Using AddBlazorWasmApp in AppHost results in 404 despite correct path

[MdeBruin93]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I've followed the official docs on how to host a standalone Blazor Webassembly app using Aspire. This came up with the following snippet:

var wasm=builder.AddBlazorWasmApp("blazor", "../BlazorApp1/BlazorApp1.csproj");

var gateway = builder.AddBlazorGateway("gateway")
    .WithExternalHttpEndpoints();

gateway.WithBlazorClientApp(wasm);

This, however, results in a 404 when clicking on the URL in the Aspire dashboard.

Expected Behavior

Opening up the Blazor Webassembly application.

Steps To Reproduce

The app I used to test the integration.
https://github.com/MdeBruin93/BlazorWasmAspire

Exceptions (if any)

No response

Aspire doctor output

No response

Anything else?

I'm using Aspire 13.4.2 and .NET 10

[joperezr]

🤖 Aspire triage pass — 2026-06-08 14:23 UTC

Area: area-integrations
Kind: bug
Repro: yes-large — reporter links an external repro repo; AppHost snippet shown in the issue body
Affected version(s): 13.4.2 (per reporter; latest stable: 13.4.0 — package Aspire.Hosting.Blazor is currently marked preview)
Severity: S2 — primary scenario of a shipped (preview) integration is broken with no workaround

Urgency: 🟡 vNext — recommend milestone 13.5
Reason: Aspire.Hosting.Blazor was newly added in 13.4 (introduced by #15691, marked preview in #17694) so technically this is a 13.4 primary-scenario break, but because the package is explicitly preview the servicing bar is unclear. Defaulting to 13.5 and flagging for human escalation to 13.4.x if desired.

Suggested owner(s): @javiercn (score ~9), backup @sebastienros (score ~3)
Evidence:

• javiercn authored Add Aspire.Hosting.Blazor integration for Blazor WebAssembly apps #15691 ("Add Aspire.Hosting.Blazor integration for Blazor WebAssembly apps", merged 2026-05-22) and Address remaining Blazor integration PR feedback #17384 ("Address remaining Blazor integration PR feedback", merged 2026-05-27) — primary author of this integration
• sebastienros merged Expose Blazor gateway and Dockerfile factory APIs to ATS #17420 ("Expose Blazor gateway and Dockerfile factory APIs to ATS")
• CODEOWNERS has no specific entry for src/Aspire.Hosting.Blazor/**

Confidence: medium
Needs human?: yes — preview-integration servicing call (13.4.x vs 13.5) requires human judgment; reporter's "13.4.2" version string also needs sanity-check against the actually-released 13.4.x patch

Dry-run mode — no labels or milestone applied. Reply /triage apply to promote, or edit and reapply.

[javiercn]

@MdeBruin93 You need to set the base tag in your app to <base href="/blazor/" /> otherwise relative references don't work correctly