feat: replace Claude with GPT for AI chat integration

- Replace ClaudeAdapter with GPTAdapter using OpenAI GPT (gpt-5-mini)
- Implement function calling for calendar operations (getDay, proposeCreate/Update/Delete, searchEvents)
- Add provider-agnostic AI utilities in ai/utils/ (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
- Add USE_TEST_RESPONSES env var to toggle between real AI and test responses
- Switch ChatService.processMessage to use real AI provider
- Add npm run format command for Prettier
- Update CLAUDE.md with new architecture

CLAUDE.md

@@ -13,6 +13,7 @@ This is a fullstack TypeScript monorepo with npm workspaces.
 ### Root (monorepo)
 ```bash
 npm install          # Install all dependencies for all workspaces
+npm run format       # Format all TypeScript files with Prettier
 ```
 
 ### Client (apps/client) - Expo React Native app
@@ -44,7 +45,7 @@ npm run start -w @caldav/server        # Run compiled server (port 3000)
 | Backend | Express.js | Web framework |
 | | MongoDB | Database |
 | | Mongoose | ODM |
-| | Claude (Anthropic) | AI/LLM for chat |
+| | GPT (OpenAI) | AI/LLM for chat |
 | | JWT | Authentication |
 | Planned | iCalendar | Event export/import |
 
@@ -91,7 +92,7 @@ src/
 └── stores/                     # Zustand state management
     ├── index.ts                # Re-exports all stores
     ├── AuthStore.ts            # user, token, isAuthenticated, login(), logout(), setToken()
-    ├── ChatStore.ts            # messages[], addMessage(), updateMessage(), clearMessages()
+    ├── ChatStore.ts            # messages[], addMessage(), addMessages(), updateMessage(), clearMessages(), chatMessageToMessageData()
     └── EventsStore.ts          # events[], setEvents(), addEvent(), updateEvent(), deleteEvent()
 ```
 
@@ -134,7 +135,14 @@ src/
 │       ├── MongoEventRepository.ts
 │       └── MongoChatRepository.ts
 ├── ai/
-│   └── ClaudeAdapter.ts      # Implements AIProvider
+│   ├── GPTAdapter.ts         # Implements AIProvider using OpenAI GPT
+│   ├── index.ts              # Re-exports GPTAdapter
+│   └── utils/                # Shared AI utilities (provider-agnostic)
+│       ├── index.ts          # Re-exports
+│       ├── eventFormatter.ts # formatExistingEvents() for system prompt
+│       ├── systemPrompt.ts   # buildSystemPrompt() - German calendar assistant prompt
+│       ├── toolDefinitions.ts # TOOL_DEFINITIONS - provider-agnostic tool specs
+│       └── toolExecutor.ts   # executeToolCall() - handles getDay, proposeCreate/Update/Delete, searchEvents
 └── utils/
     ├── jwt.ts                # signToken(), verifyToken()
     ├── password.ts           # hash(), compare()
@@ -172,7 +180,7 @@ src/
 │   ├── CalendarEvent.ts  # CalendarEvent, CreateEventDTO, UpdateEventDTO, ExpandedEvent
 │   ├── ChatMessage.ts    # ChatMessage, Conversation, SendMessageDTO, CreateMessageDTO,
 │   │                     # GetMessagesOptions, ChatResponse, ConversationSummary,
-│   │                     # ProposedEventChange, EventAction
+│   │                     # ProposedEventChange, EventAction, RespondedAction, UpdateMessageDTO
 │   └── Constants.ts      # DAYS, MONTHS, Day, Month, DAY_INDEX, DAY_INDEX_TO_DAY,
 │                         # DAY_TO_GERMAN, DAY_TO_GERMAN_SHORT, MONTH_TO_GERMAN
 └── utils/
@@ -184,12 +192,14 @@ src/
 - `User`: id, email, displayName, passwordHash?, createdAt?, updatedAt?
 - `CalendarEvent`: id, userId, title, description?, startTime, endTime, note?, isRecurring?, recurrenceRule?
 - `ExpandedEvent`: Extends CalendarEvent with occurrenceStart, occurrenceEnd (for recurring event instances)
-- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChange?
+- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChange?, respondedAction?
 - `ProposedEventChange`: action ('create' | 'update' | 'delete'), eventId?, event?, updates?
 - `Conversation`: id, userId, createdAt?, updatedAt? (messages loaded separately via lazy loading)
 - `CreateEventDTO`: Used for creating events AND for AI-proposed events
 - `GetMessagesOptions`: Cursor-based pagination with `before?: string` and `limit?: number`
 - `ConversationSummary`: id, lastMessage?, createdAt? (for conversation list)
+- `UpdateMessageDTO`: respondedAction? (for marking messages as confirmed/rejected)
+- `RespondedAction`: 'confirm' | 'reject' (tracks user response to proposed events)
 - `Day`: "Monday" | "Tuesday" | ... | "Sunday"
 - `Month`: "January" | "February" | ... | "December"
 
@@ -262,6 +272,8 @@ Server requires `.env` file in `apps/server/`:
 JWT_SECRET=your-secret-key
 JWT_EXPIRES_IN=1h
 MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
+OPENAI_API_KEY=sk-proj-...
+USE_TEST_RESPONSES=false    # true = static test responses, false = real GPT AI
 ```
 
