@harness-one/langfuse

Langfuse exporter, prompt backend, and cost tracker for harness-one observability. Maps harness-one traces/spans to Langfuse traces and generations.

Install

pnpm add @harness-one/langfuse langfuse

Peer Dependencies

• langfuse >= 3.0.0
• harness-one (workspace)

Quick Start

import { Langfuse } from 'langfuse';
import { createLangfuseExporter, createLangfuseCostTracker } from '@harness-one/langfuse';
import { createTraceManager } from 'harness-one/observe';

const client = new Langfuse({
  publicKey: process.env.LANGFUSE_PUBLIC_KEY!,
  secretKey: process.env.LANGFUSE_SECRET_KEY!,
  baseUrl: 'https://cloud.langfuse.com',
});

const exporter = createLangfuseExporter({ client });
const traces = createTraceManager({ exporters: [exporter] });

const traceId = traces.startTrace('user-request', { userId: 'u123' });
const spanId = traces.startSpan(traceId, 'llm-call');
traces.setSpanAttributes(spanId, {
  model: 'claude-sonnet-4-20250514',
  inputTokens: 150,
  outputTokens: 80,
});
traces.endSpan(spanId);
traces.endTrace(traceId);

await traces.flush(); // Ensure events reach Langfuse before exit

The package also exports createLangfusePromptBackend(config) (prompt versioning via Langfuse) and createLangfuseCostTracker(config) (cost aggregation that forwards totals as Langfuse scores).

Spans containing model or inputTokens attributes are mapped to Langfuse generations; other spans map to generic Langfuse spans.

See the main repository README.