langfuse-observability

What this skill does

# Langfuse Observability

## Overview

Set up monitoring for your Langfuse integration: Prometheus metrics for trace/generation throughput, Grafana dashboards, alert rules, and integration with Langfuse's built-in analytics dashboards and Metrics API.

## Prerequisites

- Langfuse SDK integrated and producing traces
- For custom metrics: Prometheus + Grafana (or compatible stack)
- For Langfuse analytics: access to the Langfuse UI dashboard

## Instructions

### Step 1: Langfuse Built-In Dashboards

Langfuse provides pre-built dashboards in the UI at `https://cloud.langfuse.com` (or your self-hosted URL):

- **Overview**: Total traces, generations, scores, and errors
- **Cost Dashboard**: Token usage and costs over time, broken down by model, user, session
- **Latency Dashboard**: Response times across models and user segments
- **Custom Dashboards**: Build your own with the query engine (multi-level aggregations, filters by user/model/tag)

**Accessing via Metrics API:**

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

const langfuse = new LangfuseClient();

// Fetch aggregated metrics programmatically
const traces = await langfuse.api.traces.list({
  fromTimestamp: new Date(Date.now() - 3600000).toISOString(), // Last hour
  limit: 100,
});

console.log(`Traces in last hour: ${traces.data.length}`);

// Get observations with cost data
const observations = await langfuse.api.observations.list({
  type: "GENERATION",
  fromTimestamp: new Date(Date.now() - 86400000).toISOString(),
  limit: 500,
});

const totalCost = observations.data.reduce(
  (sum, obs) => sum + (obs.calculatedTotalCost || 0), 0
);
console.log(`Total cost (24h): $${totalCost.toFixed(4)}`);
```

### Step 2: Prometheus Metrics for Your App

Track the health of your Langfuse integration with custom Prometheus metrics:

```typescript
// src/lib/langfuse-metrics.ts
import { Counter, Histogram, Gauge, Registry } from "prom-client";

const registry = new Registry();

export const metrics = {
  tracesCreated: new Counter({
    name: "langfuse_traces_created_total",
    help: "Total traces created",
    labelNames: ["status"],
    registers: [registry],
  }),

  generationDuration: new Histogram({
    name: "langfuse_generation_duration_seconds",
    help: "LLM generation latency",
    labelNames: ["model"],
    buckets: [0.1, 0.5, 1, 2, 5, 10, 30],
    registers: [registry],
  }),

  tokensUsed: new Counter({
    name: "langfuse_tokens_total",
    help: "Total tokens used",
    labelNames: ["model", "type"],
    registers: [registry],
  }),

  costUsd: new Counter({
    name: "langfuse_cost_usd_total",
    help: "Total LLM cost in USD",
    labelNames: ["model"],
    registers: [registry],
  }),

  flushErrors: new Counter({
    name: "langfuse_flush_errors_total",
    help: "Total flush/export errors",
    registers: [registry],
  }),
};

export { registry };
```

```typescript
// src/lib/traced-llm.ts -- Instrumented LLM wrapper
import { observe, updateActiveObservation } from "@langfuse/tracing";
import { metrics } from "./langfuse-metrics";
import OpenAI from "openai";

const openai = new OpenAI();

export const tracedLLM = observe(
  { name: "llm-call", asType: "generation" },
  async (model: string, messages: OpenAI.ChatCompletionMessageParam[]) => {
    const start = Date.now();
    updateActiveObservation({ model, input: messages });

    try {
      const response = await openai.chat.completions.create({ model, messages });

      const duration = (Date.now() - start) / 1000;
      metrics.generationDuration.observe({ model }, duration);
      metrics.tracesCreated.inc({ status: "success" });

      if (response.usage) {
        metrics.tokensUsed.inc({ model, type: "prompt" }, response.usage.prompt_tokens);
        metrics.tokensUsed.inc({ model, type: "completion" }, response.usage.completion_tokens);
      }

      updateActiveObservation({
        output: response.choices[0].message.content,
        usage: {
          promptTokens: response.usage?.prompt_tokens,
          completionTokens: response.usage?.completion_tokens,
        },
      });

      return response.choices[0].message.content;
    } catch (error) {
      metrics.tracesCreated.inc({ status: "error" });
      throw error;
    }
  }
);
```

### Step 3: Expose Metrics Endpoint