 ## Current Implementation Status
@@ -281,14 +293,16 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - `EventService`: Full CRUD with recurring event expansion via recurrenceExpander
   - `utils/eventFormatters`: getWeeksOverview(), getMonthOverview() with German localization
   - `utils/recurrenceExpander`: expandRecurringEvents() using rrule library for RRULE parsing
+  - `ChatController`: getConversations(), getConversation() with cursor-based pagination support
+  - `ChatService`: getConversations(), getConversation(), processMessage() uses real AI or test responses (via USE_TEST_RESPONSES), confirmEvent()/rejectEvent() update respondedAction and persist response messages
+  - `MongoChatRepository`: Full CRUD implemented (getConversationsByUser, createConversation, getMessages with cursor pagination, createMessage, updateMessage)
+  - `ChatRepository` interface: updateMessage() added for respondedAction tracking
+  - `GPTAdapter`: Full implementation with OpenAI GPT (gpt-5-mini model), function calling for calendar operations
+  - `ai/utils/`: Provider-agnostic shared utilities (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
 - **Stubbed (TODO):**
   - `AuthMiddleware.authenticate()`: Currently uses fake user for testing
   - `AuthController`: refresh(), logout()
   - `AuthService`: refreshToken()
-  - `ChatController`: getConversations(), getConversation()
-  - `MongoChatRepository`: Database persistence for chat
-- **Not started:**
-  - `ClaudeAdapter` (AI integration - currently using test responses)
 
 **Shared:** Types, DTOs, constants (Day, Month with German translations), ExpandedEvent type, and date utilities defined and exported.
 
@@ -302,20 +316,22 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - Tap-to-open modal overlay showing EventCards for selected day
   - Supports events from adjacent months visible in grid
   - Uses `useFocusEffect` for automatic reload on tab focus
-- Chat screen functional with FlashList, message sending, and event confirm/reject
-  - Messages persisted via ChatStore (survives tab switches)
+- Chat screen fully functional with FlashList, message sending, and event confirm/reject
+  - Messages persisted to database via ChatService and loaded on mount
+  - Tracks conversationId for message continuity across sessions
+  - ChatStore with addMessages() for bulk loading, chatMessageToMessageData() helper
   - KeyboardAvoidingView for proper keyboard handling (iOS padding, Android height)
   - Auto-scroll to end on new messages and keyboard show
   - keyboardDismissMode="interactive" and keyboardShouldPersistTaps="handled"
 - `ApiClient`: get(), post(), put(), delete() implemented
 - `EventService`: getAll(), getById(), getByDateRange(), create(), update(), delete() - fully implemented
-- `ChatService`: sendMessage(), confirmEvent(convId, msgId, action, event?, eventId?, updates?), rejectEvent() - supports create/update/delete actions
+- `ChatService`: sendMessage(), confirmEvent(), rejectEvent(), getConversations(), getConversation() - fully implemented with cursor pagination
 - `EventCardBase`: Shared base component with event layout (header, date/time/recurring icons, description) - used by both EventCard and ProposedEventCard
 - `EventCard`: Uses EventCardBase + edit/delete buttons for calendar display
 - `ProposedEventCard`: Uses EventCardBase + confirm/reject buttons for chat proposals (supports create/update/delete actions)
 - `Themes.tsx`: Centralized color definitions including textPrimary, borderPrimary, eventIndicator, secondaryBg
 - `EventsStore`: Zustand store with setEvents(), addEvent(), updateEvent(), deleteEvent() - stores ExpandedEvent[]
-- `ChatStore`: Zustand store with addMessage(), updateMessage(), clearMessages() - persists messages across tab switches
+- `ChatStore`: Zustand store with addMessage(), addMessages(), updateMessage(), clearMessages() - loads from server on mount and persists across tab switches
 - Auth screens (Login, Register), Event Detail, and Note screens exist as skeletons
 - AuthStore defined with `throw new Error('Not implemented')`
 

apps/client/src/app/(tabs)/chat.tsx

@@ -1,18 +1,29 @@
-import { View, Text, TextInput, Pressable, KeyboardAvoidingView, Platform, Keyboard } from "react-native";
+import {
+  View,
+  Text,
+  TextInput,
+  Pressable,
+  KeyboardAvoidingView,
+  Platform,
+  Keyboard,
+} from "react-native";
 import currentTheme from "../../Themes";
-import { useState, useRef, useEffect } from "react";
+import React, { useState, useRef, useEffect } from "react";
 import Header from "../../components/Header";
 import BaseBackground from "../../components/BaseBackground";
 import { FlashList } from "@shopify/flash-list";
 import { ChatService } from "../../services";
-import { useChatStore, MessageData } from "../../stores";
+import {
+  useChatStore,
+  chatMessageToMessageData,
+  MessageData,
+} from "../../stores";
 import { ProposedEventChange } from "@caldav/shared";
 import { ProposedEventCard } from "../../components/ProposedEventCard";
 
 // TODO: better shadows for everything
 // (maybe with extra library because of differences between android and ios)
 // TODO: max width for messages
-// TODO: create new messages
 
 type BubbleSide = "left" | "right";
 
@@ -30,14 +41,43 @@ type ChatInputProps = {
 };
 
 const Chat = () => {
-  const { messages, addMessage, updateMessage } = useChatStore();
-  const listRef = useRef<FlashList<MessageData>>(null);
+  const { messages, addMessage, addMessages, updateMessage } = useChatStore();
+  const listRef =
+    useRef<React.ComponentRef<typeof FlashList<MessageData>>>(null);
+  const [currentConversationId, setCurrentConversationId] = useState<
+    string | undefined
+  >();
 
   useEffect(() => {
-    const keyboardDidShow = Keyboard.addListener("keyboardDidShow", scrollToEnd);
+    const keyboardDidShow = Keyboard.addListener(
+      "keyboardDidShow",
+      scrollToEnd,
+    );
     return () => keyboardDidShow.remove();
   }, []);
 
