            Thread.AssureBlockingPossible();
+#endif
+
             if (!IsSet)
             {
                 if (millisecondsTimeout == 0)
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/Thread.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/Thread.cs
index 9d3fd7a0466d72..3ef77076a01988 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/Thread.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/Thread.cs
@@ -729,26 +729,46 @@ public static int GetCurrentProcessorId()
         [ThreadStatic]
         public static bool ThrowOnBlockingWaitOnJSInteropThread;
 
-        public static void AssureBlockingPossible()
+        [ThreadStatic]
+        public static bool WarnOnBlockingWaitOnJSInteropThread;
+
+#pragma warning disable CS3001
+        [MethodImplAttribute(MethodImplOptions.InternalCall)]
+        private static extern unsafe void WarnAboutBlockingWait(char* stack, int length);
+
+        public static unsafe void AssureBlockingPossible()
         {
             if (ThrowOnBlockingWaitOnJSInteropThread)
             {
                 throw new PlatformNotSupportedException(SR.WasmThreads_BlockingWaitNotSupportedOnJSInterop);
             }
+            else if (WarnOnBlockingWaitOnJSInteropThread)
+            {
+                var st = $"Blocking the thread with JS interop is dangerous and could lead to deadlock. ManagedThreadId: {Environment.CurrentManagedThreadId}\n{Environment.StackTrace}";
+                fixed (char* stack = st)
+                {
+                    WarnAboutBlockingWait(stack, st.Length);
+                }
+            }
         }
 
+#pragma warning restore CS3001
+
         public static void ForceBlockingWait(Action<object?> action, object? state = null)
         {
             var flag = ThrowOnBlockingWaitOnJSInteropThread;
+            var wflag = WarnOnBlockingWaitOnJSInteropThread;
             try
             {
                 ThrowOnBlockingWaitOnJSInteropThread = false;
+                WarnOnBlockingWaitOnJSInteropThread = false;
 
                 action(state);
             }
             finally
             {
                 ThrowOnBlockingWaitOnJSInteropThread = flag;
+                WarnOnBlockingWaitOnJSInteropThread = wflag;
             }
         }
 #endif
diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs
index 21920bc39b754f..d215a82cd3234f 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs
@@ -117,6 +117,10 @@ internal bool WaitOneNoCheck(
             SafeWaitHandle? waitHandle = _waitHandle;
             ObjectDisposedException.ThrowIf(waitHandle is null, this);
 
+#if FEATURE_WASM_MANAGED_THREADS
+            Thread.AssureBlockingPossible();
+#endif
+
             bool success = false;
             try
             {
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs
index 6faf786f3bd539..c7bb4a81d3bd56 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Interop/JavaScriptExports.cs
@@ -123,18 +123,25 @@ public static void CallDelegate(JSMarshalerArgument* arguments_buffer)
             // arg_2 set by JS caller when there are arguments
             // arg_3 set by JS caller when there are arguments
             // arg_4 set by JS caller when there are arguments
+#if !FEATURE_WASM_MANAGED_THREADS
             try
             {
-#if FEATURE_WASM_MANAGED_THREADS
-                // when we arrive here, we are on the thread which owns the proxies
-                // if we need to dispatch the call to another thread in the future
-                // we may need to consider how to solve blocking of the synchronous call
-                // see also https://github.com/dotnet/runtime/issues/76958#issuecomment-1921418290
-                arg_exc.AssertCurrentThreadContext();
+#else
+            // when we arrive here, we are on the thread which owns the proxies
+            var ctx = arg_exc.AssertCurrentThreadContext();
 
-                if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.AllowBlockingWaitInAsyncCode)
+            try
+            {
+                if (ctx.IsMainThread)
                 {
-                    Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                    if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.ThrowWhenBlockingWait)
+                    {
+                        Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                    }
+                    else if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.WarnWhenBlockingWait)
+                    {
+                        Thread.WarnOnBlockingWaitOnJSInteropThread = true;
+                    }
                 }
 #endif
 
@@ -156,9 +163,16 @@ public static void CallDelegate(JSMarshalerArgument* arguments_buffer)
 #if FEATURE_WASM_MANAGED_THREADS
             finally
             {
-                if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.AllowBlockingWaitInAsyncCode)
+                if (ctx.IsMainThread)
                 {
-                    Thread.ThrowOnBlockingWaitOnJSInteropThread = false;
+                    if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.ThrowWhenBlockingWait)
+                    {
+                        Thread.ThrowOnBlockingWaitOnJSInteropThread = false;
+                    }
+                    else if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.WarnWhenBlockingWait)
+                    {
+                        Thread.WarnOnBlockingWaitOnJSInteropThread = false;
+                    }
                 }
             }
 #endif
@@ -189,12 +203,9 @@ public static void CompleteTask(JSMarshalerArgument* arguments_buffer)
                     }
                 }
 
-                if (holder.CallbackReady != null)
-                {
-#pragma warning disable CA1416 // Validate platform compatibility
-                    Thread.ForceBlockingWait(static (callbackReady) => ((ManualResetEventSlim)callbackReady!).Wait(), holder.CallbackReady);
-#pragma warning restore CA1416 // Validate platform compatibility
-                }
+                // this is always running on I/O thread, so it will not throw PNSE
+                // it's also OK to block here, because we know we will only block shortly, as this is just race with the other thread.
+                holder.CallbackReady?.Wait();
 
                 lock (ctx)
                 {
@@ -247,21 +258,17 @@ public static void GetManagedStackTrace(JSMarshalerArgument* arguments_buffer)
 
         // this is here temporarily, until JSWebWorker becomes public API
         [DynamicDependency(DynamicallyAccessedMemberTypes.NonPublicMethods, "System.Runtime.InteropServices.JavaScript.JSWebWorker", "System.Runtime.InteropServices.JavaScript")]
-        // the marshaled signature is: GCHandle InstallMainSynchronizationContext(nint jsNativeTID, JSThreadBlockingMode jsThreadBlockingMode, JSThreadInteropMode jsThreadInteropMode, MainThreadingMode mainThreadingMode)
+        // the marshaled signature is: GCHandle InstallMainSynchronizationContext(nint jsNativeTID, JSThreadBlockingMode jsThreadBlockingMode)
         public static void InstallMainSynchronizationContext(JSMarshalerArgument* arguments_buffer)
         {
             ref JSMarshalerArgument arg_exc = ref arguments_buffer[0]; // initialized by caller in alloc_stack_frame()
             ref JSMarshalerArgument arg_res = ref arguments_buffer[1];// initialized and set by caller
             ref JSMarshalerArgument arg_1 = ref arguments_buffer[2];// initialized and set by caller
             ref JSMarshalerArgument arg_2 = ref arguments_buffer[3];// initialized and set by caller
-            ref JSMarshalerArgument arg_3 = ref arguments_buffer[4];// initialized and set by caller
-            ref JSMarshalerArgument arg_4 = ref arguments_buffer[5];// initialized and set by caller
 
