Blazor WebAssembly Release mode fails with mono/runtime out of sync

_This issue has been moved from [a ticket on Developer Community](https://developercommunity.visualstudio.com/t/Blazor-WebAssembly-Release-mode-fails-wi/11104923)._

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[severity:It's more difficult to complete my work] [regression] [worked-in:18.6.0]
Detected: OS: 10.0.26200.0 VS: 18.7.0 [18.7.11903.348], Project type: C# / Blazor WebAssembly.

I just updated from 18.6.0 to 18.7.0 and was then not able to run my Blazor Wasm app in Release mode. I _was_ able to run it in Debug mode.
First I tried to fix it with
- Unregister service worker
- Clear site data

but that did not do the trick.

I fixed it, with
- dotnet clean
- dotnet restore --no-cache
- git clean -xdf

for each of my projects in the solution.

Here'e the browser console of Release mode from before above steps. I know, it says, "Do not report this as a bug", but I did not change anything, so, the VS update was the cause.

Fetch event handler is recognized as no-op. No-op fetch handler may bring overhead during navigation. Consider removing the handler if possible.
sw-registrator.js:11 Service worker registration successful (scope: https://localhost:5001/)
blazor.webassembly.js:1 Debugging hotkey: Shift+Alt+D (when application has focus)
logging.ts:147 [MONO] /__w/1/s/src/runtime/src/mono/mono/metadata/icall.c:6989 <disabled>
(anonymous) @ logging.ts:147
blazor.webassembly.js:1
blazor.webassembly.js:1 Your mono runtime and class libraries are out of sync.
blazor.webassembly.js:1 The out of sync library is: System.Private.CoreLib.dll
blazor.webassembly.js:1
blazor.webassembly.js:1 When you update one from git you need to update, compile and install
blazor.webassembly.js:1 the other too.
blazor.webassembly.js:1 Do not report this as a bug unless you're sure you have updated correctly:
blazor.webassembly.js:1 you probably have a broken mono install.
blazor.webassembly.js:1 If you see other errors or faults after this message they are probably related
blazor.webassembly.js:1 and you need to fix your mono install first.
logging.ts:147 [MONO] /__w/1/s/src/runtime/src/mono/mono/metadata/icall.c:6989 <disabled>
(anonymous) @ logging.ts:147
blazor.webassembly.js:1
blazor.webassembly.js:1 Your mono runtime and class libraries are out of sync.
blazor.webassembly.js:1 The out of sync library is: System.Private.CoreLib.dll
blazor.webassembly.js:1
blazor.webassembly.js:1 When you update one from git you need to update, compile and install
blazor.webassembly.js:1 the other too.
blazor.webassembly.js:1 Do not report this as a bug unless you're sure you have updated correctly:
blazor.webassembly.js:1 you probably have a broken mono install.
blazor.webassembly.js:1 If you see other errors or faults after this message they are probably related
blazor.webassembly.js:1 and you need to fix your mono install first.
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Unhandled Exception:
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Nested exception detected.
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Original Exception: at (wrapper managed-to-native) System.Diagnostics.StackTrace.GetTrace (System.Runtime.CompilerServices.ObjectHandleOnStack,System.Runtime.CompilerServices.ObjectHandleOnStack,int,bool) [0x00011] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace.InitializeForException (System.Exception,int,bool) [0x00012] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace..ctor (System.Exception,bool) [0x00015] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.GetStackTrace () [0x00002] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.get_StackTrace () [0x00025] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.ToString () [0x00030] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Runtime.InteropServices.JavaScript.JavaScriptExports.CallEntrypoint (System.Runtime.InteropServices.JavaScript.JSMarshalerArgument*) [0x000bf] in <ab398d8fddd140b4b0a185449416618f>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Nested exception:at (wrapper managed-to-native) System.Diagnostics.StackTrace.GetTrace (System.Runtime.CompilerServices.ObjectHandleOnStack,System.Runtime.CompilerServices.ObjectHandleOnStack,int,bool) [0x00011] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace.InitializeForException (System.Exception,int,bool) [0x00012] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace..ctor (System.Exception,bool) [0x00015] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.GetStackTrace () [0x00002] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.get_StackTrace () [0x00025] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.ToString () [0x00030] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Unhandled Exception:
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Nested exception detected.
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Original Exception: at (wrapper managed-to-native) System.Diagnostics.StackTrace.GetTrace (System.Runtime.CompilerServices.ObjectHandleOnStack,System.Runtime.CompilerServices.ObjectHandleOnStack,int,bool) [0x00011] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace.InitializeForException (System.Exception,int,bool) [0x00012] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace..ctor (System.Exception,bool) [0x00015] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.GetStackTrace () [0x00002] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.get_StackTrace () [0x00025] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.ToString () [0x00030] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 Nested exception:at (wrapper managed-to-native) System.Diagnostics.StackTrace.GetTrace (System.Runtime.CompilerServices.ObjectHandleOnStack,System.Runtime.CompilerServices.ObjectHandleOnStack,int,bool) [0x00011] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace.InitializeForException (System.Exception,int,bool) [0x00012] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Diagnostics.StackTrace..ctor (System.Exception,bool) [0x00015] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.GetStackTrace () [0x00002] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.get_StackTrace () [0x00025] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 at System.Exception.ToString () [0x00030] in <89c77bd99917482fb11dd299cdcb0f36>:0
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
blazor.webassembly.js:1 (null)
Ot @ blazor.webassembly.js:1
logging.ts:129 [MONO] mono_wasm_invoke_jsexport unexpected double fault
Error
at Mc (dotnet.runtime.a6jcqbs390.js:3:172283)
at :5001/_framework/dotnet.native.oyl0uo52ql.wasm
at Module._mono_wasm_invoke_jsexport [as mono_wasm_invoke_jsexport] (dotnet.native.qndy5qpbvy.js:6490:195)
at wn (dotnet.runtime.a6jcqbs390.js:3:29439)
at dotnet.runtime.a6jcqbs390.js:3:178842
at Object.Vc [as runMain] (dotnet.runtime.a6jcqbs390.js:3:178995)
at async Object.callEntryPoint (blazor.webassembly.js:1:50711)
(anonymous) @ logging.ts:129
blazor.webassembly.js:1 ExitStatus
callEntryPoint @ blazor.webassembly.js:1

