Promptfoo

Overview

Promptfoo is an open-source framework for testing LLM prompts — define test cases, run them against one or more models, and assert on outputs. Think "unit tests for prompts." Compare models side-by-side, catch regressions when you change a prompt, and run red-team attacks to find vulnerabilities. Web UI for viewing results, CLI for CI integration.

When to Use

• Changing a prompt and want to make sure it doesn't break existing behavior
• Comparing model performance (GPT-4o vs Claude vs Gemini on your use case)
• Red-teaming an LLM application for prompt injection and harmful outputs
• Building a prompt evaluation suite for CI/CD
• Systematic prompt engineering (not vibes-based)

Instructions

Setup

npm install -g promptfoo
# Or: npx promptfoo@latest

Basic Evaluation

# promptfooconfig.yaml — Eval configuration
prompts:
  - |
    You are a customer support agent for a SaaS product.
    Answer the following customer question concisely and helpfully.

    Question: {{question}}

providers:
  - openai:gpt-4o
  - anthropic:messages:claude-sonnet-4-20250514

tests:
  - vars:
      question: "How do I reset my password?"
    assert:
      - type: contains
        value: "password"
      - type: llm-rubric
        value: "Response should include step-by-step instructions"
      - type: similar
        value: "Go to Settings > Security > Reset Password"
        threshold: 0.7

  - vars:
      question: "Can I get a refund?"
    assert:
      - type: contains-any
        value: ["refund", "return", "money back"]
      - type: llm-rubric
        value: "Response should mention the refund policy and timeline"
      - type: not-contains
        value: "I don't know"

  - vars:
      question: "Your product sucks and I hate it"
    assert:
      - type: llm-rubric
        value: "Response should be professional and empathetic, not defensive"
      - type: not-contains-any
        value: ["sorry you feel that way", "I understand your frustration but"]

# Run evaluation
promptfoo eval

# View results in web UI
promptfoo view

Assertion Types

tests:
  - vars: { input: "Translate 'hello' to French" }
    assert:
      # Exact/partial match
      - type: equals
        value: "Bonjour"
      - type: contains
        value: "bonjour"
      - type: icontains           # Case-insensitive
        value: "bonjour"

      # Regex
      - type: regex
        value: "\\b[Bb]onjour\\b"

      # LLM-as-judge
      - type: llm-rubric
        value: "Translation is accurate and natural-sounding"

      # Semantic similarity
      - type: similar
        value: "Hello in French is Bonjour"
        threshold: 0.8

      # JSON validation
      - type: is-json
      - type: javascript
        value: "output.length < 500"

      # Safety
      - type: not-contains
        value: "I cannot"
      - type: llm-rubric
        value: "Response does not contain harmful content"

      # Latency and cost
      - type: latency
        threshold: 3000           # Max 3 seconds
      - type: cost
        threshold: 0.01           # Max $0.01 per call

Model Comparison

# compare.yaml — Side-by-side model comparison
prompts:
  - "Summarize this article in 3 bullet points:\n\n{{article}}"

providers:
  - openai:gpt-4o
  - openai:gpt-4o-mini
  - anthropic:messages:claude-sonnet-4-20250514
  - anthropic:messages:claude-haiku-4-20250514

tests:
  - vars:
      article: "{{file://test-articles/ai-regulation.txt}}"
    assert:
      - type: llm-rubric
        value: "Summary captures the 3 most important points"
      - type: javascript
        value: "output.split('\\n').filter(l => l.startsWith('•')).length === 3"
      - type: latency
        threshold: 5000

Red-Teaming

# Auto-generate adversarial test cases
promptfoo redteam init
promptfoo redteam run

# Tests for: prompt injection, jailbreaks, PII leakage,
# harmful content, bias, and more

CI Integration

# .github/workflows/prompt-eval.yml
name: Prompt Evaluation
on:
  pull_request:
    paths: ["prompts/**", "promptfooconfig.yaml"]

jobs:
  eval:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npx promptfoo@latest eval --ci
        env:
          OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
      - run: npx promptfoo@latest eval --output results.json
      - uses: actions/upload-artifact@v4
        with:
          name: eval-results
          path: results.json

Examples

Example 1: Test a customer support chatbot

User prompt: "Create an eval suite for our support chatbot — test common questions, edge cases, and angry customers."

The agent will create test cases across categories (FAQ, billing, technical, hostile), with LLM-rubric assertions for quality and safety checks.

Example 2: Choose the best model for my use case

User prompt: "I need to pick between GPT-4o, Claude Sonnet, and Gemini Flash for code review. Help me decide."

The agent will set up a comparison eval with code review prompts, assertions for accuracy and helpfulness, and cost/latency thresholds.

Guidelines

• llm-rubric is the most flexible assertion — uses an LLM to judge quality
• similar for semantic matching — doesn't require exact text match
• vars for test data — parameterize prompts with different inputs
• File-based test data — {{file://path}} for long test inputs
• Red-team before production — promptfoo redteam finds injection vulnerabilities
• CI integration catches regressions — run on every prompt change
• Web UI for analysis — promptfoo view shows results side-by-side
• Cost assertions — prevent expensive prompts from slipping into production
• Multiple providers = comparison — run same tests across models
• Start with 10-20 test cases — cover happy path, edge cases, and adversarial inputs