fix: improve exception handling and update chat deployment name

- Replace UseExceptionHandler('/Error') with lambda handler that:
  - Logs exceptions via IExceptionHandlerFeature (was silently swallowed)
  - Returns JSON 500 for /api/* requests (fixes chat returning 404 HTML)
  - Redirects non-API requests to /Home/Error?statusCode=500 (route exists)
- Add try/catch to ChatController.StreamMessage with:
  - OperationCanceledException guard for clean client disconnect handling
  - Pre-response-start catch: returns JSON 500 with CancellationToken.None
  - Mid-stream catch: best-effort SSE error event, wrapped in try/catch
- Update ChatDeploymentName default: gpt-4o -> gpt-5 (matches ecs-cog-prod)

Author: Benjamin Michaelis

EssentialCSharp.Web/Controllers/ChatController.cs

@@ -72,29 +72,53 @@ public async Task StreamMessage([FromBody] ChatMessageRequest request, Cancellat
         Response.Headers.CacheControl = "no-cache";
         Response.Headers.Connection = "keep-alive";
 
-        await foreach (var (text, responseId) in _AiChatService.GetChatCompletionStream(
-            prompt: request.Message,
-            systemPrompt: request.SystemPrompt,
-            previousResponseId: request.PreviousResponseId,
-            enableContextualSearch: request.EnableContextualSearch,
-            cancellationToken: cancellationToken))
+        try
         {
-            if (!string.IsNullOrEmpty(text))
+            await foreach (var (text, responseId) in _AiChatService.GetChatCompletionStream(
+                prompt: request.Message,
+                systemPrompt: request.SystemPrompt,
+                previousResponseId: request.PreviousResponseId,
+                enableContextualSearch: request.EnableContextualSearch,
+                cancellationToken: cancellationToken))
             {
-                var eventData = JsonSerializer.Serialize(new { type = "text", data = text });
-                await Response.WriteAsync($"data: {eventData}\n\n", cancellationToken);
-                await Response.Body.FlushAsync(cancellationToken);
+                if (!string.IsNullOrEmpty(text))
+                {
+                    var eventData = JsonSerializer.Serialize(new { type = "text", data = text });
+                    await Response.WriteAsync($"data: {eventData}\n\n", cancellationToken);
+                    await Response.Body.FlushAsync(cancellationToken);
+                }
+
+                if (!string.IsNullOrEmpty(responseId))
+                {
+                    var eventData = JsonSerializer.Serialize(new { type = "responseId", data = responseId });
+                    await Response.WriteAsync($"data: {eventData}\n\n", cancellationToken);
+                    await Response.Body.FlushAsync(cancellationToken);
+                }
             }
 
-            if (!string.IsNullOrEmpty(responseId))
+            await Response.WriteAsync("data: [DONE]\n\n", cancellationToken);
+            await Response.Body.FlushAsync(cancellationToken);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested || HttpContext.RequestAborted.IsCancellationRequested)
+        {
+            _Logger.LogDebug("Chat stream cancelled for user {User}", User.Identity?.Name);
+        }
+        catch (Exception ex) when (!Response.HasStarted)
+        {
+            _Logger.LogError(ex, "Chat streaming error before response started for user {User}", User.Identity?.Name);
+            Response.StatusCode = 500;
+            Response.ContentType = "application/json";
+            await Response.WriteAsJsonAsync(new { error = "Chat service unavailable" }, CancellationToken.None);
+        }
+        catch (Exception ex)
+        {
+            _Logger.LogError(ex, "Chat streaming error mid-stream for user {User}", User.Identity?.Name);
+            try
             {
-                var eventData = JsonSerializer.Serialize(new { type = "responseId", data = responseId });
-                await Response.WriteAsync($"data: {eventData}\n\n", cancellationToken);
-                await Response.Body.FlushAsync(cancellationToken);
+                await Response.WriteAsync("data: {\"type\":\"error\",\"message\":\"Stream interrupted\"}\n\n", CancellationToken.None);
+                await Response.Body.FlushAsync(CancellationToken.None);
             }
+            catch { /* client already disconnected */ }
         }
-
-        await Response.WriteAsync("data: [DONE]\n\n", cancellationToken);
-        await Response.Body.FlushAsync(cancellationToken);
     }
 }

EssentialCSharp.Web/Program.cs

@@ -15,6 +15,7 @@
 using Microsoft.AspNetCore.RateLimiting;
 using Azure.Monitor.OpenTelemetry.AspNetCore;
 using Microsoft.AspNetCore.DataProtection;
+using Microsoft.AspNetCore.Diagnostics;
 using Microsoft.AspNetCore.Diagnostics.HealthChecks;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
@@ -352,7 +353,26 @@ await context.HttpContext.Response.WriteAsync(
         // Configure the HTTP request pipeline.
         if (!app.Environment.IsDevelopment())
         {
-            app.UseExceptionHandler("/Error");
+            app.UseExceptionHandler(exceptionApp =>
+            {
+                exceptionApp.Run(async context =>
+                {
+                    var exceptionFeature = context.Features.Get<IExceptionHandlerFeature>();
+                    var logger = context.RequestServices.GetRequiredService<ILogger<Program>>();
+                    logger.LogError(exceptionFeature?.Error, "Unhandled exception on {Path}", context.Request.Path);
+
+                    if (context.Request.Path.StartsWithSegments("/api"))
+                    {
+                        context.Response.StatusCode = 500;
+                        context.Response.ContentType = "application/json";
+                        await context.Response.WriteAsJsonAsync(new { error = "An unexpected error occurred" });
+                    }
+                    else
+                    {
+                        context.Response.Redirect("/Home/Error?statusCode=500");
+                    }
+                });
+            });
             app.UseForwardedHeaders();
             app.UseSecurityHeadersMiddleware(new SecurityHeadersBuilder()
                 .AddDefaultSecurePolicy());

EssentialCSharp.Web/appsettings.json

@@ -20,7 +20,7 @@
   },
   "AIOptions": {
     "VectorGenerationDeploymentName": "text-embedding-3-large-v1",
-    "ChatDeploymentName": "gpt-4o",
+    "ChatDeploymentName": "gpt-5",
     "SystemPrompt": "You are a helpful AI assistant with expertise in C# programming and the Essential C# book content. You can help users understand C# concepts, answer programming questions, and provide guidance based on the Essential C# book materials. Be concise but thorough in your explanations.",
     "Endpoint": "",
     "ApiKey": ""