             try
             {
                 JSProxyContext.ThreadBlockingMode = (JSHostImplementation.JSThreadBlockingMode)arg_2.slot.Int32Value;
-                JSProxyContext.ThreadInteropMode = (JSHostImplementation.JSThreadInteropMode)arg_3.slot.Int32Value;
-                JSProxyContext.MainThreadingMode = (JSHostImplementation.MainThreadingMode)arg_4.slot.Int32Value;
                 var jsSynchronizationContext = JSSynchronizationContext.InstallWebWorkerInterop(true, CancellationToken.None);
                 jsSynchronizationContext.ProxyContext.JSNativeTID = arg_1.slot.IntPtrValue;
                 arg_res.slot.GCHandle = jsSynchronizationContext.ProxyContext.ContextHandle;
@@ -283,9 +290,16 @@ public static void BeforeSyncJSExport(JSMarshalerArgument* arguments_buffer)
             {
                 var ctx = arg_exc.AssertCurrentThreadContext();
                 ctx.IsPendingSynchronousCall = true;
-                if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.AllowBlockingWaitInAsyncCode)
+                if (ctx.IsMainThread)
                 {
-                    Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                    if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.ThrowWhenBlockingWait)
+                    {
+                        Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                    }
+                    else if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.WarnWhenBlockingWait)
+                    {
+                        Thread.WarnOnBlockingWaitOnJSInteropThread = true;
+                    }
                 }
             }
             catch (Exception ex)
@@ -305,9 +319,16 @@ public static void AfterSyncJSExport(JSMarshalerArgument* arguments_buffer)
             {
                 var ctx = arg_exc.AssertCurrentThreadContext();
                 ctx.IsPendingSynchronousCall = false;
-                if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.AllowBlockingWaitInAsyncCode)
+                if (ctx.IsMainThread)
                 {
-                    Thread.ThrowOnBlockingWaitOnJSInteropThread = false;
+                    if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.ThrowWhenBlockingWait)
+                    {
+                        Thread.ThrowOnBlockingWaitOnJSInteropThread = false;
+                    }
+                    else if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.WarnWhenBlockingWait)
+                    {
+                        Thread.WarnOnBlockingWaitOnJSInteropThread = false;
+                    }
                 }
             }
             catch (Exception ex)
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
index 61ea2a85467966..f2d908d947074f 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSFunctionBinding.cs
@@ -230,11 +230,7 @@ internal static unsafe void InvokeJSFunction(JSObject jsFunction, Span<JSMarshal
         internal static unsafe void InvokeJSFunctionCurrent(JSObject jsFunction, Span<JSMarshalerArgument> arguments)
         {
 #if FEATURE_WASM_MANAGED_THREADS
-            if (JSProxyContext.ThreadInteropMode == JSHostImplementation.JSThreadInteropMode.NoSyncJSInterop)
-            {
-                throw new PlatformNotSupportedException("Cannot call synchronous JS functions.");
-            }
-            else if (jsFunction.ProxyContext.IsPendingSynchronousCall)
+            if (jsFunction.ProxyContext.IsPendingSynchronousCall && jsFunction.ProxyContext.IsMainThread)
             {
                 throw new PlatformNotSupportedException("Cannot call synchronous JS function from inside a synchronous call to a C# method.");
             }
@@ -260,11 +256,7 @@ internal static unsafe void InvokeJSFunctionCurrent(JSObject jsFunction, Span<JS
         internal static unsafe void DispatchJSFunctionSync(JSObject jsFunction, Span<JSMarshalerArgument> arguments)
         {
 #if FEATURE_WASM_MANAGED_THREADS
-            if (JSProxyContext.ThreadInteropMode == JSHostImplementation.JSThreadInteropMode.NoSyncJSInterop)
-            {
-                throw new PlatformNotSupportedException("Cannot call synchronous JS functions.");
-            }
-            else if (jsFunction.ProxyContext.IsPendingSynchronousCall)
+            if (jsFunction.ProxyContext.IsPendingSynchronousCall && jsFunction.ProxyContext.IsMainThread)
             {
                 throw new PlatformNotSupportedException("Cannot call synchronous JS function from inside a synchronous call to a C# method.");
             }
@@ -274,10 +266,8 @@ internal static unsafe void DispatchJSFunctionSync(JSObject jsFunction, Span<JSM
 
             // we already know that we are not on the right thread
             // this will be blocking until resolved by that thread
-            // we don't have to disable ThrowOnBlockingWaitOnJSInteropThread, because this is lock in native code
-            // we also don't throw PNSE here, because we know that the target has JS interop installed and that it could not block
+            // we know that the target has JS interop installed and that it could not block
             // so it could take some time, while target is CPU busy, but not forever
-            // see also https://github.com/dotnet/runtime/issues/76958#issuecomment-1921418290
             Interop.Runtime.InvokeJSFunctionSend(jsFunction.ProxyContext.JSNativeTID, functionHandle, args);
 
             ref JSMarshalerArgument exceptionArg = ref arguments[0];
@@ -317,11 +307,7 @@ internal static unsafe void InvokeJSImportImpl(JSFunctionBinding signature, Span
 #if FEATURE_WASM_MANAGED_THREADS
             else
             {
-                if (JSProxyContext.ThreadInteropMode == JSHostImplementation.JSThreadInteropMode.NoSyncJSInterop)
-                {
-                    throw new PlatformNotSupportedException("Cannot call synchronous JS functions.");
-                }
-                else if (targetContext.IsPendingSynchronousCall)
+                if (targetContext.IsPendingSynchronousCall && targetContext.IsMainThread)
                 {
                     throw new PlatformNotSupportedException("Cannot call synchronous JS function from inside a synchronous call to a C# method.");
                 }
@@ -407,10 +393,6 @@ internal static unsafe void DispatchJSImportSyncSend(JSFunctionBinding signature
 
             // we already know that we are not on the right thread
             // this will be blocking until resolved by that thread
-            // we don't have to disable ThrowOnBlockingWaitOnJSInteropThread, because this is lock in native code
-            // we also don't throw PNSE here, because we know that the target has JS interop installed and that it could not block
-            // so it could take some time, while target is CPU busy, but not forever
-            // see also https://github.com/dotnet/runtime/issues/76958#issuecomment-1921418290
             Interop.Runtime.InvokeJSImportSyncSend(targetContext.JSNativeTID, sig, args);
 
             if (exc.slot.Type != MarshalerType.None)
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.Types.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.Types.cs
index 1e1e74b711090d..d65421592248a2 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.Types.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.Types.cs
@@ -63,41 +63,13 @@ public struct IntPtrAndHandle
             internal RuntimeTypeHandle typeHandle;
         }
 
-        // keep in sync with types\internal.ts
-        public enum MainThreadingMode : int
-        {
-            // Running the managed main thread on UI thread.
-            // Managed GC and similar scenarios could be blocking the UI.
-            // Easy to deadlock. Not recommended for production.
-            UIThread = 0,
-            // Running the managed main thread on dedicated WebWorker. Marshaling all JavaScript calls to and from the main thread.
-            DeputyThread = 1,
-            // TODO comments
-            DeputyAndIOThreads = 2,
-        }
-
         // keep in sync with types\internal.ts
         public enum JSThreadBlockingMode : int
         {
-            // throw PlatformNotSupportedException if blocking .Wait is called on threads with JS interop, like JSWebWorker and Main thread.
-            // Avoids deadlocks (typically with pending JS promises on the same thread) by throwing exceptions.
-            NoBlockingWait = 0,
-            // TODO comments
-            AllowBlockingWaitInAsyncCode = 1,
-            // allow .Wait on all threads.
-            // Could cause deadlocks with blocking .Wait on a pending JS Task/Promise on the same thread or similar Task/Promise chain.
-            AllowBlockingWait = 100,
-        }
-
-        // keep in sync with types\internal.ts
-        public enum JSThreadInteropMode : int
-        {
-            // throw PlatformNotSupportedException if synchronous JSImport/JSExport is called on threads with JS interop, like JSWebWorker and Main thread.
-            // calling synchronous JSImport on thread pool or new threads is allowed.
-            NoSyncJSInterop = 0,
-            // allow non-re-entrant synchronous blocking calls to and from JS on JSWebWorker on threads with JS interop, like JSWebWorker and Main thread.
-            // calling synchronous JSImport on thread pool or new threads is allowed.
-            SimpleSynchronousJSInterop = 1,
+            PreventSynchronousJSExport = 0,
+            ThrowWhenBlockingWait = 1,
+            WarnWhenBlockingWait = 2,
+            DangerousAllowBlockingWait = 100,
         }
     }
 }
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSProxyContext.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSProxyContext.cs
index 02618e778bc823..e8ce55892f746b 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSProxyContext.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSProxyContext.cs
@@ -42,9 +42,7 @@ private JSProxyContext()
         public JSSynchronizationContext SynchronizationContext;
         public JSAsyncTaskScheduler? AsyncTaskScheduler;
 
