Run Codex and Claude Code Side by Side
Why run Codex and Claude Code together?
Codex and Claude Code are both powerful AI coding assistants, but they shine in slightly different workflows. Codex is commonly used by developers who already work with OpenAI-compatible tooling, automation scripts, and GPT-based coding agents. Claude Code is popular for interactive software engineering tasks, large-context reasoning, refactoring, and project-wide changes. Running them side by side lets you choose the best assistant for each job instead of committing to only one tool.
The practical challenge is cost and setup. If you connect each tool directly to different model providers, you may need separate billing accounts, different API formats, and separate environment variables. 59API simplifies this by acting as an AI API relay with cheap, pay-as-you-go access to Claude models such as Opus, Sonnet, Haiku, and Fable, plus GPT models. It is compatible with Claude Code, Codex, and any OpenAI SDK, using the base URL https://api.59api.com.
What you need before starting
Before configuring anything, make sure you have three basics ready: Node.js or your preferred runtime, the Claude Code and Codex tools you plan to use, and a 59API key. Because 59API uses official-quality native models without downgrade, you can test serious development workflows without worrying that you are evaluating a weaker model. Its low pricing and referral rebate also make it friendly for solo developers, small teams, and students experimenting with AI coding agents.
- 59API account: Create an account and copy your API key from the dashboard.
- Claude Code installed: Install it according to the official Claude Code instructions for your operating system.
- Codex or OpenAI-compatible tool installed: Use the Codex CLI, an OpenAI SDK script, or another agent that lets you set a custom API base URL.
- A test project: Pick a small repository first, such as a utility library, sample web app, or documentation site.
Step 1: Point Claude Code to 59API
Claude Code can work with Claude-compatible API providers when configured with the right endpoint and key. Set your API key from 59API as the authentication value and point the base URL to https://api.59api.com. The exact variable names can vary depending on your Claude Code version and shell, so check the current Claude Code configuration docs if needed. The important idea is simple: Claude Code should send its Claude model requests through 59API instead of a separate direct account.
For model selection, start with a balanced Claude model such as Sonnet for everyday coding. Use Opus for difficult architecture, migration, or debugging work where deeper reasoning is worth the extra cost. Use Haiku for quick edits, test generation, commit message drafting, or fast explanations. This model routing habit is one of the easiest ways to keep costs low while still getting strong results.
Step 2: Point Codex or OpenAI SDK tools to 59API
Codex-style tools and OpenAI SDK applications usually need two values: an API key and a base URL. Use your 59API key and set the base URL to https://api.59api.com. Because 59API is compatible with OpenAI SDKs, you can often keep the same client code and change only the configuration. That means existing scripts for code review, test creation, or documentation generation can continue working while using models through a lower-cost relay.
A beginner-friendly approach is to keep two terminal sessions open. In one terminal, run Claude Code for conversational project work. In the other, run Codex or your OpenAI-compatible automation for focused tasks such as generating unit tests, reviewing a diff, or creating a migration checklist. This separation avoids confusion and makes it clear which assistant produced which change.
Step 3: Use each assistant for the right task
Running both tools side by side is most useful when you assign them different roles. Do not ask both assistants to blindly edit the same files at the same time. Instead, treat them like two specialized teammates.
- Use Claude Code for project understanding: Ask it to inspect the repository, explain architecture, plan refactors, and reason through multi-file changes.
- Use Codex for repeatable automation: Run scripts that generate tests, summarize pull requests, create type definitions, or apply structured transformations.
- Use Claude for review: After Codex generates a change, ask Claude Code to review it for edge cases, security concerns, and maintainability.
- Use GPT models for alternatives: When you want a second opinion or a different implementation style, route a GPT model through your OpenAI-compatible setup.
A simple real-world workflow
Imagine you maintain a small TypeScript API. First, open Claude Code and ask it to map the project structure and identify where authentication logic lives. Next, ask it for a safe refactor plan, but do not apply changes yet. Then switch to your Codex or OpenAI SDK workflow and ask it to generate missing unit tests for the authentication module. After tests are created, return to Claude Code and ask it to review the tests and implement the refactor step by step.
This workflow gives you the benefit of planning, automation, and review without letting one tool run unchecked. It also helps control spending. Use cheaper models for mechanical tasks and reserve stronger models for the moments where reasoning quality matters most. With 59API, that cost control is easier because Claude and GPT access are available behind one relay and one billing flow.
Tips to avoid common mistakes
- Commit before AI edits: Start from a clean Git state so you can inspect and revert changes easily.
- Do not overlap file edits: Let one tool finish before asking the other to modify the same files.
- Ask for plans first: For large changes, request a plan before allowing edits.
- Set model defaults: Choose a cost-effective default model, then upgrade only for hard tasks.
- Review generated code: AI output should still pass tests, linting, and human review.
Why 59API is a practical choice
For developers who want both Codex and Claude Code, 59API removes a lot of friction. You get access to Claude and GPT models through a low-cost, pay-as-you-go relay, with compatibility for Claude Code, Codex, and OpenAI SDK-based projects. The base URL is simple to remember, the models are native official-quality rather than downgraded substitutes, and the referral rebate can reduce costs further as your team grows.
If you are experimenting with AI-assisted development, this setup is a smart starting point: connect both tools to https://api.59api.com, begin with a small repository, and compare how each assistant helps. When you are ready, sign up for 59API and try running Claude Code and Codex side by side on your next coding task.