diff --git a/src/AltGen.Console.Tests/AltGen.Console.Tests.csproj b/src/AltGen.Console.Tests/AltGen.Console.Tests.csproj
index 7995a27..94aed1c 100644
--- a/src/AltGen.Console.Tests/AltGen.Console.Tests.csproj
+++ b/src/AltGen.Console.Tests/AltGen.Console.Tests.csproj
@@ -10,10 +10,11 @@
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.2" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
     <PackageReference Include="RichardSzalay.MockHttp" Version="7.0.0" />
-    <PackageReference Include="FluentAssertions" Version="7.1.0" />
+    <PackageReference Include="FluentAssertions" Version="7.2.0" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/src/AltGen.Console.Tests/Unit/AddConfigCommandTests.cs b/src/AltGen.Console.Tests/Unit/AddConfigCommandTests.cs
index 2e68060..dc5fb23 100644
--- a/src/AltGen.Console.Tests/Unit/AddConfigCommandTests.cs
+++ b/src/AltGen.Console.Tests/Unit/AddConfigCommandTests.cs
@@ -2,5 +2,15 @@ namespace AltGen.Console.Tests.Unit;
 
 public class AddConfigCommandTests
 {
+  [Fact]
+  public Task ExecuteAsync_WhenSettingsDoNotExist_ItShouldCreateSettings()
+  {
+    throw new NotImplementedException();
+  }
 
+  [Fact]
+  public Task ExecuteAsync_WhenSettingsExist_ItShouldUpdateSettings()
+  {
+    throw new NotImplementedException();
+  }
 }
\ No newline at end of file
diff --git a/src/AltGen.Console.Tests/Unit/AppSettingsTests.cs b/src/AltGen.Console.Tests/Unit/AppSettingsTests.cs
index c7a67d0..bb5b868 100644
--- a/src/AltGen.Console.Tests/Unit/AppSettingsTests.cs
+++ b/src/AltGen.Console.Tests/Unit/AppSettingsTests.cs
@@ -2,4 +2,82 @@ namespace AltGen.Console.Tests.Unit;
 
 public class AppSettingsTests
 {
+  readonly Mock<IFileSystem> _fileSystemMock = new();
+
+  [Fact]
+  public void AddOrUpdateProvider_WhenProviderDoesNotExist_ItShouldAddProvider()
+  {
+    var providerSettings = new ProviderSettings("provider", "key", true);
+
+    var commandSettings = new AddConfigCommand.Settings(_fileSystemMock.Object)
+    {
+      Provider = providerSettings.Provider,
+      Key = providerSettings.Key,
+      Default = providerSettings.Default,
+    };
+
+    var appSettings = new AppSettings([]);
+
+    var updatedAppSettings = appSettings.AddOrUpdateProvider(commandSettings);
+
+    updatedAppSettings.Should().BeEquivalentTo(new AppSettings([providerSettings]));
+  }
+
+  [Fact]
+  public void AddOrUpdateProvider_WhenProviderDoesExist_ItShouldUpdateProvider()
+  {
+    var providerSettings = new ProviderSettings("provider", "key", true);
+    var existingProviderSettings = new ProviderSettings("provider", "key", false);
+
+    var commandSettings = new AddConfigCommand.Settings(_fileSystemMock.Object)
+    {
+      Provider = providerSettings.Provider,
+      Key = providerSettings.Key,
+      Default = providerSettings.Default,
+    };
+
+    var appSettings = new AppSettings([existingProviderSettings]);
+    var updatedAppSettings = appSettings.AddOrUpdateProvider(commandSettings);
+
+    updatedAppSettings.Should().BeEquivalentTo(new AppSettings([providerSettings]));
+  }
+
+  [Fact]
+  public void AddOrUpdateProvider_WhenExistingProviderIsSetToDefaultAndNewDefaultGiven_ItShouldUpdateProvidersCorrectly()
+  {
+    var providerSettings = new ProviderSettings("claude", "key", true);
+    var existingProviderSettings = new ProviderSettings("gemini", "key", true);
+
+    var commandSettings = new AddConfigCommand.Settings(_fileSystemMock.Object)
+    {
+      Provider = providerSettings.Provider,
+      Key = providerSettings.Key,
+      Default = providerSettings.Default,
+    };
+
+    var appSettings = new AppSettings([existingProviderSettings]);
+    var updatedAppSettings = appSettings.AddOrUpdateProvider(commandSettings);
+
+    updatedAppSettings.Should().BeEquivalentTo(new AppSettings([
+      providerSettings,
+      existingProviderSettings with { Default = false }
+    ]));
+  }
+
+  [Fact]
+  public void RemoveProvider_WhenProviderExists_ItShouldRemoveProvider()
+  {
+    var existingProviderSettings = new ProviderSettings("provider", "key", true);
+
+    var commandSettings = new RemoveConfigCommand.Settings(_fileSystemMock.Object)
+    {
+      Provider = existingProviderSettings.Provider,
+    };
+
+    var appSettings = new AppSettings([existingProviderSettings]);
+
+    var updatedAppSettings = appSettings.RemoveProvider(commandSettings);
+
+    updatedAppSettings.Should().BeEquivalentTo(new AppSettings([]));
+  }
 }
\ No newline at end of file
diff --git a/src/AltGen.Console.Tests/Unit/RemoveConfigCommandTests.cs b/src/AltGen.Console.Tests/Unit/RemoveConfigCommandTests.cs
index 2a52faf..50dea56 100644
--- a/src/AltGen.Console.Tests/Unit/RemoveConfigCommandTests.cs
+++ b/src/AltGen.Console.Tests/Unit/RemoveConfigCommandTests.cs
@@ -2,4 +2,22 @@ namespace AltGen.Console.Tests.Unit;
 
