If you've been tinkering with Android for a while, you've probably heard of Project Treble and the GSI imagesEspecially when we're talking about updates, custom ROMs, or maximizing a phone's lifespan. What might not be so clear is exactly what they are, why they exist, what their limitations are, and how far you can go with them today without turning your phone into a pretty paperweight.
In the following lines we will delve deeper into What is Project Treble, and how did it change Android's architecture?This guide explains what Generic System Images (GSIs) are, what Google uses them for, how the community leverages them to install "pure" versions of Android or custom ROMs, what requirements your device must meet, and the most important steps and precautions when flashing them. The goal is that, by the time you finish reading, you'll have a clear understanding of what you can and can't do, and whether it's worth getting involved in this process.
What is Project Treble and why did it change Android?
Before discussing GSI, we need to understand what it solved Project Treble on Android architectureUp until Android 7.x, much of the manufacturer-specific and hardware vendor-specific code (Qualcomm, MediaTek, etc.) was mixed in with the system itself. This meant that every time Google released a new version, manufacturers had to redo a significant portion of their work to adapt drivers, layers, and customizations.
With Treble, initially introduced along with Android 8.0 OreoGoogle restructured the system to clearly separate two blocks: on one hand, the Android operating system framework (framework, system services, base apps); on the other, the vendor-specific layer, which includes the HAL (Hardware Abstraction Layers)hardware controllers and other components dependent on the SoC and the manufacturer.
To help these two worlds understand each other without fighting, Google defined a well-documented interface called VINTF (Vendor Interface)This determines how the system and the provider communicate, usually through HALs written with HIDL (up to Android 9) or AIDL (from Android 10 onwards). This contract establishes what the system can expect from the provider layer and vice versa.
In practice, this means that the system image becomes more generic and it is reduced to "pure" Android, while the vendor code resides in the vendor partition. If the interface is well-defined and adheres to, a new version of Android can boot on an older vendor implementation without needing to update it at the same time.
A simple analogy to understand Treble
A very graphic way to understand Project Treble is to think of a car, its driver and the driving schoolImagine that the car is the device, the driver is the Android operating system, and the car's controls (steering wheel, brakes, pedals) are the HALs and the rest of the provider's software.
The driver doesn't need to know how the braking system is constructed; he only needs to know that the brake pedal responds in a standard wayThis standard way of using the controls would be the vendor's interface (VINTF), which the system learns "at driving school." As long as all cars (vendor implementations) use the same type of standardized steering wheel and brakes, any driver (Android version) who has been through that driving school will be able to drive them without a problem.
Thanks to this separation, that common “driving school”, Treble makes Android more modular And that updating only the system without touching the vendor layer is technically possible in many cases. And that's where GSIs come into play.
What is a GSI (Generic System Image)?
A GSI is, essentially, a generic system image based on AOSP (Android Open Source Project) without manufacturer modifications or extra additions. It's the cleanest "pure Android" implementation you'll find, compiled by Google (or the community) to work on different Treble-compliant devices.
The idea is that this system image It is not designed for a specific hardware.but for any modern Android device that respects the interface contract between the system and the vendor. That's why it's called generic: instead of adapting to a particular phone, it relies entirely on the device's vendor layer to interact with the hardware.
Google maintains GSIs as part of its strategy to Treble compatibility and verification testingIn fact, manufacturers have an obligation to properly boot certain GSIs on their devices to pass official tests such as VTS (Vendor Test Suite) and CTS-on-GSI (a subset of the Compatibility Test Suite executed on the GSI).
What are GSIs used for and who benefits from them?
Within the Android ecosystem, GSIs have three main uses. The first, and most important for Google and manufacturers, is to serve as basis for official compatibility testingOEMs replace their firmware system image with a GSI and run VTS and CTS-on-GSI to verify that their vendor implementation respects the defined interface.
Secondly, GSIs are a very valuable tool for application and system developersA developer can install a GSI based on a future version of Android (e.g., a Developer Preview or beta) on a real Treble-compatible device and test their app with the latest Android, without relying on emulators or their model receiving an official beta.
Finally, there are the GSIs that Use the custom ROM communityAlthough Google conceived GSIs solely as a development and certification tool, many developers saw the potential of having a pure Android image that, in theory, could boot on any fully Treble-compatible device. This has led to the emergence of community projects that distribute "tuned" GSIs and complete ROMs packaged as GSIs for all kinds of devices.
