Jacky Zhao - Replit Engineering

Dec 15, 2025

By Peter Zhong, Jacky Zhao, and Ryan Carelli

12m read

Cover artwork for the article “Enabling Agent 3 to Self-Test at Scale with REPL-Based Verification”.

Enabling Agent 3 to Self-Test at Scale with REPL-Based Verification

How Replit built a novel REPL-based verification system that combines code execution with browser automation to catch "Potemkin interfaces", enabling Agent 3 to work autonomously for 200+ minutes.

Engineering

Oct 11, 2024

JZ SK

By Jacky Zhao and Szymon Kaliski

9m read

Cover artwork for the article “Shell2: 200× faster, persisted, multiplayer-native Shells”.

Shell2: 200× faster, persisted, multiplayer-native Shells

Back when computers were still in their infancy, the “computer” usually was a big cabinet in the middle of a room. They typically exposed some internals via a front panel with lots of knobs and ports. Users would connect a terminal device (sometimes called a teletype or a TTY) to this front panel in order to input text and print the corresponding output.

Engineering

Writing

Enabling Agent 3 to Self-Test at Scale with REPL-Based Verification

Shell2: 200× faster, persisted, multiplayer-native Shells