Refactor: externalize OpenAI receipt parser prompt to text file

Move hardcoded prompt from OpenAIReceiptParser.cs to Prompts/ReceiptParserPrompt.txt for easier maintenance and editing without recompilation. Add caching to LoadPromptTemplateAsync for performance. Configure .csproj to copy Prompts folder to output directory.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap/MoneyMap.csproj

@@ -30,4 +30,10 @@
     <Folder Include="wwwroot\receipts\" />
   </ItemGroup>
 
+  <ItemGroup>
+    <None Update="Prompts\**\*">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
 </Project>

MoneyMap/Prompts/ReceiptParserPrompt.txt

@@ -0,0 +1,28 @@
+Analyze this receipt image and extract the following information as JSON:
+{
+  "merchant": "store name",
+  "receiptDate": "YYYY-MM-DD" (or null if not found),
+  "dueDate": "YYYY-MM-DD" (or null if not found - for bills only),
+  "subtotal": 0.00 (or null if not found),
+  "tax": 0.00 (or null if not found),
+  "total": 0.00,
+  "confidence": 0.95,
+  "lineItems": [
+    {
+      "description": "item name",
+      "quantity": 1.0 (or null),
+      "unitPrice": 0.00 (or null),
+      "lineTotal": 0.00
+    }
+  ]
+}
+
+Extract all line items you can see on the receipt. For each item:
+- description: The item or service name
+- quantity: Only include if this is an actual countable product (like groceries). For services, fees, charges, or taxes, set to null.
+- unitPrice: Price per unit if quantity applies, otherwise null
+- lineTotal: The total amount for this line (required)
+
+For utility bills, service charges, fees, and taxes - these are NOT products with quantities, so set quantity and unitPrice to null.
+
+If this is a bill (utility, credit card, etc.), look for a due date, payment due date, or deadline and extract it as dueDate. For regular receipts, dueDate should be null.

MoneyMap/Services/OpenAIReceiptParser.cs

@@ -28,6 +28,7 @@ namespace MoneyMap.Services
         private readonly HttpClient _httpClient;
         private readonly IMerchantService _merchantService;
         private readonly IServiceProvider _serviceProvider;
+        private string? _promptTemplate;
 
         public OpenAIReceiptParser(
             MoneyMapContext db,
@@ -205,38 +206,26 @@ namespace MoneyMap.Services
             });
         }
 
+        private async Task<string> LoadPromptTemplateAsync()
+        {
+            if (_promptTemplate != null)
+                return _promptTemplate;
+
+            var promptPath = Path.Combine(AppContext.BaseDirectory, "Prompts", "ReceiptParserPrompt.txt");
+
+            if (!File.Exists(promptPath))
+                throw new FileNotFoundException($"Receipt parser prompt template not found at: {promptPath}");
+
+            _promptTemplate = await File.ReadAllTextAsync(promptPath);
+            return _promptTemplate;
+        }
+
         private async Task<ParsedReceiptData> CallOpenAIVisionAsync(string apiKey, string base64Image, string mediaType, string? transactionName = null)
         {
-            // Build the prompt with optional transaction context
-            var promptText = @"Analyze this receipt image and extract the following information as JSON:
-{
-  ""merchant"": ""store name"",
-  ""receiptDate"": ""YYYY-MM-DD"" (or null if not found),
-  ""dueDate"": ""YYYY-MM-DD"" (or null if not found - for bills only),
-  ""subtotal"": 0.00 (or null if not found),
-  ""tax"": 0.00 (or null if not found),
-  ""total"": 0.00,
-  ""confidence"": 0.95,
-  ""lineItems"": [
-    {
-      ""description"": ""item name"",
-      ""quantity"": 1.0 (or null),
-      ""unitPrice"": 0.00 (or null),
-      ""lineTotal"": 0.00
-    }
-  ]
-}
-
-Extract all line items you can see on the receipt. For each item:
-- description: The item or service name
-- quantity: Only include if this is an actual countable product (like groceries). For services, fees, charges, or taxes, set to null.
-- unitPrice: Price per unit if quantity applies, otherwise null
-- lineTotal: The total amount for this line (required)
-
-For utility bills, service charges, fees, and taxes - these are NOT products with quantities, so set quantity and unitPrice to null.
-
-If this is a bill (utility, credit card, etc.), look for a due date, payment due date, or deadline and extract it as dueDate. For regular receipts, dueDate should be null.";
+            // Load the prompt template from file
+            var promptText = await LoadPromptTemplateAsync();
 
+            // Add transaction context if available
             if (!string.IsNullOrWhiteSpace(transactionName))
             {
                 promptText += $"\n\nNote: This transaction was recorded as \"{transactionName}\" in the bank statement, which may help identify the merchant if the receipt is unclear.";