How did Stephen Toub's incremental Copilot port compare with Bun's big-bang Rust rewrite?

Stephen Toub noted Bun ported about 500,000 lines from Zig to Rust in 11 days, reaching roughly 98% of tests passing, but then shipped no npm release for about two and a half months. The Copilot port took about 14 and a half weeks, a similar span, but replaced components one at a time in main while shipping daily, because an agent harness in 2026 cannot pause for three months without falling behind.

• Copilot shipped about 130 releases during the port, so each regression traced to a small diff.
• Interop through napi-rs peaked at two to three thousand signatures between TypeScript and Rust, then fell to zero.

Source: our own transcription of the episode. The full conversation is on Ryan Peterman (2026-11-02). Read the whole episode summary →

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