ManagedError: AggregateException_ctor_DefaultMessage

[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]
When I try to run my Blazor WASM project with the use of devtunnel it crash on error:

ManagedError: AggregateException_ctor_DefaultMessage (Format_InvalidStringWithOffsetAndReason, 9, Format_ExpectedAsciiDigit) {superStack: {…}, stack: <accessor>, message: 'AggregateException_ctor_DefaultMessage (Form…fsetAndReason, 9, Format_ExpectedAsciiDigit)', Symbol(wasm js_owned_gc_handle): 38}

but without the tunnel everything works as expected.

Default blazor WASM template project seems to work correctly.

Problem is in both
VS 2022: 17.14.23
VS 2026: 11312.210

[oroztocil]

Hello, thank you for contacting us.

Can you provide more information about your scenario? Any of the following can help us investigate the problem:

• A repository with a reproduction project
• A more complete stack trace
• Screenshots

[Kebechet]

For some unknown reason I am not able to reproduce this problem today.
Recently I was migrating to NET10. So idk if I resolved it by workload updates, bin/obj cleanup or something else but the dev-tunnel for our wasm app works.

In case I will experience this again I will try to investigate it further and will provide more info

[omer-repo]

Issue with modifying appsettings.json after publish (.NET 10 – Blazor Hosted WebAssembly)

I’m experiencing what seems to be the same issue on .NET 10 with Blazor Hosted WebAssembly (I had a similar situation in .NET 9 as well).

What I observe

• If I publish the project and deploy the entire publish folder to IIS without modifying anything, everything works fine.
• However, if I modify appsettings.json after publish (inside the publish folder) and redeploy without touching the compressed files, I get the following error:

AggregateException_ctor_DefaultMessage (ZeroDepthAtEnd, LineNumber: 28 | BytePositionInLine: 12.)

Important details

• If I modify appsettings.json before publishing, everything works perfectly.
• The .br file generated by Visual Studio during publish matches the JSON content and there are no issues.
• The problem only appears when I change appsettings.json after publish but do not regenerate the corresponding appsettings.json.br.

.NET 9 behavior

In .NET 9, I was able to fix this by manually recompressing appsettings.json using Brotli (via PeaZip) and regenerating the .br file with a high compression level (9 or 11 worked for me). After that, the app ran correctly.

.NET 10 behavior

In .NET 10:

• Manually recompressing does not fix the issue anymore.
• Even when using Brotli, the generated .br file size does not exactly match the one produced during publish (there is always a small difference).
• It seems .NET 10 might be using a different Brotli configuration or additional metadata.

My assumption

My assumption is that during publish, Blazor records or expects a specific relationship between the uncompressed and compressed files (possibly related to static asset fingerprinting, integrity checks, or precomputed metadata).

If appsettings.json is changed without regenerating the compressed asset exactly the same way the SDK does, the runtime fails while parsing.

Summary

• Modifying appsettings.json after publish breaks the app.
• Modifying it before publish works.
• Manually recreating .br worked in .NET 9 but no longer works in .NET 10.

Is there a documented way to safely update configuration files post-publish in Blazor WebAssembly, or is this behavior expected due to static asset processing?

[ilonatommy]

@omer-repo, can you reproduce the issue on an empty blazor template and without VS?