+  // Load existing messages from database on mount
+  useEffect(() => {
+    const fetchMessages = async () => {
+      try {
+        const conversationSummaries = await ChatService.getConversations();
+        if (conversationSummaries.length > 0) {
+          const conversationId = conversationSummaries[0].id;
+          setCurrentConversationId(conversationId);
+
+          const serverMessages =
+            await ChatService.getConversation(conversationId);
+          const clientMessages = serverMessages.map(chatMessageToMessageData);
+          addMessages(clientMessages);
+          scrollToEnd();
+        }
+      } catch (error) {
+        console.error("Failed to load messages:", error);
+      }
+    };
+    fetchMessages();
+  }, []);
+
   const scrollToEnd = () => {
     setTimeout(() => {
       listRef.current?.scrollToEnd({ animated: true });
@@ -87,13 +127,22 @@ const Chat = () => {
       id: Date.now().toString(),
       side: "right",
       content: text,
+      conversationId: currentConversationId,
     };
     addMessage(userMessage);
     scrollToEnd();
 
     try {
-      // Fetch server response
-      const response = await ChatService.sendMessage({ content: text });
+      // Fetch server response (include conversationId for existing conversations)
+      const response = await ChatService.sendMessage({
+        content: text,
+        conversationId: currentConversationId,
+      });
+
+      // Track conversation ID for subsequent messages
+      if (!currentConversationId) {
+        setCurrentConversationId(response.conversationId);
+      }
 
       // Show bot response
       const botMessage: MessageData = {

apps/client/src/services/ChatService.ts

@@ -47,13 +47,20 @@ export const ChatService = {
   },
 
   getConversations: async (): Promise<ConversationSummary[]> => {
-    throw new Error("Not implemented");
+    return ApiClient.get<ConversationSummary[]>("/chat/conversations");
   },
 
   getConversation: async (
-    _id: string,
-    _options?: GetMessagesOptions,
+    id: string,
+    options?: GetMessagesOptions,
   ): Promise<ChatMessage[]> => {
-    throw new Error("Not implemented");
+    const params = new URLSearchParams();
+    if (options?.before) params.append("before", options.before);
+    if (options?.limit) params.append("limit", options.limit.toString());
+
+    const queryString = params.toString();
+    const url = `/chat/conversations/${id}${queryString ? `?${queryString}` : ""}`;
+
+    return ApiClient.get<ChatMessage[]>(url);
   },
 };

apps/client/src/stores/ChatStore.ts

@@ -1,5 +1,5 @@
 import { create } from "zustand";
-import { ProposedEventChange } from "@caldav/shared";
+import { ChatMessage, ProposedEventChange } from "@caldav/shared";
 
 type BubbleSide = "left" | "right";
 
@@ -14,6 +14,7 @@ export type MessageData = {
 
 interface ChatState {
   messages: MessageData[];
+  addMessages: (messages: MessageData[]) => void;
   addMessage: (message: MessageData) => void;
   updateMessage: (id: string, updates: Partial<MessageData>) => void;
   clearMessages: () => void;
@@ -21,6 +22,9 @@ interface ChatState {
 
 export const useChatStore = create<ChatState>((set) => ({
   messages: [],
+  addMessages(messages) {
+    set((state) => ({ messages: [...state.messages, ...messages] }));
+  },
   addMessage: (message: MessageData) => {
     set((state) => ({ messages: [...state.messages, message] }));
   },
@@ -35,3 +39,15 @@ export const useChatStore = create<ChatState>((set) => ({
     set({ messages: [] });
   },
 }));
+
+// Helper to convert server ChatMessage to client MessageData
+export function chatMessageToMessageData(msg: ChatMessage): MessageData {
+  return {
+    id: msg.id,
+    side: msg.sender === "assistant" ? "left" : "right",
+    content: msg.content,
+    proposedChange: msg.proposedChange,
+    respondedAction: msg.respondedAction,
+    conversationId: msg.conversationId,
+  };
+}

apps/client/src/stores/index.ts

@@ -1,3 +1,7 @@
 export { useAuthStore } from "./AuthStore";
-export { useChatStore, type MessageData } from "./ChatStore";
+export {
+  useChatStore,
+  chatMessageToMessageData,
+  type MessageData,
+} from "./ChatStore";
 export { useEventsStore } from "./EventsStore";

apps/server/package.json

@@ -8,13 +8,13 @@
     "start": "node dist/app.js"
   },
   "dependencies": {
-    "@anthropic-ai/sdk": "^0.71.2",
     "@caldav/shared": "*",
     "bcrypt": "^6.0.0",
     "dotenv": "^16.4.7",
     "express": "^5.2.1",
     "jsonwebtoken": "^9.0.3",
     "mongoose": "^9.1.1",
+    "openai": "^6.15.0",
     "rrule": "^2.8.1"
   },
   "devDependencies": {

apps/server/src/ai/ClaudeAdapter.ts

@@ -1,21 +0,0 @@
-import Anthropic from "@anthropic-ai/sdk";
-import { AIProvider, AIContext, AIResponse } from "../services/interfaces";
-
-export class ClaudeAdapter implements AIProvider {
-  private client: Anthropic;
-  private model: string;
-
-  constructor(apiKey?: string, model: string = "claude-3-haiku-20240307") {
-    this.client = new Anthropic({
-      apiKey: apiKey || process.env.ANTHROPIC_API_KEY,
-    });
-    this.model = model;
-  }
-
-  async processMessage(
-    message: string,
-    context: AIContext,
-  ): Promise<AIResponse> {
-    throw new Error("Not implemented");
-  }
-}

