API AI Langfuse

Langfuse Observability Patterns

Quick Guide: Use the Langfuse TypeScript SDK (built on OpenTelemetry) to add observability to LLM applications. Install @langfuse/tracing, @langfuse/otel, and @opentelemetry/sdk-node for core tracing. Use startActiveObservation() for automatic context propagation or observe() to wrap functions. Use @langfuse/openai with observeOpenAI() for zero-config OpenAI tracing. Use LangfuseClient from @langfuse/client for prompt management, scores, and datasets. Always call forceFlush() or sdk.shutdown() in short-lived processes.

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST import and register instrumentation.ts at the top of your entry point BEFORE any other imports -- OpenTelemetry must instrument modules before they are loaded)

(You MUST call forceFlush() or sdk.shutdown() in short-lived processes (serverless, scripts, CLI tools) -- events are batched and will be lost without explicit flushing)

(You MUST use @langfuse/openai with observeOpenAI() for OpenAI SDK tracing -- do NOT manually create generation observations for OpenAI calls when the wrapper handles it automatically)

(You MUST set LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, and LANGFUSE_BASE_URL via environment variables -- never hardcode credentials)

(You MUST use startActiveObservation() or observe() for nested tracing -- manual startObservation() requires explicit .end() calls and does NOT propagate context automatically)

Auto-detection: Langfuse, langfuse, @langfuse/tracing, @langfuse/otel, @langfuse/client, @langfuse/openai, LangfuseSpanProcessor, LangfuseClient, startActiveObservation, startObservation, observeOpenAI, langfuse.score, langfuse.prompt, langfuse.dataset, LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, forceFlush

When to use:

• Adding observability and tracing to LLM application code (any provider)
• Wrapping OpenAI SDK calls for automatic token/cost tracking
• Managing prompt templates with versioning, labels, and variable compilation
• Evaluating LLM output quality with scores (numeric, categorical, boolean)
• Running experiments against datasets for regression testing
• Tracking sessions, users, and metadata across multi-turn conversations
• Monitoring LLM costs and token usage in production

Key patterns covered:

• OpenTelemetry setup with LangfuseSpanProcessor
• Tracing with startActiveObservation, observe, and manual startObservation
• Observation types (span, generation, agent, tool, retriever, evaluator, embedding, chain, guardrail)
• OpenAI SDK auto-instrumentation with observeOpenAI()
• Prompt management (get, compile, text vs chat prompts, versioning)
• Scores and evaluations (numeric, categorical, boolean)
• Datasets and experiments for testing
• Flush, shutdown, and lifecycle management

When NOT to use:

• You only need basic console.log debugging -- Langfuse is for structured production observability
• You want provider-specific tracing built into an AI SDK -- check if your framework has native observability
• You need APM/infrastructure monitoring (CPU, memory, HTTP latency) -- use a general-purpose observability tool

Examples Index

• Core: Setup & Configuration -- OpenTelemetry setup, instrumentation file, client init, flush/shutdown
• Tracing -- startActiveObservation, observe, manual tracing, nesting, observation types, metadata
• OpenAI Integration -- observeOpenAI wrapper, streaming, token tracking, custom attributes
• Prompt Management -- getPrompt, compile, text vs chat, versioning, caching
• Scores & Datasets -- Numeric/categorical/boolean scores, datasets, experiments
• Quick API Reference -- Package index, environment variables, observation types, score methods

Philosophy

Langfuse provides open-source LLM observability built on OpenTelemetry. The SDK (v4+, August 2025) is a ground-up rewrite using OTel as the tracing backbone, meaning traces integrate naturally with the broader observability ecosystem.

Core principles:

1. OpenTelemetry-native -- Built on OTel spans and context propagation. Langfuse observations are wrappers around OTel spans with LLM-specific attributes (model, tokens, cost). This means any OTel-compatible instrumentation library works alongside Langfuse.
2. Zero-latency tracing -- All trace events are queued locally and flushed in background batches. Your application's response time is not affected by observability.
3. Modular packages -- @langfuse/tracing for instrumentation, @langfuse/client for prompts/scores/datasets, @langfuse/openai for OpenAI auto-instrumentation. Install only what you need.
4. Context-first -- startActiveObservation() automatically propagates parent-child relationships. Nested observations inherit context without manual ID threading.
5. Observation types -- LLM-specific types (generation, agent, tool, retriever, evaluator, embedding) provide semantic meaning to traces, enabling richer dashboard views and filtering.

