Langfuse Core Workflow A: Tracing LLM Calls

Langfuse Core Workflow A: Tracing LLM Calls

Overview

Primary workflow for Langfuse: end-to-end tracing of LLM calls, chains, and agents.

Prerequisites

• Completed langfuse-install-auth setup
• Understanding of Langfuse trace hierarchy
• LLM provider SDK (OpenAI, Anthropic, etc.)

Instructions

Step 1: Set Up Automatic OpenAI Tracing

import { observeOpenAI } from "langfuse";
import OpenAI from "openai";

// Wrap OpenAI client for automatic tracing
const openai = observeOpenAI(new OpenAI());

// All calls are now automatically traced
const response = await openai.chat.completions.create({
  model: "gpt-4",
  messages: [
    { role: "system", content: "You are a helpful assistant." },
    { role: "user", content: "What is Langfuse?" },
  ],
});

Step 2: Manual Tracing for Complex Flows

import { Langfuse } from "langfuse";

const langfuse = new Langfuse();

async function ragPipeline(query: string) {
  // Create top-level trace
  const trace = langfuse.trace({
    name: "rag-pipeline",
    input: { query },
    metadata: { pipeline: "rag-v1" },
  });

  // Span: Query embedding
  const embedSpan = trace.span({
    name: "embed-query",
    input: { text: query },
  });

  const queryEmbedding = await embedText(query);

  embedSpan.end({
    output: { dimensions: queryEmbedding.length },
    metadata: { model: "text-embedding-ada-002" },
  });

  // Span: Vector search
  const searchSpan = trace.span({
    name: "vector-search",
    input: { embedding: "vector[1536]" },
  });

  const documents = await searchVectorDB(queryEmbedding);

  searchSpan.end({
    output: {
      documentCount: documents.length,
      topScore: documents[0]?.score,
    },
  });

  // Generation: LLM call with context
  const generation = trace.generation({
    name: "generate-answer",
    model: "gpt-4",
    modelParameters: { temperature: 0.7, maxTokens: 500 },
    input: {
      query,
      context: documents.map((d) => d.content),
    },
  });

  const answer = await generateAnswer(query, documents);

  generation.end({
    output: answer.content,
    usage: {
      promptTokens: answer.usage.prompt_tokens,
      completionTokens: answer.usage.completion_tokens,
      totalTokens: answer.usage.total_tokens,
    },
  });

  // Update trace with final output
  trace.update({
    output: { answer: answer.content },
  });

  return answer.content;
}

Step 3: Trace Streaming Responses

async function streamingChat(messages: ChatMessage[]) {
  const trace = langfuse.trace({
    name: "streaming-chat",
    input: { messages },
  });

  const generation = trace.generation({
    name: "stream-response",
    model: "gpt-4",
    input: messages,
  });

  const stream = await openai.chat.completions.create({
    model: "gpt-4",
    messages,
    stream: true,
  });

  let fullContent = "";
  let usage = { promptTokens: 0, completionTokens: 0 };

  for await (const chunk of stream) {
    const content = chunk.choices[0]?.delta?.content || "";
    fullContent += content;

    // Track usage from final chunk
    if (chunk.usage) {
      usage = {
        promptTokens: chunk.usage.prompt_tokens,
        completionTokens: chunk.usage.completion_tokens,
      };
    }

    yield content;
  }

  // End generation after stream completes
  generation.end({
    output: fullContent,
    usage,
  });

  trace.update({ output: { response: fullContent } });
}

Step 4: Link Traces to Parent Operations

// API endpoint that creates parent trace
async function handleChatRequest(req: Request) {
  const trace = langfuse.trace({
    name: "api/​chat",
    userId: req.userId,
    sessionId: req.sessionId,
    input: req.body,
  });

  // Pass trace to child operations
  const response = await processChat(trace, req.body.message);

  return response;
}

// Child function receives parent trace
async function processChat(
  parentTrace: ReturnType<typeof langfuse.trace>,
  message: string
) {
  // Create child span under parent trace
  const span = parentTrace.span({
    name: "process-chat",
    input: { message },
  });

  // OpenAI call linked to parent
  const response = await openai.chat.completions.create(
    {
      model: "gpt-4",
      messages: [{ role: "user", content: message }],
    },
    { langfuseParent: span } // Link generation to span
  );

  span.end({ output: response.choices[0].message });
  return response.choices[0].message.content;
}

Output

• Automatic OpenAI tracing with zero code changes
• Manual tracing for RAG and complex pipelines
• Streaming response tracking
• Linked parent-child trace hierarchy

Error Handling

Examples

Anthropic Claude Tracing

import Anthropic from "@anthropic-ai/​sdk";
import { Langfuse } from "langfuse";

const anthropic = new Anthropic();
const langfuse = new Langfuse();

async function callClaude(prompt: string) {
  const trace = langfuse.trace({ name: "claude-call", input: { prompt } });

  const generation = trace.generation({
    name: "claude-response",
    model: "claude-3-sonnet-20240229",
    input: [{ role: "user", content: prompt }],
  });

  const response = await anthropic.messages.create({
    model: "claude-3-sonnet-20240229",
    max_tokens: 1024,
    messages: [{ role: "user", content: prompt }],
  });

  generation.end({
    output: response.content[0].text,
    usage: {
      promptTokens: response.usage.input_tokens,
      completionTokens: response.usage.output_tokens,
    },
  });

  return response.content[0].text;
}

LangChain Integration

from langchain.callbacks import LangfuseCallbackHandler
from langchain.chat_models import ChatOpenAI
from langchain.schema import HumanMessage

# Create callback handler
langfuse_handler = LangfuseCallbackHandler()

# Pass to LLM
llm = ChatOpenAI(callbacks=[langfuse_handler])

# All LangChain operations are now traced
response = llm([HumanMessage(content="Hello!")])

Tool/​Function Calling Tracing

async function tracedToolCall(trace: Trace, toolName: string, args: any) {
  const span = trace.span({
    name: `tool/​${toolName}`,
    input: args,
    metadata: { type: "tool-call" },
  });

  try {
    const result = await executeTool(toolName, args);
    span.end({ output: result });
    return result;
  } catch (error) {
    span.end({
      level: "ERROR",
      statusMessage: String(error),
    });
    throw error;
  }
}

Resources

• Langfuse Tracing Guide
• OpenAI Integration
• LangChain Integration
• Streaming Guide

Next Steps

For evaluation and scoring workflows, see langfuse-core-workflow-b.