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;
@@ -28,8 +29,8 @@ function createCachedStreamResponse(xml: string): Response {
   return createUIMessageStreamResponse({ stream });
 }
 
-export async function POST(req: Request) {
+// Inner handler function
-  try {
+async function handleChatRequest(req: Request): Promise<Response> {
   const { messages, xml, sessionId } = await req.json();
 
   // Get user IP for Langfuse tracking
@@ -41,6 +42,17 @@ export async function POST(req: Request) {
     ? sessionId
     : undefined;
 
+  // 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);
@@ -266,23 +278,15 @@ ${lastMessageText}
       messages: [systemMessageWithCache, ...enhancedMessages],
       ...(providerOptions && { providerOptions }),
       ...(headers && { headers }),
-      // Only enable telemetry if Langfuse is configured
+      // Langfuse telemetry config (returns undefined if not configured)
-      ...(process.env.LANGFUSE_PUBLIC_KEY && {
+      ...(getTelemetryConfig({ sessionId: validSessionId, userId }) && {
-        experimental_telemetry: {
+        experimental_telemetry: getTelemetryConfig({ sessionId: validSessionId, userId }),
-          isEnabled: true,
-          metadata: {
-            sessionId: validSessionId,
-            userId: userId,
-          },
-        },
       }),
-      onFinish: ({ usage, providerMetadata }) => {
+      onFinish: ({ text, usage, providerMetadata }) => {
-        console.log('[Cache] Usage:', JSON.stringify({
+        console.log('[Cache] Full providerMetadata:', JSON.stringify(providerMetadata, null, 2));
-          inputTokens: usage?.inputTokens,
+        console.log('[Cache] Usage:', JSON.stringify(usage, null, 2));
-          outputTokens: usage?.outputTokens,
+        // Update Langfuse trace with output
-          cachedInputTokens: usage?.cachedInputTokens,
+        setTraceOutput(text);
-        }, null, 2));
-        console.log('[Cache] Provider metadata:', JSON.stringify(providerMetadata, null, 2));
       },
       tools: {
         // Client-side tool that will be executed on the client
@@ -369,11 +373,21 @@ IMPORTANT: Keep edits concise:
   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(
+    return Response.json({ error: 'Internal server error' }, { status: 500 });
-      { 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);
+}

instrumentation.ts

@@ -12,11 +12,24 @@ export function register() {
     publicKey: process.env.LANGFUSE_PUBLIC_KEY,
     secretKey: process.env.LANGFUSE_SECRET_KEY,
     baseUrl: process.env.LANGFUSE_BASEURL,
+    // Filter out Next.js HTTP request spans so AI SDK spans become root traces
+    shouldExportSpan: ({ otelSpan }) => {
+      const spanName = otelSpan.name;
+      // Skip Next.js HTTP infrastructure spans
+      if (spanName.startsWith('POST /') ||
+          spanName.startsWith('GET /') ||
+          spanName.includes('BaseServer') ||
+          spanName.includes('handleRequest')) {
+        return false;
+      }
+      return true;
+    },
   });
 
   const tracerProvider = new NodeTracerProvider({
     spanProcessors: [langfuseSpanProcessor],
   });
 
+  // Register globally so AI SDK's telemetry also uses this processor
   tracerProvider.register();
 }

lib/langfuse.ts

@@ -0,0 +1,63 @@
+import { observe, updateActiveTrace } from '@langfuse/tracing';
+import * as api from '@opentelemetry/api';
+
+// Check if Langfuse is configured
+export function isLangfuseEnabled(): boolean {
+  return !!process.env.LANGFUSE_PUBLIC_KEY;
+}
+
+// Update trace with input data at the start of request
+export function setTraceInput(params: {
+  input: string;
+  sessionId?: string;
+  userId?: string;
+}) {
+  if (!isLangfuseEnabled()) return;
+
+  updateActiveTrace({
+    name: 'chat',
+    input: params.input,
+    sessionId: params.sessionId,
+    userId: params.userId,
+  });
+}
+
+// Update trace with output and end the span
+export function setTraceOutput(output: string) {
+  if (!isLangfuseEnabled()) return;
+
+  updateActiveTrace({ output });
+  const activeSpan = api.trace.getActiveSpan();
+  if (activeSpan) {
+    activeSpan.end();
+  }
+}
+
+// Get telemetry config for streamText
+export function getTelemetryConfig(params: {
+  sessionId?: string;
+  userId?: string;
+}) {
+  if (!isLangfuseEnabled()) return undefined;
+
+  return {
+    isEnabled: true,
+    recordInputs: true,
+    recordOutputs: true,
+    metadata: {
+      sessionId: params.sessionId,
+      userId: params.userId,
+    },
+  };
+}
+
+// Wrap a handler with Langfuse observe
+export function wrapWithObserve<T>(
+  handler: (req: Request) => Promise<T>
+): (req: Request) => Promise<T> {
+  if (!isLangfuseEnabled()) {
+    return handler;
+  }
+
+  return observe(handler, { name: 'chat', endOnExit: false });
+}

