Why does F# configuration binding work locally but fail when deployed to Azure App Service?

Answer

The most common cause is that Azure App Service runs on a Linux-based container where the file system is case-sensitive, while local Windows development is case-insensitive. If the referenced configuration file name does not match the exact casing on disk, the JSON configuration provider cannot find the file, falls back to environment variables (or defaults), and the bound record receives missing or wrong values.

Confirmed facts from the research

• Locally on Windows, appsettings.json, appsettings.Development.json or any differently-cased reference resolves correctly.
• Azure App Service Linux containers use a case-sensitive file system, so a mismatch in casing causes the provider to miss the file.
• If the project's CopyToOutputDirectory setting is missing or set to PreserveNewest, the file may not be copied to the site's wwwroot folder, leading to a FileNotFoundException that is silently ignored when optional:true.
• Azure App Service application settings (environment variables) override configuration keys; an existing setting with an empty or incorrect value will win over the file-based value.
• F#/.NET Core configuration binding respects the order of providers: if AddEnvironmentVariables is called before AddJsonFile, environment values overwrite file values.

Steps to guarantee consistent binding

1. Verify the file exists with the exact expected casing in the deployed app.
   • Open Kudu console: https://<app-name>.scm.azurewebsites.net/DebugConsole.
   • Navigate to /home/site/wwwroot and confirm appsettings.json (and any environment-specific files) are present with the correct case.
2. Ensure the file is copied during publish.
   • In the .fsproj (or .csproj if using SDK-style), set:

   <None Update="appsettings.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
   </None>

   • For environment-specific files, add similar entries or rely on CopyToPublishDirectory.
3. Order configuration providers correctly.
   • Call AddJsonFile (with optional:false for troubleshooting) before AddEnvironmentVariables.
   • Example in F#:

   let config = ConfigurationBuilder()
                   .SetBasePath(env.ContentRootPath)
                   .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                   .AddJsonFile(sprintf "appsettings.%s.json" env.EnvironmentName, optional: true, reloadOnChange: true)
                   .AddEnvironmentVariables()
                   .Build()

4. Check for conflicting Azure App Service application settings.
   • In the Azure Portal, go to Configuration → Application settings.
   • List any settings whose keys match the configuration paths you bind (e.g., MySection:MyProperty).
   • Temporarily remove or correct those values to see if the file-based values are then used.

Missing diagnostic detail

To confirm whether case sensitivity is the culprit, please verify the underlying OS of your Azure App Service (Windows or Linux). If it is Windows, the issue is likely elsewhere (e.g., missing copy or provider order).