When to use Langfuse:

• You need production-grade LLM observability with tracing, cost tracking, and evaluations
• You want prompt management with versioning, A/B testing via labels, and variable compilation
• You need dataset-driven testing and experiment tracking for LLM quality assurance
• You want an open-source, self-hostable alternative to proprietary LLM observability platforms

When NOT to use:

• Simple debugging -- console.log is sufficient for local development
• Infrastructure monitoring -- use Datadog, Grafana, etc. for APM
• You need a complete AI agent framework -- Langfuse is observability, not orchestration

Core Patterns

Pattern 1: OpenTelemetry Setup

Create an instrumentation.ts file and import it at the top of your entry point.

// instrumentation.ts
import { NodeSDK } from "@opentelemetry/sdk-node";
import { LangfuseSpanProcessor } from "@langfuse/otel";

const sdk = new NodeSDK({
  spanProcessors: [new LangfuseSpanProcessor()],
});

sdk.start();

export { sdk };

// index.ts -- import instrumentation FIRST
import "./instrumentation";
// All other imports AFTER instrumentation
import { startActiveObservation } from "@langfuse/tracing";

Why good: OTel must instrument modules before they are loaded; importing instrumentation first ensures all subsequent imports are traced automatically

// BAD: importing instrumentation after other modules
import { startActiveObservation } from "@langfuse/tracing";
import "./instrumentation"; // TOO LATE -- tracing won't capture earlier imports

Why bad: OpenAI/LangChain auto-instrumentation requires OTel to be initialized before those SDKs are imported

See: examples/core.md for environment variables, sampling, masking, and production configuration

Pattern 2: Tracing with startActiveObservation

The primary instrumentation pattern. Creates an observation, makes it the active context, and automatically ends it when the callback completes.

import { startActiveObservation } from "@langfuse/tracing";

async function handleRequest(query: string): Promise<string> {
  return await startActiveObservation("handle-request", async (span) => {
    span.update({ input: { query } });

    // Nested observation -- automatically becomes a child
    const result = await startActiveObservation(
      "process-query",
      async (child) => {
        child.update({ input: { query } });
        const answer = await callLLM(query);
        child.update({ output: { answer } });
        return answer;
      },
    );

    span.update({ output: { result } });
    return result;
  });
}

Why good: Automatic context propagation, automatic end on callback completion, nesting creates parent-child hierarchy without manual ID management

// BAD: using startObservation without ending it
import { startObservation } from "@langfuse/tracing";

const span = startObservation("my-span");
await doWork();
// span.end() never called -- observation stays open forever

Why bad: Manual startObservation requires explicit .end() calls; forgetting creates open-ended observations

See: examples/tracing.md for observe wrapper, observation types, metadata, and manual tracing

Pattern 3: The observe() Wrapper

Wraps a function to automatically capture inputs, outputs, timings, and errors.

import { observe } from "@langfuse/tracing";

const classifyIntent = observe(
  async (query: string) => {
    const result = await callLLM(query);
    return result.intent;
  },
  { name: "classify-intent", asType: "generation" },
);

// Usage -- automatically traced
const intent = await classifyIntent("Book a flight to Paris");

Why good: Declarative tracing, inputs/outputs captured automatically, asType tags the observation type for richer dashboard filtering

Pattern 4: OpenAI Auto-Instrumentation

Use observeOpenAI() to wrap the OpenAI client for automatic tracing of all calls.

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

const openai = observeOpenAI(new OpenAI());

// All calls automatically traced with model, tokens, cost
const completion = await openai.chat.completions.create({
  model: "gpt-4o",
  messages: [{ role: "user", content: "Hello" }],
});

Why good: Zero manual instrumentation, captures model name, token counts, estimated costs, latency, and streaming metrics automatically

// BAD: manually creating generation observations for OpenAI calls
await startActiveObservation("openai-call", async (span) => {
  const result = await rawOpenai.chat.completions.create({ ... });
  span.update({
    model: "gpt-4o",
    input: messages,
    output: result.choices[0].message.content,
  });
}, { asType: "generation" });

Why bad: observeOpenAI handles all of this automatically with more accurate token/cost data; manual tracking is error-prone and duplicates effort

See: examples/openai-integration.md for streaming, custom attributes, and token tracking on streams

Pattern 5: Prompt Management

Fetch versioned prompts, compile with variables, and link to traces.

import { LangfuseClient } from "@langfuse/client";

const langfuse = new LangfuseClient();

// Fetch a text prompt (production label by default)
const prompt = await langfuse.prompt.get("summarize-article");
const compiled = prompt.compile({ topic: "AI safety", length: "brief" });
// -> "Write a brief summary about AI safety."

