Make AI categorization prompts user-configurable

Add SystemPrompt and UserPromptTemplate fields to AI categorization
settings so users can customize the prompts sent to Ollama without
code changes. Null/empty values fall back to built-in defaults.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/index.ts

@@ -493,6 +493,8 @@ export interface AiCategorizationSettings {
   confidenceThreshold: number
   timeoutSeconds: number
   updatePayeeDefaults: boolean
+  systemPrompt: string | null
+  userPromptTemplate: string | null
 }
 
 export interface OllamaConnectionTestResult {

frontend/src/views/settings/SettingsView.vue

@@ -466,6 +466,44 @@
           </v-card-text>
         </v-card>
 
+        <v-card class="mt-4">
+          <v-card-title>Prompt Templates</v-card-title>
+          <v-card-text>
+            <p class="text-body-2 text-medium-emphasis mb-4">
+              Customize the prompts sent to the AI model. Leave empty to use built-in defaults.
+              The category list is always appended to the system prompt, and the transaction list is always appended to the user prompt.
+            </p>
+
+            <v-textarea
+              v-model="aiSettings.systemPrompt"
+              label="System Prompt"
+              variant="outlined"
+              rows="8"
+              :placeholder="defaultSystemPrompt"
+              persistent-placeholder
+              class="mb-4"
+            />
+
+            <v-textarea
+              v-model="aiSettings.userPromptTemplate"
+              label="User Prompt Template"
+              variant="outlined"
+              rows="3"
+              :placeholder="defaultUserPromptTemplate"
+              persistent-placeholder
+              class="mb-4"
+            />
+
+            <v-btn
+              variant="tonal"
+              color="secondary"
+              @click="resetPromptsToDefaults"
+            >
+              Reset to Defaults
+            </v-btn>
+          </v-card-text>
+        </v-card>
+
         <v-snackbar v-model="showAiSuccess" :timeout="4000" color="success">
           AI categorization settings saved
         </v-snackbar>
@@ -801,7 +839,9 @@ const aiSettings = reactive<AiCategorizationSettings>({
   modelName: 'llama3.1:8b',
   confidenceThreshold: 0.7,
   timeoutSeconds: 30,
-  updatePayeeDefaults: true
+  updatePayeeDefaults: true,
+  systemPrompt: null,
+  userPromptTemplate: null
 })
 const aiSaving = ref(false)
 const aiTesting = ref(false)
@@ -811,6 +851,31 @@ const showAiSuccess = ref(false)
 const showAiError = ref(false)
 const aiErrorMessage = ref('')
 
+const defaultSystemPrompt = `You are a personal finance transaction categorizer. Classify each transaction into the most appropriate category.
+
+You MUST respond with valid JSON in exactly this format:
+{
+  "classifications": [
+    {"index": 0, "categoryName": "Groceries", "confidence": 0.9},
+    {"index": 1, "categoryName": "Utilities", "confidence": 0.85}
+  ]
+}
+
+Rules:
+- "index" must be the transaction number (integer) from the list below
+- "categoryName" must be one of the exact category names listed below
+- "confidence" must be a number between 0 and 1
+- Include one entry for every transaction
+
+Available categories:`
+
+const defaultUserPromptTemplate = 'Classify these uncategorized transactions:'
+
+function resetPromptsToDefaults() {
+  aiSettings.systemPrompt = null
+  aiSettings.userPromptTemplate = null
+}
+
 async function loadAiSettings() {
   try {
     const { data } = await aiCategorizationApi.getSettings()

src/Purrse.Api/Services/AiCategorizationService.cs

@@ -16,6 +16,29 @@ public class AiCategorizationService : IAiCategorizationService
     private readonly IHttpClientFactory _httpClientFactory;
     private readonly ILogger<AiCategorizationService> _logger;
 