-        public static MainThreadingMode MainThreadingMode = MainThreadingMode.DeputyThread;
-        public static JSThreadBlockingMode ThreadBlockingMode = JSThreadBlockingMode.AllowBlockingWaitInAsyncCode;
-        public static JSThreadInteropMode ThreadInteropMode = JSThreadInteropMode.SimpleSynchronousJSInterop;
+        public static JSThreadBlockingMode ThreadBlockingMode = JSThreadBlockingMode.PreventSynchronousJSExport;
         public bool IsPendingSynchronousCall;
 
 #if !DEBUG
@@ -52,7 +50,7 @@ private JSProxyContext()
 #endif
         public bool IsCurrentThread()
         {
-            return ManagedTID == Environment.CurrentManagedThreadId && (!IsMainThread || MainThreadingMode == MainThreadingMode.UIThread);
+            return ManagedTID == Environment.CurrentManagedThreadId && !IsMainThread;
         }
 
         [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "thread_id")]
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSSynchronizationContext.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSSynchronizationContext.cs
index 2a2ff0f6f02226..e93b1774d6871a 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSSynchronizationContext.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSSynchronizationContext.cs
@@ -50,9 +50,18 @@ public static unsafe JSSynchronizationContext InstallWebWorkerInterop(bool isMai
             ctx.previousSynchronizationContext = SynchronizationContext.Current;
             SynchronizationContext.SetSynchronizationContext(ctx);
 
-            if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.NoBlockingWait)
+            if (!isMainThread)
             {
-                Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.ThrowWhenBlockingWait)
+                {
+                    Thread.ThrowOnBlockingWaitOnJSInteropThread = true;
+                }
+                else if (JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.WarnWhenBlockingWait
+                    || JSProxyContext.ThreadBlockingMode == JSHostImplementation.JSThreadBlockingMode.PreventSynchronousJSExport
+                    )
+                {
+                    Thread.WarnOnBlockingWaitOnJSInteropThread = true;
+                }
             }
 
             var proxyContext = ctx.ProxyContext;
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.Http.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.Http.cs
index 16ef9f72cf5573..489120ee0e4772 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.Http.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.Http.cs
@@ -82,7 +82,7 @@ await Assert.ThrowsAsync<TaskCanceledException>(async () =>
                 {
                     CancellationTokenSource cts = new CancellationTokenSource();
                     var promise = response.Content.ReadAsStringAsync(cts.Token);
-                    Console.WriteLine("HttpClient_CancelInDifferentThread: ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
+                    WebWorkerTestHelper.Log("HttpClient_CancelInDifferentThread: ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
                     cts.Cancel();
                     var res = await promise;
                     throw new Exception("This should be unreachable: " + res);
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.cs
index c67bac997294b9..0a2ae44142dc35 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTest.cs
@@ -11,15 +11,9 @@ namespace System.Runtime.InteropServices.JavaScript.Tests
 
     // TODO test:
     // JSExport 2x
-    // JSExport async
-    // lock
-    // thread allocation, many threads
     // ProxyContext flow, child thread, child task
     // use JSObject after JSWebWorker finished, especially HTTP
-    // WS on JSWebWorker
-    // HTTP continue on TP
     // event pipe
-    // FS
     // JS setTimeout till after JSWebWorker close
     // synchronous .Wait for JS setTimeout on the same thread -> deadlock problem **7)**
 
@@ -159,7 +153,7 @@ public async Task JSSynchronizationContext_Send_Post_Items_Cancellation()
                 }
                 catch (Exception ex)
                 {
-                    Console.WriteLine("Unexpected exception " + ex);
+                    WebWorkerTestHelper.Log("Unexpected exception " + ex);
                     postReady.SetException(ex);
                     return Task.FromException(ex);
                 }
@@ -344,7 +338,7 @@ public async Task ManagedConsole(Executor executor)
             using var cts = CreateTestCaseTimeoutSource();
             await executor.Execute(() =>
             {
-                Console.WriteLine("C# Hello from ManagedThreadId: " + Environment.CurrentManagedThreadId);
+                WebWorkerTestHelper.Log("C# Hello from ManagedThreadId: " + Environment.CurrentManagedThreadId);
                 Console.Clear();
                 return Task.CompletedTask;
             }, cts.Token);
@@ -392,7 +386,7 @@ public async Task ThreadingTimer(Executor executor)
             await executor.Execute(async () =>
             {
                 TaskCompletionSource tcs = new TaskCompletionSource();
-                Console.WriteLine("ThreadingTimer: Start Time: " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
+                WebWorkerTestHelper.Log("ThreadingTimer: Start Time: " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
 
                 using var timer = new Timer(_ =>
                 {
@@ -405,7 +399,7 @@ await executor.Execute(async () =>
                 await tcs.Task;
             }, cts.Token);
 
-            Console.WriteLine("ThreadingTimer: End Time: " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
+            WebWorkerTestHelper.Log("ThreadingTimer: End Time: " + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
             Assert.True(hit);
         }
 
@@ -496,7 +490,7 @@ await executor.Execute(async () =>
         }
 
         [Theory, MemberData(nameof(GetTargetThreadsAndBlockingCalls))]
-        public async Task WaitDoesNotAssertInAsyncCode(Executor executor, NamedCall method)
+        public async Task WaitInAsyncAssertsOnlyOnJSWebWorker(Executor executor, NamedCall method)
         {
             using var cts = CreateTestCaseTimeoutSource();
             await executor.Execute(async () =>
@@ -513,7 +507,15 @@ await executor.Execute(async () =>
                     exception = ex;
                 }
 
-                Assert.Null(exception);
+                if (method.IsBlocking && executor.Type == ExecutorType.JSWebWorker)
+                {
+                    Assert.NotNull(exception);
+                    Assert.IsType<PlatformNotSupportedException>(exception);
+                }
+                else
+                {
+                    Assert.Null(exception);
+                }
             }, cts.Token);
         }
 
@@ -527,7 +529,8 @@ await executor.Execute(async () =>
 
                 Exception? exception = null;
                 // the callback will hit Main or JSWebWorker, not the original executor thread
-                await WebWorkerTestHelper.CallMeBackSync(() => {
+                await WebWorkerTestHelper.CallMeBackSync(() =>
+                {
                     // when we are inside of synchronous callback, all blocking .Wait is forbidden
                     try
                     {
@@ -539,9 +542,15 @@ await WebWorkerTestHelper.CallMeBackSync(() => {
                     }
                 });
 
-                Console.WriteLine("WaitAssertsOnJSInteropThreads: ExecuterType: " + executor.Type + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
-                Assert.NotNull(exception);
-                Assert.IsType<PlatformNotSupportedException>(exception);
+                if (method.IsBlocking)
+                {
+                    Assert.NotNull(exception);
+                    Assert.IsType<PlatformNotSupportedException>(exception);
+                }
+                else
+                {
+                    Assert.Null(exception);
+                }
             }, cts.Token);
         }
 
@@ -558,9 +567,15 @@ await executor.Execute(async () =>
                 // the callback will hit Main or JSWebWorker, not the original executor thread
                 await WebWorkerTestHelper.CallExportBackSync(nameof(WebWorkerTestHelper.CallCurrentCallback));
 
