1. Reset Bluetooth (Big Sur and earlier)
Open Terminal, run: sudo defaults delete com.apple.Bluetooth ; sudo killall -HUP blued. Or hold Shift + Option, click the Bluetooth menu icon → Reset the Bluetooth module. This forgets all pairings and starts fresh.
On Sonoma/Sequoia/Tahoe, the reset menu was removed. Use System Settings → Bluetooth → unpair the problem device → re-pair.
2. Check for Wi-Fi interference
Both Wi-Fi and Bluetooth use 2.4GHz. If your router is 2.4GHz only and you have many devices, Bluetooth can struggle. Switch your router to 5GHz Wi-Fi or move the Mac/device closer.
3. Test in Safe Mode
Boot in Safe Mode (Shift on Intel, power-hold on Apple Silicon). If Bluetooth works perfectly there, a third-party app or extension is the culprit — usually old DisplayLink drivers or remote-control apps.
4. When it's a hardware fault
If Bluetooth disconnects from EVERY device after exhausting software fixes, the Bluetooth/Wi-Fi card has a fault. On most Macs this is integrated to the logic board — fix is component-level rework, PKR 12,000–25,000.