Build formally verified web apps using Claude and Lean

The author used Claude Opus 5.5 under high-level guidance to develop Lasm, bringing the formally verifiable Lean language to browsers, Node, serverless, and edge environments. This enables developers to build end-to-end web applications with formally verified specifications (such as Express or Hono servers running on Cloudflare Workers).

Hacker Newslastmjs

LeanLasmClaude Opus 5.5NodeCloudflare WorkersExpressHono

Added 2026-10-31 · originally published 2026-10-30

Steps

1Set up an environment that supports Lean development and understand Lean's core features as a general-purpose programming language and formal verification tool.
2Use an AI agent that supports high-level guidance, such as Claude Opus 5.5, and provide it with the overarching direction for building a formally verified web application.
3Use Lasm to extend Lean code to run in browser, Node, serverless, or edge environments.
4Write end-to-end web application logic and apply formal verification via Lean to ensure specification correctness.

Copy-ready prompt

Please help me write a server-side web application running on [Cloudflare Workers / Node] using the Lean language and Lasm tool with [Express / Hono], and write formal verification code for the application's specifications.

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#AI Agent#Formal Verification#Lean#Web Development

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