-                Console.WriteLine("WaitAssertsOnJSInteropThreads: ExecuterType: " + executor.Type + " ManagedThreadId: " + Environment.CurrentManagedThreadId + " NativeThreadId: " + WebWorkerTestHelper.NativeThreadId);
-                Assert.NotNull(WebWorkerTestHelper.LastException);
-                Assert.IsType<PlatformNotSupportedException>(WebWorkerTestHelper.LastException);
+                if (method.IsBlocking)
+                {
+                    Assert.NotNull(WebWorkerTestHelper.LastException);
+                    Assert.IsType<PlatformNotSupportedException>(WebWorkerTestHelper.LastException);
+                }
+                else
+                {
+                    Assert.Null(WebWorkerTestHelper.LastException);
+                }
             }, cts.Token);
         }
 
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestBase.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestBase.cs
index 1df4c61e6bcbc6..77aef0857a8d2e 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestBase.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestBase.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.IO;
 using System.Threading.Tasks;
 using System.Threading;
 using Xunit;
@@ -35,7 +36,7 @@ protected CancellationTokenSource CreateTestCaseTimeoutSource([CallerMemberName]
             cts.Token.Register(() =>
             {
                 var end = DateTime.Now;
-                Console.WriteLine($"Unexpected test case {memberName} timeout after {end - start} ManagedThreadId:{Environment.CurrentManagedThreadId}");
+                WebWorkerTestHelper.Log($"Unexpected test case {memberName} timeout after {end - start} ManagedThreadId:{Environment.CurrentManagedThreadId}");
             });
             return cts;
         }
@@ -90,7 +91,7 @@ async Task ActionsInDifferentThreads1()
                 }
                 catch (Exception ex)
                 {
-                    Console.WriteLine("ActionsInDifferentThreads1 failed\n" + ex);
+                    WebWorkerTestHelper.Log("ActionsInDifferentThreads1 failed\n" + ex);
                     job1ReadyTCS.SetResult(default);
                     e1Failed = true;
                     throw;
@@ -137,36 +138,74 @@ async Task ActionsInDifferentThreads2()
                 }
                 if (!e1Done || !e2Done)
                 {
-                    Console.WriteLine("ActionsInDifferentThreads canceling because of unexpected fail: \n" + ex);
+                    WebWorkerTestHelper.Log("ActionsInDifferentThreads canceling because of unexpected fail: \n" + ex);
                     cts.Cancel();
                 }
                 else
                 {
-                    Console.WriteLine("ActionsInDifferentThreads failed with: \n" + ex);
+                    WebWorkerTestHelper.Log("ActionsInDifferentThreads failed with: \n" + ex);
                 }
                 throw;
             }
         }
 
+        static void LocalCtsIgnoringCall(Action<CancellationToken> action)
+        {
+            var cts = new CancellationTokenSource(8);
+            try
+            {
+                action(cts.Token);
+            }
+            catch (OperationCanceledException exception)
+            {
+                if (exception.CancellationToken != cts.Token)
+                {
+                    throw;
+                }
+                /* ignore the local one */
+            }
+        }
+
         public static IEnumerable<NamedCall> BlockingCalls = new List<NamedCall>
         {
-                new NamedCall { Name = "Task.Wait", Call = delegate (CancellationToken ct) { Task.Delay(10, ct).Wait(ct); }},
-                new NamedCall { Name = "Task.WaitAll", Call = delegate (CancellationToken ct) { Task.WaitAll(Task.Delay(10, ct)); }},
-                new NamedCall { Name = "Task.WaitAny", Call = delegate (CancellationToken ct) { Task.WaitAny(Task.Delay(10, ct)); }},
-                new NamedCall { Name = "ManualResetEventSlim.Wait", Call = delegate (CancellationToken ct) {
-                    using var mr = new ManualResetEventSlim(false);
-                    using var cts = new CancellationTokenSource(8);
-                    try {
-                        mr.Wait(cts.Token);
-                    } catch (OperationCanceledException) { /* ignore */ }
-                }},
-                new NamedCall { Name = "SemaphoreSlim.Wait", Call = delegate (CancellationToken ct) {
-                    using var sem = new SemaphoreSlim(2);
-                    var cts = new CancellationTokenSource(8);
-                    try {
-                        sem.Wait(cts.Token);
-                    } catch (OperationCanceledException) { /* ignore */ }
-                }},
+            // things that should NOT throw PNSE
+            new NamedCall { IsBlocking = false, Name = "Console.WriteLine", Call = delegate (CancellationToken ct) { Console.WriteLine("Blocking"); }},
+            new NamedCall { IsBlocking = false, Name = "Directory.GetCurrentDirectory", Call = delegate (CancellationToken ct) { Directory.GetCurrentDirectory(); }},
+            new NamedCall { IsBlocking = false, Name = "CancellationTokenSource.ctor", Call = delegate (CancellationToken ct) {
+                using var cts = new CancellationTokenSource(8);
+            }},
+            new NamedCall { IsBlocking = false, Name = "Task.Delay", Call = delegate (CancellationToken ct) {
+                Task.Delay(30, ct);
+            }},
+            new NamedCall { IsBlocking = false, Name = "new Timer", Call = delegate (CancellationToken ct) {
+                new Timer((_) => { }, null, 1, -1);
+            }},
+
+            // things which should throw PNSE on sync JSExport and JSWebWorker
+            new NamedCall { IsBlocking = true, Name = "Task.Wait", Call = delegate (CancellationToken ct) { Task.Delay(30, ct).Wait(ct); }},
+            new NamedCall { IsBlocking = true, Name = "Task.WaitAll", Call = delegate (CancellationToken ct) { Task.WaitAll(Task.Delay(30, ct)); }},
+            new NamedCall { IsBlocking = true, Name = "Task.WaitAny", Call = delegate (CancellationToken ct) { Task.WaitAny(Task.Delay(30, ct)); }},
+            new NamedCall { IsBlocking = true, Name = "ManualResetEventSlim.Wait", Call = delegate (CancellationToken ct) {
+                using var mr = new ManualResetEventSlim(false);
+                LocalCtsIgnoringCall(mr.Wait);
+            }},
+            new NamedCall { IsBlocking = true, Name = "SemaphoreSlim.Wait", Call = delegate (CancellationToken ct) {
+                using var sem = new SemaphoreSlim(2);
+                LocalCtsIgnoringCall(sem.Wait);
+            }},
+            new NamedCall { IsBlocking = true, Name = "Mutex.WaitOne", Call = delegate (CancellationToken ct) {
+                using var mr = new ManualResetEventSlim(false);
+                var mutex = new Mutex();
+                var thread = new Thread(() => {
+                    mutex.WaitOne();
+                    mr.Set();
+                    Thread.Sleep(50);
+                    mutex.ReleaseMutex();
+                });
+                thread.Start();
+                Thread.ForceBlockingWait(static (b) => ((ManualResetEventSlim)b).Wait(), mr);
+                mutex.WaitOne();
+            }},
         };
 
         public static IEnumerable<object[]> GetTargetThreadsAndBlockingCalls()
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestHelper.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestHelper.cs
index 9a1856780d5d5b..fa83846f149291 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestHelper.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/WebWorkerTestHelper.cs
@@ -385,10 +385,10 @@ public static Task RunOnTargetAsync(SynchronizationContext ctx, Func<Task> job,
     public class NamedCall
     {
         public string Name { get; set; }
+        public bool IsBlocking { get; set; }
         public delegate void Method(CancellationToken ct);
         public Method Call { get; set; }
 
