# Feature Recipe: Semantic Kernel Chat with Tool Calling

**Source**: migrate-to-semantic-kernel + wire-responses-api | **Lines to type**: ~120
**Included**: wire-responses-api (SSE streaming), migrate-to-semantic-kernel (SK + plugins)
**Skipped**: wire-responses-api's manual HttpClient proxy (superseded by SK)
**Skipped**: basic-chat-interface (UI — not selected), multi-turn-conversations (not selected)

---

## Prerequisites

Add to your API `.csproj`:
```xml
<PackageReference Include="Microsoft.SemanticKernel" Version="1.74.0" />
<PackageReference Include="Microsoft.SemanticKernel.Connectors.OpenAI" Version="1.74.0" />
```

---

## Step 1: New file — Shared/Models/ChatMessage.cs (~6 lines)

```csharp
namespace __YOUR_NAMESPACE__.Shared.Models
{
    public class ChatMessage
    {
        public string Role { get; set; } = string.Empty;
        public string Content { get; set; } = string.Empty;
        public DateTime Timestamp { get; set; }
    }
}
```

## Step 2: New file — Shared/Models/ChatRequest.cs (~6 lines)

```csharp
namespace __YOUR_NAMESPACE__.Shared.Models
{
    public class ChatRequest
    {
        public List<ChatMessage> Messages { get; set; } = new();
    }
}
```

## Step 3: New file — Shared/Models/ExtractedFields.cs (~12 lines)

```csharp
// ADAPT: Replace these fields with your domain's extraction schema
namespace __YOUR_NAMESPACE__.Shared.Models
{
    public class ExtractedFields
    {
        public string? Client { get; set; }
        public string? Project { get; set; }
        public decimal? Hours { get; set; }
        public decimal? Rate { get; set; }
        public string? Currency { get; set; }
        public string? Date { get; set; }
        public string? Description { get; set; }
        public string? PoNumber { get; set; }
    }
}
```

## Step 4: New file — Shared/Models/ValidationResult.cs (~5 lines)

```csharp
namespace __YOUR_NAMESPACE__.Shared.Models
{
    public class ValidationResult
    {
        public bool IsValid { get; set; }
        public List<string> Errors { get; set; } = new();
    }
}
```

## Step 5: New file — Plugins/ExtractionPlugin.cs (~35 lines)

```csharp
// ADAPT: Change RequiredFields and validation logic to match your ExtractedFields
using System.ComponentModel;
using System.Text.Json;
using __YOUR_NAMESPACE__.Shared.Models;
using Microsoft.SemanticKernel;

namespace __YOUR_NAMESPACE__.Api.Plugins
{
    public class ExtractionPlugin
    {
        private static readonly string[] RequiredFields =
            { "Client", "Project", "Hours", "Rate", "Currency", "Date" };

        [KernelFunction("validate_extracted_fields")]
        [Description("Validates extracted key-value fields against the required schema. " +
            "Call this after extracting fields from natural language text to check " +
            "that all required fields (Client, Project, Hours, Rate, Currency, Date) " +
            "are present and correctly typed. Returns validation result with any errors.")]
        public string ValidateExtractedFields(
            [Description("JSON string of extracted fields")] string fieldsJson)
        {
            var result = new ValidationResult();
            ExtractedFields? fields;
            try
            {
                fields = JsonSerializer.Deserialize<ExtractedFields>(fieldsJson,
                    new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
            }
            catch (JsonException ex)
            {
                result.IsValid = false;
                result.Errors.Add($"Invalid JSON: {ex.Message}");
                return JsonSerializer.Serialize(result);
            }

            if (fields == null)
            {
                result.IsValid = false;
                result.Errors.Add("Deserialized fields object is null");
                return JsonSerializer.Serialize(result);
            }

            if (string.IsNullOrWhiteSpace(fields.Client))
                result.Errors.Add("Missing required field: Client");
            if (string.IsNullOrWhiteSpace(fields.Project))
                result.Errors.Add("Missing required field: Project");
            if (fields.Hours == null || fields.Hours <= 0)
                result.Errors.Add("Missing or invalid required field: Hours (must be positive)");
            if (fields.Rate == null || fields.Rate <= 0)
                result.Errors.Add("Missing or invalid required field: Rate (must be positive)");
            if (string.IsNullOrWhiteSpace(fields.Currency))
                result.Errors.Add("Missing required field: Currency");
            if (string.IsNullOrWhiteSpace(fields.Date))
                result.Errors.Add("Missing required field: Date");

            result.IsValid = result.Errors.Count == 0;
            return JsonSerializer.Serialize(result);
        }
    }
}
```