+    internal const string DefaultSystemPrompt =
+        """
+        You are a personal finance transaction categorizer. Classify each transaction into the most appropriate category.
+
+        You MUST respond with valid JSON in exactly this format:
+        {
+          "classifications": [
+            {"index": 0, "categoryName": "Groceries", "confidence": 0.9},
+            {"index": 1, "categoryName": "Utilities", "confidence": 0.85}
+          ]
+        }
+
+        Rules:
+        - "index" must be the transaction number (integer) from the list below
+        - "categoryName" must be one of the exact category names listed below
+        - "confidence" must be a number between 0 and 1
+        - Include one entry for every transaction
+
+        Available categories:
+        """;
+
+    internal const string DefaultUserPromptTemplate = "Classify these uncategorized transactions:";
+
     private static readonly JsonSerializerOptions JsonOptions = new()
     {
         PropertyNameCaseInsensitive = true,
@@ -39,12 +62,13 @@ public class AiCategorizationService : IAiCategorizationService
         if (settings == null)
         {
             return new AiCategorizationSettingsResponse(
-                false, "http://localhost:11434", "llama3.1:8b", 0.7, 30, true);
+                false, "http://localhost:11434", "llama3.1:8b", 0.7, 30, true, null, null);
         }
 
         return new AiCategorizationSettingsResponse(
             settings.IsEnabled, settings.OllamaUrl, settings.ModelName,
-            settings.ConfidenceThreshold, settings.TimeoutSeconds, settings.UpdatePayeeDefaults);
+            settings.ConfidenceThreshold, settings.TimeoutSeconds, settings.UpdatePayeeDefaults,
+            settings.SystemPrompt, settings.UserPromptTemplate);
     }
 
     public async Task<AiCategorizationSettingsResponse> UpdateSettingsAsync(Guid userId, UpdateAiCategorizationSettingsRequest request)
@@ -67,13 +91,16 @@ public class AiCategorizationService : IAiCategorizationService
         settings.ConfidenceThreshold = request.ConfidenceThreshold;
         settings.TimeoutSeconds = request.TimeoutSeconds;
         settings.UpdatePayeeDefaults = request.UpdatePayeeDefaults;
+        settings.SystemPrompt = string.IsNullOrWhiteSpace(request.SystemPrompt) ? null : request.SystemPrompt;
+        settings.UserPromptTemplate = string.IsNullOrWhiteSpace(request.UserPromptTemplate) ? null : request.UserPromptTemplate;
         settings.UpdatedAt = DateTime.UtcNow;
 
         await _db.SaveChangesAsync();
 
         return new AiCategorizationSettingsResponse(
             settings.IsEnabled, settings.OllamaUrl, settings.ModelName,
-            settings.ConfidenceThreshold, settings.TimeoutSeconds, settings.UpdatePayeeDefaults);
+            settings.ConfidenceThreshold, settings.TimeoutSeconds, settings.UpdatePayeeDefaults,
+            settings.SystemPrompt, settings.UserPromptTemplate);
     }
 
     public async Task<OllamaConnectionTestResponse> TestConnectionAsync(Guid userId)
@@ -158,8 +185,8 @@ public class AiCategorizationService : IAiCategorizationService
         List<Transaction> batch,
         int globalOffset)
     {
-        var systemMessage = BuildSystemMessage(categories);
+        var systemMessage = BuildSystemMessage(settings, categories);
-        var userMessage = BuildUserMessage(recentCategorized, batch);
+        var userMessage = BuildUserMessage(settings, recentCategorized, batch);
 
         var chatRequest = new OllamaChatRequest(
             settings.ModelName,
@@ -179,6 +206,8 @@ public class AiCategorizationService : IAiCategorizationService
             var requestJson = JsonSerializer.Serialize(chatRequest, JsonOptions);
             var content = new StringContent(requestJson, Encoding.UTF8, "application/json");
 
+            _logger.LogInformation("Sending {Count} transactions to Ollama for classification (batch offset {Offset})", batch.Count, globalOffset);
+
             var response = await client.PostAsync($"{settings.OllamaUrl.TrimEnd('/')}/api/chat", content);
             response.EnsureSuccessStatusCode();
 
@@ -186,7 +215,12 @@ public class AiCategorizationService : IAiCategorizationService
             var chatResponse = JsonSerializer.Deserialize<OllamaChatResponse>(responseJson, JsonOptions);
 
             if (chatResponse?.Message?.Content == null)
+            {
+                _logger.LogWarning("Ollama returned null message content for batch at offset {Offset}", globalOffset);
                 return new List<TransactionClassification>();
+            }
+
+            _logger.LogDebug("Ollama raw response for batch at offset {Offset}: {Response}", globalOffset, chatResponse.Message.Content);
 
             return ParseClassifications(chatResponse.Message.Content, categories, batch.Count, globalOffset, settings.ConfidenceThreshold);
         }
@@ -197,24 +231,21 @@ public class AiCategorizationService : IAiCategorizationService
         }
     }
 