         override public string ToString() => Name;
     }
-
 }
diff --git a/src/libraries/System.Threading.Thread/src/CompatibilitySuppressions.Threading.xml b/src/libraries/System.Threading.Thread/src/CompatibilitySuppressions.Threading.xml
index 5fc41e30ed4415..8eb2f78e6a7066 100644
--- a/src/libraries/System.Threading.Thread/src/CompatibilitySuppressions.Threading.xml
+++ b/src/libraries/System.Threading.Thread/src/CompatibilitySuppressions.Threading.xml
@@ -20,6 +20,10 @@
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>F:System.Threading.Thread.ThrowOnBlockingWaitOnJSInteropThread</Target>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>F:System.Threading.Thread.WarnOnBlockingWaitOnJSInteropThread</Target>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:System.Threading.Thread.AssureBlockingPossible</Target>
diff --git a/src/mono/browser/runtime/corebindings.c b/src/mono/browser/runtime/corebindings.c
index 37a8598ec6c4eb..485ba3189af6e2 100644
--- a/src/mono/browser/runtime/corebindings.c
+++ b/src/mono/browser/runtime/corebindings.c
@@ -51,6 +51,7 @@ void mono_wasm_invoke_js_function_send (pthread_t target_tid, int function_js_ha
 extern void mono_threads_wasm_async_run_in_target_thread_vi (pthread_t target_thread, void (*func) (gpointer), gpointer user_data1);
 extern void mono_threads_wasm_async_run_in_target_thread_vii (pthread_t target_thread, void (*func) (gpointer, gpointer), gpointer user_data1, gpointer user_data2);
 extern void mono_threads_wasm_sync_run_in_target_thread_vii (pthread_t target_thread, void (*func) (gpointer, gpointer), gpointer user_data1, gpointer args);
+extern void mono_wasm_warn_about_blocking_wait (void* ptr, int32_t length);
 #else
 extern void* mono_wasm_bind_js_import_ST (void *signature);
 extern void mono_wasm_invoke_jsimport_ST (int function_handle, void *args);
@@ -86,6 +87,7 @@ void bindings_initialize_internals (void)
 	mono_add_internal_call ("Interop/Runtime::InvokeJSImportAsyncPost", mono_wasm_invoke_jsimport_async_post);
 	mono_add_internal_call ("Interop/Runtime::InvokeJSFunctionSend", mono_wasm_invoke_js_function_send);
 	mono_add_internal_call ("Interop/Runtime::CancelPromisePost", mono_wasm_cancel_promise_post);
+	mono_add_internal_call ("System.Threading.Thread::WarnAboutBlockingWait", mono_wasm_warn_about_blocking_wait);
 #else
 	mono_add_internal_call ("Interop/Runtime::BindJSImportST", mono_wasm_bind_js_import_ST);
 	mono_add_internal_call ("Interop/Runtime::InvokeJSImportST", mono_wasm_invoke_jsimport_ST);
diff --git a/src/mono/browser/runtime/exports-binding.ts b/src/mono/browser/runtime/exports-binding.ts
index bcecfec8b10ea4..f5420a3a729e14 100644
--- a/src/mono/browser/runtime/exports-binding.ts
+++ b/src/mono/browser/runtime/exports-binding.ts
@@ -29,12 +29,11 @@ import { mono_wasm_cancel_promise } from "./cancelable-promise";
 import {
     mono_wasm_start_deputy_thread_async,
     mono_wasm_pthread_on_pthread_attached, mono_wasm_pthread_on_pthread_unregistered,
-    mono_wasm_pthread_on_pthread_registered, mono_wasm_pthread_set_name, mono_wasm_install_js_worker_interop, mono_wasm_uninstall_js_worker_interop, mono_wasm_start_io_thread_async
+    mono_wasm_pthread_on_pthread_registered, mono_wasm_pthread_set_name, mono_wasm_install_js_worker_interop, mono_wasm_uninstall_js_worker_interop, mono_wasm_start_io_thread_async, mono_wasm_warn_about_blocking_wait
 } from "./pthreads";
 import { mono_wasm_dump_threads } from "./pthreads/ui-thread";
 import { mono_wasm_schedule_synchronization_context } from "./pthreads/shared";
 
-
 // the JS methods would be visible to EMCC linker and become imports of the WASM module
 
 export const mono_wasm_threads_imports = !WasmEnableThreads ? [] : [
@@ -58,6 +57,7 @@ export const mono_wasm_threads_imports = !WasmEnableThreads ? [] : [
     mono_wasm_install_js_worker_interop,
     mono_wasm_uninstall_js_worker_interop,
     mono_wasm_invoke_jsimport_MT,
+    mono_wasm_warn_about_blocking_wait,
 ];
 
 export const mono_wasm_imports = [
diff --git a/src/mono/browser/runtime/interp-pgo.ts b/src/mono/browser/runtime/interp-pgo.ts
index 0a5a6e6c728534..14697385800b16 100644
--- a/src/mono/browser/runtime/interp-pgo.ts
+++ b/src/mono/browser/runtime/interp-pgo.ts
@@ -206,9 +206,7 @@ export async function getCacheKey (prefix: string): Promise<string | null> {
     delete inputs.enableDownloadRetry;
     delete inputs.extensions;
     delete inputs.runtimeId;
-    delete inputs.mainThreadingMode;
     delete inputs.jsThreadBlockingMode;
-    delete inputs.jsThreadInteropMode;
 
     inputs.GitHash = loaderHelpers.gitHash;
     inputs.ProductVersion = ProductVersion;
diff --git a/src/mono/browser/runtime/loader/config.ts b/src/mono/browser/runtime/loader/config.ts
index 8fd7f00fe3152d..5ff185827c106b 100644
--- a/src/mono/browser/runtime/loader/config.ts
+++ b/src/mono/browser/runtime/loader/config.ts
@@ -4,7 +4,7 @@
 import BuildConfiguration from "consts:configuration";
 import WasmEnableThreads from "consts:wasmEnableThreads";
 
-import { MainThreadingMode, type DotnetModuleInternal, type MonoConfigInternal, JSThreadBlockingMode, JSThreadInteropMode } from "../types/internal";
+import { type DotnetModuleInternal, type MonoConfigInternal, JSThreadBlockingMode } from "../types/internal";
 import type { DotnetModuleConfig, MonoConfig, ResourceGroups, ResourceList } from "../types";
 import { exportedRuntimeAPI, loaderHelpers, runtimeHelpers } from "./globals";
 import { mono_log_error, mono_log_debug } from "./logging";
@@ -12,7 +12,6 @@ import { importLibraryInitializers, invokeLibraryInitializers } from "./libraryI
 import { mono_exit } from "./exit";
 import { makeURLAbsoluteWithApplicationBase } from "./polyfills";
 import { appendUniqueQuery } from "./assets";
-import { mono_log_warn } from "./logging";
 