Real limitations of GSI in daily use
On paper, a GSI should boot on any device that complies with Project Treble. In practice, Things are much more complicated.When the first Android 8.0 Oreo phones were launched, many of them were not fully "treble-ized": although they theoretically supported Treble, their compatibility with GSI was partial or non-existent.
It was not until Android 9 Pie When the first devices truly compatible with Treble at a strict level began to appear. However, even today many official Google GSIs only partially work on certain models: audio may be missing during calls, Bluetooth audio may fail, specific sensors may not be recognized correctly, or the system may simply not boot on devices with uncommon hardware.
Furthermore, AOSP for years It did not include generic HALs for certain increasingly common componentssuch as in-display fingerprint readers or certain ultra-wideband (UWB) radio modules. Google has been adding generic support as it has introduced that hardware into its own Pixel phones, but gaps remain.
Therefore, a GSI compiled as is from AOSP It is not usually suitable for daily use on a primary mobile phone.Google makes it quite clear: these apps are intended for validating Treble compliance and for app testing, not for the average user to install on their personal phone. Making them reasonably usable requires a lot of extra work from the community.
The role of the community: Enhanced GSIs and generic ROMs
Within the custom ROM scene, one of the key names is phhussonA developer set out to realistically test the promise of "a generic image that boots on any Treble device." After many hours of testing and debugging, he achieved the first community GSI that booted on phones from various manufacturers with different SoCs.
His work has evolved into a reference GSI within the communityThis update corrects bugs present in Google builds, adds compatibility with problematic hardware, and offers numerous specific adjustments for particular models. For example, its builds resolve classic call audio and Bluetooth A2DP issues that still affect some official GSI devices.
In addition to patches for general bugs, these community GSIs include overlays and configurations They adjust things like automatic brightness, refresh rate on high-frequency displays, and sensor detection. They often also include a dedicated settings app for enabling or disabling features for specific devices.
Many popular ROMs, such as LineageOS or derivative projectsThey leverage the foundation of these GSIs and the build scripts to offer their own generic images. Instead of maintaining individual ports for hundreds of models, they rely on Treble to publish a single GSI that can run on any compatible device with minor adjustments.
Requirements to install a GSI
Although Project Treble and the GSI open many doors, Having a relatively modern Android is not enough.In order to flash a GSI with certain guarantees, your device must meet several specific technical requirements.
First of all, it is essential that the mobile phone or tablet has already been launched with Treble supportIt typically runs Android 9 Pie or higher out of the box. Many models that were updated to Android 9 from an earlier version do not achieve full compatibility, even if the system reports that Treble is enabled.
The first basic check can be done via ADB with the command:
adb shell getprop ro.treble.enabled
If the returned value is falseForget it: that device isn't compatible with GSI. If you return trueIt does support Treble, but the compatibility level still needs to be validated.
The second step is to check if your phone is compatible with multiple Android versions via GSI or only with the same version. This involves consulting a configuration file from the vendor's implementation. Depending on the version, the following is used:
- /system/etc/ld.config.version_API.txt (in some earlier versions, with “API” replaced by the API number) or
- /system/etc/ld.config.version_identifier.txt (in newer branches of Android).
The typical command is usually something like:
adb shell cat /system/etc/ld.config.version_identifier.txt | grep -A 20 «[vendor]»
At the exit, inside the block vendorYou need to look at the value of namespace.default.isolated:
- Yes it is trueThe device is fully compatible with VNDK and can boot GSI based on Android versions later than the one it has installed (provided the architecture matches).
- Yes it is falseThe device does not meet all the VNDK requirements and can only load a GSI of the same Android version as the current system. For example, a phone with Android 10 will only be able to boot Android 10 GSIs.
Another key requirement is that the bootloader is unlockedWithout an unlocked bootloader, there's no way to flash the system image from fastboot. There are usually options in the developer menu, such as "OEM Unlocking," that allow you to enable this process and then execute the corresponding command (for example, fastboot oem unlock o FastBoot Flashing Unlock, according to the manufacturer).
It is also essential that the The GSI's CPU architecture matches that of the device.To find out which GSI to download, you can run:
adb shell getprop ro.product.cpu.abi
On a Pixel 5, for example, this command returns arm64-v8a, so you would need to use an arm64 GSI. If your device is x86 or x86_64 (more common in emulators and some very specific devices), you will need to download the corresponding variant.
