Langfuse Cost Tuning

Overview

Track, analyze, and optimize LLM costs using Langfuse's built-in token/cost tracking, the Metrics API for programmatic cost analysis, model routing for cost reduction, and automated budget alerts.

Prerequisites

• Langfuse tracing with token usage captured (via observeOpenAI or manual usage fields)
• For Metrics API: @langfuse/client installed
• Understanding of LLM pricing models

How Langfuse Tracks Costs

Langfuse automatically calculates costs for supported models (OpenAI, Anthropic, Google) when token usage is captured. For custom models, you can configure pricing in the Langfuse UI under Settings > Model Definitions.

Cost tracking works on observations of type generation and embedding. The observeOpenAI wrapper captures usage automatically; for manual tracing, include usage in your observation updates.

Instructions

Step 1: Ensure Token Usage is Captured

// Automatic: observeOpenAI captures everything
import { observeOpenAI } from "@langfuse/openai";
const openai = observeOpenAI(new OpenAI());
// Tokens, model, latency, and cost are all auto-tracked

// Manual: include usage in generation observations
import { startActiveObservation, updateActiveObservation } from "@langfuse/tracing";

await startActiveObservation(
  { name: "llm-call", asType: "generation" },
  async () => {
    updateActiveObservation({ model: "gpt-4o" }); // Model required for cost calc

    const response = await openai.chat.completions.create({
      model: "gpt-4o",
      messages: [{ role: "user", content: prompt }],
    });

    updateActiveObservation({
      output: response.choices[0].message.content,
      usage: {
        promptTokens: response.usage?.prompt_tokens,
        completionTokens: response.usage?.completion_tokens,
        totalTokens: response.usage?.total_tokens,
      },
      // Optional: override inferred cost (in USD)
      // costInUsd: 0.0015,
    });
  }
);

Step 2: Query Costs via Metrics API

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

const langfuse = new LangfuseClient();

// Fetch aggregated cost metrics
async function getCostReport(days: number) {
  const fromTimestamp = new Date(Date.now() - days * 86400000).toISOString();

  // Use the API to list traces with cost data
  const traces = await langfuse.api.traces.list({
    fromTimestamp,
    limit: 1000,
    orderBy: "timestamp",
  });

  const costByModel = new Map<string, { cost: number; tokens: number; count: number }>();

  for (const trace of traces.data) {
    const observations = await langfuse.api.observations.list({
      traceId: trace.id,
      type: "GENERATION",
    });

    for (const obs of observations.data) {
      const model = obs.model || "unknown";
      const existing = costByModel.get(model) || { cost: 0, tokens: 0, count: 0 };
      existing.cost += obs.calculatedTotalCost || 0;
      existing.tokens += obs.totalTokens || 0;
      existing.count += 1;
      costByModel.set(model, existing);
    }
  }

  console.log("\n=== LLM Cost Report ===");
  console.log(`Period: Last ${days} days\n`);

  let totalCost = 0;
  for (const [model, data] of costByModel.entries()) {
    console.log(`${model}:`);
    console.log(`  Calls: ${data.count}`);
    console.log(`  Tokens: ${data.tokens.toLocaleString()}`);
    console.log(`  Cost: $${data.cost.toFixed(4)}`);
    totalCost += data.cost;
  }
  console.log(`\nTotal: $${totalCost.toFixed(4)}`);
}

getCostReport(7);

Step 3: Implement Smart Model Routing

Route requests to cheaper models when appropriate:

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

interface ModelConfig {
  model: string;
  costPer1MInput: number;
  costPer1MOutput: number;
  maxComplexity: "simple" | "moderate" | "complex";
}

const MODELS: ModelConfig[] = [
  { model: "gpt-4o-mini", costPer1MInput: 0.15, costPer1MOutput: 0.60, maxComplexity: "simple" },
  { model: "gpt-4o", costPer1MInput: 2.50, costPer1MOutput: 10.00, maxComplexity: "moderate" },
  { model: "claude-sonnet-4-20250514", costPer1MInput: 3.00, costPer1MOutput: 15.00, maxComplexity: "complex" },
];

function selectModel(task: string, inputLength: number): ModelConfig {
  const simpleTasks = ["classify", "extract", "summarize-short", "translate"];
  const isSimple = simpleTasks.some((t) => task.includes(t));
  const isShort = inputLength < 500;

  if (isSimple && isShort) return MODELS[0]; // gpt-4o-mini
  if (isSimple || inputLength < 2000) return MODELS[1]; // gpt-4o
  return MODELS[2]; // claude-sonnet-4
}

const costOptimizedLLM = observe(
  { name: "cost-optimized-llm", asType: "generation" },
  async (task: string, input: string) => {
    const config = selectModel(task, input.length);

    updateActiveObservation({
      model: config.model,
      metadata: {
        task,
        selectedReason: `${config.maxComplexity} tier`,
        estimatedCostPer1M: config.costPer1MInput,
      },
    });

    const response = await callModel(config.model, input);
    updateActiveObservation({
      output: response.content,
      usage: response.usage,
    });

    return response;
  }
);

Step 4: Budget Alerts

// scripts/cost-alert.ts -- run as cron job
import { LangfuseClient } from "@langfuse/client";

const langfuse = new LangfuseClient();

const ALERT_THRESHOLDS = {
  dailyWarn: 50,    // $50/day warning
  dailyCritical: 200, // $200/day critical
  perRequestWarn: 1,  // $1/request warning
};

async function checkCostAlerts() {
  const since = new Date(Date.now() - 86400000).toISOString(); // Last 24h

  const traces = await langfuse.api.traces.list({
    fromTimestamp: since,
    limit: 500,
  });

  let dailyCost = 0;
  let maxRequestCost = 0;

  for (const trace of traces.data) {
    const observations = await langfuse.api.observations.list({
      traceId: trace.id,
      type: "GENERATION",
    });

    const traceCost = observations.data.reduce(
      (sum, obs) => sum + (obs.calculatedTotalCost || 0), 0
    );

    dailyCost += traceCost;
    maxRequestCost = Math.max(maxRequestCost, traceCost);
  }

  console.log(`Daily cost: $${dailyCost.toFixed(2)}`);
  console.log(`Max request cost: $${maxRequestCost.toFixed(4)}`);

  if (dailyCost > ALERT_THRESHOLDS.dailyCritical) {
    await sendAlert("CRITICAL", `Daily LLM cost: $${dailyCost.toFixed(2)}`);
  } else if (dailyCost > ALERT_THRESHOLDS.dailyWarn) {
    await sendAlert("WARNING", `Daily LLM cost: $${dailyCost.toFixed(2)}`);
  }
}

checkCostAlerts();

Langfuse Dashboard Features

Langfuse provides built-in cost analytics in the UI:

• Cost Dashboard: Tracks token usage and costs over time by model, user, and session
• Latency Dashboard: Response times across models and user segments
• Custom Dashboards: Build custom views with multi-level aggregations
• Pricing Tiers: Supports complex pricing (cached tokens, audio tokens, per-model tiers)

Cost Optimization Strategies

Error Handling

Output

Produce a dated cost report showing total spend, calls, tokens, and cost by model, plus the selected budget threshold and any alert state. When routing changes, record the before/after model mix and quality guardrail used to ensure savings did not reduce acceptable output quality.

Examples

Run getCostReport(7) after a deployment, compare the report with the prior seven-day baseline, and investigate any model whose cost per request rises unexpectedly. For a simple classification path, route a sampled cohort to the lower-cost model and retain the quality evaluation result before making the route the default.

Resources

• Token & Cost Tracking
• Metrics API
• Custom Dashboards
• Model Pricing Tiers