Simon Willison
★ 9/10 signal
Rewriting Bun in Rust
agenticevaltool-use
What happened
Simon Willison details Jarred Sumner's rewrite of the Bun runtime from Zig to Rust, heavily accelerated by an advanced agentic engineering harness. The agent leveraged Bun's existing TypeScript test suite as a strict conformance suite to validate its ports. The workflow featured dynamic planning, trial runs, and adversarial review to handle complex memory management transitions (safe Rust vs. Zig's manual memory).
Why it matters
Robust test suites are the ultimate enabler for autonomous coding agents, turning software rewrite projects from impossible risks into structured, iterative agent tasks.
The take
This is a landmark case study for coding agents. It proves that large-scale, cross-language rewrites of complex systems are now viable when paired with a robust, automated test suite (conformance suite) that acts as an objective evaluator for the agent. The use of adversarial review and dynamic workflows is a blueprint for enterprise-grade agentic coding.
Do this
Read the full post and design your coding agent workflows to leverage existing test suites as automated feedback loops (conformance-driven development).
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