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Run Codex and Claude Code Side by Side with 59API

Why run both tools through one relay?

Running Codex and Claude Code side by side is a fast way to compare coding styles, catch weak suggestions, and choose the best answer for each task. The trick is to give both tools the same reliable backend so your results are comparable and your costs stay low. That is where 59API fits well: it is a pay-as-you-go AI API relay with cheap access to Claude and GPT models, native official-quality output, and compatibility with Claude Code, Codex, and any OpenAI SDK.

Using 59API also simplifies billing. Instead of managing multiple direct subscriptions, you can route both tools through one API base URL, monitor usage in one place, and benefit from referral rebates if you share the link with teammates or clients.

Step 1: Create your 59API account and key

Start by signing up for 59API and generating an API key for your project. Keep the key in a password manager or secret vault. If you plan to compare prompts often, create separate keys or labels for each workspace so you can tell which tool used what during testing.

The main endpoint you will use is https://api.59api.com. Treat it as the shared base URL for both tools. Because 59API supports multiple model families, you can test Claude Opus, Sonnet, Haiku, Fable, and GPT models without changing providers.

Step 2: Point Codex at 59API

Open your terminal and set the OpenAI-compatible settings that Codex reads. In most setups, that means using your 59API key and assigning https://api.59api.com as the base URL. If your Codex environment uses a config file, paste the same values there instead of hardcoding them in the shell.

After saving the settings, run a simple coding request such as: explain this function, add tests for this component, or refactor this loop for clarity. The goal is not to solve a hard problem yet. First confirm that Codex can authenticate, make a request, and return a normal response through 59API.

Step 3: Point Claude Code at the same relay

Now open a second terminal and configure Claude Code to use the same relay endpoint. Claude Code should also use your 59API key and the https://api.59api.com base URL. If you already use Claude Code locally, the change is usually just replacing the provider endpoint, not rewriting your workflow.

Run a different prompt from the one you used in Codex. For example, ask Claude Code to inspect a failing test, suggest a safer API design, or summarize a large file before editing it. Keeping the prompts slightly different helps you see where each model is strongest.

Step 4: Test both tools on the same repository

Open one codebase and give both assistants access to the same files. Use a task with a clear expected outcome, such as:

• fixing a bug in a route handler
• writing unit tests for a utility module
• documenting a public API
• explaining a complex refactor before you merge it

Ask Codex first, then Claude Code, or reverse the order. What matters is that both see the same context. This makes it easier to compare correctness, speed, and how much cleanup each suggestion needs.

Step 5: Compare output, not just style

When you run them side by side, do not judge only by tone. Look at practical details: does the patch compile, does the explanation match the code, and does the assistant preserve existing patterns in the repository? A model that sounds polished but misses a dependency is less useful than a more direct one that gets the change right.

Because 59API gives you access to official-quality models at low cost, you can compare more prompts without worrying about expensive tokens. That makes side-by-side testing realistic for daily development, not just a one-time experiment.

Step 6: Use cost controls and pick the right model

Use smaller models for quick edits, lint fixes, and simple explanations. Save larger models for architecture decisions, tricky debugging, and large refactors. With 59API, you can move between Claude and GPT families without changing your workflow, so you can choose the cheapest model that still solves the task well.

A practical rule is to start with a fast, lower-cost model, then escalate only if the answer is incomplete. This keeps your bill predictable and helps you learn which tool is most efficient for each kind of work.

Step 7: Make side-by-side use part of your normal workflow

Once both tools are wired to 59API, create a repeatable routine. Use Codex for one pass on implementation, Claude Code for a second pass on review or edge cases, and then combine the strongest parts of each answer. Over time, you will build a clear sense of which model is best for planning, patching, testing, and cleanup.

If you want a low-cost way to try this setup today, sign up for 59API, connect both tools to the same base URL, and run the same repo through each assistant. It is one of the cheapest relay options available, and the referral rebate can make it even more economical for teams.

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