#engineering

Jarred Sumner announced the successful rewrite of the Bun JavaScript runtime from Zig to Rust, a process that took 11 days of intensive agentic engineering using Claude and cost approximately $165,000 in API tokens. This rewrite demonstrates that modern AI coding agents can make large-scale rewrites feasible, challenging the long-held 'never rewrite' wisdom. It also shows how Rust's memory safety can eliminate common bugs like use-after-free that plagued Zig's manual memory management. The Bun test suite, written in TypeScript, served as a conformance suite to validate the port. The new Rust-based Bun has been running in Claude Code since June 17, 2026, with 10% faster startup on Linux and no noticeable changes for users.

A fintech engineering handbook titled 'Fintech Engineering Handbook' was published, but drew criticism from the community for recommending practices like storing monetary values as decimals or floats instead of integers, and for oversimplifying foreign exchange handling. This discussion highlights critical engineering decisions in fintech, such as monetary representation and FX handling, which have significant consequences for accuracy and compliance. The debate underscores the need for rigorous best practices in financial software, affecting developers and companies building financial systems. Critics pointed out that storing monetary values as integers in the smallest currency unit (e.g., cents) is safer to avoid floating-point rounding errors. The handbook's advice on using decimals or floats for JSON interchange was specifically called out as risky, especially when dealing with currencies having different minor unit counts.