Blazor Server app in .NET 10 fails to reconnect after server restart, unlike .NET 9

[tossnet]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When testing Blazor Server apps created from the default Microsoft templates, I noticed a difference in reconnection behavior between .NET 9 and .NET 10 (RC2).

In .NET 9, when the server is stopped and restarted, the client shows the reconnection panel and successfully reconnects after the server restarts.

In .NET 10, the client also shows the reconnection panel but never reconnects, and eventually displays the message:

"Failed to resume the session. Please reload the page."

This seems like a regression because both templates use the same default setup, and the expected behavior should be identical between .NET 9 and .NET 10.

Expected Behavior

After restarting the Blazor Server app, the client should automatically reconnect once the server becomes available again — just like it does in .NET 9.

Steps To Reproduce

You can reproduce this issue using the default Blazor Server + Aspire templates:

1. Create two new Aspire projects using the default Microsoft templates:

bash
   dotnet new aspire-blazorserver -f net9.0
   dotnet new aspire-blazorserver -f net10.0

2. Run each solution using the AppHost project:
3. Open the Blazor Server frontend in a browser.
4. Stop the 2 projets and restart them.
5. Observe the reconnection behavior in the browser:

peek_10.mp4

Exceptions (if any)

Template: Blazor Server (Aspire)
OS: Windows 11
Browsers tested: Chrome

.NET Version

10.0.100-rc.2.25502.107

Anything else?

No response

[tossnet]

this is my minimal repo : https://github.com/tossnet/BlazorServerReconnectIssue64228/

1. Run the BlazorServerReconnectIssue64228.AppHost project
2. Open the 2 apps in different tabs
3. From Aspire , stop the 2 projects
4. Restart the 2 projects
5. The dotnet 9 project restarts
6. The dotnet 10 project does not restart

[oroztocil]

@javiercn This is caused by the new logic in ResumeCircuit. The client receives this error and stops trying to reconnect.

It is the same issue as #62901. Only back then it only ocurred when you had multiple client tabs. Now it seems to occur reliably with just one client tab. (It is not Aspire related, it happens with a basic dotnet new blazor project as well.)

[Edit: The following paragraph is misleading, see my comment below.]
Unless I am missing something, we should not treat the situation when the server does not have the persisted state as an error and we should simply create a new circuit with a blank state instead (maybe log a warning). This is how it worked before state persistence got introduced and allowed for the reconnection after the server is restarted during local development.

[ryanbuening]

I installed .NET 10 and the first thing I tried was the new Blazor reconnect UI/logic and I was met with this issue. This is easily reproducible in a new .NET 10 app.

[ryanbuening]

Unless I am missing something, we should not treat the situation when the server does not have the persisted state as an error and we should simply create a new circuit with a blank state instead (maybe log a warning). This is how it worked before state persistence got introduced and allowed for the reconnection after the server is restarted during local development.

☝️+1

This new reconnect logic (or better stated, lack thereof) will be a rude awakening for Blazor Server apps upgrading to .NET 10. It fundamentally changes how the default reconnect logic has worked previously.