 export function deep_merge_config (target: MonoConfigInternal, source: MonoConfigInternal): MonoConfigInternal {
     // no need to merge the same object
@@ -198,38 +197,8 @@ export function normalizeConfig () {
         if (!Number.isInteger(config.finalizerThreadStartDelayMs)) {
             config.finalizerThreadStartDelayMs = 200;
         }
-        if (config.mainThreadingMode == undefined) {
-            config.mainThreadingMode = MainThreadingMode.DeputyAndIOThreads;
-        }
         if (config.jsThreadBlockingMode == undefined) {
-            config.jsThreadBlockingMode = JSThreadBlockingMode.AllowBlockingWaitInAsyncCode;
-        }
-        if (config.jsThreadInteropMode == undefined) {
-            config.jsThreadInteropMode = JSThreadInteropMode.SimpleSynchronousJSInterop;
-        }
-        let validModes = false;
-        if (config.mainThreadingMode == MainThreadingMode.DeputyThread
-            && config.jsThreadBlockingMode == JSThreadBlockingMode.NoBlockingWait
-            && config.jsThreadInteropMode == JSThreadInteropMode.SimpleSynchronousJSInterop
-        ) {
-            validModes = true;
-        } else if (config.mainThreadingMode == MainThreadingMode.DeputyAndIOThreads
-            && config.jsThreadBlockingMode == JSThreadBlockingMode.AllowBlockingWaitInAsyncCode
-            && config.jsThreadInteropMode == JSThreadInteropMode.SimpleSynchronousJSInterop
-        ) {
-            validModes = true;
-        } else if (config.mainThreadingMode == MainThreadingMode.DeputyThread
-            && config.jsThreadBlockingMode == JSThreadBlockingMode.AllowBlockingWait
-            && config.jsThreadInteropMode == JSThreadInteropMode.SimpleSynchronousJSInterop
-        ) {
-            validModes = true;
-        }
-        if (!validModes) {
-            mono_log_warn("Unsupported threading configuration", {
-                mainThreadingMode: config.mainThreadingMode,
-                jsThreadBlockingMode: config.jsThreadBlockingMode,
-                jsThreadInteropMode: config.jsThreadInteropMode
-            });
+            config.jsThreadBlockingMode = JSThreadBlockingMode.PreventSynchronousJSExport;
         }
     }
 
diff --git a/src/mono/browser/runtime/managed-exports.ts b/src/mono/browser/runtime/managed-exports.ts
index f90fbb049d1504..065136faaba476 100644
--- a/src/mono/browser/runtime/managed-exports.ts
+++ b/src/mono/browser/runtime/managed-exports.ts
@@ -3,7 +3,7 @@
 
 import WasmEnableThreads from "consts:wasmEnableThreads";
 
-import { GCHandle, GCHandleNull, JSMarshalerArguments, JSThreadInteropMode, MarshalerToCs, MarshalerToJs, MarshalerType, MonoMethod, PThreadPtr } from "./types/internal";
+import { GCHandle, GCHandleNull, JSMarshalerArguments, JSThreadBlockingMode, MarshalerToCs, MarshalerToJs, MarshalerType, MonoMethod, PThreadPtr } from "./types/internal";
 import cwraps, { threads_c_functions as twraps } from "./cwraps";
 import { runtimeHelpers, Module, loaderHelpers, mono_assert } from "./globals";
 import { JavaScriptMarshalerArgSize, alloc_stack_frame, get_arg, get_arg_gc_handle, is_args_exception, set_arg_i32, set_arg_intptr, set_arg_type, set_gc_handle, set_receiver_should_free } from "./marshal";
@@ -165,10 +165,12 @@ export function complete_task (holder_gc_handle: GCHandle, error?: any, data?: a
 export function call_delegate (callback_gc_handle: GCHandle, arg1_js: any, arg2_js: any, arg3_js: any, res_converter?: MarshalerToJs, arg1_converter?: MarshalerToCs, arg2_converter?: MarshalerToCs, arg3_converter?: MarshalerToCs) {
     loaderHelpers.assert_runtime_running();
     if (WasmEnableThreads) {
-        if (runtimeHelpers.config.jsThreadInteropMode == JSThreadInteropMode.NoSyncJSInterop) {
-            throw new Error("Cannot call synchronous C# methods.");
-        } else if (runtimeHelpers.isPendingSynchronousCall) {
-            throw new Error("Cannot call synchronous C# method from inside a synchronous call to a JS method.");
+        if (monoThreadInfo.isUI) {
+            if (runtimeHelpers.config.jsThreadBlockingMode == JSThreadBlockingMode.PreventSynchronousJSExport) {
+                throw new Error("Cannot call synchronous C# methods.");
+            } else if (runtimeHelpers.isPendingSynchronousCall) {
+                throw new Error("Cannot call synchronous C# method from inside a synchronous call to a JS method.");
+            }
         }
     }
     const sp = Module.stackSave();
@@ -225,26 +227,39 @@ export function get_managed_stack_trace (exception_gc_handle: GCHandle) {
     }
 }
 
-// GCHandle InstallMainSynchronizationContext(nint jsNativeTID, JSThreadBlockingMode jsThreadBlockingMode, JSThreadInteropMode jsThreadInteropMode, MainThreadingMode mainThreadingMode)
-export function install_main_synchronization_context (jsThreadBlockingMode: number, jsThreadInteropMode: number, mainThreadingMode: number): GCHandle {
+// GCHandle InstallMainSynchronizationContext(nint jsNativeTID, JSThreadBlockingMode jsThreadBlockingMode)
+export function install_main_synchronization_context (jsThreadBlockingMode: JSThreadBlockingMode): GCHandle {
     if (!WasmEnableThreads) return GCHandleNull;
     assert_c_interop();
 
     try {
         // this block is like alloc_stack_frame() but without set_args_context()
-        const bytes = JavaScriptMarshalerArgSize * 6;
+        const bytes = JavaScriptMarshalerArgSize * 4;
         const args = Module.stackAlloc(bytes) as any;
         _zero_region(args, bytes);
 
         const res = get_arg(args, 1);
         const arg1 = get_arg(args, 2);
         const arg2 = get_arg(args, 3);
-        const arg3 = get_arg(args, 4);
-        const arg4 = get_arg(args, 5);
         set_arg_intptr(arg1, mono_wasm_main_thread_ptr() as any);
-        set_arg_i32(arg2, jsThreadBlockingMode);
-        set_arg_i32(arg3, jsThreadInteropMode);
-        set_arg_i32(arg4, mainThreadingMode);
+
+        // sync with JSHostImplementation.Types.cs
+        switch (jsThreadBlockingMode) {
+            case JSThreadBlockingMode.PreventSynchronousJSExport:
+                set_arg_i32(arg2, 0);
+                break;
+            case JSThreadBlockingMode.ThrowWhenBlockingWait:
+                set_arg_i32(arg2, 1);
+                break;
+            case JSThreadBlockingMode.WarnWhenBlockingWait:
+                set_arg_i32(arg2, 2);
+                break;
+            case JSThreadBlockingMode.DangerousAllowBlockingWait:
+                set_arg_i32(arg2, 100);
+                break;
+            default:
+                throw new Error("Invalid jsThreadBlockingMode");
+        }
 
