JSSE
A from-scratch JavaScript engine in Rust that the author says was built entirely by Claude Code and became the first engine to pass 100% of test262 non-staging tests.
Paulo Matos's 17 March 2026 post reports 42 days (27 January to 9 March 2026), about 170,000 lines of Rust, 592 commits, and roughly four hours of hands-on time. Claude Code ran in YOLO mode. The author says he did not write a single line of Rust.
Human involvement
Person directed, agent wrote the code. About four hours of human hands-on time over six weeks, mostly launching loops, checking progress, and writing prompts. The author set up CLAUDE.md, PLAN.md, and skills at the start.
Evidence
- Author blog
A from-scratch engine in Rust, built entirely by Claude Code running in YOLO mode. I didn’t write a single line of Rust. Not one.
- Author README
An agent-coded JS engine in Rust. I didn't touch a single line of code here. Not one.
Notes
The exact Claude model version is not clearly named in the excerpts reviewed. Test262 claims are the author's; this catalog does not independently re-run the suite.