feat: add trace-level input/output to Langfuse observability (#69)

* feat: add trace-level input/output to Langfuse observability

- Add @langfuse/client and @langfuse/tracing dependencies
- Wrap POST handler with observe() for proper tracing
- Use updateActiveTrace() to set trace input, output, sessionId, userId
- Filter Next.js HTTP spans in shouldExportSpan so AI SDK spans become root traces
- Enable recordInputs/recordOutputs in experimental_telemetry

* refactor: extract Langfuse logic to separate lib/langfuse.ts module

app/api/chat/route.ts

@@ -1,6 +1,7 @@
 import { streamText, convertToModelMessages, createUIMessageStream, createUIMessageStreamResponse } from 'ai';
 import { getAIModel } from '@/lib/ai-providers';
 import { findCachedResponse } from '@/lib/cached-responses';
+import { setTraceInput, setTraceOutput, getTelemetryConfig, wrapWithObserve } from '@/lib/langfuse';
 import { z } from "zod";
 
 export const maxDuration = 300;
@@ -28,22 +29,33 @@ function createCachedStreamResponse(xml: string): Response {
   return createUIMessageStreamResponse({ stream });
 }
 
-export async function POST(req: Request) {
-  try {
-    const { messages, xml, sessionId } = await req.json();
+// Inner handler function
+async function handleChatRequest(req: Request): Promise<Response> {
+  const { messages, xml, sessionId } = await req.json();
 
-    // Get user IP for Langfuse tracking
-    const forwardedFor = req.headers.get('x-forwarded-for');
-    const userId = forwardedFor?.split(',')[0]?.trim() || 'anonymous';
+  // Get user IP for Langfuse tracking
+  const forwardedFor = req.headers.get('x-forwarded-for');
+  const userId = forwardedFor?.split(',')[0]?.trim() || 'anonymous';
 
-    // Validate sessionId for Langfuse (must be string, max 200 chars)
-    const validSessionId = sessionId && typeof sessionId === 'string' && sessionId.length <= 200
-      ? sessionId
-      : undefined;
+  // Validate sessionId for Langfuse (must be string, max 200 chars)
+  const validSessionId = sessionId && typeof sessionId === 'string' && sessionId.length <= 200
+    ? sessionId
+    : undefined;
 
-    // === CACHE CHECK START ===
-    const isFirstMessage = messages.length === 1;
-    const isEmptyDiagram = !xml || xml.trim() === '' || isMinimalDiagram(xml);
+  // Extract user input text for Langfuse trace
+  const currentMessage = messages[messages.length - 1];
+  const userInputText = currentMessage?.parts?.find((p: any) => p.type === 'text')?.text || '';
+
+  // Update Langfuse trace with input, session, and user
+  setTraceInput({
+    input: userInputText,
+    sessionId: validSessionId,
+    userId: userId,
+  });
+
+  // === CACHE CHECK START ===
+  const isFirstMessage = messages.length === 1;
+  const isEmptyDiagram = !xml || xml.trim() === '' || isMinimalDiagram(xml);
 
     if (isFirstMessage && isEmptyDiagram) {
       const lastMessage = messages[0];
@@ -266,23 +278,15 @@ ${lastMessageText}
       messages: [systemMessageWithCache, ...enhancedMessages],
       ...(providerOptions && { providerOptions }),
       ...(headers && { headers }),
-      // Only enable telemetry if Langfuse is configured
-      ...(process.env.LANGFUSE_PUBLIC_KEY && {
-        experimental_telemetry: {
-          isEnabled: true,
-          metadata: {
-            sessionId: validSessionId,
-            userId: userId,
-          },
-        },
+      // Langfuse telemetry config (returns undefined if not configured)
+      ...(getTelemetryConfig({ sessionId: validSessionId, userId }) && {
+        experimental_telemetry: getTelemetryConfig({ sessionId: validSessionId, userId }),
       }),
-      onFinish: ({ usage, providerMetadata }) => {
-        console.log('[Cache] Usage:', JSON.stringify({
-          inputTokens: usage?.inputTokens,
-          outputTokens: usage?.outputTokens,
-          cachedInputTokens: usage?.cachedInputTokens,
-        }, null, 2));
-        console.log('[Cache] Provider metadata:', JSON.stringify(providerMetadata, null, 2));
+      onFinish: ({ text, usage, providerMetadata }) => {
+        console.log('[Cache] Full providerMetadata:', JSON.stringify(providerMetadata, null, 2));
+        console.log('[Cache] Usage:', JSON.stringify(usage, null, 2));
+        // Update Langfuse trace with output
+        setTraceOutput(text);
       },
       tools: {
         // Client-side tool that will be executed on the client
@@ -366,14 +370,24 @@ IMPORTANT: Keep edits concise:
       return errorString;
     }
 
-    return result.toUIMessageStreamResponse({
-      onError: errorHandler,
-    });
+  return result.toUIMessageStreamResponse({
+    onError: errorHandler,
+  });
+}
+
+// Wrap handler with error handling
+async function safeHandler(req: Request): Promise<Response> {
+  try {
+    return await handleChatRequest(req);
   } catch (error) {
     console.error('Error in chat route:', error);
-    return Response.json(
-      { error: 'Internal server error' },
-      { status: 500 }
-    );
+    return Response.json({ error: 'Internal server error' }, { status: 500 });
   }
 }
+
+// Wrap with Langfuse observe (if configured)
+const observedHandler = wrapWithObserve(safeHandler);
+
+export async function POST(req: Request) {
+  return observedHandler(req);
+}