Table of Contents

• What is GPT-5-Codex?

• Key Features and Improvements

  • Dynamic Thinking Time

  • Enhanced Code Refactoring and Review

  • Seamless Integration and Accessibility

  • Multimodal and Front-End Prowess

• Performance Benchmarks

• Community and Industry Reactions

• Implications for Developers

• Getting Started with GPT-5-Codex: A Step-by-Step G …

  • Step 1: Verify Your Subscription

  • Step 2: Install Codex CLI (Terminal Usage)

  • Step 3: Set Up Codex IDE Extension (VS Code and Be …

  • Step 4: Explore Web and Mobile Access

  • Step 5: Enable GitHub Code Reviews

  • Step 6: Customize and Experiment

  • Looking Ahead

OpenAI has just released GPT-5-Codex, a specialized version of its already impressive GPT-5 model, optimized for agentic coding. Announced yesterday, September 15, 2025, via a post on X and detailed further on the OpenAI blog, this new tool promises to revolutionize how developers write, refactor, and review code. Whether you're a seasoned software engineer or an AI enthusiast, GPT-5-Codex is poised to become an indispensable part of your toolkit. Let’s dive into what makes this release so exciting, what it means for the future of coding, and how you can get started with it today.

What is GPT-5-Codex?

GPT-5-Codex is not just an incremental update; it’s a purpose-built evolution of the GPT-5 model, tailored specifically for agentic coding within the Codex ecosystem. Unlike its predecessor, which serves as a general-purpose AI, GPT-5-Codex is designed to handle the nuances of software engineering tasks with remarkable autonomy. Available across multiple platforms—Codex CLI, IDE Extension, web, mobile, and integrated with GitHub code reviews—this model is accessible to developers wherever they work.

According to OpenAI’s blog post, "Introducing upgrades to Codex," GPT-5-Codex was trained on real-world software engineering challenges, including building projects from scratch, adding features and tests, debugging, performing large-scale refactors, and conducting code reviews. This focus on practical, hands-on coding tasks sets it apart, making it a versatile partner for both quick fixes and complex, multi-hour projects.

Key Features and Improvements

Dynamic Thinking Time

One of the standout features of GPT-5-Codex is its ability to dynamically adjust its thinking time based on task complexity. For simple requests, it responds with lightning speed, using 93.7% fewer tokens than GPT-5 for the easiest 10% of tasks. Conversely, for complex challenges, it can think and iterate for over seven hours, as demonstrated during internal testing where it successfully completed large-scale refactoring tasks. This adaptability ensures efficiency without sacrificing depth, a balance that’s critical for professional coding environments.

Enhanced Code Refactoring and Review

The model shines in code refactoring and review capabilities. OpenAI reports a 51.3% accuracy rate on refactoring tasks, a significant leap from the 33.9% achieved by GPT-5. Additionally, its code review functionality has reduced false positives by 68%, with a misidentification rate dropping to just 4.4%. This means developers can trust GPT-5-Codex to catch critical bugs and provide meaningful feedback, acting as a reliable “safety net” that complements human oversight.

Seamless Integration and Accessibility

GPT-5-Codex is integrated into the Codex ecosystem, unifying the developer experience across terminals, IDEs, web platforms, and mobile devices. The revamped Codex CLI, now open-source, and the new IDE extension allow for real-time collaboration and context retention, whether you’re working locally or in the cloud. For GitHub users, the model’s code review capabilities are immediately available, streamlining pull request processes. Best of all, it’s included at no additional cost for ChatGPT Plus and Pro users, making it widely accessible.

Multimodal and Front-End Prowess

Beyond traditional coding, GPT-5-Codex excels in front-end development, creating aesthetically pleasing desktop apps and mobile websites. It can interpret screenshots, wireframes, and diagrams shared via the CLI, turning visual inputs into functional code. This multimodal capability enhances its utility, bridging the gap between design and implementation.

Performance Benchmarks

OpenAI’s blog highlights GPT-5-Codex’s performance on the SWE-bench Verified dataset, achieving a 74.9% accuracy rate across all 500 tasks—a slight improvement over GPT-5’s 74.4% on a subset of 477 tasks. While its edge is more pronounced in refactoring (51.3% vs. 33.9%), it maintains parity with GPT-5 on general coding benchmarks, proving its specialization doesn’t compromise breadth.

Community and Industry Reactions

The X post announcing GPT-5-Codex (@OpenAI, 17:09 UTC, September 15, 2025) has sparked a flurry of excitement and discussion. Developers like @danshipper praised its dynamic thinking time and local-to-web handoff, noting it’s “quickly closing the gap with Claude Code.” Meanwhile, @flavioAd shared benchmark comparisons, and @every highlighted its potential for marathon coding sessions, with one test run lasting 35 minutes. Humorous takes, such as @boneGPT’s quip about a 50% chance of rewriting everything in Rust, underscore the community’s engagement.

