Promptfoo

One-liner — Popular open-source tool for testing, evaluating, and red-teaming LLM apps; acquired by OpenAI in March 2026 and folded into OpenAI Frontier (stays open source).

Categories — ai-red-teaming, llm-observability

What it does

Promptfoo is a developer-first, open-source framework for systematically testing AI applications: prompt/model evals (compare prompts, models, and RAG configs against test cases and graded assertions), automated red teaming (generates adversarial probes for prompt injection, jailbreaks, PII leakage, etc., mapped to frameworks like OWASP LLM Top 10), and static scanning of models/configs. You wire it into CI so a model or prompt change can’t ship without passing your eval and security suite. It runs locally from a config file (no data leaves your environment by default), which is a big part of why it spread bottom-up among engineers.

Where it sits in the stack

Primary fit is ai-red-teaming (build-time adversarial testing / guardrail validation) with heavy overlap into llm-observability (the eval half). Its focus is untrusted input — it helps you find where prompt injection and jailbreaks get through before production. It’s a testing/assurance tool, not an inline runtime control, so it complements rather than replaces an ai-runtime-security firewall.

Deployment & architecture

Open-source CLI/SDK (npm), run locally or in CI/CD; config-as-code (YAML). Self-hosted by default; a hosted/enterprise offering adds collaboration, sharing, and continuous monitoring. Integrates with most model providers and gateways via API. No inline proxy.

Positioning & differentiators

Known as the de-facto open-source eval + red-team tool — huge bottom-up adoption (350k+ developers used it, ~130k monthly active, teams at 25%+ of the Fortune 500, per OpenAI’s acquisition post; vendor/acquirer figures). Differs from commercial red-teaming peers like mindgard, splxai, and managed services in that it’s free, code-native, and self-hosted; differs from pure eval/observability tools like langfuse, braintrust, and patronus-ai by bundling security red-teaming into the same harness. Now OpenAI-owned, which colors neutrality for buyers standardizing on non-OpenAI models — though it remains multi-model and open source.

Ownership, funding & M&A

Seed flag CONFIRMED. OpenAI announced it would acquire Promptfoo on 2026-03-09 (primary: OpenAI’s “OpenAI to acquire Promptfoo” post and Promptfoo’s own “joining OpenAI” blog; corroborated by CNBC/Bloomberg). Terms undisclosed. Promptfoo’s tech integrates into OpenAI Frontier (OpenAI’s enterprise agent platform) to strengthen agentic security testing; the open-source suite stays open source under its current license and existing customers continue to be supported. Founded 2024 by Ian Webster (CEO) and Michael D’Angelo (CTO). Pre-acquisition funding ~$23M, including an $18.4M Series A (Jul 2025, Insight Partners lead, a16z participation, ~$86M post-money). ownership_confidence: high.

CTO / hedge-fund lens

Day-2, but a cheap and high-value Day-2. If you build or heavily customize any LLM/agent app, an eval+red-team gate in CI is one of the best-leverage controls you can add, and the OSS version costs nothing to trial. SR 11-7 / model-risk angle: the eval harness produces repeatable, versioned evidence that a model was tested before promotion — useful for promotion-gates and model-validation documentation. Caveats for a hedge fund: (1) it’s a build-time assurance tool, not a runtime guardrail, so it doesn’t substitute for ai-runtime-security or ai-access-governance; (2) OpenAI ownership is a procurement/neutrality consideration if you’re standardizing on competing models. Fit is medium — valuable if you develop AI in-house, largely irrelevant if you only consume SaaS assistants.

Competitors / alternatives

mindgard, splxai, patronus-ai, giskard, braintrust, langfuse, guardrails-ai, lakera.

Open questions / to verify

• Acquisition close date and any post-close rebrand/integration specifics (only the announcement was public as of 2026-06-28).
• Long-term governance of the open-source license now that it’s OpenAI-owned.
• Exact HQ (San Francisco inferred from team/press; confirm).

Sources

• Promptfoo is joining OpenAI — fetched 2026-06-28 — supports: acquisition 2026-03-09, founders, OSS commitment, OpenAI Frontier integration; confidence: high
• OpenAI to acquire Promptfoo — fetched 2026-06-28 (403 to bot; content via search + Promptfoo blog) — supports: acquisition, adoption stats; confidence: high
• CNBC: OpenAI to buy Promptfoo — fetched 2026-06-28 — supports: deal, date; confidence: high
• (cached: raw/sources/2026-06-28--promptfoo--openai-acquisition.md)

History

• [2026-06-28] Stub created from seed registry.
• [2026-06-28] Researched; CONFIRMED OpenAI acquisition (announced 2026-03-09, terms undisclosed, stays open source → OpenAI Frontier); set ownership_confidence high. Founded 2024 (Webster/D’Angelo), ~$23M raised. Added llm-observability as secondary category (eval half). hedge_fund_fit = medium (build-time assurance tool).