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### Original Comments

#### Feedback Bot on 10/06/2026, 11:40 AM:

Based on similar issues resolved with Microsoft in the last 6 months, the following can help resolve this issue faster.

Possible workarounds

• Try cleaning and restoring all projects in the solution:
  
  
  
  • dotnet clean
  
  
  • dotnet restore --no-cache
  
  
  • git clean -xdf
  
  
  
  


• If the issue appears after a Visual Studio update, re-run the build after the cleanup steps and confirm whether Release mode works again.

If not already included, please provide:

Detected: OS: 10.0.26200.0 VS: 18.7.0 [18.7.11903.348], Project type: C# / Blazor WebAssembly. Please share:

• Whether this happens consistently or only once after the VS update.


• The exact .NET SDK version in use.


• Whether the issue reproduces in a brand-new Blazor WebAssembly project.


• Whether Release mode fails immediately after a clean restore/build, and whether Debug mode still works in the same session.


• If available, a short trace or recording captured while reproducing the Release-mode failure.

#### Feedback Bot on 12/06/2026, 07:22 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

No repository file or test is named. Start by reproducing the Release-mode failure in a C# Blazor WebAssembly project on Visual Studio 18.7.0, compare it with Debug mode, and then repeat after dotnet clean, dotnet restore --no-cache, and git clean -xdf; done means Release mode runs without the mono/System.Private.CoreLib mismatch.

Written by the indexing model from the issue text.