apps/server/src/ai/GPTAdapter.ts

@@ -0,0 +1,116 @@
+import OpenAI from "openai";
+import { ProposedEventChange } from "@caldav/shared";
+import { AIProvider, AIContext, AIResponse } from "../services/interfaces";
+import {
+  buildSystemPrompt,
+  TOOL_DEFINITIONS,
+  executeToolCall,
+  ToolDefinition,
+} from "./utils";
+
+/**
+ * Convert tool definitions to OpenAI format.
+ */
+function toOpenAITools(
+  defs: ToolDefinition[],
+): OpenAI.Chat.Completions.ChatCompletionTool[] {
+  return defs.map((def) => ({
+    type: "function" as const,
+    function: {
+      name: def.name,
+      description: def.description,
+      parameters: def.parameters,
+    },
+  }));
+}
+
+export class GPTAdapter implements AIProvider {
+  private client: OpenAI;
+  private model: string;
+  private tools: OpenAI.Chat.Completions.ChatCompletionTool[];
+
+  constructor(apiKey?: string, model: string = "gpt-5-mini") {
+    this.client = new OpenAI({
+      apiKey: apiKey || process.env.OPENAI_API_KEY,
+    });
+    this.model = model;
+    this.tools = toOpenAITools(TOOL_DEFINITIONS);
+  }
+
+  async processMessage(
+    message: string,
+    context: AIContext,
+  ): Promise<AIResponse> {
+    const systemPrompt = buildSystemPrompt(context);
+
+    // Build messages array with conversation history
+    const messages: OpenAI.Chat.Completions.ChatCompletionMessageParam[] = [
+      { role: "developer", content: systemPrompt },
+    ];
+
+    // Add conversation history
+    for (const msg of context.conversationHistory) {
+      messages.push({
+        role: msg.sender === "user" ? "user" : "assistant",
+        content: msg.content,
+      });
+    }
+
+    // Add current user message
+    messages.push({ role: "user", content: message });
+
+    let proposedChange: ProposedEventChange | undefined;
+
+    // Tool calling loop
+    while (true) {
+      const response = await this.client.chat.completions.create({
+        model: this.model,
+        messages,
+        tools: this.tools,
+      });
+
+      const assistantMessage = response.choices[0].message;
+
+      // If no tool calls, return the final response
+      if (
+        !assistantMessage.tool_calls ||
+        assistantMessage.tool_calls.length === 0
+      ) {
+        return {
+          content:
+            assistantMessage.content || "Ich konnte keine Antwort generieren.",
+          proposedChange,
+        };
+      }
+
+      // Process tool calls
+      for (const toolCall of assistantMessage.tool_calls) {
+        // Skip non-function tool calls
+        if (toolCall.type !== "function") continue;
+
+        const { name, arguments: argsRaw } = toolCall.function;
+        const args = JSON.parse(argsRaw);
+
+        const result = executeToolCall(name, args, context);
+
+        // If the tool returned a proposedChange, capture it
+        if (result.proposedChange) {
+          proposedChange = result.proposedChange;
+        }
+
+        // Add assistant message with tool call
+        messages.push({
+          role: "assistant",
+          tool_calls: [toolCall],
+        });
+
+        // Add tool result
+        messages.push({
+          role: "tool",
+          tool_call_id: toolCall.id,
+          content: result.content,
+        });
+      }
+    }
+  }
+}

apps/server/src/ai/index.ts

@@ -1 +1 @@
-export * from "./ClaudeAdapter";
+export * from "./GPTAdapter";

apps/server/src/ai/utils/eventFormatter.ts

@@ -0,0 +1,29 @@
+import { CalendarEvent } from "@caldav/shared";
+
+// German date/time formatting helpers
+export const formatDate = (d: Date) => d.toLocaleDateString("de-DE");
+export const formatTime = (d: Date) =>
+  d.toLocaleTimeString("de-DE", { hour: "2-digit", minute: "2-digit" });
+export const formatDateTime = (d: Date) =>
+  `${formatDate(d)} ${d.toLocaleTimeString("de-DE")}`;
+
+/**
+ * Format a list of events for display in the system prompt.
+ * Output is in German with date/time formatting.
+ */
+export function formatExistingEvents(events: CalendarEvent[]): string {
+  if (events.length === 0) {
+    return "Keine Termine vorhanden.";
+  }
+
+  return events
+    .map((e) => {
+      const start = new Date(e.startTime);
+      const end = new Date(e.endTime);
+      const timeStr = `${formatTime(start)} - ${formatTime(end)}`;
+      const recurring = e.isRecurring ? " (wiederkehrend)" : "";
+      const desc = e.description ? ` | ${e.description}` : "";
+      return `- ${e.title} (ID: ${e.id}) | ${formatDate(start)} ${timeStr}${recurring}${desc}`;
+    })
+    .join("\n");
+}

apps/server/src/ai/utils/index.ts

@@ -0,0 +1,13 @@
+export {
+  formatExistingEvents,
+  formatDate,
+  formatTime,
+  formatDateTime,
+} from "./eventFormatter";
+export { buildSystemPrompt } from "./systemPrompt";
+export {
+  TOOL_DEFINITIONS,
+  type ToolDefinition,
+  type ParameterDef,
+} from "./toolDefinitions";
+export { executeToolCall, type ToolResult } from "./toolExecutor";

apps/server/src/ai/utils/systemPrompt.ts

@@ -0,0 +1,37 @@
+import { AIContext } from "../../services/interfaces";
+import { formatExistingEvents } from "./eventFormatter";
+
+/**
+ * Build the system prompt for the AI assistant.
+ * This prompt is provider-agnostic and can be used with any LLM.
+ */
+export function buildSystemPrompt(context: AIContext): string {
+  const currentDate = context.currentDate.toLocaleDateString("de-DE", {
+    weekday: "long",
+    year: "numeric",
+    month: "long",
+    day: "numeric",
+    hour: "2-digit",
+    minute: "2-digit",
+  });
+
+  const eventsText = formatExistingEvents(context.existingEvents);
+
+  return `Du bist ein hilfreicher Kalender-Assistent für die App "CalChat".
+Du hilfst Benutzern beim Erstellen, Ändern und Löschen von Terminen.
+Antworte immer auf Deutsch.
+
+Aktuelles Datum und Uhrzeit: ${currentDate}
+
+Wichtige Regeln:
+- Nutze getDay() für relative Datumsberechnungen (z.B. "nächsten Freitag um 14 Uhr")
+- Wenn der Benutzer einen Termin erstellen will, nutze proposeCreateEvent
+- Wenn der Benutzer einen Termin ändern will, nutze proposeUpdateEvent mit der Event-ID
+- Wenn der Benutzer einen Termin löschen will, nutze proposeDeleteEvent mit der Event-ID
+- Du kannst NUR EINEN Event-Vorschlag pro Antwort machen
+- Wenn der Benutzer nach seinen Terminen fragt, nutze die unten stehende Liste
+- Nutze searchEvents um nach Terminen zu suchen, wenn du die genaue ID brauchst
+
+Existierende Termine des Benutzers:
+${eventsText}`;
+}

