three.dev

Key facts: three.dev became generally available on November 10, 2026, and launched on Product Hunt on November 12. It finds how your AI agent fails in your live traffic, on its own: you write no tests first and tell it nothing to look for. It scores your requests and groups the issues it finds into failure modes, ranked by severity. Setup is one prompt to Claude Code, Cursor or Codex, and your requests pass through the three.dev proxy to your provider: OpenAI, Anthropic, Gemini, Azure OpenAI and LiteLLM work today. Open a failure mode to see the failing requests, then hand the fix to your coding agent over the three.dev MCP server. It does not train on your data.

Plans: Sign up by November 22, 2026 to get the Starter plan free for six months. No paid price list was published at launch: check the official site for current plans.

Best for: Teams with AI features in production who hear about failures from users first. Who should skip it: anyone with no AI in production yet, or who tests everything by hand already.

Pros

• Finds failures on its own: no tests to write first
• Failure modes ranked by severity, so the worst comes first
• Hands the fix to your coding agent over MCP
• Works as a proxy: keep your SDK and provider

Watch out for

• The free Starter is time-limited: six months if you sign up by November 22
• Paid pricing is not published yet
• It watches live traffic, so it needs your requests to flow through it

Best for

Teams with AI features in production who hear about failures from users first. Who should skip it: anyone with no AI in production yet, or someone who tests everything by hand already.

Pricing

Starter free for six months if you sign up by November 22, 2026. Paid pricing not published yet.

FAQs

What is three.dev? A tool that finds how your AI agent fails in live traffic: it scores your requests, groups the failures into modes ranked by severity, and hands the fix to your coding agent.

Do I have to write tests? No. You write no tests first and tell it nothing to look for: it finds the failures in your live traffic on its own.

How does it connect to my app? Your requests pass through the three.dev proxy to your provider. OpenAI, Anthropic, Gemini, Azure OpenAI and LiteLLM work today.

Is there a free plan? Sign up by November 22, 2026 to get the Starter plan free for six months.