Mengqi Chen implemented OAuth 2.0 login protocol for Wrangler, the Workers command line interface (CLI), to improve developer experience and security. Previously, wrangler login used API tokens which required manual copying and pasting. The new OAuth 2.0 integration allows users to directly choose permissions or scopes from Wrangler, simplifying the login process while making it more secure. It also enables users to revoke access at any time using the command "wrangler logout". The security features include Cross-Site Request Forgery (CSRF) states, Proof Key for Code Exchange (PKCE) challenges, and short-lived access tokens. OAuth 2.0 integration is now available in the 1.19.3 version release of Wrangler.