apps/server/src/ai/utils/toolDefinitions.ts

@@ -0,0 +1,170 @@
+/**
+ * Parameter definition for tool parameters.
+ */
+export interface ParameterDef {
+  type: "string" | "number" | "boolean" | "object" | "array";
+  description?: string;
+  enum?: string[];
+}
+
+/**
+ * Provider-agnostic tool definition format.
+ * Can be converted to OpenAI, Claude, or other provider formats.
+ */
+export interface ToolDefinition {
+  name: string;
+  description: string;
+  parameters: {
+    type: "object";
+    properties: Record<string, ParameterDef>;
+    required: string[];
+  };
+}
+
+/**
+ * All available tools for the calendar assistant.
+ */
+export const TOOL_DEFINITIONS: ToolDefinition[] = [
+  {
+    name: "getDay",
+    description:
+      "Get a date for a specific weekday relative to today. Returns an ISO date string.",
+    parameters: {
+      type: "object",
+      properties: {
+        day: {
+          type: "string",
+          enum: [
+            "Monday",
+            "Tuesday",
+            "Wednesday",
+            "Thursday",
+            "Friday",
+            "Saturday",
+            "Sunday",
+          ],
+          description: "The target weekday",
+        },
+        offset: {
+          type: "number",
+          description:
+            "1 = next occurrence, 2 = the one after, -1 = last occurrence, etc.",
+        },
+        hour: {
+          type: "number",
+          description: "Hour of day (0-23)",
+        },
+        minute: {
+          type: "number",
+          description: "Minute (0-59)",
+        },
+      },
+      required: ["day", "offset", "hour", "minute"],
+    },
+  },
+  {
+    name: "getCurrentDateTime",
+    description: "Get the current date and time as an ISO string",
+    parameters: {
+      type: "object",
+      properties: {},
+      required: [],
+    },
+  },
+  {
+    name: "proposeCreateEvent",
+    description:
+      "Propose creating a new calendar event. The user must confirm before it's saved. Call this when the user wants to create a new appointment.",
+    parameters: {
+      type: "object",
+      properties: {
+        title: {
+          type: "string",
+          description: "Event title",
+        },
+        startTime: {
+          type: "string",
+          description: "Start time as ISO date string",
+        },
+        endTime: {
+          type: "string",
+          description: "End time as ISO date string",
+        },
+        description: {
+          type: "string",
+          description: "Optional event description",
+        },
+        isRecurring: {
+          type: "boolean",
+          description: "Whether this is a recurring event",
+        },
+        recurrenceRule: {
+          type: "string",
+          description: "RRULE format string for recurring events",
+        },
+      },
+      required: ["title", "startTime", "endTime"],
+    },
+  },
+  {
+    name: "proposeUpdateEvent",
+    description:
+      "Propose updating an existing event. The user must confirm. Use this when the user wants to modify an appointment.",
+    parameters: {
+      type: "object",
+      properties: {
+        eventId: {
+          type: "string",
+          description: "ID of the event to update",
+        },
+        title: {
+          type: "string",
+          description: "New title (optional)",
+        },
+        startTime: {
+          type: "string",
+          description: "New start time as ISO date string (optional)",
+        },
+        endTime: {
+          type: "string",
+          description: "New end time as ISO date string (optional)",
+        },
+        description: {
+          type: "string",
+          description: "New description (optional)",
+        },
+      },
+      required: ["eventId"],
+    },
+  },
+  {
+    name: "proposeDeleteEvent",
+    description:
+      "Propose deleting an event. The user must confirm. Use this when the user wants to remove an appointment.",
+    parameters: {
+      type: "object",
+      properties: {
+        eventId: {
+          type: "string",
+          description: "ID of the event to delete",
+        },
+      },
+      required: ["eventId"],
+    },
+  },
+  {
+    name: "searchEvents",
+    description:
+      "Search for events by title in the user's calendar. Returns matching events.",
+    parameters: {
+      type: "object",
+      properties: {
+        query: {
+          type: "string",
+          description: "Search query to match against event titles",
+        },
+      },
+      required: ["query"],
+    },
+  },
+];

