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Coding Guide 4 — Code your Butter Bot with OpenAI Codex

The same describe-it-and-the-AI-programs-it workflow as Guide 3, using ChatGPT's Codex CLI instead.

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Written by Karla Trstenjak

What is Codex?

Codex is OpenAI's AI coding agent. Like Claude Code (Guide 3), the Codex CLI runs in your terminal: you describe a change in plain English, it reads the Butter Bot's source code, writes the C++, and can build and flash the firmware for you. It's open source, written in Rust, and included with ChatGPT paid plans.

(Codex also exists as a VS Code IDE extension and as a cloud agent inside ChatGPT — this guide uses the CLI, which is the simplest way to work on firmware that lives on your computer.)

The Codex CLI docs

The Codex CLI docs

What you need

Item

Notes

Everything from Guide 2

Working ESP-IDF setup + the cloned ButterBot-Firmware project, built and flashed once.

A ChatGPT account

ChatGPT Plus, Pro, Business, Edu, and Enterprise plans include Codex (Plus is ~$20/month). Alternatively, pay-as-you-go with an OpenAI API key.

15 minutes

Same as Guide 3.

Step 1 — Install the Codex CLI

The official setup lives at https://developers.openai.com/codex/cli — here's the short version:

Codex CLI setup from the official docs

Codex CLI setup from the official docs

macOS/Linux

Easiest, in Terminal (official installer):

curl -fsSL https://chatgpt.com/codex/install.sh | sh

Or, if you are on macOS and use Homebrew: brew install --cask codex

Windows

Codex runs natively on Windows (Windows 11 recommended). Open PowerShell (Start menu → type powershell → Enter) and paste the official installer:

powershell -ExecutionPolicy ByPass -c "irm https://chatgpt.com/codex/install.ps1 | iex"

(Alternatives: npm install -g @openai/codex if you have Node.js; or run Codex inside WSL2 with the macOS/Linux command above — see OpenAI's Windows setup guide.)

Check it worked

Open a new terminal and run:

codex --version

You should see something like codex-cli 0.142.5.

To update Codex later: codex update (or re-run the installer / npm update -g @openai/codex).

Step 2 — Sign in

  1. Go to your Butter Bot project folder: - macOS/Linux: cd ~/Projects/ButterBot-Firmware - Windows: cd C:\Projects\ButterBot-Firmware

  2. Run: codex

  3. On first run, Codex asks you to sign in — choose Sign in with ChatGPT. Your browser opens; log in and approve. (You can also use an OpenAI API key instead.)

  4. You're now chatting with Codex inside your project. To leave later, press Ctrl+C (or type /exit).

If anything seems off, codex doctor checks your installation, login and configuration.

Approvals: Codex starts in Auto mode — it works freely inside the project folder, but asks for your approval before touching anything outside it or using the network (flashing the robot over USB may trigger such a prompt — that's expected, approve it). Type /permissions to see or change what Codex may do without asking; /status shows the current setup. Same philosophy as Claude Code: you review, you approve, git can undo.

Step 3 — Open the right terminal (so Codex can build)

Codex can only compile the firmware if idf.py works in the terminal you launched it from:

  1. Open the ButterBot-Firmware folder in VS Code (as in Guide 2).

  2. Press F1 → "ESP-IDF: Open ESP-IDF Terminal".

  3. Run codex in that terminal.

(Terminal purists: any shell with the ESP-IDF environment loaded works too — source the export.sh from your ESP-IDF install; the path is version-specific, e.g. ~/esp/esp-idf-v5.5.3/export.sh.)

Step 4 — The fun part: teach the robot to be funnier

The Butter Bot picks its jokes from a list in the firmware and speaks them with on-device text-to-speech. Paste this prompt into Codex (put your own joke in!):

Add a new joke to the Butter Bot's joke list: "I asked the toaster for a joke. It was too dark". Look at how the existing jokes are defined in components/ButterBot-Common/src/Phrases.cpp and follow the same pattern exactly, including the trailing comma. Write it the way a text-to-speech engine can pronounce it.

What you'll see (example session, shortened):

Example Codex session

Example Codex session

Codex will: 1. Search and read Phrases.cpp to find JokePhrases. 2. Edit the file: your joke added, formatted exactly like its neighbors and written so the text-to-speech engine pronounces it right — the little details that trip up humans. 3. Show you the diff of what it changed.

Not sure about the change? Ask: "show me exactly what you changed and why".

Step 5 — Build and flash, hands-free

Still in the same session:

Build the firmware with idf.py and fix any compile errors you caused. Then flash it to the robot on port <YOUR-PORT> and tell me when it's done.

(Replace the port with yours from Guide 2, Step 7 — e.g. COM5 on Windows or /dev/ttyUSB0 on Linux.)

Codex runs idf.py build (~20 seconds for a one-file change), asks for approval to flash, then runs idf.py -p <port> flash.

Step 6 — Test it on the robot

  1. Short-press the power button — the robot starts listening.

  2. Say: "Tell me a joke."

  3. Repeat until your joke comes up (they're picked at random). Take a bow. 🎭

More prompt ideas

Identical playground as Guide 3 — these all work with Codex too:

  • "Add three new responses for when someone tells the robot to pass the butter, matching the existing deadpan tone."

  • "List every voice command this robot understands, in a table."

  • "Make the robot grumpier when it gets picked up."

  • "Walk me through what happens in the code when I say 'tell me a joke', step by step, for a beginner."

And the same note: adding new spoken commands (not responses) involves phoneme strings in ListenState.cpp — Guide 5 — Program Your Butter Bot explains that whole system step by step, and its sections work as specs you can paste straight into Codex.

Troubleshooting

Symptom

Fix

codex: command not found

Open a new terminal so PATH refreshes; if it still fails, re-run the Step 1 installer. (Only if you installed via npm: confirm Node.js is present first.)

Codex can't run idf.py

Launch codex from the ESP-IDF Terminal in VS Code (Step 3)

Login loops or errors

Run codex doctor, then codex login again

Build fails after the change

Paste the error into the chat: "the build failed with this error, fix it"

Flash fails

Guide 2, Step 8 checklist: close the serial monitor, verify the port, power-cycle, direct USB

Want a clean slate

Ask Codex: "show me everything that changed". Full reset to stock: git checkout -- . && git submodule foreach --recursive git checkout -- . (the phrase files live in a submodule, so the second half matters)

Claude Code or Codex — which one?

Both do this job well, and both follow the same loop: describe → review the diff → approve → build → flash → play. Use whichever assistant you already pay for: Claude Pro/Max gets you Claude Code (Guide 3), ChatGPT Plus/Pro gets you Codex. Firmware tinkering is a fantastic, low-risk way to learn either — your robot can always be restored to stock (Guide 2, FAQ).

Ready to go deeper than jokes? Guide 5 — Program Your Butter Bot walks through new voice commands, object detection, and controller mods in full detail.

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