GPT-5 High's Performance in Codex CLI Marks Significant Leap in AI-Assisted Coding

Recent observations from developers highlight a notable improvement in the performance of OpenAI's Codex Command Line Interface (CLI) when paired with the "gpt5-high" reasoning model. Thomas Schranz, a prominent voice in the tech community, shared his positive assessment on social media, stating, "codex cli w/ gpt5-high is quite good by now." He further noted being "positively surprised compared to a few weeks back," despite acknowledging that "ux has a few sharp corners and editing sometimes needs more than one attempt."

The "gpt5-high" model, a reasoning level within the recently launched GPT-5, is designed to maximize reasoning depth for complex and ambiguous problems. OpenAI officially rolled out GPT-5 on August 8, 2025, touting it as possessing "PhD-Level Intelligence" and significantly enhancing capabilities across writing, coding, and health applications. This new model has become the default Large Language Model (LLM) within ChatGPT, replacing its predecessors.

User discussions across developer forums echo Schranz's sentiment, often comparing the Codex CLI with GPT-5's advanced models to competing AI coding tools like Claude Code. Many users report that GPT-5 demonstrates superior instruction following and problem-solving abilities for intricate coding challenges. While the Codex CLI's user experience is still evolving, its integration with GPT-5's enhanced reasoning appears to be a game-changer for developers seeking more accurate and coherent code generation.

The availability of "gpt5-high" to ChatGPT Plus subscribers, alongside unlimited access for Pro users, indicates OpenAI's strategy to democratize access to its most advanced coding AI. This development suggests a competitive shift in the landscape of AI-powered developer tools, pushing the boundaries of what command-line interfaces can achieve in complex software development workflows. The ongoing improvements signal a future where AI assistants play an increasingly central role in coding efficiency and accuracy.