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

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/services/ChatService.ts

@@ -265,12 +265,26 @@ export class ChatService {
       content: data.content,
     });
 
-    const response = await getTestResponse(
-      responseIndex,
-      this.eventRepo,
-      userId,
-    );
-    responseIndex++;
+    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, {
@@ -292,7 +306,9 @@ export class ChatService {
     updates?: UpdateEventDTO,
   ): Promise<ChatResponse> {
     // Update original message with respondedAction
-    await this.chatRepo.updateMessage(messageId, { respondedAction: "confirm" });
+    await this.chatRepo.updateMessage(messageId, {
+      respondedAction: "confirm",
+    });
 
     // Perform the actual event operation
     let content: string;