The two-model coding stack: Claude Code and Codex for 40 dollars a month, Quentin IA Marketing

The reflex when you start coding seriously with AI is to move up the ladder on a single subscription. You jump to the 100-dollar plan, then the 200-dollar one, and you keep sending every prompt to the most expensive model you can afford. Including to rename a variable.

A single plan forces you to pay the price of deep reasoning on tasks that require none.

The alternative fits in one sentence: two 20-dollar subscriptions, two agents in the same terminal, and a routing rule that sends each task to the model that handles it best. You get two separate quota pools instead of one, which changes everything when you push hard from Tuesday to Thursday.

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✅What you need

A Claude Pro subscription, 20 dollars a month. It gives you Claude Code in the terminal, with Opus access.

A ChatGPT Plus subscription, 20 dollars a month. It gives you Codex, OpenAI's CLI agent.

Node.js installed on your machine. Both CLIs ship through npm.

A real project at hand. Routing does not click in the abstract, it clicks on code that already exists.

Time required

The setup takes five minutes. The routing rule takes two or three sessions to become a reflex. That is the part that matters.

Why two plans beat one big plan

🧠 The reasoning before the setup

A single high-end plan gives you one counter. When you drain it on a Thursday afternoon, you are stopped until the reset, no matter how much you paid. Two subscriptions with two different providers give you two independent counters that do not drain at the same rate, because they do not receive the same tasks.

The second argument is more interesting than the first. The two models are not strong in the same places. Claude Opus 4.8 holds long reasoning, the big picture of a codebase, architecture and critical review. GPT-5.5 inside Codex takes the repetitive implementation, the high-volume passes across many files, the ground work once the plan is clear.

⚙️The math that decides

Count your tasks, not your prompts. Over a normal week, design is a small share of the volume. Execution is all the rest.

Look at where your quota goes. On a single plan, most of your usage goes into execution billed at deep-reasoning rates.

Split the two flows. Reasoning goes to Opus, execution goes to Codex. Each counter gets what it is efficient at.

The honest caveat

This stack is not magic, it is discipline. If you install both agents and keep sending everything to the same one, you have simply spent 40 dollars instead of 20. The value lives entirely in the routing.

Installing both agents

⌨️ npm, two commands

Both CLIs install globally and live in the same terminal. You do not switch tabs, you switch commands.

npm install -g @anthropic-ai/claude-code
npm install -g @openai/codex

Then you launch either one from your project root:

claude     # Claude Code, connected to your Claude Pro account
codex      # Codex, connected to your ChatGPT Plus account

⚙️Three things to watch

Authenticate each agent separately. On first launch, each opens its own login flow to its own provider. Two accounts, two sessions, no interference.

Stay in the same project folder. Both agents read the current directory. That is what lets them work on the same code without you copying anything across.

Do not let them write at the same time. Two agents editing the same files in parallel is the fastest way to lose work. You alternate, you do not parallelise.

Check it is in place

Run claude then quit, run codex then quit. If both start from the same folder with no auth error, the stack is up.

The routing rule

🎯 The heart of the system

This is the only part of the guide worth learning by heart. Everything else is installation.

The question to ask before each task is not "which model is best", it is "does this task require deciding, or executing?". Deciding goes to Opus. Executing goes to Codex.

🏗️

Architecture and scoping

Opus. Structuring a feature, choosing a data model, arbitrating between two approaches.

🔍

Understanding existing code

Opus. Reading a codebase you do not know and producing a reliable map of it.

🐛

A bug that resists

Opus. When the cause is not obvious and you need to hold several hypotheses at once.

👀

Critical review

Opus. Reading a diff looking for what will break, not for what looks tidy.

⚙️

Scoped implementation

Codex. The plan exists, the files need writing.

🔁

High-volume repetitive work

Codex. Migrating twenty components to a new pattern, applying the same transform everywhere.

🧪

Writing tests

Codex. The expected behaviour is defined, coverage is ground work.

🧹

Mechanical refactoring

Codex. Rename, extract, move, clean up.

The sequence that works best is always the same: Opus scopes, Codex executes, Opus reviews. You open Claude Code to frame the problem and get a plan. You hand the plan to Codex to implement. You come back to Claude Code for review before you commit.

⚙️How to hand the plan from one agent to the other

Ask for the plan in writing, not in the chat. Have Opus write a PLAN.md file at the root, listing the files to touch and the expected behaviour.

Open Codex and point at the file. "Read PLAN.md and implement it. Do not deviate from the plan, if a point is ambiguous, stop and ask."

Come back to Opus for review. "Here is the diff. Look for what breaks, not for what is correct."

Why go through a file

The file is what makes the handoff possible. The two agents share no memory, they only share a folder. The disk is your communication channel between them.

The anti-token-burn rules

🔋 What drains a quota without producing anything

A quota rarely drains because of useful work. It drains because of the context you drag along and the round trips you could have avoided.

⚙️The five rules that hold

One session, one task. Do not let a conversation drift across three subjects. Every message resends the whole history. A session that lingers costs more with each turn.

Drop down a model when the task is simple. In Claude Code, /model lets you switch to a lighter model. On mechanical work the difference in output is nil, the difference in consumption is not.

Give the full spec on the first shot. Three clarification round trips cost more than one long, precise prompt. Write what you want, the constraints, and what counts as done.

Compact instead of continuing. On a long session, /compact summarises the history instead of resending it whole. The moment to do it is when you switch subtask.

Do not make it search for what you can point at. "Look at src/lib/auth.ts" costs one read. "Find where authentication is handled" costs a full sweep of the project.

The rule worth the other four

Context is the real bill. It is not the length of your question that costs, it is everything the agent has to re-read in order to answer it.

The workflow on a real case

🚀 One feature end to end

Take a concrete feature: adding magic-link authentication to an app that already has an account system.

⚙️The full run

Claude Code, scoping. "Read the current auth system. Propose how to graft magic links onto it without breaking what exists. Write the plan into PLAN.md, with the files touched and the edge cases." You read it, you challenge it, you have it corrected.

Codex, implementation. "Read PLAN.md and implement it fully. If a point is ambiguous, stop and ask rather than guess." This is the longest phase by volume of code, and exactly the one where you do not want to pay deep-reasoning rates.

Codex, tests. "Write the tests covering the edge cases listed in PLAN.md." Still ground work, still Codex.

Claude Code, review. "Here is the full diff. Look for security holes, uncovered cases and what will break in production. Be harsh." This is where the expensive model earns its price.

Codex, fixes. You hand the list of review comments back to Codex to apply. Decide with one, execute with the other, all the way through.

On that feature, the typical split lands around one fifth of the volume on Opus and the rest on Codex. That is precisely what the single 200-dollar plan cannot do: it bills all five fifths at the rate of the first.

The signal your routing is right

If you end the week with both quotas drawn down comparably, you are routing well. If one is dry and the other untouched, you are sending everything to the same place and paying for a subscription you do not use.

What this stack does not solve

Two points of honesty, because a guide that only shows the upside helps no one.

First: you manage two tools instead of one. Two logins, two update cycles, two behaviours to learn. If you code two hours a month, the simple 20-dollar plan is still the right call and this guide is not for you.

Second: the file handoff stays manual. The two agents do not talk to each other, you are the link between them. That is a real attention cost, only worth paying if you push enough volume for the quota question to genuinely come up.

Where to go from here

Install both CLIs today, it takes five minutes. But do not judge the stack on the install, judge it on your next real feature. Take it, force yourself to write the plan into a file, have Codex implement, have Opus review.

The takeaway

The lever is not owning two subscriptions. It is refusing to pay the price of thinking on work that requires none.