apps/server/src/ai/utils/toolExecutor.ts

@@ -0,0 +1,156 @@
+import {
+  ProposedEventChange,
+  getDay,
+  Day,
+  DAY_TO_GERMAN,
+} from "@caldav/shared";
+import { AIContext } from "../../services/interfaces";
+import { formatDate, formatTime, formatDateTime } from "./eventFormatter";
+
+/**
+ * Result of executing a tool call.
+ */
+export interface ToolResult {
+  content: string;
+  proposedChange?: ProposedEventChange;
+}
+
+/**
+ * Execute a tool call and return the result.
+ * This function is provider-agnostic and can be used with any LLM.
+ */
+export function executeToolCall(
+  name: string,
+  args: Record<string, unknown>,
+  context: AIContext,
+): ToolResult {
+  switch (name) {
+    case "getDay": {
+      const date = getDay(
+        args.day as Day,
+        args.offset as number,
+        args.hour as number,
+        args.minute as number,
+      );
+      const dayName = DAY_TO_GERMAN[args.day as Day];
+      return {
+        content: `${date.toISOString()} (${dayName}, ${formatDate(date)} um ${formatTime(date)} Uhr)`,
+      };
+    }
+
+    case "getCurrentDateTime": {
+      const now = context.currentDate;
+      return {
+        content: `${now.toISOString()} (${formatDateTime(now)})`,
+      };
+    }
+
+    case "proposeCreateEvent": {
+      const event = {
+        title: args.title as string,
+        startTime: new Date(args.startTime as string),
+        endTime: new Date(args.endTime as string),
+        description: args.description as string | undefined,
+        isRecurring: args.isRecurring as boolean | undefined,
+        recurrenceRule: args.recurrenceRule as string | undefined,
+      };
+      const dateStr = formatDate(event.startTime);
+      const startStr = formatTime(event.startTime);
+      const endStr = formatTime(event.endTime);
+      return {
+        content: `Event-Vorschlag erstellt: "${event.title}" am ${dateStr} von ${startStr} bis ${endStr} Uhr`,
+        proposedChange: {
+          action: "create",
+          event,
+        },
+      };
+    }
+
+    case "proposeUpdateEvent": {
+      const eventId = args.eventId as string;
+      const existingEvent = context.existingEvents.find(
+        (e) => e.id === eventId,
+      );
+
+      if (!existingEvent) {
+        return { content: `Event mit ID ${eventId} nicht gefunden.` };
+      }
+
+      const updates: Record<string, unknown> = {};
+      if (args.title) updates.title = args.title;
+      if (args.startTime)
+        updates.startTime = new Date(args.startTime as string);
+      if (args.endTime) updates.endTime = new Date(args.endTime as string);
+      if (args.description) updates.description = args.description;
+
+      // Build event object for display (merge existing with updates)
+      const displayEvent = {
+        title: (updates.title as string) || existingEvent.title,
+        startTime: (updates.startTime as Date) || existingEvent.startTime,
+        endTime: (updates.endTime as Date) || existingEvent.endTime,
+        description:
+          (updates.description as string) || existingEvent.description,
+        isRecurring: existingEvent.isRecurring,
+      };
+
+      return {
+        content: `Update-Vorschlag für "${existingEvent.title}" erstellt.`,
+        proposedChange: {
+          action: "update",
+          eventId,
+          updates,
+          event: displayEvent,
+        },
+      };
+    }
+
+    case "proposeDeleteEvent": {
+      const eventId = args.eventId as string;
+      const existingEvent = context.existingEvents.find(
+        (e) => e.id === eventId,
+      );
+
+      if (!existingEvent) {
+        return { content: `Event mit ID ${eventId} nicht gefunden.` };
+      }
+
+      return {
+        content: `Lösch-Vorschlag für "${existingEvent.title}" erstellt.`,
+        proposedChange: {
+          action: "delete",
+          eventId,
+          event: {
+            title: existingEvent.title,
+            startTime: existingEvent.startTime,
+            endTime: existingEvent.endTime,
+            description: existingEvent.description,
+            isRecurring: existingEvent.isRecurring,
+          },
+        },
+      };
+    }
+
+    case "searchEvents": {
+      const query = (args.query as string).toLowerCase();
+      const matches = context.existingEvents.filter((e) =>
+        e.title.toLowerCase().includes(query),
+      );
+
+      if (matches.length === 0) {
+        return { content: `Keine Termine mit "${args.query}" gefunden.` };
+      }
+
+      const results = matches
+        .map((e) => {
+          const start = new Date(e.startTime);
+          return `- ${e.title} (ID: ${e.id}) am ${formatDate(start)} um ${formatTime(start)} Uhr`;
+        })
+        .join("\n");
+
+      return { content: `Gefundene Termine:\n${results}` };
+    }
+
+    default:
+      return { content: `Unbekannte Funktion: ${name}` };
+  }
+}

apps/server/src/app.ts

@@ -10,7 +10,7 @@ import {
   MongoEventRepository,
   MongoChatRepository,
 } from "./repositories";
-import { ClaudeAdapter } from "./ai";
+import { GPTAdapter } from "./ai";
 
 const app = express();
 const port = process.env.PORT || 3000;
@@ -43,7 +43,7 @@ const eventRepo = new MongoEventRepository();
 const chatRepo = new MongoChatRepository();
 
 // Initialize AI provider
-const aiProvider = new ClaudeAdapter();
+const aiProvider = new GPTAdapter();
 
 // Initialize services
 const authService = new AuthService(userRepo);

apps/server/src/controllers/ChatController.ts

@@ -4,6 +4,7 @@ import {
   CreateEventDTO,
   UpdateEventDTO,
   EventAction,
+  GetMessagesOptions,
 } from "@caldav/shared";
 import { ChatService } from "../services";
 import { AuthenticatedRequest } from "../middleware";
@@ -66,13 +67,43 @@ export class ChatController {
     req: AuthenticatedRequest,
     res: Response,
   ): Promise<void> {
-    throw new Error("Not implemented");
+    try {
+      const userId = req.user!.userId;
+      const conversations = await this.chatService.getConversations(userId);
+      res.json(conversations);
+    } catch (error) {
+      res.status(500).json({ error: "Failed to get conversations" });
+    }
   }
 
   async getConversation(
     req: AuthenticatedRequest,
     res: Response,
   ): Promise<void> {
-    throw new Error("Not implemented");
+    try {
+      const userId = req.user!.userId;
+      const { id } = req.params;
+      const { before, limit } = req.query as {
+        before?: string;
+        limit?: string;
+      };
+
+      const options: GetMessagesOptions = {};
+      if (before) options.before = before;
+      if (limit) options.limit = parseInt(limit, 10);
+
+      const messages = await this.chatService.getConversation(
+        userId,
+        id,
+        options,
+      );
+      res.json(messages);
+    } catch (error) {
+      if ((error as Error).message === "Conversation not found") {
+        res.status(404).json({ error: "Conversation not found" });
+      } else {
+        res.status(500).json({ error: "Failed to get conversation" });
+      }
+    }
   }
 }

apps/server/src/repositories/mongo/MongoChatRepository.ts

@@ -3,6 +3,7 @@ import {
   Conversation,
   CreateMessageDTO,
   GetMessagesOptions,
+  UpdateMessageDTO,
 } from "@caldav/shared";
 import { ChatRepository } from "../../services/interfaces";
 import { ChatMessageModel, ConversationModel } from "./models";
@@ -10,11 +11,15 @@ import { ChatMessageModel, ConversationModel } from "./models";
 export class MongoChatRepository implements ChatRepository {
   // Conversations
   async getConversationsByUser(userId: string): Promise<Conversation[]> {
-    throw new Error("Not implemented");
+    const conversations = await ConversationModel.find({ userId });
+    return conversations.map((c) => c.toJSON() as unknown as Conversation);
   }
 
   async createConversation(userId: string): Promise<Conversation> {
-    throw new Error("Not implemented");
+    const conversation = await ConversationModel.create({
+      userId,
+    });
+    return conversation.toJSON() as unknown as Conversation;
   }
 
