Codex turning your MacBook into a furnace: how to fix it

Codex is not the hot part. The commands it runs for you are.

By Andrew Wilson · Updated 2026-10-26

Short answerCodex CLI heats a MacBook by running builds, test suites and scripts in your terminal, often several at once. To fix it, find the Codex session that owns the busy processes, lower how many run in parallel, and move the running work to the efficiency cores with taskpolicy -b or a tool such as WhoIsCooking, so the job keeps going at lower heat.

Why Codex makes a Mac hot

Codex CLI sends your task to a model, then executes commands locally: package installs, compilers, linters, test runners, formatters. Each is a normal CPU-heavy process. When Codex runs a test suite after every edit, or starts a watcher, your performance cores stay at full load for as long as the session goes on. On a MacBook Pro this shows up as fan noise. On a MacBook Air, which has no fan, it shows up as the chassis getting warm and the system slowing down.

Step 1: confirm it is the commands, not Codex

Open Activity Monitor, CPU tab, sort by % CPU.
If the top entries are node, rustc, cc1, python, swift-frontend or similar rather than codex itself, the heat is from the tasks Codex launched.
Use View → All Processes, Hierarchically to see which terminal and which Codex session started them.

Step 2: reduce what runs at the same time

Ask Codex to run a single test file instead of the full suite while iterating.
Set worker limits in your tools (for example --maxWorkers=2 for Jest, -j2 for make).
Stop watchers and dev servers when you are done. Codex often leaves them running after its session ends.

Step 3: keep the work running but cooler

Start commands in background mode so macOS schedules them on the efficiency cores:

taskpolicy -b npm test

For work that is already running, WhoIsCooking shows one row per Codex session (with everything it started grouped under it), the chip temperature, and a Cool down my Mac button that moves busy processes to the efficiency cores and keeps catching new ones. In a measured render, 64°C at full speed became 50°C cooled.

Why the temperature can be high while the fan stays slow

Apple Silicon fan curves are conservative, so a MacBook Pro can read hot while the fans stay quiet. See why your MacBook is hot but the fan is not spinning.

Frequently asked questions

Why does Codex make my MacBook so hot?

Codex itself is light. The heat comes from the builds, tests and scripts it runs locally, which can keep all CPU cores busy for long periods.

How do I stop Codex from overheating my M-series Mac?

Limit parallel work, stop leftover dev servers, and run heavy commands at background priority so they use the efficiency cores. Tools like WhoIsCooking do this with one click.

Will cooling make Codex slower?

Slightly. Work on the efficiency cores takes longer but continues, with less heat and fan noise.