```typescript
// src/routes/metrics.ts
import { registry } from "../lib/langfuse-metrics";

app.get("/metrics", async (req, res) => {
  res.set("Content-Type", registry.contentType);
  res.end(await registry.metrics());
});
```

### Step 4: Prometheus Scrape Config

```yaml
# prometheus.yml
scrape_configs:
  - job_name: "llm-app"
    scrape_interval: 15s
    static_configs:
      - targets: ["llm-app:3000"]
```

### Step 5: Grafana Dashboard

```json
{
  "panels": [
    {
      "title": "LLM Requests/min",
      "type": "graph",
      "targets": [{ "expr": "rate(langfuse_traces_created_total[5m]) * 60" }]
    },
    {
      "title": "Generation Latency P95",
      "type": "graph",
      "targets": [{ "expr": "histogram_quantile(0.95, rate(langfuse_generation_duration_seconds_bucket[5m]))" }]
    },
    {
      "title": "Cost/Hour",
      "type": "stat",
      "targets": [{ "expr": "rate(langfuse_cost_usd_total[1h]) * 3600" }]
    },
    {
      "title": "Error Rate",
      "type": "graph",
      "targets": [{ "expr": "rate(langfuse_traces_created_total{status='error'}[5m]) / rate(langfuse_traces_created_total[5m])" }]
    }
  ]
}
```

### Step 6: Alert Rules

```yaml
# alertmanager-rules.yml
groups:
  - name: langfuse
    rules:
      - alert: HighLLMErrorRate
        expr: rate(langfuse_traces_created_total{status="error"}[5m]) / rate(langfuse_traces_created_total[5m]) > 0.05
        for: 5m
        labels: { severity: critical }
        annotations:
          summary: "LLM error rate above 5%"

      - alert: HighLLMLatency
        expr: histogram_quantile(0.95, rate(langfuse_generation_duration_seconds_bucket[5m])) > 10
        for: 5m
        labels: { severity: warning }
        annotations:
          summary: "LLM P95 latency above 10s"

      - alert: HighDailyCost
        expr: rate(langfuse_cost_usd_total[1h]) * 24 > 100
        for: 15m
        labels: { severity: warning }
        annotations:
          summary: "Projected daily LLM cost exceeds $100"
```

## Key Metrics Reference

| Metric | Type | Purpose |
|--------|------|---------|
| `langfuse_traces_created_total` | Counter | LLM request throughput + error rate |
| `langfuse_generation_duration_seconds` | Histogram | Latency percentiles |
| `langfuse_tokens_total` | Counter | Token usage tracking |
| `langfuse_cost_usd_total` | Counter | Budget monitoring |
| `langfuse_flush_errors_total` | Counter | SDK health |

## Error Handling

| Issue | Cause | Solution |
|-------|-------|----------|
| Missing metrics | No instrumentation | Use the `tracedLLM` wrapper |
| High cardinality | Too many label values | Limit to model + status only |
| Alert storms | Thresholds too low | Start conservative, tune over time |
| Metrics endpoint slow | Large registry | Use summary instead of histogram for high-volume |

## Resources

- [Langfuse Metrics Overview](https://langfuse.com/docs/metrics/overview)
- [Custom Dashboards](https://langfuse.com/docs/metrics/features/custom-dashboards)
- [Metrics API](https://langfuse.com/docs/metrics/features/metrics-api)
- [Token & Cost Tracking](https://langfuse.com/docs/observability/features/token-and-cost-tracking)