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

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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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.

[MihuBot]

I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• Publising returns this error: "Your mono runtime and class libraries are out of sync." #13882

[github-actions]

Triage Summary

Area: area-blazor (Blazor WebAssembly Release-mode startup failure involving the mono runtime and System.Private.CoreLib.dll)
Type: Bug (reporter states the app worked before a Visual Studio update and now fails to run in Release mode)

Regression Info

• Previously working version: VS 18.6.0
• Broken since: VS 18.7.0 [18.7.11903.348]
• Reporter states the Blazor WebAssembly app ran successfully in Release mode before updating Visual Studio from 18.6.0 to 18.7.0; after the update, Release mode fails with a mono runtime/class library mismatch, while Debug mode continued to work.

Potential Duplicates

• None found

Notes

• Missing: .NET SDK version in use, and whether the issue reproduces in a brand-new Blazor WebAssembly project.

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