// Fetch a chat prompt
const chatPrompt = await langfuse.prompt.get("assistant-v2", { type: "chat" });
const messages = chatPrompt.compile({ userName: "Alice" });
// -> [{ role: "system", content: "You are helping Alice..." }, ...]

Why good: Centralized prompt management with versioning, labels for A/B testing, variable compilation, and built-in caching

See: examples/prompt-management.md for versioning, labels, cache control, and linking prompts to traces

Pattern 6: Scores and Evaluations

Attach quality measurements to traces and observations.

import { LangfuseClient } from "@langfuse/client";

const langfuse = new LangfuseClient();

// Numeric score
langfuse.score.create({
  traceId: "trace-123",
  name: "relevance",
  value: 0.95,
  dataType: "NUMERIC",
});

// Categorical score
langfuse.score.create({
  traceId: "trace-123",
  name: "quality",
  value: "good",
  dataType: "CATEGORICAL",
});

// Boolean score (0 or 1)
langfuse.score.create({
  traceId: "trace-123",
  name: "contains-hallucination",
  value: 0,
  dataType: "BOOLEAN",
});

// Score a specific observation within a trace
langfuse.score.create({
  traceId: "trace-123",
  observationId: "obs-456",
  name: "accuracy",
  value: 0.88,
  dataType: "NUMERIC",
});

// Flush in short-lived processes
await langfuse.score.flush();

Why good: Three data types cover all evaluation needs, scores attach at trace or observation level, fire-and-forget API with batching

See: examples/scores-datasets.md for active observation scoring, session scores, datasets, and experiments

Pattern 7: Flush and Shutdown

Always flush in short-lived processes. The SDK batches events and sends them asynchronously.

import { sdk } from "./instrumentation";
import { LangfuseClient } from "@langfuse/client";
import { LangfuseSpanProcessor } from "@langfuse/otel";

const langfuse = new LangfuseClient();

async function main() {
  // ... do work ...

  // Flush scores
  await langfuse.score.flush();

  // Shutdown OTel SDK (flushes all pending spans)
  await sdk.shutdown();
}

main();

Why good: Explicit flush/shutdown ensures all events are sent before the process exits; without this, data is silently lost in serverless and scripts

// BAD: exiting without flushing
async function handler() {
  await startActiveObservation("my-trace", async (span) => {
    span.update({ output: "done" });
  });
  // Process exits -- batched events never sent
}

Why bad: Langfuse batches events locally; if the process exits before the flush interval, events are lost

Performance Optimization

Sampling for High-Volume Applications

Reduce costs by sampling a subset of traces:

import { TraceIdRatioBasedSampler } from "@opentelemetry/sdk-trace-base";

const sdk = new NodeSDK({
  sampler: new TraceIdRatioBasedSampler(0.2), // Sample 20% of traces
  spanProcessors: [new LangfuseSpanProcessor()],
});

Or via environment variable: LANGFUSE_SAMPLE_RATE=0.2

Key Optimization Patterns

• Batch flush tuning -- Configure LANGFUSE_FLUSH_AT (default 10) and LANGFUSE_FLUSH_INTERVAL (default 1s) for your workload
• Span filtering -- Use shouldExportSpan on LangfuseSpanProcessor to drop noisy non-LLM spans
• Data masking -- Redact PII before transmission with the mask option to avoid storing sensitive data
• Stream token tracking -- Set stream_options: { include_usage: true } on OpenAI streaming calls so observeOpenAI captures token counts

Decision Framework

Which Packages to Install

What do you need?
+-- Tracing LLM calls?
|   +-- YES -> npm install @langfuse/tracing @langfuse/otel @opentelemetry/sdk-node
|   +-- Also using OpenAI SDK?
|       +-- YES -> npm install @langfuse/openai
+-- Prompt management, scores, or datasets?
|   +-- YES -> npm install @langfuse/client
+-- Both tracing AND client features?
    +-- YES -> Install all: @langfuse/tracing @langfuse/otel @opentelemetry/sdk-node @langfuse/client