package-lock.json

@@ -7,6 +7,7 @@
         "": {
             "name": "next-ai-draw-io",
             "version": "0.2.0",
+            "license": "Apache-2.0",
             "dependencies": {
                 "@ai-sdk/amazon-bedrock": "^3.0.62",
                 "@ai-sdk/anthropic": "^2.0.44",
@@ -15,7 +16,9 @@
                 "@ai-sdk/google": "^2.0.0",
                 "@ai-sdk/openai": "^2.0.19",
                 "@ai-sdk/react": "^2.0.22",
+                "@langfuse/client": "^4.4.9",
                 "@langfuse/otel": "^4.4.4",
+                "@langfuse/tracing": "^4.4.9",
                 "@next/third-parties": "^16.0.6",
                 "@openrouter/ai-sdk-provider": "^1.2.3",
                 "@opentelemetry/sdk-trace-node": "^2.2.0",
@@ -1594,10 +1597,24 @@
                 "@jridgewell/sourcemap-codec": "^1.4.14"
             }
         },
+        "node_modules/@langfuse/client": {
+            "version": "4.4.9",
+            "resolved": "https://registry.npmjs.org/@langfuse/client/-/client-4.4.9.tgz",
+            "integrity": "sha512-Y7bU70tMx/lYOU/A7NGvXXVZoL3AiFigbf7EwS5PVFc0xd34eRUmvwdLwEtuK7CnYTyxIZTzVVP2KEaicWCYZg==",
+            "license": "MIT",
+            "dependencies": {
+                "@langfuse/core": "^4.4.9",
+                "@langfuse/tracing": "^4.4.9",
+                "mustache": "^4.2.0"
+            },
+            "peerDependencies": {
+                "@opentelemetry/api": "^1.9.0"
+            }
+        },
         "node_modules/@langfuse/core": {
-            "version": "4.4.4",
+            "version": "4.4.9",
-            "resolved": "https://registry.npmjs.org/@langfuse/core/-/core-4.4.4.tgz",
+            "resolved": "https://registry.npmjs.org/@langfuse/core/-/core-4.4.9.tgz",
-            "integrity": "sha512-hmtMNAOIsvDwT/xld0CJPXrIsakETbelSmAOGEY07faKtKdJy/BGjxexBbfAWLPgAC6wqC2fK2ByaYCGgC7MBw==",
+            "integrity": "sha512-9Hz/eH6dkOP8E/FLt1fsAQR8RE/TF8Ag/39GmY8JjN1o/Tl/MFJfK2QvqRGrkjDkIkMJGOSD+iQmV2pYm4upDA==",
             "license": "MIT",
             "peerDependencies": {
                 "@opentelemetry/api": "^1.9.0"
@@ -1621,6 +1638,21 @@
                 "@opentelemetry/sdk-trace-base": "^2.0.1"
             }
         },
+        "node_modules/@langfuse/tracing": {
+            "version": "4.4.9",
+            "resolved": "https://registry.npmjs.org/@langfuse/tracing/-/tracing-4.4.9.tgz",
+            "integrity": "sha512-if+G/v9NsyTKj40KKX96bRSdMXwyDbVL4GJQvmwQ9SxvGYF+d99pGFB7L6QOeCd1KBHMdmDe733ncmvCnSHJ9w==",
+            "license": "MIT",
+            "dependencies": {
+                "@langfuse/core": "^4.4.9"
+            },
+            "engines": {
+                "node": ">=20"
+            },
+            "peerDependencies": {
+                "@opentelemetry/api": "^1.9.0"
+            }
+        },
         "node_modules/@napi-rs/wasm-runtime": {
             "version": "0.2.12",
             "resolved": "https://registry.npmjs.org/@napi-rs/wasm-runtime/-/wasm-runtime-0.2.12.tgz",
@@ -7690,6 +7722,15 @@
             "integrity": "sha512-6FlzubTLZG3J2a/NVCAleEhjzq5oxgHyaCU9yYXvcLsvoVaHJq/s5xXI6/XXP6tz7R9xAOtHnSO/tXtF3WRTlA==",
             "license": "MIT"
         },
+        "node_modules/mustache": {
+            "version": "4.2.0",
+            "resolved": "https://registry.npmjs.org/mustache/-/mustache-4.2.0.tgz",
+            "integrity": "sha512-71ippSywq5Yb7/tVYyGbkBggbU8H3u5Rz56fH60jGFgr8uHwxs+aSKeqmluIVzM0m0kB7xQjKS6qPfd0b2ZoqQ==",
+            "license": "MIT",
+            "bin": {
+                "mustache": "bin/mustache"
+            }
+        },
         "node_modules/nanoid": {
             "version": "3.3.11",
             "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-3.3.11.tgz",

package.json

@@ -17,7 +17,9 @@
         "@ai-sdk/google": "^2.0.0",
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.22",
+        "@langfuse/client": "^4.4.9",
         "@langfuse/otel": "^4.4.4",
+        "@langfuse/tracing": "^4.4.9",
         "@next/third-parties": "^16.0.6",
         "@openrouter/ai-sdk-provider": "^1.2.3",
         "@opentelemetry/sdk-trace-node": "^2.2.0",