-    private static string BuildSystemMessage(List<Category> categories)
+    private static string BuildSystemMessage(AiCategorizationSettings settings, List<Category> categories)
     {
         var sb = new StringBuilder();
-        sb.AppendLine("You are a personal finance transaction categorizer. Classify each transaction into the most appropriate category.");
+        sb.AppendLine(settings.SystemPrompt ?? DefaultSystemPrompt);
-        sb.AppendLine("Respond with JSON: {\"classifications\": [{\"index\": 0, \"categoryId\": \"...\", \"categoryName\": \"...\", \"confidence\": 0.9}, ...]}");
-        sb.AppendLine();
-        sb.AppendLine("Available categories:");
 
         foreach (var category in categories)
         {
-            var parentInfo = category.Parent != null ? $", under \"{category.Parent.Name}\"" : "";
+            var parentInfo = category.Parent != null ? $" (under \"{category.Parent.Name}\")" : "";
-            sb.AppendLine($"- {category.Id} \"{category.Name}\" ({category.Type}{parentInfo})");
+            sb.AppendLine($"- \"{category.Name}\"{parentInfo}");
         }
 
         return sb.ToString();
     }
 
-    private static string BuildUserMessage(List<Transaction> recentCategorized, List<Transaction> batch)
+    private static string BuildUserMessage(AiCategorizationSettings settings, List<Transaction> recentCategorized, List<Transaction> batch)
     {
         var sb = new StringBuilder();
 
@@ -232,7 +263,7 @@ public class AiCategorizationService : IAiCategorizationService
             sb.AppendLine();
         }
 
-        sb.AppendLine("Classify these uncategorized transactions:");
+        sb.AppendLine(settings.UserPromptTemplate ?? DefaultUserPromptTemplate);
         for (int i = 0; i < batch.Count; i++)
         {
             var t = batch[i];
@@ -243,7 +274,7 @@ public class AiCategorizationService : IAiCategorizationService
         return sb.ToString();
     }
 
-    private static List<TransactionClassification> ParseClassifications(
+    private List<TransactionClassification> ParseClassifications(
         string responseContent,
         List<Category> categories,
         int batchSize,
@@ -255,55 +286,130 @@ public class AiCategorizationService : IAiCategorizationService
             using var doc = JsonDocument.Parse(responseContent);
             var root = doc.RootElement;
 
-            if (!root.TryGetProperty("classifications", out var classificationsArray))
+            // Try "classifications" wrapper first, then bare array
-                return new List<TransactionClassification>();
+            JsonElement classificationsArray;
+            if (root.TryGetProperty("classifications", out classificationsArray))
+            {
+                // good
+            }
+            else if (root.ValueKind == JsonValueKind.Array)
+            {
+                classificationsArray = root;
+            }
+            else
+            {
+                // Try common alternative wrapper names
+                if (!root.TryGetProperty("results", out classificationsArray) &&
+                    !root.TryGetProperty("data", out classificationsArray))
+                {
+                    _logger.LogWarning("Ollama response has no 'classifications' array. Root properties: {Props}",
+                        string.Join(", ", root.EnumerateObject().Select(p => p.Name)));
+                    return new List<TransactionClassification>();
+                }
+            }
 
             var categoryLookup = categories.ToDictionary(c => c.Id);
             var categoryNameLookup = categories
-                .GroupBy(c => c.Name.ToLowerInvariant())
+                .GroupBy(c => c.Name.Trim().ToLowerInvariant())
                 .ToDictionary(g => g.Key, g => g.First());
 
             var results = new List<TransactionClassification>();
+            int skippedNoIndex = 0, skippedBadIndex = 0, skippedLowConf = 0, skippedNoMatch = 0;
 
             foreach (var item in classificationsArray.EnumerateArray())
             {
-                if (!item.TryGetProperty("index", out var indexProp))
+                // Parse index — handle both number and string
-                    continue;
+                int localIndex;
+                if (item.TryGetProperty("index", out var indexProp))
+                {
+                    if (indexProp.ValueKind == JsonValueKind.Number)
+                        localIndex = indexProp.GetInt32();
+                    else if (indexProp.ValueKind == JsonValueKind.String && int.TryParse(indexProp.GetString(), out var parsed))
+                        localIndex = parsed;
+                    else { skippedNoIndex++; continue; }
+                }
+                else { skippedNoIndex++; continue; }
 
