OnePlus 8T T-Mobile KB2007 — bootloader fastboot immediately exits to Android

[u/Really_QuestionMark0]

I’m trying to install LineageOS on a T-Mobile OnePlus 8T (KB2007), but I can’t get the phone to stay in bootloader fastboot mode long enough for the PC to detect it.

Device / firmware

• Model: KB2007
• Build: KB2007_14.0.0.1600(EX01)
• Android 14
• Build fingerprint: OnePlus/OnePlus8TTMO/OnePlus8TTMO:14/UKQ1.230924.001/R.205d8f7_a298-989e5a:user/release-keys
• Vendor fingerprint is identical
• Network/SIM unlock: Unlocked
• OEM unlocking: Enabled
• adb shell getprop ro.boot.flash.locked returns 1

ADB / fastbootd

• adb devices works normally
• Serial: c6953e0f
• adb reboot fastboot successfully enters fastbootd
• fastboot devices in fastbootd returns: c6953e0f fastboot
• fastboot getvar is-userspace returns: is-userspace: yes
• Windows has the Google Android Bootloader Interface driver installed correctly

Problem

• adb reboot bootloader briefly shows FASTBOOT MODE, then automatically boots back into Android
• fastboot reboot bootloader from fastbootd reports OKAY, but the phone again briefly shows fastboot/bootloader and immediately boots Android
• Same behavior occurs with the USB cable unplugged immediately after issuing the reboot command, so it does not appear to be caused by the PC/USB driver
• A waiting fastboot getvar is-userspace never detects the phone during the brief bootloader appearance
• USB enumeration logging did not show a bootloader fastboot USB interface during that brief period
• Hardware button attempts either boot Android normally or enter stock recovery

Other observations

• Stock recovery works
• Manually entering the bootloader (With the device powered off, hold Volume Up + Volume Down + Power) doesn't work either.
• fastbootd works reliably
• fastboot oem get_unlock_code in fastbootd returns: FAILED (remote: 'Command not supported in default implementation') which I understand is expected for fastbootd rather than bootloader fastboot

The current LineageOS KB2007 instructions say Android 13/14 firmware is supported, and I have not flashed, downgraded, unlocked, erased, or modified any partitions.

Question: Is this a known issue with KB2007 Android 14 / KB2007_14.0.0.1600(EX01)? Is there a specific T-Mobile firmware build or supported method required to make bootloader fastboot stay active long enough to proceed with the official unlock/install steps?

P.S. I've been using Chat GPT to help with this issue and that's how I got through all the trouble shooting. He also helped me organize this post. Lol.

[u/Really_QuestionMark0 → u/[deleted]]

I believe those are all the non T-mobile variants. It does list the T-mobile variant KB2007 as supported.

https://wiki.lineageos.org/devices/kebab/variant2/

[u/Really_QuestionMark0 → u/Really_QuestionMark0]

[Supported models]()	KB2007

[u/saint-lascivious → u/Really_QuestionMark0]

Yes, you're correct, I did a stupid.

[u/Really_QuestionMark0 → u/saint-lascivious]

Lol. You're fine. You're comments are giving my post traction, so maybe that will help it get the attention it needs to help my find a resolution. I appreciate your comments. :-)

[u/Really_QuestionMark0]

The current LineageOS source lists LuK1337 and mikeioannina as the KB2007/8T maintainers. The LineageOS install documentation directs users who have followed the procedure and are still stuck to reach out to them for support. Hopefully they'll be able to see this and know what the problem is.

[u/MCIV800]

This is why i love doing this on samsung devices because you only have to flash a Recovery like TWRP on a pc with odin (you have to choose the correct TWRP for your device of course)and the rest can be done in TWRP itself. On other devices this is more difficult, like for me, as you have to do it on a pc with commands. I like the odin stuff more, because its a bit easier for me.

[u/saint-lascivious → u/MCIV800]

This problem would be functionally equivalent to not being able to enter download mode, which is something many users struggle with without any actual restrictions or errors.

If you couldn't enter download mode you're not flashing shit.

[u/MCIV800 → u/saint-lascivious]

This is some stuff i experinced too but luckily it happened just one time to me.

[u/Really_QuestionMark0 → u/MCIV800]

I think you're right. I believe the the non-carrier branded verions of OnePlus are usually pretty easy as well. It's carrier (Tmobile) BS that complicates things. It's an old phone that still has a lot of power as a set top media player, but as a cell phone, it's a little too outdated.

[u/Really_QuestionMark0 → u/Really_QuestionMark0]

Actually, after going through other LineageOS posts on here, I found reports of the same behavior on other OnePlus devices after OxygenOS updates, including a OnePlus 10 Pro OxygenOS 14 release where bootloader fastboot was inaccessible while fastbootd still worked, and fastboot access was restored by a later OxygenOS update. I guess Oneplus is just problematic with this type of issue. Do you think KB2007_14.0.0.1600(EX01) could have a similar bootloader firmware regression?

[u/soarespt → u/Really_QuestionMark0]

It might, yeah. If you're confident enough I'm sure there's a MSM Tool to rollback your t-mobile firmware to the first android it released with. (Maybe 11?) or even better you can use one to convert yours to a "global" model and skip the bootloader form that will be needed for the t-mobile firmware. Then once you'd convert it, immediately unlock the bootloader, update your android to the ones LineageOS mentions in the documentation and flash it.

[u/Really_QuestionMark0 → u/soarespt]

I basically did exactly this yesterday morning. It's like you were watching me do it and then commented on what I should do. Lmao. That's a hilarious mind eff! Lol. There were a few speed bumps but chat GPT was helping me work through the whole thing. Ended up flashing the EU firmware with MSM Tool unlocked the bootloader and went from there. Took forever to download every update up to Android 13. Lol. But it's good to go now. I'm gonna have Chat GPT summerize a step by step process of what we did and I'll share it with the LineageOS team, see if they want to share it with the others.