diff --git a/src/Cellm/AddIn/ArgumentParser.cs b/src/Cellm/AddIn/ArgumentParser.cs
index 8684adf..168c463 100644
--- a/src/Cellm/AddIn/ArgumentParser.cs
+++ b/src/Cellm/AddIn/ArgumentParser.cs
@@ -3,11 +3,10 @@
 using Cellm.Models.Providers;
 using ExcelDna.Integration;
 using Microsoft.Extensions.Configuration;
-using Microsoft.Office.Interop.Excel;
 
 namespace Cellm.AddIn;
 
-public record Arguments(string Provider, string Model, string Context, string Instructions, double Temperature);
+public record Arguments(Provider Provider, string Model, string Context, string Instructions, double Temperature);
 
 public class ArgumentParser
 {
@@ -71,11 +70,16 @@ public ArgumentParser AddTemperature(object temperature)
 
     public Arguments Parse()
     {
-        var provider = _provider ?? _configuration
+        var providerAsString = _provider ?? _configuration
             .GetSection(nameof(ProviderConfiguration))
             .GetValue<string>(nameof(ProviderConfiguration.DefaultProvider))
             ?? throw new ArgumentException(nameof(ProviderConfiguration.DefaultProvider));
 
+        if (!Enum.TryParse<Provider>(providerAsString, true, out var provider))
+        {
+            throw new ArgumentException($"Unsupported default provider: {providerAsString}");
+        }
+
         var model = _model ?? _configuration
             .GetSection($"{provider}Configuration")
             .GetValue<string>(nameof(IProviderConfiguration.DefaultModel))
@@ -147,9 +151,9 @@ private static string ParseCells(ExcelReference reference)
     {
         try
         {
-            var app = (Application)ExcelDnaUtil.Application;
+            var app = ExcelDnaUtil.Application;
             var sheetName = (string)XlCall.Excel(XlCall.xlSheetNm, reference);
-            sheetName = sheetName[(sheetName.LastIndexOf("]") + 1)..];
+            sheetName = sheetName[(sheetName.LastIndexOf(']') + 1)..];
             var worksheet = app.Sheets[sheetName];
 
             var tableBuilder = new StringBuilder();
diff --git a/src/Cellm/AddIn/ExcelFunctions.cs b/src/Cellm/AddIn/ExcelFunctions.cs
index fe50294..8f5fda6 100644
--- a/src/Cellm/AddIn/ExcelFunctions.cs
+++ b/src/Cellm/AddIn/ExcelFunctions.cs
@@ -93,11 +93,10 @@ public static object PromptWith(
                 .AddUserMessage(userMessage)
                 .Build();
 
-            // ExcelAsyncUtil yields Excel's main thread, Task.Run enables async/await in inner code
-            return ExcelAsyncUtil.Run(nameof(PromptWith), new object[] { providerAndModel, instructionsOrContext, instructionsOrTemperature, temperature }, () =>
-            {
-                return Task.Run(async () => await CallModelAsync(prompt, arguments.Provider)).GetAwaiter().GetResult();
-            });
+            return ExcelAsyncUtil.RunTask(
+                nameof(PromptWith),
+                new object[] { providerAndModel, instructionsOrContext, instructionsOrTemperature, temperature },
+                () => CompleteAsync(prompt, arguments.Provider, null));
         }
         catch (CellmException ex)
         {
@@ -116,7 +115,7 @@ public static object PromptWith(
     /// <returns>A task that represents the asynchronous operation. The task result contains the model's response as a string.</returns>
     /// <exception cref="CellmException">Thrown when an unexpected error occurs during the operation.</exception>
 
-    internal static async Task<string> CallModelAsync(Prompt prompt, string? provider = null, Uri? baseAddress = null)
+    internal static async Task<string> CompleteAsync(Prompt prompt, Provider? provider = null, Uri? baseAddress = null)
     {
         var client = ServiceLocator.Get<Client>();
         var response = await client.Send(prompt, provider, baseAddress, CancellationToken.None);
diff --git a/src/Cellm/Models/Client.cs b/src/Cellm/Models/Client.cs
index feffdcb..c7ed174 100644
--- a/src/Cellm/Models/Client.cs
+++ b/src/Cellm/Models/Client.cs
@@ -7,34 +7,24 @@
 using Cellm.Models.Providers.OpenAi;
 using Cellm.Models.Providers.OpenAiCompatible;
 using MediatR;
-using Microsoft.Extensions.Options;
 using Polly.Timeout;
 
 namespace Cellm.Models;
 
-public class Client(ISender sender, IOptions<ProviderConfiguration> providerConfiguration)
+public class Client(ISender sender)
 {
-    private readonly ProviderConfiguration _providerConfiguration = providerConfiguration.Value;
-
-    public async Task<Prompt> Send(Prompt prompt, string? provider, Uri? baseAddress, CancellationToken cancellationToken)
+    public async Task<Prompt> Send(Prompt prompt, Provider? provider, Uri? baseAddress, CancellationToken cancellationToken)
     {
         try
         {
-            provider ??= _providerConfiguration.DefaultProvider;
-
-            if (!Enum.TryParse<Provider>(provider, true, out var parsedProvider))
-            {
-                throw new ArgumentException($"Unsupported provider: {provider}");
-            }
-
-            IModelResponse response = parsedProvider switch
+            IModelResponse response = provider switch
             {
-                Provider.Anthropic => await sender.Send(new AnthropicRequest(prompt, provider, baseAddress), cancellationToken),
+                Provider.Anthropic => await sender.Send(new AnthropicRequest(prompt, provider.ToString(), baseAddress), cancellationToken),
                 Provider.Llamafile => await sender.Send(new LlamafileRequest(prompt), cancellationToken),
                 Provider.Ollama => await sender.Send(new OllamaRequest(prompt), cancellationToken),
                 Provider.OpenAi => await sender.Send(new OpenAiRequest(prompt), cancellationToken),
-                Provider.OpenAiCompatible => await sender.Send(new OpenAiCompatibleRequest(prompt, baseAddress), cancellationToken),
-                _ => throw new InvalidOperationException($"Provider {parsedProvider} is defined but not implemented")
+                Provider.OpenAiCompatible => await sender.Send(new OpenAiCompatibleRequest(prompt, baseAddress ?? throw new NullReferenceException($"{nameof(Provider.OpenAiCompatible)} requires BaseAddress")), cancellationToken),
+                _ => throw new NotSupportedException($"Provider {provider} is not supported")
             };
 
             return response.Prompt;