Fix “Please free up some pty devices” on macOS

If you’re running tools that spawn many subprocesses (AI agents, tmux sessions, SSH connections), you may hit the default pseudo-terminal (pty) limit on macOS.

Check your current limit

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sysctl kern.tty.ptmx_max

Default is 511. Check how many are in use:

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ls /dev/ttys* | wc -l

If the count is near or above the max, you’re out of ptys.

Increase the limit

The maximum accepted value depends on your macOS / Darwin version. Try in order:

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sudo sysctl -w kern.tty.ptmx_max=999

If that returns “Invalid argument”, fall back to 512:

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sudo sysctl -w kern.tty.ptmx_max=512

Note: On Darwin 24.x (macOS 15 Sequoia), the kernel rejects values above a hard cap (e.g. 1023 fails). Use the highest value your system accepts.

This takes effect immediately. No reboot needed.

Make it persistent

Create or append to /etc/sysctl.conf so the setting survives reboots (use whichever value worked above):

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echo 'kern.tty.ptmx_max=999' | sudo tee -a /etc/sysctl.conf

Clean up stale processes

Orphaned processes can hold ptys open. Find them:

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ps aux | grep -E 'node|tmux|ssh|claude' | head -20

Kill anything you don’t recognize or no longer need:

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kill <PID>

Verify

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sysctl kern.tty.ptmx_max
# kern.tty.ptmx_max: 999

That’s it. You’re back in business.

R