Does Caffeine Keep Slack Active? It Depends Which Caffeine

The short answer: two completely different tools are called “Caffeine,” and only one of them holds your Slack status green. The Windows tray utility simulates a keypress, so Slack sees activity. The macOS caffeinate command only stops your Mac sleeping — it never pretends to be a user, so Slack still marks you away.

That distinction explains most of the confusion. People try “Caffeine” on a Mac, watch their Slack green dot disappear anyway, and assume the tool is broken. It isn’t — it was never solving the problem they had.

Sleeping and Being Away Are Two Different Things

Slack says it normally marks you away after 10 minutes of desktop inactivity. The signal it watches is input — whether the keyboard and mouse have been touched. Sleep prevention is a different mechanism entirely: it tells the operating system not to power down the display or suspend the machine.

A Mac can sit wide awake, screen fully lit, for hours, with the idle counter climbing the entire time. That is the exact state caffeinate produces, and it is why the Slack away status still appears. The computer is awake. You, as far as the input layer is concerned, are not.

Why Caffeine on Windows Does Work

Zhorn Software’s Caffeine for Windows takes the other approach. By default it sends an F15 key-up event every 59 seconds — a key chosen because virtually nothing responds to it. Windows counts that as genuine keyboard use, the idle timer resets, and applications watching for activity, Slack included, treat you as present.

The developer documents the trade-off: a synthetic keypress can have side effects in software that listens for key events, with PuTTY, PowerPoint, Google Docs, and Smartsheet named as examples. Caffeine also offers a mode that prevents sleep without the fake keystroke — and in that mode, by the developer’s own note, applications tracking user input will not see any activity. Which returns you to the macOS situation: awake, but idle.

What Each caffeinate Flag Actually Does

macOS ships caffeinate as a standard command. Its flags are worth reading closely, because none of them simulate input:

• -d — prevent the display from sleeping.
• -i — prevent the system from idle sleeping.
• -m — prevent the disk from idle sleeping.
• -s — prevent system sleep while running on AC power.
• -u — declare that the user is active, keeping the display awake.
• -t — a timeout in seconds, used with -u.
• -w — wait for a given process ID to exit, then stop.

The -u flag is the one people reach for, since “declare that the user is active” sounds like exactly the right thing. It asserts activity to power management — enough to keep the screen lit. It does not move a cursor or press a key, and users consistently report Slack going away regardless. See Apple’s caffeinate documentation for the full man page.

The Limit Both Approaches Share

Even where a keystroke simulator works, it works only while the machine is on, unlocked, awake, and running Slack. Close the laptop and presence ends. Reboot and it ends. Lock the screen and, depending on configuration, it ends. Anything running on your computer inherits your computer’s availability — which is usually the thing you were trying to work around.

Neither approach knows anything about Slack specifically, either. If a workspace’s credentials expire, a local jiggler keeps happily jiggling while your status quietly stops updating. Nothing tells you.

When Caffeine Is Genuinely the Right Answer

If you are on Windows, want something free, and only need to stop the screen locking during a long read or a presentation, Caffeine is hard to beat. It is tiny, free for commercial use, and needs no account or credentials. For that job, reach for it.

It is the wrong tool when the actual requirement is that your Slack presence reflect your working hours rather than your laptop’s power state. NotAway maintains the Slack connection from the cloud on a schedule you set, across up to seven workspaces, from $3.99/month — and emails you if a workspace needs its credentials refreshed. Your machine can be closed, asleep, or off. If what you want is to keep Slack active during the hours you are actually available, that is a different problem from keeping a computer awake, and it wants a different tool.