Try a Codex alternative on a real task in Reason
Open Reason on the web or the Mac app
Sign in at reasonmachines.com, or download the Reason app for macOS from reasonmachines.com/download. Both open the same Sessions. Sending work to Reason needs a Pro or Max plan.
Connect the plans you already pay for
In Settings › Models, connect subscriptions such as Claude (Pro or Max), ChatGPT (Plus, Pro or Business), Grok (SuperGrok or X Premium+) or GitHub Copilot, or add API keys. Reason runs on your own accounts first; a Reason plan's credits cover the rest.
Give it a task you know the answer to
In the composer, name the repository and paste a task you've already solved, such as a failing test. Reason works on its own cloud computer, runs the tests and opens a pull request, so you can compare its diff with your other agent's.
Switch the model and compare
Start a second Session on the same branch with a model from another provider, and ask it to review the first diff. Each Session shows the model and account it ran on.
Codex alternatives at a glance
| Made by | Open source | Where it runs | Price | |
|---|---|---|---|---|
| Claude Code | Anthropic | Not open source | Terminal, IDE, desktop, web; cloud sessions keep running with the laptop closed | Claude Pro $20, Max $100, Ultra $200 a month |
| Cursor | Anysphere | Not open source | AI code editor; Cloud Agents run in cloud VMs, from the editor, web, iOS or Slack | Free; Plus $8 (Grok models only); Premium $30; Super $100; Ultra $200 a month |
| Devin | Cognition | Not open source | Cloud workspace with shell, IDE and browser; CLI, desktop; Slack and Teams | Free; Pro $20; Max $200 a month; Teams $80 a month plus $40 per seat |
| GitHub Copilot cloud agent | GitHub | Not open source | Works in the background in a cloud environment; sessions on Actions run at most 59 minutes | Copilot paid plans from $10 to $100 a month; Business $19 per seat |
| Jules | Not open source | Experimental; runs tasks in a cloud VM | 15 tasks a day free; more with Google AI Pro and Ultra | |
| opencode | The opencode project | MIT | Terminal, desktop and IDE; 75+ providers and local models | Free; optional OpenCode Go from $10 a month |
| Aider | Aider-AI (GitHub) | Apache-2.0 | Pair programming in your terminal; almost any LLM, local ones included | Free; you pay your model provider |
| Reason | Reason Machines | Not open source | Web, Mac and iOS apps, Slack, command line; its own cloud computer per Session, or yours | Pro $50, Max $200 a month; credits |
From each vendor's own pages, checked Nov 2, 2026; prices are US list prices. For reference, Codex itself is included in ChatGPT Free and Go, Plus is $20 a month, and its CLI is open source under Apache-2.0.
Which alternative for which reason
Stronger benchmark scores: Claude Code. On Artificial Analysis's Coding Agent Index v2.0 (October 2026: FrontierCode v1.1, Terminal-Bench 4.0 and SWE-Atlas-QnA-AA), Claude Code on Opus 5.5 scores 62, Codex on GPT-6 Astra 55 and Grok Build on Grok 4.7 47, as listed on its comparison pages, checked Nov 2, 2026. Scores measure a harness and model together; try the same task in each on your own repository.
An editor instead of a terminal: Cursor. A teammate you tag in Slack that works in its own cloud workspace: Devin, or Reason. Already pay for GitHub: Copilot's cloud agent. Free and open source with any model, local ones included: opencode or Aider.
Reason is an always-on teammate that is also a frontier coding agent: its own cloud computer per Session, memory across Sessions, Loops and Slack, and the model picked per task from Reason's models or the Claude, ChatGPT, Grok and Copilot plans you connect. Hit your limit? Reason keeps going on your other connected accounts or Reason credits. It isn't open source and has no free plan. In Reason's published runs on GPT-6 Astra, Reason 1.1 scored 65.1 against Codex's 62.5 on the earlier Coding Agent Index v1.5.
Sources (checked Nov 2, 2026)
- OpenAI: Codex pricing
- openai/codex on GitHub (Apache-2.0)
- Anthropic: Claude plans and pricing
- Cursor: pricing
- Cursor docs: Cloud Agents
- Devin: pricing
- GitHub Docs: Copilot plans
- GitHub Docs: about Copilot cloud agent
- Jules docs: usage limits
- anomalyco/opencode on GitHub (MIT)
- Aider-AI/aider on GitHub (Apache-2.0)
- Artificial Analysis: Claude Code vs Codex
- Reason: Reason 1.1 Coding Agent Index results (Oct 10, 2026)
- Reason docs: plans
Questions
What is the best alternative to Codex?
For coding quality on public benchmarks, Claude Code: on Artificial Analysis's Coding Agent Index v2.0 it scores 62 on Opus 5.5 against Codex's 55 on GPT-6 Astra. For an editor, Cursor; for a free open-source tool, opencode or Aider.
Is there a free open-source Codex alternative?
Yes. opencode (MIT) and Aider (Apache-2.0) are free and work with almost any model provider or a local model. Codex's own CLI is also open source under Apache-2.0.
Is Claude Code better than Codex?
On Artificial Analysis's Coding Agent Index v2.0, Claude Code on Opus 5.5 scores 62 and Codex on GPT-6 Astra 55. Codex is cheaper to start, since ChatGPT Free and Go include it.
Can I keep using my ChatGPT plan in another agent?
In Reason, yes: Settings › Models has Sign in with ChatGPT for Plus, Pro and Business plans. opencode also supports ChatGPT Plus. That plan's own limits still apply.
What happens when an agent hits its usage limit?
Claude Code, Codex and Cursor stop or offer paid usage at their plan's limits. Hit your limit? Reason keeps going on your other connected accounts or Reason credits.
Is Reason affiliated with these vendors?
No. Reason is built by Reason Machines and is not affiliated with or endorsed by Anthropic, OpenAI, Anysphere, Cognition, GitHub, Google or xAI. Facts about their products come from their own pages, linked under Sources.