-                var localIndex = indexProp.GetInt32();
                 if (localIndex < 0 || localIndex >= batchSize)
-                    continue;
+                { skippedBadIndex++; continue; }
 
                 var globalIndex = globalOffset + localIndex;
 
-                double confidence = 0;
+                // Parse confidence — handle number, string, or missing (default 1.0)
+                double confidence = 1.0;
                 if (item.TryGetProperty("confidence", out var confProp))
-                    confidence = confProp.GetDouble();
+                {
+                    if (confProp.ValueKind == JsonValueKind.Number)
+                        confidence = confProp.GetDouble();
+                    else if (confProp.ValueKind == JsonValueKind.String && double.TryParse(confProp.GetString(), out var parsedConf))
+                        confidence = parsedConf;
+                }
 
                 if (confidence < confidenceThreshold)
-                    continue;
+                { skippedLowConf++; continue; }
 
                 Guid? categoryId = null;
                 string? categoryName = null;
 
-                // Try to resolve by categoryId first
+                // Try to resolve by categoryId (also check snake_case variant)
-                if (item.TryGetProperty("categoryId", out var catIdProp) && catIdProp.ValueKind == JsonValueKind.String)
+                foreach (var propName in new[] { "categoryId", "category_id" })
                 {
-                    if (Guid.TryParse(catIdProp.GetString(), out var parsedId) && categoryLookup.ContainsKey(parsedId))
+                    if (categoryId.HasValue) break;
+                    if (item.TryGetProperty(propName, out var catIdProp) && catIdProp.ValueKind == JsonValueKind.String)
                     {
-                        categoryId = parsedId;
+                        if (Guid.TryParse(catIdProp.GetString(), out var parsedId) && categoryLookup.ContainsKey(parsedId))
-                        categoryName = categoryLookup[parsedId].Name;
+                        {
+                            categoryId = parsedId;
+                            categoryName = categoryLookup[parsedId].Name;
+                        }
                     }
                 }
 
-                // Fall back to name matching
+                // Fall back to name matching (also check snake_case variant)
-                if (!categoryId.HasValue && item.TryGetProperty("categoryName", out var catNameProp) && catNameProp.ValueKind == JsonValueKind.String)
+                if (!categoryId.HasValue)
                 {
-                    var name = catNameProp.GetString()?.ToLowerInvariant();
+                    string? rawName = null;
-                    if (name != null && categoryNameLookup.TryGetValue(name, out var matched))
+                    foreach (var propName in new[] { "categoryName", "category_name", "category" })
                     {
-                        categoryId = matched.Id;
+                        if (item.TryGetProperty(propName, out var catNameProp) && catNameProp.ValueKind == JsonValueKind.String)
-                        categoryName = matched.Name;
+                        {
+                            rawName = catNameProp.GetString();
+                            break;
+                        }
+                    }
+
+                    if (rawName != null)
+                    {
+                        var name = rawName.Trim().ToLowerInvariant();
+
+                        // Exact match
+                        if (categoryNameLookup.TryGetValue(name, out var matched))
+                        {
+                            categoryId = matched.Id;
+                            categoryName = matched.Name;
+                        }
+                        else
+                        {
+                            // Handle "Parent > Child" format — try the last segment
+                            var lastSegment = name.Contains('>') ? name.Split('>').Last().Trim() : null;
+                            if (lastSegment != null && categoryNameLookup.TryGetValue(lastSegment, out var segmentMatch))
+                            {
+                                categoryId = segmentMatch.Id;
+                                categoryName = segmentMatch.Name;
+                            }
+                            else
+                            {
+                                // Contains-based fuzzy match as last resort
+                                var fuzzy = categories.FirstOrDefault(c =>
+                                    c.Name.Trim().ToLowerInvariant().Contains(name) ||
+                                    name.Contains(c.Name.Trim().ToLowerInvariant()));
+                                if (fuzzy != null)
+                                {
+                                    categoryId = fuzzy.Id;
+                                    categoryName = fuzzy.Name;
+                                }
+                            }
+                        }
                     }
                 }
 
@@ -311,12 +417,21 @@ public class AiCategorizationService : IAiCategorizationService
                 {
                     results.Add(new TransactionClassification(globalIndex, categoryId, categoryName, confidence));
                 }
+                else
+                {
+                    skippedNoMatch++;
+                }
             }
 
