Why does my Blazor app serve empty CSS and JavaScript files with a 200 status?

Because the app is running in the Production environment and MapStaticAssets is serving from the manifest of published, optimized assets, which do not exist in a development checkout. Each request matches an endpoint but there is no file behind it, so the response is 200 with a zero Content-Length. Running in Development fixes it.

Does dotnet run --no-launch-profile change ASPNETCORE_ENVIRONMENT?

Yes, indirectly. The flag skips launchSettings.json, which is where the Development environment variable is normally set, so the app starts with no environment and ASP.NET Core defaults to Production. Set ASPNETCORE_ENVIRONMENT=Development yourself or run with the launch profile.

How do I set ASPNETCORE_ENVIRONMENT for dotnet run?

Either let the launch profile set it, or set the variable in the shell before running: $env:ASPNETCORE_ENVIRONMENT = "Development" in PowerShell or export ASPNETCORE_ENVIRONMENT=Development in bash, then dotnet run.

Blazor serves empty CSS and JS files: MapStaticAssets without ASPNETCORE_ENVIRONMENT=Development

Every stylesheet and script your Blazor app serves comes back 200 OK with Content-Length: 0, and the page renders like 1996 — unstyled text, no icon. The app is running as Production, and MapStaticAssets is looking for published assets that a development checkout doesn't have.

dotnet run --no-launch-profile skips launchSettings.json, which is where ASPNETCORE_ENVIRONMENT=Development normally comes from. With no environment set, ASP.NET Core defaults to Production, and in Production MapStaticAssets serves from the optimized asset manifest that dotnet publish produces. Nothing published, nothing to serve — so each request matches an endpoint and returns an empty body. Set the environment explicitly, or run with the launch profile.

The error

What the screenshot harness saw, abridged — the web host's real asset paths, every one empty:

GET /_content/ClinicLive.Pocket.Shared/pocket.css   200 OK   Content-Length: 0   (no Content-Type)
GET /ClinicLive.Pocket.Web.styles.css               200 OK   Content-Length: 0
GET /_content/ClinicLive.Pocket.Shared/pocket.js    200 OK   Content-Length: 0

Why it happens

MapStaticAssets (the .NET 9 replacement for UseStaticFiles in the templates) doesn't walk wwwroot at request time. It reads a manifest the build generates, listing every static web asset with its fingerprint and precompressed variants, and maps each to an endpoint. In Development the manifest points at the files in your source tree, including the _content/ assets that Razor class libraries contribute.

Outside Development the manifest expects the processed copies a publish lays down. In a checkout that has never been published, the endpoints exist but the files behind them don't, and the response is a successful request with no body and no content type. Kestrel isn't wrong; it served exactly what the manifest described. The environment is the only variable — the same command, with Development set, serves every file.

The fix

Say which environment you mean. The profile the app ships with already does, in Properties/launchSettings.json:

"http": {
  "commandName": "Project",
  "applicationUrl": "http://localhost:5057",
  "environmentVariables": {
    "ASPNETCORE_ENVIRONMENT": "Development"
  }
}

So the simplest fix is to stop passing --no-launch-profile. If you need it — a harness that wants its own port and no browser popping open — set the variable yourself before the run: $env:ASPNETCORE_ENVIRONMENT = "Development" in PowerShell, export ASPNETCORE_ENVIRONMENT=Development in a shell. And watch the second bite: on the server side of this app, Production also meant no demo data, because the demo seed only runs in Development.

What the AI got wrong: it added --no-launch-profile to keep the harness quiet, and didn't know the flag silently changes which environment the app thinks it is in. A flag that removes configuration removes all of it.

Where it bit us

Season three, Part 2: one UI, three hosts, when the season-two screenshot harness first photographed the new web host — tag pocket-02 in the repo. It bit again in Part 3, on the clinic server, as an API with no demo visits in it. The lesson: --no-launch-profile means Production, and Production means whatever your app does differently there.

Frequently asked

Why does my Blazor app serve empty CSS and JavaScript files with a 200 status?: Because the app is running in the Production environment and MapStaticAssets is serving from the manifest of published, optimized assets, which do not exist in a development checkout. Each request matches an endpoint but there is no file behind it, so the response is 200 with a zero Content-Length. Running in Development fixes it.
Does dotnet run --no-launch-profile change ASPNETCORE_ENVIRONMENT?: Yes, indirectly. The flag skips launchSettings.json, which is where the Development environment variable is normally set, so the app starts with no environment and ASP.NET Core defaults to Production. Set ASPNETCORE_ENVIRONMENT=Development yourself or run with the launch profile.
How do I set ASPNETCORE_ENVIRONMENT for dotnet run?: Either let the launch profile set it, or set the variable in the shell before running: $env:ASPNETCORE_ENVIRONMENT = "Development" in PowerShell or export ASPNETCORE_ENVIRONMENT=Development in bash, then dotnet run.

More decoded errors in the Fixes category; the season starts at Part 1.