promptfoo/examples/langfuse-labels

Langfuse Prompt Management with Labels

This example demonstrates how to use Langfuse prompt management with labels in promptfoo.

Why use labels?

Labels allow you to:

• Deploy new prompt versions without changing your code
• Use different prompts for different environments (production, staging, development)
• A/B test different prompt versions
• Roll back to previous versions quickly

Setup

1. Install dependencies:

   npm install langfuse
   

2. Set environment variables:

   export LANGFUSE_PUBLIC_KEY="your-public-key"
   export LANGFUSE_SECRET_KEY="your-secret-key"
   export LANGFUSE_HOST="https://cloud.langfuse.com"
   

3. Create prompts in Langfuse:

   • Log into your Langfuse dashboard
   • Create prompts named customer-support and email-writer
   • Add prompt templates with variables like {{customer_name}}, {{issue}}, etc.
   • Assign labels to your prompt versions (e.g., production, staging, latest)

Initialize this example

To get started with this example:

npx promptfoo@latest init --example langfuse-labels

Running the example

npx promptfoo@latest eval

Expected outputs

After running the eval, you should see:

• Comparison of responses from different labeled prompts (production vs staging)
• Performance metrics for each prompt version
• Test results showing how different providers handle the same prompts
• A detailed evaluation report demonstrating label-based prompt management

Prompt format reference

Two syntax options

Promptfoo supports two ways to reference Langfuse prompts with labels:

1. Explicit @ syntax (recommended for clarity)

   langfuse://prompt-name@label:type
   

2. Auto-detection with : syntax

   langfuse://prompt-name:version-or-label:type
   

   • Numeric values (e.g., 1, 2, 3) are treated as versions
   • String values (e.g., production, staging) are treated as labels

Format components

• prompt-name: The name of your prompt in Langfuse
• version: Specific version number (1, 2, 3, etc.)
• label: Label assigned to a prompt version
• type: Either text or chat (defaults to text)

Example prompt references

# Explicit @ syntax for labels (recommended)
langfuse://customer-support@production:chat
langfuse://customer-support@staging:chat
langfuse://email-writer@latest

# Auto-detection with : syntax
langfuse://customer-support:production:chat  # String → label
langfuse://customer-support:staging          # String → label
langfuse://email-writer:latest:text          # "latest" → label

# Version references (numeric values)
langfuse://customer-support:1:chat           # Numeric → version
langfuse://email-writer:3:text               # Numeric → version

Best practices

1. Use labels for production - Avoid hardcoding version numbers
2. Test in staging first - Use a staging label to test changes
3. Use descriptive labels - production, staging, experiment-a, tenant-xyz
4. Monitor performance - Track metrics for different prompt versions