Version: 2019.4
Unity's Package Manager
Dependency and resolution


This section explains many of the concepts surrounding the Unity Package Manager functionality:


Multiple versions of each package are available, marking changes to that package along its life cycle. Every time a developer updates the package, they give it a new version number. A change in package version tells you whether it contains a breaking change (major), new backward-compatible functionality (minor), or bug fixes only (patch), following Semantic Versioning.

To view the list of versions available for a specific package, see Finding a specific version.


There are two types of manifest files:

  • Project manifestsEach Unity project has a project manifest, which acts as an entry point for the Package Manager. This file must be available in the <project>/Packages directory. The Package Manager uses it to configure many things, including a list of dependencies for that project, as well as any package repository to query for packages. More info
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    (manifest.json) store information that the Package Manager needs to locate and load the right packages, including a list of packages and versions declared as dependencies
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  • Package manifestsEach package has a manifest, which provides information about the package to the Package Manager. The manifest contains information such as the name of the package, its version, a description for users, dependencies on other packages (if any), and other details. More info
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    (package.json) store information about a specific package, and a list of packages and versions that the package requires.

Both files use JSON (JavaScript Object Notation) syntax.


In the domain of Unity’s Package Manager, a package registry is a server that stores package contents and information (metadata) on each package version. Unity maintains a central registry of official packages that are available for distribution. By default, all projects use the official Unity package registry, but you can add additional registries to store and distribute private packages or stage custom packages while you are developing them.

Package Management

The Unity Package Manager is a tool that manages the entire package system. Its primary tasks include the following:

The Unity Package Manager installs samples, tools, and assets on a per-project basis, rather than installing them across all projects for a specific machine or device. It uses a global cache to store downloaded package metadata and contents. Once installed in a project, Unity treats package assets just like any other asset in the project, except that these assets are stored inside the package folder and are immutableYou cannot change the contents of an immutable (read-only) package. This is the opposite of mutable. Most packages are immutable, including packages downloaded from the package registry or by Git URL.
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. You can only permanently change content from Local and Embedded package sources.

Package states and lifecycle

After developing a package to the point where it is ready for Unity users to test it and provide feedback, a package enters the Preview state. Packages in this state usually appear in the Unity Editor with the preview label and often use preview as part of their version.

Unity’s release management only grants a package the verified status after it passes several testing stages and validation procedures, which also include checks for appropriate documentation, changelog, and license files. Packages in this state may appear in the Unity Editor with the 2019.4 verified label and never use preview as part of their version.

If the package developer introduces changing or breaking changes to a package (either by introducing a change that breaks an API or affects its usage), that package returns to the Preview state and the Package Manager does not suggest the new preview package as a possible update for the verified version.

Package lifecycle with Unity Package Manager
Package lifecycle with Unity Package Manager

These states indicate where the package is in the development cycle:

State Description
Custom The package developer creates the package. Usually this corresponds to having the package embedded in the developer’s project.
Preview When the package is ready for testing, the Unity package developer makes sure there is some basic test coverage, and that the package passes testing with the package validation suite. The package also needs at least preliminary documentation, an updated changelog, and licensing before the Unity package developer can apply to publish the package on Unity’s official package registry.

At this point, the package is available for testing, so that any eligible Unity user can provide the developer with feedback. Most packages are available to any Unity users, although a few packages are limited to Unity IDs with specific entitlements.

Preview packages can go through many changes before they are ready to be verified for a specific version of Unity. At some point in the future, they might pass the verification requirements; however, they might also be deprecated instead. Because there is no guarantee for future support, you should not use preview packages in production. For a list of preview packages available for this version, see Preview packagesA preview package is in development and not yet ready for production. A package in preview might be at any stage of development, from the initial stages to near completion.
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Verified The package has undergone rigorous testing and has been verified to work safely with this specific version of Unity, and all other packages verified for the same version. This state is only for packages that Unity develops internally. Contact third-party package developers to ask about their specific processes.

When a package is verified for a version of Unity, Unity guarantees to support that verified package for the duration of that version of Unity, throughout the period of long-term support, and does not allow any major or minor updates (that is, changes that break or change an API). With each new version of Unity, each verified package must go through the verification process again before the Package Manager considers it verified.

To see a list of packages that are verified for this version of Unity, see Verified packagesWhen a package passes release cycle testing for a specific version of Unity, it receives the Verified For designation. This means that these packages are guaranteed to work with the designated version of Unity.
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Note: Some packages are neither verified nor preview packages. In many cases, this is because a package was verified in a previous version of Unity but has not completed the verification process yet for the current version. For a few packages, they were published to the Unity Package Manager registry because they were considered safe to use, but were never verified. In both of these cases, these packages are safe to use in production.

The Package Manager window displays a tagA reference word which you can assign to one or more GameObjects to help you identify GameObjects for scripting purposes. For example, you might define and “Edible” Tag for any item the player can eat in your game. More info
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that corresponds to some of these states.

Package sources

Sources describe where the package came from:

Source Description
Registry The Unity Package Manager downloads most packages from a package registry server into a global cache on your computer as you request them. These packages are immutable, so you can use them in your project, but you cannot modify them or change their package manifests.
Built-in These packages allow you to enable or disable Unity features (for example, Terrain Physics, Animation, etc.). They are immutable. For more information, see Built-in packagesBuilt-in packages allow users to toggle Unity features on or off through the Package Manager. Enabling or disabling a package reduces the run-time build size. For example, most projects don’t use the legacy Particle System. By removing the abstracted package of this feature, the related code and resources are not part of the final built product. Typically, these packages contain only the package manifest and are bundled with Unity (rather than available on the package registry).
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Embedded Any package stored inside your project folder is embedded. This source corresponds with the Custom state because you typically put all the scripts, libraries, samples, and other assets your new package needs in a folder under your project folder when you begin development on a custom package.
Local You can install a package from any folder on your computer (for example, if you have cloned a development repository locally).
Tarball (local) You can install a package from a tarball file on your computer. The Package Manager extracts the package from the tarball and stores it in the cache. However, these packages are immutable, unlike installations from a local folder.
Git The Package Manager installs Git-based packages directly from a Git repository instead of from the package registry server.

To edit the package manifest for a package, see Inspecting packages.

The Package Manager window displays a tag that corresponds to some of these sources. For more information, see Tags.

Note: The Package Manager stores packages that you download from the Asset StoreA growing library of free and commercial assets created by Unity and members of the community. Offers a wide variety of assets, from textures, models and animations to whole project examples, tutorials and Editor extensions. More info
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in a cache which is separate from the global cache. See Asset Store packages for more information.

Unity's Package Manager
Dependency and resolution
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