+            _logger.LogInformation(
+                "Parsed {Matched} classifications from {Total} items. Skipped: {NoIndex} no index, {BadIndex} bad index, {LowConf} low confidence, {NoMatch} no category match",
+                results.Count, classificationsArray.GetArrayLength(), skippedNoIndex, skippedBadIndex, skippedLowConf, skippedNoMatch);
+
             return results;
         }
-        catch (JsonException)
+        catch (JsonException ex)
         {
+            _logger.LogWarning(ex, "Failed to parse Ollama JSON response: {Content}", responseContent.Length > 500 ? responseContent[..500] : responseContent);
             return new List<TransactionClassification>();
         }
     }

src/Purrse.Core/DTOs/AiCategorizationDtos.cs

@@ -9,7 +9,9 @@ public record AiCategorizationSettingsResponse(
     string ModelName,
     double ConfidenceThreshold,
     int TimeoutSeconds,
-    bool UpdatePayeeDefaults);
+    bool UpdatePayeeDefaults,
+    string? SystemPrompt,
+    string? UserPromptTemplate);
 
 public record UpdateAiCategorizationSettingsRequest(
     bool IsEnabled,
@@ -17,7 +19,9 @@ public record UpdateAiCategorizationSettingsRequest(
     string ModelName,
     double ConfidenceThreshold,
     int TimeoutSeconds,
-    bool UpdatePayeeDefaults);
+    bool UpdatePayeeDefaults,
+    string? SystemPrompt,
+    string? UserPromptTemplate);
 
 // Connection test
 public record OllamaConnectionTestResponse(

src/Purrse.Core/Models/AiCategorizationSettings.cs

@@ -10,6 +10,8 @@ public class AiCategorizationSettings
     public double ConfidenceThreshold { get; set; } = 0.7;
     public int TimeoutSeconds { get; set; } = 30;
     public bool UpdatePayeeDefaults { get; set; } = true;
+    public string? SystemPrompt { get; set; }
+    public string? UserPromptTemplate { get; set; }
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 
     public User User { get; set; } = null!;

src/Purrse.Data/Configurations/AiCategorizationSettingsConfiguration.cs

@@ -12,6 +12,8 @@ public class AiCategorizationSettingsConfiguration : IEntityTypeConfiguration<Ai
         builder.HasKey(s => s.Id);
         builder.Property(s => s.OllamaUrl).HasMaxLength(500);
         builder.Property(s => s.ModelName).HasMaxLength(100);
+        builder.Property(s => s.SystemPrompt).HasMaxLength(4000);
+        builder.Property(s => s.UserPromptTemplate).HasMaxLength(4000);
 
         builder.HasOne(s => s.User)
             .WithOne(u => u.AiCategorizationSettings)

src/Purrse.Data/Migrations/20260209002801_AddAiPromptTemplates.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Purrse.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddAiPromptTemplates : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "SystemPrompt",
+                table: "ai_categorization_settings",
+                type: "character varying(4000)",
+                maxLength: 4000,
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "UserPromptTemplate",
+                table: "ai_categorization_settings",
+                type: "character varying(4000)",
+                maxLength: 4000,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "SystemPrompt",
+                table: "ai_categorization_settings");
+
+            migrationBuilder.DropColumn(
+                name: "UserPromptTemplate",
+                table: "ai_categorization_settings");
+        }
+    }
+}

src/Purrse.Data/Migrations/PurrseDbContextModelSnapshot.cs

@@ -107,6 +107,10 @@ namespace Purrse.Data.Migrations
                         .HasMaxLength(500)
                         .HasColumnType("character varying(500)");
 
+                    b.Property<string>("SystemPrompt")
+                        .HasMaxLength(4000)
+                        .HasColumnType("character varying(4000)");
+
                     b.Property<int>("TimeoutSeconds")
                         .HasColumnType("integer");
 
@@ -119,6 +123,10 @@ namespace Purrse.Data.Migrations
                     b.Property<Guid>("UserId")
                         .HasColumnType("uuid");
 
+                    b.Property<string>("UserPromptTemplate")
+                        .HasMaxLength(4000)
+                        .HasColumnType("character varying(4000)");
+
                     b.HasKey("Id");
 
                     b.HasIndex("UserId")