   // Messages (cursor-based pagination)
@@ -22,13 +27,44 @@ export class MongoChatRepository implements ChatRepository {
     conversationId: string,
     options?: GetMessagesOptions,
   ): Promise<ChatMessage[]> {
-    throw new Error("Not implemented");
+    const limit = options?.limit ?? 20;
+    const query: Record<string, unknown> = { conversationId };
+
+    // Cursor: load messages before this ID (for "load more" scrolling up)
+    if (options?.before) {
+      query._id = { $lt: options.before };
+    }
+
+    // Fetch newest first, then reverse for chronological order
+    const docs = await ChatMessageModel.find(query)
+      .sort({ _id: -1 })
+      .limit(limit);
+
+    return docs.reverse().map((doc) => doc.toJSON() as unknown as ChatMessage);
   }
 
   async createMessage(
     conversationId: string,
     message: CreateMessageDTO,
   ): Promise<ChatMessage> {
-    throw new Error("Not implemented");
+    const repoMessage = await ChatMessageModel.create({
+      conversationId: conversationId,
+      sender: message.sender,
+      content: message.content,
+      proposedChange: message.proposedChange,
+    });
+    return repoMessage.toJSON() as unknown as ChatMessage;
+  }
+
+  async updateMessage(
+    messageId: string,
+    updates: UpdateMessageDTO,
+  ): Promise<ChatMessage | null> {
+    const doc = await ChatMessageModel.findByIdAndUpdate(
+      messageId,
+      { $set: updates },
+      { new: true },
+    );
+    return doc ? (doc.toJSON() as unknown as ChatMessage) : null;
   }
 }

apps/server/src/repositories/mongo/models/ChatModel.ts

@@ -80,6 +80,10 @@ const ChatMessageSchema = new Schema<
     proposedChange: {
       type: ProposedChangeSchema,
     },
+    respondedAction: {
+      type: String,
+      enum: ["confirm", "reject"],
+    },
   },
   {
     timestamps: true,

apps/server/src/services/ChatService.ts

@@ -9,6 +9,7 @@ import {
   CreateEventDTO,
   UpdateEventDTO,
   EventAction,
+  CreateMessageDTO,
 } from "@caldav/shared";
 import { ChatRepository, EventRepository, AIProvider } from "./interfaces";
 import { getWeeksOverview, getMonthOverview } from "../utils/eventFormatters";
@@ -252,22 +253,47 @@ export class ChatService {
     userId: string,
     data: SendMessageDTO,
   ): Promise<ChatResponse> {
-    const response = await getTestResponse(
-      responseIndex,
-      this.eventRepo,
-      userId,
-    );
-    responseIndex++;
+    let conversationId = data.conversationId;
+    if (!conversationId) {
+      const conversation = await this.chatRepo.createConversation(userId);
+      conversationId = conversation.id;
+    }
 
-    const message: ChatMessage = {
-      id: Date.now().toString(),
-      conversationId: data.conversationId || "temp-conv-id",
+    // Save user message
+    await this.chatRepo.createMessage(conversationId, {
+      sender: "user",
+      content: data.content,
+    });
+
+    let response: TestResponse;
+
+    if (process.env.USE_TEST_RESPONSES === "true") {
+      // Test mode: use static responses
+      response = await getTestResponse(responseIndex, this.eventRepo, userId);
+      responseIndex++;
+    } else {
+      // Production mode: use real AI
+      const events = await this.eventRepo.findByUserId(userId);
+      const history = await this.chatRepo.getMessages(conversationId, {
+        limit: 20,
+      });
+
+      response = await this.aiProvider.processMessage(data.content, {
+        userId,
+        conversationHistory: history,
+        existingEvents: events,
+        currentDate: new Date(),
+      });
+    }
+
+    // Save and then return assistant response
+    const answerMessage = await this.chatRepo.createMessage(conversationId, {
       sender: "assistant",
       content: response.content,
       proposedChange: response.proposedChange,
-    };
+    });
 
-    return { message, conversationId: message.conversationId };
+    return { message: answerMessage, conversationId: conversationId };
   }
 
   async confirmEvent(
@@ -279,6 +305,12 @@ export class ChatService {
     eventId?: string,
     updates?: UpdateEventDTO,
   ): Promise<ChatResponse> {
+    // Update original message with respondedAction
+    await this.chatRepo.updateMessage(messageId, {
+      respondedAction: "confirm",
+    });
+
+    // Perform the actual event operation
     let content: string;
 
     if (action === "create" && event) {
@@ -298,12 +330,12 @@ export class ChatService {
       content = "Ungültige Aktion.";
     }
 
-    const message: ChatMessage = {
-      id: Date.now().toString(),
-      conversationId,
+    // Save response message to DB
+    const message = await this.chatRepo.createMessage(conversationId, {
       sender: "assistant",
       content,
-    };
+    });
+
     return { message, conversationId };
   }
 
