Simon Willison 11/9/2025

Reverse engineering Codex CLI to get GPT-5-Codex-Mini to draw me a pelican

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This article details the process of reverse-engineering OpenAI's Codex CLI, written in Rust, to gain direct access to the GPT-5-Codex-Mini model before its official API release. The author explains how they modified the CLI to add a new 'prompt' subcommand, allowing them to send custom prompts and system messages directly to the model, demonstrating the technique by having it draw a pelican.

Reverse engineering Codex CLI to get GPT-5-Codex-Mini to draw me a pelican

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