Simon Willison: Reco Vibe-Ported JSONata from JS to Go with AI in 7 Hours, Saving $500K/Year on $400 in Tokens

Simon Willison·medium signal

Simon Willison highlighted Reco's case study of 'vibe porting' — using AI to rewrite the JSONata JSON expression language from JavaScript to a custom Go implementation. The first working Go version was completed in 7 hours with approximately $400 in AI token costs, projected to save $500K/year in operational costs. A week-long shadow deployment ran both versions in parallel to verify behavioral equivalence. JSONata's existing comprehensive test suite was the key enabler, guiding the AI to produce a functionally equivalent implementation.