Another data point in favor of this: PPPoE-based fiber connections have a hard path MTU of 1492 bytes. When ICMP "frag needed" messages are filtered anywhere along the path (common ISP behavior), the app's default tunnel MTU fails silently; sites partially load, requests time out, and users conclude the VPN is broken.

Currently the only workaround is manual WireGuard config files with MTU = 1400 under [Interface], losing all app conveniences (server switching, Secure Core, kill switch, split tunneling) just to set one number.

I'd also second Juha's suggestion above of simplifying the title to "Adjustable MTU"; both auto-negotiation and a manual override field solve the same user problem from different ends, and a broader title would aggregate these votes instead of splitting them.

Concrete ask: Settings → Connection → "Override tunnel MTU" (desktop apps, Linux particularly), or auto-clamping via path-MTU discovery. The config-file ecosystem already supports the concept, so the GUI just needs to expose it.

Absolutely this!

To be honest, I don’t mind whether it is automatic or manual, as long as it is possible to adjust the MTU. Come to think of it, I would prefer it to be a manual and permanent setting to avoid any unnecessary lag when negotiating (I’m not entirely sure, but I imagine that could be the case).

As it is now, I am forced to use OpenVPN or the native WireGuard client, which is far from optimal.

I suggest simplifying the title of this request to “Adjustable MTU” or something similar to give it a wee punch.

Best regards,
juha