However, not all feedback is universally positive. @flavioAd also voiced concerns about losing the “human touch” of GPT-4o, suggesting OpenAI balance technological advances with emotional resonance—a reminder of the human element in AI development.

Implications for Developers

This release marks a significant step toward autonomous coding agents. As @masahirochaen noted in a related X trend, OpenAI’s internal use of Codex for most code reviews signals a future where AI handles routine tasks, freeing humans to focus on creative and strategic work. The model’s ability to adapt to task complexity and integrate with existing tools positions it as a leader in the agentic coding space, challenging competitors like xAI and Anthropic’s Claude.

For developers, now is the perfect time to experiment with GPT-5-Codex. @kenn reported using it to automate pull requests over the weekend, complete with unit tests, while @kamui_qai recommends opting for the “high” version for optimal results. The open-source Codex CLI (installable via npm i -g @openai/codex) invites community contributions, ensuring continuous improvement.

Getting Started with GPT-5-Codex: A Step-by-Step Guide

Ready to harness the power of GPT-5-Codex? Follow these steps to set it up and start using it across different platforms. Note that you’ll need a ChatGPT Plus or Pro account for full access.

Step 1: Verify Your Subscription

• Ensure you have an active ChatGPT Plus or Pro subscription, as GPT-5-Codex is included with these plans.

• Visit chatgpt.com and sign in to confirm your subscription status.

Step 2: Install Codex CLI (Terminal Usage)

The Codex CLI is perfect for developers who prefer working in the terminal.

1. Install Node.js: Ensure Node.js (version 16 or later) is installed. Download it from nodejs.org if needed.

2. Install Codex CLI: Open your terminal and run:

   text

   npm i -g @openai/codex

   Alternatively, download the binary from the latest GitHub Release and rename it to codex after extraction (e.g., codex-x86_64-unknown-linux-musl to codex).

3. Sign In: Run codex in your terminal and select “Sign in with ChatGPT” to link your account.

4. Test It: Try a simple command, e.g., codex -m gpt-5-codex "Write a Python function to calculate factorial", and review the output.

Step 3: Set Up Codex IDE Extension (VS Code and Beyond)

For a seamless IDE experience, use the Codex VS Code extension.

1. Install the Extension:

   • Open VS Code (or forks like Cursor/Windsurf).

   • Go to the Extensions marketplace (Ctrl+Shift+X or Cmd+Shift+X on Mac).

   • Search for “Codex” and install the official OpenAI Codex extension.

   • Alternatively, download it manually from developers.openai.com.

2. Configure the Extension:

   • Sign in with your ChatGPT account when prompted.

   • Pin the Codex sidebar for easy access (especially in Cursor, where it may be collapsed by default).

3. Use It:

   • Open a project and select code to chat with Codex.

   • Try a prompt like “Refactor this function” or “Add a test for @example.ts”.

   • Adjust reasoning effort (low for speed, high for complex tasks) via the switcher.

Step 4: Explore Web and Mobile Access

• Web: Visit chatgpt.com/codex, sign in, and start coding directly in your browser.

• Mobile: Download the ChatGPT app (iOS/Android), navigate to the Codex section, and use it on the go.

Step 5: Enable GitHub Code Reviews

1. Set Up Codex Cloud: Ensure you’re signed into Codex via CLI or IDE.

2. Enable Code Review:

   • Go to your GitHub repository settings.

   • Enable “Code review” under Codex settings.

3. Trigger a Review: In a pull request, comment with @codex review. Codex will respond with 👀 and leave a detailed review.

Step 6: Customize and Experiment

• Edit ~/.codex/config.toml to tweak settings (e.g., enable MCP servers).

• Experiment with multimodal inputs (e.g., upload a wireframe via CLI with codex -m gpt-5-codex --image wireframe.png "Convert to HTML/CSS").

• Share your results on X with #GPT5Codex for community feedback.

Looking Ahead

OpenAI’s vision extends beyond the current release. The blog hints at future developments, including cloud-based agent swarms supervised by humans and enhanced security and alignment research. GPT-5-Codex’s goal isn’t to generate more code but to elevate its quality, simplifying systems and automating tasks like COBOL migrations and security patches.

As we move into this AI-driven era, foundational skills in software engineering remain crucial. Pairing human expertise with GPT-5-Codex’s capabilities could unlock unprecedented productivity gains. We encourage you to try it out, share your experiences, and join the conversation on X using #GPT5Codex.

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