How to Run OpenAI’s GPT-5.6 Sol Inside Anthropic’s Claude Code

OpenAI recently dropped GPT-5.6 Sol, and it is already looking like a very capable model for agentic coding. Its release also came at an interesting time. Anthropic extended access to Claude Fable 5 twice, first through July 12 and then through July 19, while temporarily raising Claude Code usage limits.

Maybe it was demand. Maybe it was capacity. Maybe the timing was completely unrelated. Still, I can smell a little fear in Anthropic.

One thing many developers may not know is that it is possible to combine GPT-5.6 Sol with Claude Code.

Yes, you can access OpenAI’s coding model without abandoning Claude Code’s terminal interface. Depending on the method, Sol can either work beside Claude as a reviewer or replace Claude as the main model behind the session.

In this guide, I will show both approaches, explain how they differ, and go over why someone might choose this setup instead of simply running Sol through Codex.

Let’s get started.

What is new in GPT-5.6 Sol?

The biggest change for coding is not just raw benchmark performance. GPT-5.6 introduces stronger tool coordination, higher reasoning settings, and better support for long-running agent workflows.

OpenAI’s internal benchmarks show that GPT‑5.6 Sol sets a new state of the art on Terminal‑Bench 2.1.

GPT 5.6 in TerminalBench 2.1. Bemchmark from OpenAI

The new max reasoning level goes beyond the previous xhigh setting. Sol Ultra goes further by coordinating four agents by default, trading higher token usage for stronger results and faster parallel investigation.

GPT-5.6 also supports Programmatic Tool Calling. Instead of repeatedly sending individual tool requests back and forth, the model can write small programs that call tools, process intermediate results, and return the final output.

For coding agents, that can reduce unnecessary round trips when searching a repository, comparing files, or processing large amounts of structured output.

How Sol compares with GPT-5.5 and Claude

OpenAI’s benchmark results show a real improvement over GPT-5.5, but the comparison with Claude is more complicated.

Fable 5 and GPT 5.6 Sol in Artificial Analysis benchmark

Here are three useful results from OpenAI’s published evaluations:

• On the Artificial Analysis Coding Agent Index, Sol scored 80. GPT-5.5 scored 76.4, Fable 5 scored 77.2, and Opus 4.8 scored 72.5.
• On Terminal-Bench 2.1, Sol reached 88.8%, while Sol Ultra reached 91.9%. GPT-5.5 scored 85.6%, Fable 5 scored 83.1%, and Opus 4.8 scored 78.9%.
• On SWE-Bench Pro, Fable 5 remained ahead at 80%. Opus 4.8 scored 69.2%, Sol scored 64.6%, and GPT-5.5 scored 59.4%.

That last result is important. Sol does not automatically beat Claude on every software engineering benchmark.

It appears especially strong on terminal tasks, tool-heavy workflows, and broader agentic coding evaluations. Fable 5 still performs better on some repository-level bug-fixing tests.

Sol also costs $5 per million input tokens and $30 per million output tokens. That places it above Opus 4.8’s $5 and $25 pricing, but below Fable 5’s $10 and $50 pricing.

In practice, I would not choose between them based on one leaderboard. The task, agent harness, reasoning level, and number of tool calls can change the result just as much as the model itself. That is where Claude Code becomes interesting.

Two ways to use Sol inside Claude Code

There are currently two practical methods.

1. The first uses OpenAI’s official Codex plugin for Claude Code. Claude remains the main agent, but it can call Codex for reviews, adversarial checks, and rescue tasks.
2. The second uses a local proxy. This routes Claude Code’s Anthropic-style requests to OpenAI and makes GPT-5.6 Sol the main model behind the terminal session.

These approaches may look similar from the outside, but they behave very differently.

Option 1: Use the official Codex plugin

For most people, this is the method I would recommend starting with.

OpenAI maintains an official Claude Code plugin called codex-plugin-cc. It adds Codex commands directly to Claude Code without replacing Claude as the primary model.

Before installing it, make sure the following are available:

• Claude Code
• Node.js 18.18 or newer
• A ChatGPT subscription with Codex access or an OpenAI API key
• A recent version of the Codex CLI

OpenAI currently recommends Codex CLI version 0.144.0 or newer for GPT-5.6. Open Claude Code and add the OpenAI plugin marketplace:

/plugin marketplace add openai/codex-plugin-cc

Installing Codex plugin in Claude Code. Image by Jim Clyde Monge

Make sure that you see the “Successfully added marketplace: openai-codex” message before you continue. Next, install the Codex plugin:

/plugin install codex@openai-codex

Installing Codex plugin in Claude Code. Image by Jim Clyde Monge

Notice that you will be asked to choose which scope you want the plugin to be installed.

