Langfuse Observability

Overview

Langfuse is the open-source LLM observability platform that SkillForge uses for tracing, monitoring, evaluation, and prompt management. Unlike LangSmith (deprecated), Langfuse is self-hosted, free, and designed for production LLM applications.

When to use this skill:

• Setting up LLM observability from scratch
• Debugging slow or incorrect LLM responses
• Tracking token usage and costs
• Managing prompts in production
• Evaluating LLM output quality
• Migrating from LangSmith to Langfuse

SkillForge Integration:

• Status: Migrated from LangSmith (Dec 2025)
• Location: backend/app/shared/services/langfuse/
• MCP Server: skillforge-langfuse (optional)

Quick Start

Setup

# backend/app/shared/services/langfuse/client.py
from langfuse import Langfuse
from app.core.config import settings

langfuse_client = Langfuse(
    public_key=settings.LANGFUSE_PUBLIC_KEY,
    secret_key=settings.LANGFUSE_SECRET_KEY,
    host=settings.LANGFUSE_HOST  # Self-hosted or cloud
)

Basic Tracing with @observe

from langfuse.decorators import observe, langfuse_context

@observe()  # Automatic tracing
async def analyze_content(content: str):
    langfuse_context.update_current_observation(
        metadata={"content_length": len(content)}
    )
    return await llm.generate(content)

Session & User Tracking

langfuse.trace(
    name="analysis",
    user_id="user_123",
    session_id="session_abc",
    metadata={"content_type": "article", "agent_count": 8},
    tags=["production", "skillforge"]
)

Core Features Summary

References

Tracing Setup

See: references/tracing-setup.md

Key topics covered:

• Initializing Langfuse client with @observe decorator
• Creating nested traces and spans
• Tracking LLM generations with metadata
• LangChain/LangGraph CallbackHandler integration
• Workflow integration patterns

Cost Tracking

See: references/cost-tracking.md

Key topics covered:

• Automatic cost calculation from token usage
• Custom model pricing configuration
• Monitoring dashboard SQL queries
• Cost tracking per analysis/user
• Daily cost trend analysis

Prompt Management

See: references/prompt-management.md

Key topics covered:

• Prompt versioning and labels (production/staging/draft)
• Template variables with Jinja2 syntax
• A/B testing prompt versions
• SkillForge 4-level caching architecture (L1-L4)
• Linking prompts to generation spans

LLM Evaluation

See: references/evaluation-scores.md

Key topics covered:

• Custom scoring with numeric/categorical values
• G-Eval automated quality assessment
• Score trends and comparisons
• Filtering traces by score thresholds

Session Tracking

See: references/session-tracking.md

Key topics covered:

• Grouping traces by session_id
• Multi-turn conversation tracking
• User and metadata analytics

Experiments API

See: references/experiments-api.md

Key topics covered:

• Creating test datasets in Langfuse
• Running automated evaluations
• Regression testing for LLMs
• Benchmarking prompt versions

Multi-Judge Evaluation

See: references/multi-judge-evaluation.md

Key topics covered:

• Multiple LLM judges for quality assessment
• Weighted scoring across judges
• SkillForge langfuse_evaluators.py integration

Best Practices

1. Always use @observe decorator for automatic tracing
2. Set user_id and session_id for better analytics
3. Add meaningful metadata (content_type, analysis_id, etc.)
4. Score all production traces for quality monitoring
5. Use prompt management instead of hardcoded prompts
6. Monitor costs daily to catch spikes early
7. Create datasets for regression testing
8. Tag production vs staging traces

LangSmith Migration Notes

Key Differences:

External References

• Langfuse Docs
• Python SDK
• Decorators Guide
• Prompt Management
• Self-Hosting

Capability Details

distributed-tracing

Keywords: trace, tracing, observability, span, nested, parent-child, observe Solves:

• How do I trace LLM calls across my application?
• How to debug slow LLM responses?
• Track execution flow in multi-agent workflows
• Create nested trace spans

cost-tracking

Keywords: cost, token usage, pricing, budget, spend, expense Solves:

• How do I track LLM costs?
• Calculate token usage and pricing
• Monitor AI budget and spending
• Track cost per user or session

prompt-management

Keywords: prompt version, prompt template, prompt control, prompt registry Solves:

• How do I version control prompts?
• Manage prompts in production
• A/B test different prompt versions
• Link prompts to traces

llm-evaluation

Keywords: score, quality, evaluation, rating, assessment, g-eval Solves:

• How do I evaluate LLM output quality?
• Score responses with custom metrics
• Track quality trends over time
• Compare prompt versions by quality

session-tracking

Keywords: session, user tracking, conversation, group traces Solves:

• How do I group related traces?
• Track multi-turn conversations
• Monitor per-user performance
• Organize traces by session

langchain-integration

Keywords: langchain, callback, handler, langgraph integration Solves:

• How do I integrate Langfuse with LangChain?
• Use CallbackHandler for tracing
• Automatic LangGraph workflow tracing
• LangChain observability setup

datasets-evaluation

Keywords: dataset, test set, evaluation dataset, benchmark Solves:

• How do I create test datasets in Langfuse?
• Run automated evaluations
• Regression testing for LLMs
• Benchmark prompt versions

ab-testing

Keywords: a/b test, experiment, compare prompts, variant testing Solves:

• How do I A/B test prompts?
• Compare two prompt versions
• Experimental prompt evaluation
• Statistical prompt testing

monitoring-dashboard

Keywords: dashboard, analytics, metrics, monitoring, queries Solves:

• What are the most expensive traces?
• Average cost by agent type
• Quality score trends
• Custom monitoring queries

skillforge-integration

Keywords: skillforge, migration, setup, workflow integration Solves:

• How does SkillForge use Langfuse?
• Migrate from LangSmith to Langfuse
• SkillForge workflow tracing patterns
• Cost tracking per analysis

multi-judge-evaluation

Keywords: multi judge, g-eval, multiple evaluators, ensemble evaluation, weighted scoring Solves:

• How do I use multiple LLM judges to evaluate quality?
• Set up G-Eval criteria evaluation
• Configure weighted scoring across judges
• Wire SkillForge's existing langfuse_evaluators.py

experiments-api

Keywords: experiment, dataset, benchmark, regression test, prompt testing Solves:

• How do I run experiments across datasets?
• A/B test models and prompts systematically
• Track quality regression over time
• Compare experiment results