Finally, it is worth remembering the impact this process has on security: Unlocking the bootloader breaks SafetyNet and Widevine L1 may be lostThis means that many banking or payment apps may stop working, and streaming services may no longer play high-resolution content. Furthermore, except in very specific cases (such as security-focused ROMs like GrapheneOS, CalyxOS, or DivestOS), installing a generic GSI means giving up part of Android's standard security model.
Additional checks and relationship with Keymaster
Starting with Android 9, Google also adjusted the behavior of KeyMaster (the security component that manages cryptographic keys in hardware) to avoid blocking the startup of valid GSIs. In previous versions, devices with Keymaster 3 or earlier had to verify that the Android version and security patch reported by the system matched those passed by the bootloader at startup.
If you try to boot a GSI with a different version, that verification could fail and prevent the system from finishing bootingTherefore, Android 9 and later specifies that Keymaster should skip this check when dealing with a GSI, or that providers should update to Keymaster 4 and its corresponding implementation.
This change is part of Google's global effort to to make GSIs viable to start on real Treble devices without compromising the most sensitive security mechanisms, maintaining a balance between flexibility and protection.
Where to download official and community GSIs
Depending on what you want to use a GSI for, you have several possible sources. If you're an app developer or simply want to try the latest version of “pure” AndroidGoogle publishes GSI images in:
- The official page of GSI versions, where you will find GSIs of preliminary versions (Developer Preview, betas) and Android GSIs with Google services (GMS).
- The AOSP continuous integration site (ci.android.com), where pre-built compilations without GMS are uploaded, useful for tests closer to pure AOSP.
On the other hand, if custom ROMs are your thing, there are community lists of GSIs (maintained on forums like XDA or specific wikis) where projects based on phhusson and other developers are compiled. These lists usually indicate which devices they have been tested on, what known bugs they have, and what variants exist (with or without GApps, with root, with different visual layers, etc.).
Important: Google makes it clear that Manufacturers should not use public GSIs intended for developers or the community. for official conformance testing. For that purpose, they have specific channels and builds validated for the certification program.
How to compile your own GSI from AOSP
If you have a slightly higher technical level, that's also possible compile a GSI directly from Android source codeFrom Android 9 onwards, AOSP maintains dedicated branches for GSIs named DESSERT-gsi (for example, android12-gsi). These branches include Android as released, along with GSI-specific patches and security updates.
The general workflow involves initializing the repository by pointing to the appropriate branch, synchronizing the code, loading the build environment, and selecting the corresponding GSI target. For example, to compile a gsi_arm64-userdebug based on android12-gsi, you would use something similar to:
repo init -u https://android.googlesource.com/platform/manifest -b android12-gsi
repo sync -cq
source build/envsetup.sh
lunch gsi_arm64-userdebug
make -j4
Upon completion, a generic system.img in the corresponding output folder (for example, out/target/product/generic_arm64). That image can then be flashed onto compatible devices, following the standard GSI installation process.
At the variant level, GSIs are built for different Binder architectures and configurationsThe most common ones for consumer devices are:
- gsi_arm (ARM 32-bit, Binder 64-bit, system-as-root)
- gsi_arm64 (ARM64, Binder 64 bit, system-as-root)
- gsi_x86 (x86 32-bit, Binder 64-bit, system-as-root)
- gsi_x86_64 (x86_64, Binder 64 bit, system-as-root)
Each one is released in user and userdebug flavors, depending on whether the build is geared towards end-user environments or more intensive testing and debugging.
Evolution of GSI through Android versions
With each major version of Android, Google has gone adjusting the operation of the GSIs, changing build target names, adding partitions, and polishing integration with new system features.
The scheme was fully introduced in Android 9 system-as-rootwhere the system root is mounted directly as the device root, even for terminals without A/B partitions. The GSIs for this environment also merged much of the emulator's work, so products like aosp_arm64 or aosp_x86 served as a basis for generating generic images.
Also, starting with Android 9, the use of VNDK (Vendor Native Development Kit) It became mandatory. VNDK ensures that the system's native binaries interact with the vendor layer in a compatible manner, which is key for the same GSI to work across different vendor implementations.
In Android 10, GSIs were officially distributed as user-type builds For CTS-on-GSI testing, the image format was simplified (non-scatter format, convertible with tools such as img2simg) and some legacy aosp_$arch_a targets were removed, favoring modern configurations.
Android 11 introduced partitioning system_extdesigned to host system extensions. The GSIs had to adapt and place that content under system/system_ext within the system.img itself, maintaining compatibility with the new structure.