## Step 6: New file — Controllers/ChatController.cs (~40 lines)

```csharp
using System.Text.Json;
using __YOUR_NAMESPACE__.Api.Plugins;
using __YOUR_NAMESPACE__.Shared.Models;
using Microsoft.AspNetCore.Mvc;
using Microsoft.SemanticKernel;
using Microsoft.SemanticKernel.ChatCompletion;
using Microsoft.SemanticKernel.Connectors.OpenAI;

namespace __YOUR_NAMESPACE__.Api.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class ChatController : ControllerBase
    {
        private readonly Kernel _kernel;

        public ChatController(Kernel kernel) { _kernel = kernel; }

        [HttpPost]
        public async Task Post([FromBody] ChatRequest request)
        {
            Response.ContentType = "text/event-stream";
            Response.Headers["Cache-Control"] = "no-cache";
            try
            {
                var chatService = _kernel.GetRequiredService<IChatCompletionService>();
                var chatHistory = new ChatHistory();
                foreach (var msg in request.Messages)
                {
                    if (msg.Role == "user") chatHistory.AddUserMessage(msg.Content);
                    else if (msg.Role == "assistant") chatHistory.AddAssistantMessage(msg.Content);
                }

                var plugin = HttpContext.RequestServices.GetRequiredService<ExtractionPlugin>();
                _kernel.ImportPluginFromObject(plugin, "Extraction");

                var settings = new OpenAIPromptExecutionSettings
                {
                    FunctionChoiceBehavior = FunctionChoiceBehavior.Auto()
                };

                await foreach (var chunk in chatService.GetStreamingChatMessageContentsAsync(
                    chatHistory, settings, _kernel, HttpContext.RequestAborted))
                {
                    if (!string.IsNullOrEmpty(chunk.Content))
                    {
                        await WriteSSEAsync($"{{\"text\":{JsonSerializer.Serialize(chunk.Content)}}}");
                        await Response.Body.FlushAsync();
                    }
                }
                await WriteSSEAsync("[DONE]");
            }
            catch (HttpRequestException ex)
            {
                await WriteSSEAsync($"{{\"error\":{JsonSerializer.Serialize($"Failed to reach LLM service: {ex.Message}")}}}");
                await WriteSSEAsync("[DONE]");
            }
            catch (TaskCanceledException) { }
        }

        private async Task WriteSSEAsync(string data)
        {
            await Response.WriteAsync($"data: {data}\n\n");
            await Response.Body.FlushAsync();
        }
    }
}
```

## Step 7: Add to Program.cs (~12 lines)

Add `using Microsoft.SemanticKernel;` at the top.

Insert after `builder.Services.AddControllers();`:
```csharp
// ADAPT: Change BaseUrl and Model for your proxy/LLM
var baseUrl = builder.Configuration["ResponsesApi:BaseUrl"] ?? "http://localhost:8317/v1";
var model = builder.Configuration["ResponsesApi:Model"] ?? "claude-sonnet-4-6";

builder.Services.AddOpenAIChatCompletion(
    modelId: model,
    endpoint: new Uri(baseUrl),
    apiKey: builder.Configuration["ResponsesApi:ApiKey"] ?? "not-needed");
builder.Services.AddKernel();
builder.Services.AddSingleton<__YOUR_NAMESPACE__.Api.Plugins.ExtractionPlugin>();
```

## Step 8: Add to appsettings.json (~4 lines)

Add this section:
```json
"ResponsesApi": {
    "BaseUrl": "http://localhost:8317/v1",
    "Model": "claude-sonnet-4-6"
}
```

---

## Gotchas

- **Base URL must include `/v1`** — the OpenAI SDK appends `chat/completions` directly, so without `/v1` you get a 404
- **`using Microsoft.SemanticKernel;`** is required in Program.cs for the `AddOpenAIChatCompletion` extension method to resolve
- **Plugin is imported per-request** via `_kernel.ImportPluginFromObject` — do not register it in the kernel at startup
- **CORS**: If your Blazor client is on a different port, add CORS policy in Program.cs before `app.Run()`