Failed to execute "open" on "SerialPort": Failed to open serial port

Short answer: the browser found and selected a serial device, but the operating system refused to open it. Close every serial monitor, Arduino IDE, M5Burner, qFlipper, terminal, and other flasher tab; unplug and reconnect the device; then try once from a single browser tab.

Exact error

NetworkError: Failed to execute 'open' on 'SerialPort': Failed to open serial port.

The exact Web Serial string is reproduced in the Meshtastic firmware issue tracker. Espressif's official esptool troubleshooting guide classifies serial-open exceptions as OS, hardware, or driver failures and lists five concrete causes: another program owns the port, the port no longer exists, the device disconnected, the driver is missing/faulty, or the user lacks permission.

This page was checked on 2026-07-16 against esptool v5.3.0 (released 2026-06-01) and esptool-js v0.6.0 (released 2026-03-26).

Fix it in order

1. Close the owner. Quit Arduino Serial Monitor, PlatformIO monitor, M5Burner, qFlipper, lab.flipper.net, and every other browser flasher tab. One serial port cannot be used by two tools at once.
2. Disconnect cleanly. Close the failing page, unplug the USB cable, wait five seconds, reconnect, reload this site, and select the newly appeared port.
3. Do not select a stale port. On Windows, reconnect while Device Manager is open and select the COM port that actually appears. Espressif's maintainer response in issue #9414 identifies a named COM port that no longer exists as the cause of could not open port.
4. Windows Access is denied: close the owner first; if no owner remains, disable/re-enable the device in Device Manager or reinstall its vendor driver from the driver guide.
5. Linux Permission denied: inspect the group owning the device node and add your user to that serial-access group (dialout on many distributions, uucp on others), then start a new login session. Follow your distribution's group policy; do not make the device world-writable as a permanent fix.
6. Port vanishes after selection: the device reset or disconnected. Replace the cable, remove the hub, reconnect, and re-enter the board's download mode.

Do not confuse it with a bootloader error

If the port opens and the log advances to Connecting..., this page no longer applies. Use Timed out waiting for packet header or Wrong boot mode detected.

Sources

• Meshtastic firmware issue #8543 (exact Web Serial Failed to execute 'open' error reproduced; opened 2025, checked 2026-07-16).
• Espressif — esptool troubleshooting: serial exception (official cause list and Linux group guidance; checked 2026-07-16).
• Espressif ESP-IDF issue #9414 (maintainer diagnosis of a nonexistent COM port and Device Manager verification; 2022).
• Espressif — esptool v5.3.0 release (released 2026-06-01).
• Espressif — esptool-js v0.6.0 release (released 2026-03-26).