In Android 12, Google renamed the official testing build targets to gsi_$archWhile maintaining the aosp_$arch variants for app developers, it also began removing more legacy solutions for devices still stuck on a partial Treble adoption from Android 8.x. Additionally, a userdebug-specific SELinux policy was added within the GSI to facilitate VTS debugging with vendor_boot debug images.
Finally, with Android 13, 14, and 15, devices launched with those versions must use Signed GSIs gsi_$arch-user specific to each API for their compliance tests, maintaining the common base in Android 12 but with patches corresponding to each cycle.
How to install a GSI on a compatible device
Installing a GSI is still, to this day, a delicate process and not suitable for absolute beginnersThere isn't a single set of commands that works for all devices, because each manufacturer organizes its partitions and fastboot in its own way. Even so, the general flow is quite similar.
First of all, it is essential to do a full backup Back up your data and have the device's original firmware on hand, either as a factory image or an official recovery package. If something goes wrong (bootloop, brick, etc.), you'll need to flash the original system again to recover your phone.
The general steps are usually these:
- Unlock the bootloaderEach brand has its own procedure: in some cases, "OEM unlocking" is enabled in developer options, and then fastboot is used (for example, `fastboot oem unlock` or `fastboot flashing unlock`). This step erases all user data.
- Reboot into fastboot or fastbootd modeSome devices use classic fastboot, others add fastbootd (fastboot reboot fastboot) to manage dynamic partitions from the system itself.
- Delete the current system partitionCommands such as fastboot erase system or, in A/B schemes, fastboot erase system_a (and/or system_b), according to the manufacturer's instructions or the specific guide for your model.
- Temporarily disable verified boot (AVB) Flashing a suitable vbmeta image: fastboot –disable-verification flash vbmeta vbmeta.img. This vbmeta image is usually included in the official GSI packages or it can be an "empty" vbmeta generated for this purpose.
- Flash the GSI to the system partitionFor example: `fastboot flash system system.img` or `fastboot -w flash system image_name.img`. The `-w` option also deletes user data to avoid conflicts.
- On devices with dynamic partitions or limited spaceIt may be necessary to delete the product partition (or other logical partitions) to free up more space for system. A typical example: `fastboot delete-logical-partition product_a`, matching the `_a` suffix to that of `system_a`.
- Delete user data and other relevant partitions (fastboot -w, wipe in recovery, etc.) to boot into a clean environment.
- Restart device And cross your fingers. The first start usually takes much longer than subsequent ones, so be patient.
In the specific case of Pixel phones (Pixel 3 and later), Google documents a fastbootd-based workflow for flashing GSIs, which has since been demonstrated quite applicable to other devices with a similar partition design, such as some recent tablets and mobile phones with dynamic partitions.
When booting a GSI, you may find behavioral details that differ from those of the manufacturer's ROM: Changes in the user interface, absence of specific functions (for example, proprietary gestures, advanced camera modes, audio customizations, etc.) or even incompatibilities with hardware that relied on non-standard HAL.
Minor adjustments and customizations after installing a GSI
Once you have the GSI up and running, you'll probably want to adjust certain details of the user experience that come configured generically. For example, gesture navigation or virtual buttons may be defined by vendor overlays that don't match your expectations.
Android allows you to change the navigation bar mode using overlays via ADB. A typical command would look something like this:
adb exec-out cmd overlay enable-exclusive com.android.internal.systemui.navbar.mode
Where the mode can be threebutton, twobutton, gesturaletc., depending on what the specific version of the GSI you're using supports. This is useful if, for example, the GSI starts with three-button navigation and you prefer full gestures.
In advanced community GSIs, such as those based on phhusson's work, you will also find applications of specific settings with options to solve problems with brightness, audio, camera, interface scaling and a long etcetera, adapted to different manufacturers and models.
Beyond the technical aspects, this entire GSI ecosystem opens the door to extend the lifespan of many devices that no longer receive official support, experiment with newer versions of Android and reduce, even if only a little, the amount of hardware that ends up in a drawer or directly in the trash.
Project Treble and GSI images have transformed a historically monolithic and difficult-to-update system into something far more modular, verifiable, and flexible. While installing a GSI remains a process with risks, limitations, and a certain level of technical expertise, the progress is undeniable: from allowing Google and manufacturers to better test their devices to offering advanced users and the ROM community a common foundation upon which to build alternative Android experiences across a vast array of devices.