Have Claude Code build a full-stack app in Lean 4 that passes the official test suite
A developer built RealWorld, the reference full-stack app with official test suites, entirely in the proof language Lean 4. He says almost none of the code was written by hand: Claude Code wrote the app at his direction and also contributed fixes to six of the libraries it uses.
Source
saviorand on Hacker News · 2026-10-14 · Project link
Written in our own words. Credit to the original author. Link checked 2026-10-18.
What they did
A developer built RealWorld, the reference full-stack app with official test suites, entirely in the proof language Lean 4. He says almost none of the code was written by hand: Claude Code wrote the app at his direction and also contributed fixes to six of the libraries it uses.
Result (as claimed)
App passes the RealWorld test suites; 95 points on HN.
Tools they used
Cost: Claude Code: from $17/mo.
Set it up
Pick a contained repo or task
Start on a branch or side project where mistakes are cheap.
Install and configure
Set up Claude Code, Lean 4. Read what it can run on your machine and limit permissions where possible.
Run it on a real task
Give it a well-scoped issue and let it work. Review the diff like any pull request.
Add guardrails
Keep tests and CI in the loop, and never let an agent push to main or touch production secrets unattended.
Copy the full setup prompt
Paste into your agent and it will walk you through the setup, asking before anything that sends, posts or spends.
You are helping me set up this AI use case end to end in Claude Code. Goal: Have Claude Code build a full-stack app in Lean 4 that passes the official test suite What the original author did: A developer built RealWorld, the reference full-stack app with official test suites, entirely in the proof language Lean 4. He says almost none of the code was written by hand: Claude Code wrote the app at his direction and also contributed fixes to six of the libraries it uses. Reported result (as claimed by the author): App passes the RealWorld test suites; 95 points on HN. Source: https://news.ycombinator.com/item?id=50088748 Tools they used: Claude Code, Lean 4 Category: Dev & ops Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
You are helping me set up this AI use case end to end in Codex. Goal: Have Claude Code build a full-stack app in Lean 4 that passes the official test suite What the original author did: A developer built RealWorld, the reference full-stack app with official test suites, entirely in the proof language Lean 4. He says almost none of the code was written by hand: Claude Code wrote the app at his direction and also contributed fixes to six of the libraries it uses. Reported result (as claimed by the author): App passes the RealWorld test suites; 95 points on HN. Source: https://news.ycombinator.com/item?id=50088748 Tools they used: Claude Code, Lean 4 Category: Dev & ops Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
You are helping me set up this AI use case end to end in Grok Bot. Goal: Have Claude Code build a full-stack app in Lean 4 that passes the official test suite What the original author did: A developer built RealWorld, the reference full-stack app with official test suites, entirely in the proof language Lean 4. He says almost none of the code was written by hand: Claude Code wrote the app at his direction and also contributed fixes to six of the libraries it uses. Reported result (as claimed by the author): App passes the RealWorld test suites; 95 points on HN. Source: https://news.ycombinator.com/item?id=50088748 Tools they used: Claude Code, Lean 4 Category: Dev & ops Please: 1. Ask me 3 short questions about my situation (accounts I have, budget, what I want the output to look like). 2. Propose a step-by-step plan using these tools or cheaper alternatives, with the time and cost of each step. Use official pricing pages; say "check pricing" if unsure. 3. Then do the setup with me one step at a time, waiting for my OK before anything that sends messages, posts publicly, spends money or deletes data. 4. Finish with a checklist to run it again next time.
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