Promptfoo is an open-source command-line tool for systematically testing and evaluating LLM prompts across multiple models and providers — enabling developers to define test cases in YAML, run them against OpenAI, Anthropic, Ollama, and any other provider simultaneously, and get quantitative scores that replace "vibes-based" prompt engineering with data-driven iteration.

What Is Promptfoo?

• Definition: An open-source CLI tool and library (MIT license, 4,000+ GitHub stars) that runs structured evaluations of LLM prompts — taking test case inputs, running them through one or more models, applying scoring assertions (regex match, LLM-as-judge, semantic similarity, custom Python/JavaScript functions), and producing a comparison report.
• YAML-First Configuration: Evaluations are defined in a promptfooconfig.yaml file — prompts, providers, test cases, and assertions are all declarative, making evaluations version-controllable and reproducible.
• Multi-Provider Testing: Run the same prompt through GPT-4o, Claude 3.5 Sonnet, Llama-3, and a local Ollama model in a single command — compare quality and cost across providers simultaneously.
• Assertion Types: Built-in assertions include exact string match, regex, cosine similarity, LLM-based quality scoring (LLM-as-judge), and arbitrary JavaScript/Python evaluation functions.
• CI/CD Integration: Runs as a CLI command (npx promptfoo eval) — integrates into GitHub Actions, GitLab CI, or any pipeline to catch prompt regressions automatically.

Why Promptfoo Matters

• Systematic vs Ad-Hoc Testing: Most prompt development involves manually trying a few examples and deciding "that looks good." Promptfoo forces definition of test cases upfront and evaluates them all consistently — the same discipline software testing brings to code.
• Multi-Model Comparison: Evaluating GPT-4o vs Claude 3.5 Haiku on your specific use case is one command — real performance data on your actual task replaces benchmark comparisons that may not generalize.
• Red Teaming: Built-in adversarial test generation for safety testing — promptfoo can automatically generate jailbreak attempts, prompt injection attacks, and bias-revealing inputs to identify vulnerabilities before deployment.
• Cost Visibility: Each test run reports token usage and estimated cost per provider — model selection becomes a cost/quality optimization with real numbers.
• Open Source and Self-Hosted: No data leaves your environment — test proprietary prompts without concerns about model providers training on your evaluation data.

Core Usage

Basic Configuration (promptfooconfig.yaml):

prompts:
  - "Summarize the following in one sentence: {{input}}"
  - "Provide a concise one-sentence summary of: {{input}}"

providers:
  - openai:gpt-4o
  - anthropic:claude-3-5-haiku-20241022
  - ollama:llama3

tests:
  - vars:
      input: "The quick brown fox jumps over the lazy dog near the riverbank."
    assert:
      - type: contains
        value: "fox"
      - type: llm-rubric
        value: "Is the summary accurate and under 20 words?"
  - vars:
      input: "Quarterly earnings exceeded analyst expectations by 15% on strong cloud revenue."
    assert:
      - type: regex
        value: "earnings|revenue|quarter"

Run with: npx promptfoo eval

Assertion Types

• contains: Response must include a specific substring — simple factual checks.
• regex: Response must match a regular expression — structured data extraction validation.
• llm-rubric: An LLM grades the response against a natural language criterion — flexible quality assessment.
• similar: Cosine similarity above threshold vs a reference answer — semantic correctness without exact match.
• javascript: Custom JavaScript function — any logic expressible in JS.
• python: Custom Python function — leverage any Python library for evaluation.

Red Teaming:

redteam:
  plugins:
    - harmful:hate      # Test for hate speech generation
    - jailbreak         # Test prompt injection resistance
    - pii:direct        # Test PII leakage
  strategies:
    - jailbreak
    - prompt-injection

CI/CD Integration:

# .github/workflows/eval.yml
- name: Run LLM Evals
  run: npx promptfoo eval --ci
  # Fails if any assertion fails — blocks PR merge

Promptfoo vs Alternatives

Promptfoo is the open-source evaluation tool that brings test-driven development discipline to prompt engineering — by making it trivial to define test cases, run them across multiple models, and integrate evaluation into CI/CD, promptfoo enables any developer to replace subjective prompt quality judgments with objective, reproducible, data-driven iteration.