Which Tracing Method to Use

How do you want to instrument?
+-- Wrapping a function? -> observe() (declarative, auto-captures inputs/outputs)
+-- Block of code with nesting? -> startActiveObservation() (context propagation, auto-end)
+-- Need manual start/end control? -> startObservation() (requires explicit .end())
+-- OpenAI SDK calls? -> observeOpenAI() (zero-config auto-instrumentation)
+-- Update active span without reference? -> updateActiveObservation()

Which Observation Type (asType)

What is this observation?
+-- LLM call (prompt -> completion) -> "generation"
+-- AI agent decision-making step -> "agent"
+-- External API or function call -> "tool"
+-- Vector store or DB retrieval -> "retriever"
+-- Quality assessment step -> "evaluator"
+-- Embedding creation -> "embedding"
+-- Link between application steps -> "chain"
+-- Content safety / jailbreak check -> "guardrail"
+-- Generic duration operation -> "span" (default)
+-- Point-in-time event -> "event"

RED FLAGS

High Priority Issues:

• Not importing instrumentation.ts before other modules (auto-instrumentation silently fails)
• Exiting short-lived processes without forceFlush() or sdk.shutdown() (events are silently lost)
• Hardcoding LANGFUSE_SECRET_KEY or LANGFUSE_PUBLIC_KEY in source code (credential exposure)
• Using manual generation observations when observeOpenAI() would handle it automatically (duplicated effort, less accurate data)
• Using startObservation() without calling .end() (observation stays open indefinitely)

Medium Priority Issues:

• Not setting stream_options: { include_usage: true } on OpenAI streaming calls (token counts missing from observeOpenAI traces)
• Forgetting to call langfuse.score.flush() in short-lived processes (scores are batched and may be lost)
• Using startObservation() when startActiveObservation() would work (no automatic context propagation or auto-end)
• Not using asType on observations (all observations appear as generic spans, losing semantic meaning)
• Not setting LANGFUSE_BASE_URL for self-hosted instances (defaults to cloud.langfuse.com)

Common Mistakes:

• Importing @langfuse/openai without setting up the OTel NodeSDK first -- the OpenAI wrapper requires OTel context to send traces
• Confusing LangfuseClient (from @langfuse/client, for prompts/scores/datasets) with the OTel tracing functions (from @langfuse/tracing)
• Using prompt.compile() without matching all {{variable}} placeholders -- unmatched variables remain as literal {{name}} in output
• Calling langfuse.score.create() with a value of type string for NUMERIC scores or number for CATEGORICAL scores (type mismatch)
• Running dataset experiments without OTel setup -- experiment tasks run inside startActiveObservation which requires OTel

Gotchas & Edge Cases:

• observeOpenAI() does NOT support the OpenAI Assistants API -- only Chat Completions and Responses API
• The SDK's default span filter only exports Langfuse and GenAI spans. If you use a custom instrumentation library, you must configure shouldExportSpan to include it.
• LangfuseClient.prompt.get() caches prompts with a default TTL. If you update a prompt and don't see changes, set cacheTtlSeconds: 0 to bypass caching.
• Boolean scores use float values (0 or 1), not JavaScript booleans (true/false).
• Self-hosted Langfuse requires platform version >= 3.95.0 for TypeScript SDK v4 compatibility.
• score.create() is fire-and-forget (synchronous) -- it queues the score for batched delivery. You only need await on flush().
• Dataset names with slashes (evaluation/qa-dataset) must be URL-encoded when used as path parameters.
• The v4+ SDK is a complete rewrite from v3 -- Langfuse class, trace(), span(), generation() from v3 are replaced by OTel-based APIs.

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST import and register instrumentation.ts at the top of your entry point BEFORE any other imports -- OpenTelemetry must instrument modules before they are loaded)

(You MUST call forceFlush() or sdk.shutdown() in short-lived processes (serverless, scripts, CLI tools) -- events are batched and will be lost without explicit flushing)

(You MUST use @langfuse/openai with observeOpenAI() for OpenAI SDK tracing -- do NOT manually create generation observations for OpenAI calls when the wrapper handles it automatically)

(You MUST set LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, and LANGFUSE_BASE_URL via environment variables -- never hardcode credentials)

(You MUST use startActiveObservation() or observe() for nested tracing -- manual startObservation() requires explicit .end() calls and does NOT propagate context automatically)

Failure to follow these rules will produce silent data loss, missing traces, or credential exposure in LLM observability.