- If the `versionCheckPhase` (the phase defined by [`versionCheckHook`](#versioncheckhook)) fails, it triggers a failure which can't be ignored if you use the package, or if you find out about it in a [`nixpkgs-review`](https://github.com/Mic92/nixpkgs-review) report.
- Sometimes packages become silently broken - meaning they fail to launch but their build passes because they don't perform any tests in the `checkPhase`. If you use this tool infrequently, such a silent breakage may rot in your system / profile configuration, and you will not notice the failure until you will want to use this package. Testing such basic functionality ensures you have to deal with the failure when you update your system / profile.
- When you open a PR, [ofborg](https://github.com/NixOS/ofborg)'s CI _will_ run `passthru.tests` of [packages that are directly changed by your PR (according to your commits' messages)](https://github.com/NixOS/ofborg?tab=readme-ov-file#automatic-building), but if you'd want to use the [`@ofborg build`](https://github.com/NixOS/ofborg?tab=readme-ov-file#build) command for dependent packages, you won't have to specify in addition the `.tests` attribute of the packages you want to build, and nobody will be able to avoid these tests.

<!-- NOTE(@fricklerhandwerk): one may argue whether that testing guide should rather be in the user's manual -->
For more on how to write and run package tests for Nixpkgs, see the [testing section in the package contributor guide](https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#package-tests).

#### NixOS tests {#var-passthru-tests-nixos}
[]{#var-meta-tests-nixos} <!-- legacy anchor -->

Tests written for NixOS are available as the `nixosTests` argument to package recipes.
For instance, the [OpenSMTPD derivation](https://search.nixos.org/packages?show=opensmtpd) includes lines similar to:

```nix
{ nixosTests, ... }:
{
  # ...
  passthru.tests = {
    basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
  };
}
```

NixOS tests run in a virtual machine (VM), so they are slower than regular package tests.
For more information see the NixOS manual on [NixOS module tests](https://nixos.org/manual/nixos/stable/#sec-nixos-tests).

### `passthru.updateScript` {#var-passthru-updateScript}
<!-- legacy anchors -->
[]{#var-passthru-updateScript-command}
[]{#var-passthru-updateScript-set-command}
[]{#var-passthru-updateScript-set-attrPath}
[]{#var-passthru-updateScript-set-supportedFeatures}
[]{#var-passthru-updateScript-env-UPDATE_NIX_NAME}
[]{#var-passthru-updateScript-env-UPDATE_NIX_PNAME}
[]{#var-passthru-updateScript-env-UPDATE_NIX_OLD_VERSION}
[]{#var-passthru-updateScript-env-UPDATE_NIX_ATTR_PATH}
[]{#var-passthru-updateScript-execution}
[]{#var-passthru-updateScript-supported-features}
[]{#var-passthru-updateScript-commit}
[]{#var-passthru-updateScript-commit-attrPath}
[]{#var-passthru-updateScript-commit-oldVersion}
[]{#var-passthru-updateScript-commit-newVersion}
[]{#var-passthru-updateScript-commit-files}
[]{#var-passthru-updateScript-commit-commitBody}
[]{#var-passthru-updateScript-commit-commitMessage}
[]{#var-passthru-updateScript-example-commit}

Nixpkgs tries to automatically update all packages that have an `passthru.updateScript` attribute.
See the [section on automatic package updates in the package contributor guide](https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#automatic-package-updates) for details.