• Install for you (user scope)
• Install for all collaborators on this repository (project scope)
• Install for you, in this repo only (local scope)

In my case, I usually select the user scope so it applies to any of my personal projects in the future.

Installing Codex plugin in Claude Code. Image by Jim Clyde Monge

Again, make sure that you see the “Installed Codex” message before you continue. Reload the installed plugins:

/reload-plugins

Finally, run the setup command:

/codex:setup

The setup command checks whether Codex is installed and authenticated. When npm is available, it can also install the required CLI automatically.

Installing Codex plugin in Claude Code. Image by Jim Clyde Monge

Select “Install Codex” if it is not already installed on your system. Follow the on-screen instructions to properly install Codex on your system. Once done, you should see the messages below.

Installing Codex plugin in Claude Code. Image by Jim Clyde Monge

Confirm that all the checkmarks are green. That’s an indicator that you are ready to go. To install Codex manually, run this command outside Claude Code:

npm install -g @openai/codex

Then return to Claude Code and authenticate through the shell:

!codex login

Once setup is complete, several new slash commands become available.

Running Codex plugin in Claude Code. Image by Jim Clyde Monge

That unlocks /codex:review, /codex:adversarial-review, and /codex:rescue. Claude writes, GPT critiques, you ship.

You can now do something like:

Prompt: Using the Codex CLI (already authenticated on this machine), scaffold a new NextJS web application for my new personal website.

Review Claude’s code with Sol

The simplest command is:

/codex:review

This asks Codex to inspect the current uncommitted changes or compare the branch against its base.

Claude can write the implementation, and Sol can look for bugs, regressions, missing tests, or unsafe assumptions. The review runs in read-only mode, so Codex will not modify the files.

A branch-specific review can be started with:

/codex:review --base main

Longer reviews can also run in the background:

/codex:review --base main --background

You can continue working in Claude Code and check the task later using:

/codex:status

Then retrieve its output with:

/codex:result

Run an adversarial review

For a more aggressive evaluation, use:

/codex:adversarial-review

This is more interactive than the standard review. It is designed to challenge the implementation, question the architecture, and look for problems involving authentication, race conditions, data loss, rollback behavior, and reliability.

I would use this after implementing a feature that touches payments, permissions, migrations, or other sensitive parts of an application.

The value here is not that Sol is always smarter than Claude. It is that a second model may notice assumptions the first model accepted without questioning.

Hand a difficult task to Codex

The rescue command lets Claude delegate an implementation or debugging task directly to Codex:

/codex:rescue

It can also specify the model and reasoning effort:

/codex:rescue --model gpt-5.6-sol --effort high

For a difficult task, I could use:

/codex:rescue --model gpt-5.6-sol --effort max --background

The rescue command is useful when Claude has become stuck, keeps repeating the same fix, or needs a separate investigation.

It also supports resuming an existing Codex thread, starting a fresh thread, waiting for completion, or running in the background.

Enable the optional review gate

The plugin can block Claude from ending the task until Codex has reviewed the changes:

/codex:setup --enable-review-gate

This sounds useful, but I would enable it carefully. When Codex keeps finding new issues, and Claude keeps applying fixes, the session can enter a review loop. That can consume a large amount of both Claude and Codex usage before the task is considered complete.

For most projects, manually running /codex:review before committing gives me more control.

Option 2: Make Sol the main Claude Code model

The official plugin lets Claude delegate tasks to Codex, but Claude still controls the main session.

To make GPT-5.6 Sol the primary model, Claude Code needs to send its requests through a compatible proxy.

A commonly shared setup uses CLIProxyAPI. It provides compatible interfaces for Claude, OpenAI, Gemini, and Codex, and can authenticate with Codex through OAuth.

This is a community-driven method rather than an officially supported Claude Code configuration. It may break when Anthropic or OpenAI changes an API format, authentication flow, or tool schema. I would treat it as an experiment, not as the safest setup for production work.

On macOS, install CLIProxyAPI with Homebrew:

brew install cliproxyapi

Start it as a background service:

brew services start cliproxyapi

Linux users can use the installation script provided by the project, while Windows users can download the executable or use its desktop interface.