 public class RemoveConfigCommandTests
 {
+  [Fact]
+  public Task ExecuteAsync_WhenSettingsDoNotExist_ItShouldThrow()
+  {
+    throw new NotImplementedException();
+  }
+
+
+  [Fact]
+  public Task ExecuteAsync_WhenProviderDoesNotExist_ItShouldDoNothing()
+  {
+    throw new NotImplementedException();
+  }
+
+  [Fact]
+  public Task ExecuteAsync_WhenProviderExists_ItShouldRemoveProviderFromSettings()
+  {
+    throw new NotImplementedException();
+  }
 }
\ No newline at end of file
diff --git a/src/AltGen.Console.Tests/Usings.cs b/src/AltGen.Console.Tests/Usings.cs
index a5fd333..cf6c14c 100644
--- a/src/AltGen.Console.Tests/Usings.cs
+++ b/src/AltGen.Console.Tests/Usings.cs
@@ -1,3 +1,10 @@
-global using RichardSzalay.MockHttp;
+global using System.IO.Abstractions;
 
-global using AltGen.Console.Generate;
\ No newline at end of file
+global using AltGen.Console.Config;
+global using AltGen.Console.Generate;
+
+global using FluentAssertions;
+
+global using Moq;
+
+global using RichardSzalay.MockHttp;
diff --git a/src/AltGen.Console/Common/JsonOptions.cs b/src/AltGen.Console/Common/JsonOptions.cs
new file mode 100644
index 0000000..f28d252
--- /dev/null
+++ b/src/AltGen.Console/Common/JsonOptions.cs
@@ -0,0 +1,11 @@
+namespace AltGen.Console.Common;
+
+static class JsonOptions
+{
+  public static JsonSerializerOptions Default { get; } = new()
+  {
+    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+    PropertyNameCaseInsensitive = true,
+    WriteIndented = true
+  };
+}
\ No newline at end of file
diff --git a/src/AltGen.Console/Config/AddConfigCommand.cs b/src/AltGen.Console/Config/AddConfigCommand.cs
index 176b38c..597bd39 100644
--- a/src/AltGen.Console/Config/AddConfigCommand.cs
+++ b/src/AltGen.Console/Config/AddConfigCommand.cs
@@ -5,12 +5,6 @@ sealed class AddConfigCommand(
   IFileSystem fileSystem
 ) : AsyncCommand<AddConfigCommand.Settings>
 {
-  static readonly JsonSerializerOptions JsonOptions = new()
-  {
-    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
-    PropertyNameCaseInsensitive = true,
-    WriteIndented = true
-  };
   readonly IAnsiConsole _console = console;
   readonly IFileSystem _fileSystem = fileSystem;
 
@@ -43,17 +37,17 @@ sealed class AddConfigCommand(
         new(settings.Provider, settings.Key, settings.Default)
       ]);
 
-      var json = JsonSerializer.Serialize(appSettings, JsonOptions);
+      var json = JsonSerializer.Serialize(appSettings, JsonOptions.Default);
       await _fileSystem.File.WriteAllTextAsync(settings.SettingsPath, json);
       _console.MarkupLine($"[bold]{settings.Provider}[/] has been configured.");
       return 0;
     }
 
     var existingJson = await _fileSystem.File.ReadAllTextAsync(settings.SettingsPath);
-    var existingAppSettings = JsonSerializer.Deserialize<AppSettings>(existingJson, JsonOptions)
+    var existingAppSettings = JsonSerializer.Deserialize<AppSettings>(existingJson, JsonOptions.Default)
       ?? throw new ConfigException("Failed to deserialize settings.");
     var updatedAppSettings = existingAppSettings.AddOrUpdateProvider(settings);
-    var udpatedJson = JsonSerializer.Serialize(updatedAppSettings, JsonOptions);
+    var udpatedJson = JsonSerializer.Serialize(updatedAppSettings, JsonOptions.Default);
     await _fileSystem.File.WriteAllTextAsync(settings.SettingsPath, udpatedJson);
     _console.MarkupLine($"[bold]{settings.Provider}[/] has been configured.");
     return 0;
diff --git a/src/AltGen.Console/Config/RemoveConfigCommand.cs b/src/AltGen.Console/Config/RemoveConfigCommand.cs
index 47fdd8f..3af4423 100644
--- a/src/AltGen.Console/Config/RemoveConfigCommand.cs
+++ b/src/AltGen.Console/Config/RemoveConfigCommand.cs
@@ -25,14 +25,13 @@ sealed class RemoveConfigCommand(
 
     if (exists is false)
     {
-      _console.MarkupLine("[bold]No existing settings found.[/]");
-      return 1;
+      throw new ConfigException("No existing settings found.");
     }
 
     var settingsJson = await _fileSystem.File.ReadAllTextAsync(settings.SettingsPath);
-    var appSettings = JsonSerializer.Deserialize<AppSettings>(settingsJson) ?? throw new ConfigException("Failed to deserialize app settings.");
+    var appSettings = JsonSerializer.Deserialize<AppSettings>(settingsJson, JsonOptions.Default) ?? throw new ConfigException("Failed to deserialize app settings.");
     var updatedAppSettings = appSettings.RemoveProvider(settings);
-    var updatedJson = JsonSerializer.Serialize(updatedAppSettings);
+    var updatedJson = JsonSerializer.Serialize(updatedAppSettings, JsonOptions.Default);
     await _fileSystem.File.WriteAllTextAsync(settings.SettingsPath, updatedJson);
     _console.MarkupLine($"[bold]{settings.Provider}[/] has been removed.");
     return 0;