         // this block is like invoke_sync_jsexport() but without assert_js_interop()
         cwraps.mono_wasm_invoke_jsexport(managedExports.InstallMainSynchronizationContext!, args);
@@ -281,10 +296,12 @@ export function invoke_sync_jsexport (method: MonoMethod, args: JSMarshalerArgum
     if (!WasmEnableThreads) {
         cwraps.mono_wasm_invoke_jsexport(method, args as any);
     } else {
-        if (runtimeHelpers.config.jsThreadInteropMode == JSThreadInteropMode.NoSyncJSInterop) {
-            throw new Error("Cannot call synchronous C# methods.");
-        } else if (runtimeHelpers.isPendingSynchronousCall) {
-            throw new Error("Cannot call synchronous C# method from inside a synchronous call to a JS method.");
+        if (monoThreadInfo.isUI) {
+            if (runtimeHelpers.config.jsThreadBlockingMode == JSThreadBlockingMode.PreventSynchronousJSExport) {
+                throw new Error("Cannot call synchronous C# methods.");
+            } else if (runtimeHelpers.isPendingSynchronousCall) {
+                throw new Error("Cannot call synchronous C# method from inside a synchronous call to a JS method.");
+            }
         }
         if (runtimeHelpers.isManagedRunningOnCurrentThread) {
             twraps.mono_wasm_invoke_jsexport_sync(method, args as any);
diff --git a/src/mono/browser/runtime/multi-threading.md b/src/mono/browser/runtime/multi-threading.md
deleted file mode 100644
index e4b3985923d503..00000000000000
--- a/src/mono/browser/runtime/multi-threading.md
+++ /dev/null
@@ -1,52 +0,0 @@
-# Multi-threading with JavaScript interop
-
-## Meaningful configurations are:
-
- * Single-threaded mode as you know it since .Net 6
-    - default, safe, tested, supported
-    - from .Net 8 it could be easily started also as a web worker, but you need your own messaging between main and worker
- * `MainThreadingMode.DeputyThread` + `JSThreadBlockingMode.NoBlockingWait` + `JSThreadInteropMode.SimpleSynchronousJSInterop`
-    + **default threading**, safe, tested, supported
-    + blocking `.Wait` is allowed on thread pool and new threads
-    - blocking `.Wait` throws `PlatformNotSupportedException` on `JSWebWorker` and main thread
-    - DOM events like `onClick` need to be asynchronous, if the handler needs use synchronous `[JSImport]`
-    - synchronous calls to `[JSImport]`/`[JSExport]` can't synchronously call back
-
- * `MainThreadingMode.DeputyAndIOThreads` + `JSThreadBlockingMode.AllowBlockingWaitInAsyncCode` + `JSThreadInteropMode.SimpleSynchronousJSInterop`
-    + **default threading**, safe, tested, supported
-    + blocking `.Wait` is allowed on thread pool and new threads
-    - blocking `.Wait` throws `PlatformNotSupportedException` on `JSWebWorker` and main thread only when they are called from JS via synchronous `JSExport`
-    - DOM events like `onClick` need to be asynchronous, if the handler needs use synchronous `[JSImport]`
-    - synchronous calls to `[JSImport]`/`[JSExport]` can't synchronously call back
-
- * `MainThreadingMode.DeputyThread` + `JSThreadBlockingMode.AllowBlockingWait` + `JSThreadInteropMode.SimpleSynchronousJSInterop`
-    + pragmatic for legacy codebase, which contains blocking code and can't be fully executed on thread pool or new threads
-    - ** could cause deadlocks !!!**
-        - Use your own judgment before you opt in.
-    - blocking .Wait is allowed on all threads!
-    - blocking .Wait on pending JS `Task`/`Promise` (like HTTP/WS requests) could cause deadlocks!
-        - reason is that blocked thread can't process the browser event loop
-        - so it can't resolve the promises
-        - even when it's longer `Promise`/`Task` chain
-    - DOM events like `onClick` need to be asynchronous, if the handler needs use synchronous `[JSImport]`
-    - synchronous calls to `[JSImport]`/`[JSExport]` can't synchronously call back
-
-## Unsupported combinations are:
- * `MainThreadingMode.DeputyThread` + `JSThreadBlockingMode.NoBlockingWait` + `JSThreadInteropMode.NoSyncJSInterop`
-    + very safe
-    - HTTP/WS requests are not possible because it currently uses synchronous JS interop
-    - Blazor doesn't work because it currently uses synchronous JS interop
- * `MainThreadingMode.UIThread`
-    - not recommended, not tested, not supported!
-    - can deadlock on creating new threads
-    - can deadlock on blocking `.Wait` for a pending JS `Promise`/`Task`, including HTTP/WS requests
-    - .Wait is spin-waiting - it blocks debugger, network, UI rendering, ...
-    + JS interop to UI is faster, synchronous and re-entrant
-
-### There could be more JSThreadInteropModes:
- - allow re-entrant synchronous JS interop on `JSWebWorker`.
-    - This is possible because managed code is running on same thread as JS.
-    - But it's nuanced to debug it, when things go wrong.
- - allow re-entrant synchronous JS interop also on deputy thread.
-    - This is not possible for deputy, because it would deadlock on call back to different thread.
-    - The thread receiving the callback is still blocked waiting for the first synchronous call to finish.
diff --git a/src/mono/browser/runtime/pthreads/index.ts b/src/mono/browser/runtime/pthreads/index.ts
index 195df3e126ab65..3678d709d1050b 100644
--- a/src/mono/browser/runtime/pthreads/index.ts
+++ b/src/mono/browser/runtime/pthreads/index.ts
@@ -1,6 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+import { mono_log_warn } from "../logging";
+import { utf16ToString } from "../strings";
+
 export {
     mono_wasm_main_thread_ptr, mono_wasm_install_js_worker_interop, mono_wasm_uninstall_js_worker_interop,
     mono_wasm_pthread_ptr, update_thread_info, isMonoThreadMessage, monoThreadInfo,
@@ -18,3 +21,8 @@ export {
 
 export { mono_wasm_start_deputy_thread_async } from "./deputy-thread";
 export { mono_wasm_start_io_thread_async } from "./io-thread";
+
+export function mono_wasm_warn_about_blocking_wait (ptr: number, length: number) {
+    const warning = utf16ToString(ptr, ptr + (length * 2));
+    mono_log_warn(warning);
+}
diff --git a/src/mono/browser/runtime/startup.ts b/src/mono/browser/runtime/startup.ts
index 24fbb1d16a04f2..c3958289627cf3 100644
--- a/src/mono/browser/runtime/startup.ts
+++ b/src/mono/browser/runtime/startup.ts
@@ -4,7 +4,7 @@
 import WasmEnableThreads from "consts:wasmEnableThreads";
 import BuildConfiguration from "consts:configuration";
 
-import { DotnetModuleInternal, CharPtrNull, MainThreadingMode } from "./types/internal";
+import { DotnetModuleInternal, CharPtrNull } from "./types/internal";
 import { exportedRuntimeAPI, INTERNAL, loaderHelpers, Module, runtimeHelpers, createPromiseController, mono_assert } from "./globals";
 import cwraps, { init_c_exports, threads_c_functions as tcwraps } from "./cwraps";
 import { mono_wasm_raise_debug_event, mono_wasm_runtime_ready } from "./debug";
@@ -274,19 +274,14 @@ async function onRuntimeInitializedAsync (userOnRuntimeInitialized: () => void)
             mono_log_info("UI thread is alive!");
         }, 3000);
 
-        if (WasmEnableThreads &&
-            (runtimeHelpers.config.mainThreadingMode == MainThreadingMode.DeputyThread
-                || runtimeHelpers.config.mainThreadingMode == MainThreadingMode.DeputyAndIOThreads)) {
+        if (WasmEnableThreads) {
             // this will create thread and call start_runtime() on it
             runtimeHelpers.monoThreadInfo = monoThreadInfo;
             runtimeHelpers.isManagedRunningOnCurrentThread = false;
             update_thread_info();
             runtimeHelpers.managedThreadTID = tcwraps.mono_wasm_create_deputy_thread();
             runtimeHelpers.proxyGCHandle = await runtimeHelpers.afterMonoStarted.promise;
-
-            if (WasmEnableThreads && runtimeHelpers.config.mainThreadingMode == MainThreadingMode.DeputyAndIOThreads) {
-                runtimeHelpers.ioThreadTID = tcwraps.mono_wasm_create_io_thread();
-            }
+            runtimeHelpers.ioThreadTID = tcwraps.mono_wasm_create_io_thread();
 
             // TODO make UI thread not managed/attached https://github.com/dotnet/runtime/issues/100411
             tcwraps.mono_wasm_register_ui_thread();
@@ -303,7 +298,7 @@ async function onRuntimeInitializedAsync (userOnRuntimeInitialized: () => void)
             await start_runtime();
         }
 
-        if (WasmEnableThreads && runtimeHelpers.config.mainThreadingMode == MainThreadingMode.DeputyAndIOThreads) {
+        if (WasmEnableThreads) {
             await runtimeHelpers.afterIOStarted.promise;
         }
 