@@ -312,17 +344,34 @@ export class ChatService {
     conversationId: string,
     messageId: string,
   ): Promise<ChatResponse> {
-    const message: ChatMessage = {
-      id: Date.now().toString(),
-      conversationId,
+    // Update original message with respondedAction
+    await this.chatRepo.updateMessage(messageId, { respondedAction: "reject" });
+
+    // Save response message to DB
+    const message = await this.chatRepo.createMessage(conversationId, {
       sender: "assistant",
       content: "Der Vorschlag wurde abgelehnt.",
-    };
+    });
+
     return { message, conversationId };
   }
 
   async getConversations(userId: string): Promise<ConversationSummary[]> {
-    throw new Error("Not implemented");
+    const conversations = await this.chatRepo.getConversationsByUser(userId);
+
+    // For each conversation, get the last message
+    const summaries: ConversationSummary[] = await Promise.all(
+      conversations.map(async (conv) => {
+        const messages = await this.chatRepo.getMessages(conv.id, { limit: 1 });
+        return {
+          id: conv.id,
+          lastMessage: messages[0],
+          createdAt: conv.createdAt,
+        };
+      }),
+    );
+
+    return summaries;
   }
 
   async getConversation(
@@ -330,6 +379,14 @@ export class ChatService {
     conversationId: string,
     options?: GetMessagesOptions,
   ): Promise<ChatMessage[]> {
-    throw new Error("Not implemented");
+    // Verify conversation belongs to user
+    const conversations = await this.chatRepo.getConversationsByUser(userId);
+    const conversation = conversations.find((c) => c.id === conversationId);
+
+    if (!conversation) {
+      throw new Error("Conversation not found");
+    }
+
+    return this.chatRepo.getMessages(conversationId, options);
   }
 }

apps/server/src/services/interfaces/ChatRepository.ts

@@ -3,6 +3,7 @@ import {
   Conversation,
   CreateMessageDTO,
   GetMessagesOptions,
+  UpdateMessageDTO,
 } from "@caldav/shared";
 
 export interface ChatRepository {
@@ -15,8 +16,14 @@ export interface ChatRepository {
     conversationId: string,
     options?: GetMessagesOptions,
   ): Promise<ChatMessage[]>;
+
   createMessage(
     conversationId: string,
     message: CreateMessageDTO,
   ): Promise<ChatMessage>;
+
+  updateMessage(
+    messageId: string,
+    updates: UpdateMessageDTO,
+  ): Promise<ChatMessage | null>;
 }

package-lock.json

@@ -62,13 +62,13 @@
       "name": "@caldav/server",
       "version": "1.0.0",
       "dependencies": {
-        "@anthropic-ai/sdk": "^0.71.2",
         "@caldav/shared": "*",
         "bcrypt": "^6.0.0",
         "dotenv": "^16.4.7",
         "express": "^5.2.1",
         "jsonwebtoken": "^9.0.3",
         "mongoose": "^9.1.1",
+        "openai": "^6.15.0",
         "rrule": "^2.8.1"
       },
       "devDependencies": {
@@ -106,26 +106,6 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
-    "node_modules/@anthropic-ai/sdk": {
-      "version": "0.71.2",
-      "resolved": "https://registry.npmjs.org/@anthropic-ai/sdk/-/sdk-0.71.2.tgz",
-      "integrity": "sha512-TGNDEUuEstk/DKu0/TflXAEt+p+p/WhTlFzEnoosvbaDU2LTjm42igSdlL0VijrKpWejtOKxX0b8A7uc+XiSAQ==",
-      "license": "MIT",
-      "dependencies": {
-        "json-schema-to-ts": "^3.1.1"
-      },
-      "bin": {
-        "anthropic-ai-sdk": "bin/cli"
-      },
-      "peerDependencies": {
-        "zod": "^3.25.0 || ^4.0.0"
-      },
-      "peerDependenciesMeta": {
-        "zod": {
-          "optional": true
-        }
-      }
-    },
     "node_modules/@babel/code-frame": {
       "version": "7.27.1",
       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.27.1.tgz",
@@ -11252,6 +11232,27 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/openai": {
+      "version": "6.15.0",
+      "resolved": "https://registry.npmjs.org/openai/-/openai-6.15.0.tgz",
+      "integrity": "sha512-F1Lvs5BoVvmZtzkUEVyh8mDQPPFolq4F+xdsx/DO8Hee8YF3IGAlZqUIsF+DVGhqf4aU0a3bTghsxB6OIsRy1g==",
+      "license": "Apache-2.0",
+      "bin": {
+        "openai": "bin/cli"
+      },
+      "peerDependencies": {
+        "ws": "^8.18.0",
+        "zod": "^3.25 || ^4.0"
+      },
+      "peerDependenciesMeta": {
+        "ws": {
+          "optional": true
+        },
+        "zod": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/optionator": {
       "version": "0.9.4",
       "resolved": "https://registry.npmjs.org/optionator/-/optionator-0.9.4.tgz",

package.json

@@ -6,6 +6,9 @@
     "apps/*",
     "packages/*"
   ],
+  "scripts": {
+    "format": "prettier --write \"apps/*/src/**/*.{ts,tsx}\" \"packages/*/src/**/*.ts\""
+  },
   "devDependencies": {
     "eslint": "^9.25.0",
     "prettier": "^3.7.4",

packages/shared/src/models/ChatMessage.ts

@@ -4,6 +4,8 @@ export type MessageSender = "user" | "assistant";
 
 export type EventAction = "create" | "update" | "delete";
 
+export type RespondedAction = "confirm" | "reject";
+
 export interface ProposedEventChange {
   action: EventAction;
   eventId?: string; // Required for update/delete
@@ -17,6 +19,7 @@ export interface ChatMessage {
   sender: MessageSender;
   content: string;
   proposedChange?: ProposedEventChange;
+  respondedAction?: RespondedAction;
   createdAt?: Date;
 }
 
@@ -43,6 +46,10 @@ export interface GetMessagesOptions {
   limit?: number; // Default: 20
 }
 
+export interface UpdateMessageDTO {
+  respondedAction?: RespondedAction;
+}
+
 export interface ChatResponse {
   message: ChatMessage;
   conversationId: string;