fx was still working.
Your Mac didn't care.

Why your Mac sleeps while fx is still going

fx spends most of a task waiting on a model response — which barely uses any CPU. But macOS doesn't decide from CPU at all — it goes by your idle time, how long since real keyboard or mouse input, plus the power assertions running processes hold. So working hard doesn't protect the machine: a Mac mid-refactor sleeps on exactly the same schedule as one doing nothing. After your system-sleep timer runs out, macOS suspends the Mac, and the fx process is frozen along with everything else until you wake it back up.

This bites hardest on the runs worth walking away from: a long plan-then-edit pass across a repo, a big test-and-fix loop, or a fx ask you kicked off and left alone. The fix is to stop macOS from sleeping for as long as the agent is actually running — then let it sleep normally again once the run is done.

fx is a terminal agent — a single native binary, not a windowed app — so the cleanest free fix is one command, and it is genuinely the right answer for most people:

• The built-in caffeinate command. Launch your session as caffeinate -i fx instead of fx. The Mac can't idle-sleep while fx runs, and the moment fx exits, normal sleep is allowed again — nothing to remember to undo. Already on your Mac, free. For a one-shot run, wrap it the same way: caffeinate -i fx ask "explain the changes in this repository".
• Attach to a session you already started. You don't have to restart it. fx installs as a real native binary — the setup script puts a single executable at ~/.local/bin/fx by default, or wherever FX_INSTALL_DIR points — so macOS reports the process under its own name rather than under an interpreter like node or python. Because two different tools can be called fx (see the next section), list them with their full path so you can tell which is which: ps -Awwo pid,comm | grep -E '/fx$'. The Vercel agent shows as …/.local/bin/fx; the JSON viewer shows as /opt/homebrew/bin/fx on Apple silicon, or /usr/local/bin/fx on an Intel Mac. Take the PID you want, then hold sleep off until that exact process exits with caffeinate -i -w <pid>.
• Amphetamine (free, Mac App Store). A menu-bar toggle to keep the Mac awake on a timer or while your terminal app is open — handy if you'd rather flip a switch than type a command.

All of these are a fine choice if fx runs for a few minutes here and there. The catch: none of them knows when the agent actually finished a task versus when it's just sitting at an idle prompt. You either stop it yourself, or it keeps your Mac awake — and your battery draining — long after the work is done.

Vercel's coding agent installs with curl -fsSL https://fx.sh/setup.sh | bash, which writes one executable to ~/.local/bin/fx. But brew install fx installs something else entirely: a long-established terminal JSON viewer, homepage fx.wtf, currently at version 39. Same four keystrokes, completely different program. Plenty of Macs have both.

That matters for anything that decides "is my agent working?" by looking at a process name. A JSON viewer piped a large file will burn CPU for a moment and then exit. A coding agent will sit at near-zero CPU for two minutes waiting on a model, then edit twelve files. If a keep-awake tool matches the bare name fx and trusts CPU as proof of work, it gets both cases exactly backwards: it can hold your Mac awake for a pager you already closed, and let it sleep during the run you actually cared about.

The practical version, whatever tool you use: don't match on the bare name. Match on the install path, or watch what fx writes while it works. fx keeps its state under ~/.fx/, and each saved session gets its own folder holding an events.jsonl that fx appends to as the run progresses. So a file-timestamp check tells you whether the run is actually moving:

• stat -f "%Sm %N" -t "%F %T" ~/.fx/sessions/*/events.jsonl | sort | tail -5 — newest last. Updated in the last minute means something is happening; frozen for ten means decide what that means for your workload. The -t "%F %T" matters: without it macOS prints the date as Aug 24 12:00:00 2026, and sort then orders by month name — Sep above Aug above Apr — so the last line is whichever month sorts last alphabetically rather than the freshest one. If you'd rather not remember that, ls -lt ~/.fx/sessions/*/events.jsonl | head -5 gives you newest-first with no flags to get wrong.

We read that layout out of fx's own source rather than guessing it, but we have not yet run fx on a Mac of ours and watched those timestamps move — so treat it as read-from-the-source, not measured-on-hardware, and check it against your own machine before you build anything on it.