@@ -541,10 +536,7 @@ export async function start_runtime () {
             monoThreadInfo.isRegistered = true;
             runtimeHelpers.currentThreadTID = monoThreadInfo.pthreadId = runtimeHelpers.managedThreadTID = mono_wasm_pthread_ptr();
             update_thread_info();
-            runtimeHelpers.proxyGCHandle = install_main_synchronization_context(
-                runtimeHelpers.config.jsThreadBlockingMode!,
-                runtimeHelpers.config.jsThreadInteropMode!,
-                runtimeHelpers.config.mainThreadingMode!);
+            runtimeHelpers.proxyGCHandle = install_main_synchronization_context(runtimeHelpers.config.jsThreadBlockingMode!);
             runtimeHelpers.isManagedRunningOnCurrentThread = true;
 
             // start finalizer thread, lazy
diff --git a/src/mono/browser/runtime/types/internal.ts b/src/mono/browser/runtime/types/internal.ts
index 2017a86922423e..0e564f8c872827 100644
--- a/src/mono/browser/runtime/types/internal.ts
+++ b/src/mono/browser/runtime/types/internal.ts
@@ -95,9 +95,7 @@ export type MonoConfigInternal = MonoConfig & {
     GitHash?: string,
     ProductVersion?: string,
 
-    mainThreadingMode?: MainThreadingMode,
     jsThreadBlockingMode?: JSThreadBlockingMode,
-    jsThreadInteropMode?: JSThreadInteropMode,
 };
 
 export type RunArguments = {
@@ -570,36 +568,37 @@ export interface MonoThreadMessage {
     cmd: string;
 }
 
-// keep in sync with JSHostImplementation.Types.cs
-export const enum MainThreadingMode {
-    // Running the managed main thread on UI thread.
-    // Managed GC and similar scenarios could be blocking the UI.
-    // Easy to deadlock. Not recommended for production.
-    UIThread = 0,
-    // Running the managed main thread on dedicated WebWorker. Marshaling all JavaScript calls to and from the main thread.
-    DeputyThread = 1,
-    // TODO comment
-    DeputyAndIOThreads = 2,
-}
-
 // keep in sync with JSHostImplementation.Types.cs
 export const enum JSThreadBlockingMode {
-    // throw PlatformNotSupportedException if blocking .Wait is called on threads with JS interop, like JSWebWorker and Main thread.
-    // Avoids deadlocks (typically with pending JS promises on the same thread) by throwing exceptions.
-    NoBlockingWait = 0,
-    // TODO comment
-    AllowBlockingWaitInAsyncCode = 1,
-    // allow .Wait on all threads.
-    // Could cause deadlocks with blocking .Wait on a pending JS Task/Promise on the same thread or similar Task/Promise chain.
-    AllowBlockingWait = 100,
-}
-
-// keep in sync with JSHostImplementation.Types.cs
-export const enum JSThreadInteropMode {
-    // throw PlatformNotSupportedException if synchronous JSImport/JSExport is called on threads with JS interop, like JSWebWorker and Main thread.
-    // calling synchronous JSImport on thread pool or new threads is allowed.
-    NoSyncJSInterop = 0,
-    // allow non-re-entrant synchronous blocking calls to and from JS on JSWebWorker on threads with JS interop, like JSWebWorker and Main thread.
-    // calling synchronous JSImport on thread pool or new threads is allowed.
-    SimpleSynchronousJSInterop = 1,
+    /**
+     * Prevents synchronous JSExport from being called from JavaScript code in UI thread.
+     * On JSWebWorker synchronous JSExport always works.
+     * On JSWebWorker blocking .Wait always warns.
+     * This is the default mode.
+     */
+    PreventSynchronousJSExport = "PreventSynchronousJSExport",
+    /**
+     * Allows synchronous JSExport to be called from JavaScript code also in UI thread.
+     * Inside of that call blocking .Wait throws PNSE.
+     * Inside of that call nested call back to synchronous JSImport throws PNSE (because it would deadlock otherwise in 100% cases).
+     * On JSWebWorker synchronous JSExport always works.
+     * On JSWebWorker blocking .Wait always throws PNSE.
+     */
+    ThrowWhenBlockingWait = "ThrowWhenBlockingWait",
+    /**
+     * Allows synchronous JSExport to be called from JavaScript code also in UI thread.
+     * Inside of that call blocking .Wait warns.
+     * Inside of that call nested call back to synchronous JSImport throws PNSE (because it would deadlock otherwise in 100% cases).
+     * On JSWebWorker synchronous JSExport always works.
+     * On JSWebWorker blocking .Wait always warns.
+     */
+    WarnWhenBlockingWait = "WarnWhenBlockingWait",
+    /**
+     * Allows synchronous JSExport to be called from JavaScript code, and allows managed code to use blocking .Wait
+     * .Wait on Promise/Task chains could lead to deadlock because JS event loop is not processed and it can't resolve JS promises.
+     * This mode is dangerous and not supported.
+     * Allows synchronous JSExport to be called from JavaScript code also in Main thread.
+     * Inside of that call nested call back to synchronous JSImport throws PNSE (because it would deadlock otherwise in 100% cases).
+     */
+    DangerousAllowBlockingWait = "DangerousAllowBlockingWait",
 }
diff --git a/src/mono/browser/test-main.js b/src/mono/browser/test-main.js
index 3aacd8e2c67d68..1feb21ef2f796b 100644
--- a/src/mono/browser/test-main.js
+++ b/src/mono/browser/test-main.js
@@ -252,7 +252,8 @@ function configureRuntime(dotnet, runArgs) {
         .withInteropCleanupOnExit()
         .withDumpThreadsOnNonZeroExit()
         .withConfig({
-            loadAllSatelliteResources: true
+            loadAllSatelliteResources: true,
+            jsThreadBlockingMode: "ThrowWhenBlockingWait",
         });
 
     if (ENVIRONMENT_IS_NODE) {
diff --git a/src/mono/sample/wasm/browser-threads/Program.cs b/src/mono/sample/wasm/browser-threads/Program.cs
index 331f8e35de3ccf..8783ace18be7b2 100644
--- a/src/mono/sample/wasm/browser-threads/Program.cs
+++ b/src/mono/sample/wasm/browser-threads/Program.cs
@@ -38,9 +38,14 @@ public static async Task<int> Main(string[] args)
     public static void Progress2()
     {
         // both calls here are sync POSIX calls dispatched to UI thread, which is already blocked because this is synchronous method on deputy thread
-        // in should not deadlock anyway, see also invoke_later_when_on_ui_thread_sync and emscripten_yield
+        // it should not deadlock anyway, see also invoke_later_when_on_ui_thread_sync and emscripten_yield
         var cwd = Directory.GetCurrentDirectory();
         Console.WriteLine("Progress! "+ cwd); 
+
+        // below is blocking call, which means that UI will spin-lock little longer
+        // it will warn about blocking wait because of jsThreadBlockingMode: "WarnWhenBlockingWait"
+        // but it will not deadlock because underlying task chain is not JS promise
+        Task.Delay(10).Wait();
     }
 
     [JSExport]
diff --git a/src/mono/sample/wasm/browser-threads/main.js b/src/mono/sample/wasm/browser-threads/main.js
index 8da1e4fb608e6d..ea97a5ce200c8c 100644
--- a/src/mono/sample/wasm/browser-threads/main.js
+++ b/src/mono/sample/wasm/browser-threads/main.js
@@ -17,6 +17,9 @@ try {
         .withElementOnExit()
         .withExitCodeLogging()
         .withExitOnUnhandledError()
+        .withConfig({
+            jsThreadBlockingMode: "WarnWhenBlockingWait",
+        })
         .create();
 
     setModuleImports("main.js", {