Colin McDonnell

Colin McDonnell is a Seattle-based TypeScript developer best known as the creator of Zod, the TypeScript-first schema validation library, and the author of the first version of tRPC. Zod is one of the most depended-on packages in the JavaScript ecosystem, with more than 44,000 GitHub stars and over a billion npm downloads a month as of October 2026.

McDonnell first tried building a company in healthcare: Bagel, electronic medical record software for direct primary care practices, went through Y Combinator's Winter 2019 batch. While building it he grew frustrated that TypeScript validation libraries could not infer static types cleanly, and in March 2020 he published Zod with an essay on designing the ideal schema library. The idea of declaring a schema once and getting both runtime validation and a TypeScript type caught on quickly, and Zod became the default validator for form libraries, API frameworks and, later, AI tool calling and structured output.

In July 2020 he wrote the initial version of tRPC, a way to build end-to-end typesafe APIs without schemas or code generation, which grew into a major project under other maintainers. He also proposed sponsor pools as a funding model for open source. McDonnell later worked as an engineer at EdgeDB (now Gel) and at Bun, while continuing to maintain Zod largely through sponsorships.

Zod 3 shipped in May 2021, and Zod 4, a major rewrite focused on performance and smaller bundles, went stable on July 9, 2025, followed by Zod Mini, codecs and a 4.6 release in September 2026. He co-created Standard Schema, a shared interface that lets tools accept any compliant validation library, and maintains Nub, a TypeScript-first toolkit for Node.js.

His current company is Pullfrog, an agent workflow platform for GitHub that runs AI agents in a repository's GitHub Actions to review pull requests, triage issues and fix CI, and is free for personal and open-source use. McDonnell matters as one of the few individual developers whose side project became core infrastructure for much of the TypeScript world.