JSSE: A JavaScript Engine Built by an Agent

7 hours ago

An AI-driven JavaScript engine named JSSE was built in Rust by an agent with zero manual code contributions.
The project achieved 100% pass rate on test262 non-staging tests (98,426 scenarios) within six weeks.
Total hands-on time was only 4 hours over 42 days, with autonomous commits handling 170,000 lines of Rust.
Key tools included Claude Code in YOLO mode, a self-generated PLAN.md, and parallel worktrees for complex features like Temporal API.
Performance is intentionally unoptimized (tree-walking interpreter), making it up to 703x slower than Node.js on some benchmarks.
The estimated API-equivalent cost was $4,619, roughly $0.03 per line of code.
Lessons include: plan quality matters more than code, test262 is a critical feedback loop, and agents struggle with long contexts.
The code is open-source on GitHub under an MIT license.

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