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How do I debug a SAFE app?

I'm using Visual Studio

In order to debug Server code from Visual Studio, we need set the correct URLs in the project's debug properties.

Debugging the Server

1. Configure launch settings

You can do this through the Server project's Properties/Debug editor or by editing the launchSettings.json file which is in the properties folder.

After selecting the debug profile that you wish to edit (IIS Express or Server), you will need to set the App URL field to http://localhost:5000 and Launch browser field to http://localhost:8080. The process is very similar for VS Mac.

Once this is done, you can expect your launchSettings.json file to look something like this:

  "iisSettings": {
    "windowsAuthentication": false,
    "anonymousAuthentication": true,
    "iisExpress": {
      "applicationUrl": "http://localhost:5000/",
      "sslPort": 44330
  "profiles": {
    "IIS Express": {
      "commandName": "IISExpress",
      "launchBrowser": true,
      "launchUrl": "http://localhost:8080/",
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Development"
    "Server": {
      "commandName": "Project",
      "launchBrowser": true,
      "launchUrl": "http://localhost:8080",
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Development"
      "applicationUrl": "http://localhost:5000"

2. Start the Client

Since you will be running the server directly through Visual Studio, you cannot use a FAKE script to start the application, so launch the client directly using e.g. npm run start.

3. Debug the Server

Set the server as your Startup project, either using the drop-down menu at the top of the IDE or by right clicking on the project itself and selecting Set as Startup Project. Select the profile that you set up earlier and wish to launch from the drop-down at the top of the IDE. Either press the Play button at the top of the IDE or hit F5 on your keyboard to start the Server debugging and launch a browser pointing at the website.

Debugging the Client

Although we write our client-side code using F#, it is being converted into JavaScript at runtime by Fable and executed in the browser. However, we can still debug it via the magic of source mapping. If you are using Visual Studio, you cannot directly connect to the browser debugger. You can, however, debug your client F# code using the browser's development tools.

1. Set breakpoints in Client code

The exact instructions will depend on your browser, but essentially it simply involves:

  • Opening the Developer tools panel (usually by hitting F12).
  • Finding the F# file you want to add breakpoints to in the source of the website (look inside the webpack folder).
  • Add breakpoints to it in your browser inspector.

I'm using VS Code

VS Code allows "full stack" debugging i.e. both the client and server. Prerequisites that you should install:

0. Install Prerequisites

  • Install either Google Chrome or Microsoft Edge: Enables client-side debugging.
  • Configure your browser with the following extensions:
  • Configure VS Code with the following extensions:
    • Ionide: Provides F# support to Code.
    • C#: Provides .NET Core debugging support.

1. Create a launch.json file

Open the Command Palette using Ctrl+Shift+P and run Debug: Add Configuration.... This will ask you to choose a debugger; select Ionide LaunchSettings.

This will create a launch.json file in the root of your solution and also open it in the editor.

2. Update the Configuration

The only change required is to point it at the Server application, by replacing the program line with this:

"program": "${workspaceFolder}/src/Server/bin/Debug/net6.0/Server.dll",

3. Configure a build task

  • From the Command Palette, choose Tasks: Configure Task.
  • Select Create tasks.json file from template. This will show you a list of pre-configured templates.
  • Select .NET Core.
  • Update the build directory using "options": {"cwd": "src/Server"}, as shown below:
    // See
    // for the documentation about the tasks.json format
    "version": "2.0.0",
    "tasks": [
            "label": "build",
            "command": "dotnet",
            "type": "shell",
            "options": {"cwd": "src/Server"}, 
            "args": [
                // Ask dotnet build to generate full paths for file names.
                // Do not generate summary otherwise it leads to duplicate errors in Problems panel
            "group": "build",
            "presentation": {
                "reveal": "silent"
            "problemMatcher": "$msCompile"

4. Debug the Server

Either hit F5 or open the Debugging pane and press the Play button to build and launch the Server with the debugger attached. Observe that the Debug Console panel will show output from the server. The server is now running and you can set breakpoints and view the callstack etc.

5. Debug the Client

  • Start the Client by running dotnet fable watch -o output -s --run npm run start from <repo root>/src/Client/.
  • Open the Command Palette and run Debug: Open Link.
  • When prompted for a url, type http://localhost:8080/. This will launch a browser which is pointed at the URL and connect the debugger to it.
  • You can now set breakpoints in the generated .fs.js files within VS Code.
  • Select the appropriate Debug Console you wish to view.

If you find that your breakpoints aren't being hit, try stopping the Client, disconnecting the debugger and re-launching them both.

To find out more about the VS Code debugger, see here.