Insomniac: keep the Mac awake, lid closed, without cooking it

Problem

I wanted my Mac to keep running with the lid shut. Not dimmed: actually working, an AI agent overnight, an Xcode archive, a long sync, lid closed, no external monitor. macOS calls that clamshell sleep, and it does not care that your task is mid-flight.

The catch is in the mechanism. The popular keep-awake tools use IOKit power assertions, and those do not prevent clamshell sleep unless you have an external display, power, and a keyboard attached, exactly the setup I was trying to avoid. The only lever that works with nothing attached is a privileged command: pmset disablesleep. So the whole product is really one thing, a safe wrapper around a dangerous system setting, and everything else exists to make that one lever safe and pleasant to pull.

Decisions

Safety is the feature, not staying awake. A generic paid keep-awake app is dead on arrival: Amphetamine and caffeinate already do it for free. The unserved angle was heat. A closed lid chokes airflow, and no existing tool says a word about it. So Insomniac is not “stay awake”, it is “stay awake without cooking the machine”.

Measure what macOS exposes honestly, not fake degrees. The tempting version reads a CPU temperature and shows a countdown in degrees. But Apple Silicon exposes no supported way to read die temperature; anything claiming to is reading private keys that break across models. So I built on ProcessInfo.thermalState (nominal, fair, serious, critical), which needs no admin and is supported forever. And the real protection is not an up-front prediction at all: it is a live thermal cutoff that watches the state while the lid is closed and auto-stops the session if it escalates. A prediction can be wrong; a live cutoff is ground truth.

An architecture where the dangerous part is swappable. Running pmset as root has two paths: an AppleScript prompt that asks for a password on every toggle (trivial, works day one) and a privileged helper over XPC that toggles silently (production-grade, needs signing and a pinned contract). Instead of coupling the app to either, I hid both behind one PowerControlling protocol. I shipped the password-prompt version on day one and added the silent helper later with zero caller changes. The helper is locked down: as a root daemon it only accepts XPC connections pinned to this app’s identifier and Team ID.

The safety invariants are enforced by the code. A keep-awake app has one catastrophic failure: leaving sleep disabled after you are done. I treated “never leave sleep disabled” as an invariant and closed every exit. Toggle off, auto-off, thermal cutoff, and battery cutoff all restore sleep. Quit and logout defer termination until the restore completes; the app refuses to die with sleep still disabled. A hard crash cannot be intercepted, so on next launch the app detects sleep disabled with no active session and offers to reset it. There is also no “indefinite”: the longest session is a hard 8-hour cap, because “forever” is exactly the setting that leaves a dead, hot laptop in a bag.

Honest UI, honest privacy. The app is menu-bar only. The advisory shows a risk level (low, moderate, high) and a plain message, never degrees I cannot read honestly, and the weather nudge is deliberately the weakest input: a hot room can raise risk, but a cold room can never make a cooking CPU safe. The weather location comes from IP geolocation, city-level, so there is no GPS permission prompt; turn weather off and the app sends nothing. No telemetry, no analytics.

The bug I only found by living with it. V1 kept the Mac awake, but the screen stayed lit with the lid closed, because disabling sleep also stops macOS from turning the internal display off. A backlight burning against the keyboard is wasted battery and heat, the exact thing the app protects against. I added a lid monitor that fires on the open-to-closed edge and puts the display to sleep with a harmless user-level command, kept deliberately separate from the privileged helper. Least privilege for the thing that does not need it.

Ship now, own the rough edge. The core feature needs the App Sandbox off, which rules out the Mac App Store, so I shipped a direct download. Rather than spend days on Apple’s paperwork first, I shipped an un-notarized build now and made the resulting Gatekeeper warning a guided, one-minute step: the installer is a hand-drawn setup guide, the quarantine-clearing command sits right next to the app as a copyable file, and the app strips its own quarantine flag on every launch so the warning never comes back. The packaging script re-signs and notarizes with one command the moment a certificate exists.

Outcome

Insomniac is live: a menu-bar app that keeps a Mac awake lid-closed, with a live thermal cutoff, a battery cutoff, crash recovery, and silent toggling through a signed helper. The whole thing rests on two rules: be honest (the real mechanism, risk levels instead of fake degrees, a weak signal kept weak) and ship fast with the rough edges owned rather than hidden. A keep-awake app is a small thing, but it touches a setting that can damage hardware, so every shortcut that would have made the demo flashier would have made the product less trustworthy.