My Mac Pro is acting like a key is stuck

if a key is stuck, adding another keyboard will not fix that, because the Mac uses 'all devices active' model for keyboards. You can literally type every other keystroke from every other keyboard.

Use keyboard viewer to see if a key is (effectively) stuck. If it is, you will need a repair.

Use the Keyboard Viewer on Mac - Apple Support

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I had a similar problem on my 2023 Mini (Ventura), ever since I cleaned the accumulated dust, hair(!), and dandruff(!?!) out of my keyboard (a custom Unicomp, which is to say, I pay the big bucks for a keyboard that lasts long enough to require cleaning) the other day.

The system would power up with the option key active. And nothing short of unplugging the keyboard and plugging it back in would deactivate it: none of the usual remedies had any effect whatsoever. I thought that somehow, plugging in a spare keyboard while the Unicomp was down for cleaning had driven something insane, but I say again, nothing worked other than unplugging the keyboard and plugging it back in.

Turns out that when I reinstalled the "option" keycaps, the "buckling spring" on one of them had failed to seat properly, holding the contact closed. So I got out the chip-puller again (best thing I've found to date for pulling Unicomp keycaps), pulled the option keycaps, and then replaced them, and there was no further trouble.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” - Sherlock Homes

― Arthur Conan Doyle

My New MacBook pro M3 is acting like the tab or the arrow key is being pressed constantly. I disconnected my Bluetooth mouse which I've been using for the past 5 months since I've bought the laptop to see if that was the issue, bit it still happening. Anyone else had this issue?

The list of 'usual suspects' also includes a tablet, with the pen resting on it, providing stray inputs as you tap on the keyboard and wiggle the table top.

try another keyboard such as a wired USB keyboard. Even a Windows keyboard will suffice, and all alpha numeric and most modifiers work without issue.