diff --git a/.vortex/installer/tests/Functional/Prompts/Handlers/AbstractHandlerProcessTestCase.php b/.vortex/installer/tests/Functional/Prompts/Handlers/AbstractHandlerProcessTestCase.php
index 059132549..7ba62eac4 100644
--- a/.vortex/installer/tests/Functional/Prompts/Handlers/AbstractHandlerProcessTestCase.php
+++ b/.vortex/installer/tests/Functional/Prompts/Handlers/AbstractHandlerProcessTestCase.php
@@ -36,11 +36,7 @@ abstract class AbstractHandlerProcessTestCase extends FunctionalTestCase {
   protected function setUp(): void {
     parent::setUp();
 
-    static::envUnsetPrefix('VORTEX_');
-    static::envUnsetPrefix('DRUPAL_');
-    static::envUnsetPrefix('LAGOON_');
-    static::envUnset('WEBROOT');
-    static::envUnset('TZ');
+    static::envUnsetProjectVars();
 
     static::applicationInitFromCommand(InstallCommand::class);
 
diff --git a/.vortex/installer/tests/Unit/Prompts/Handlers/AbstractHandlerDiscoveryTestCase.php b/.vortex/installer/tests/Unit/Prompts/Handlers/AbstractHandlerDiscoveryTestCase.php
index 93e0116ab..1f0ae785f 100644
--- a/.vortex/installer/tests/Unit/Prompts/Handlers/AbstractHandlerDiscoveryTestCase.php
+++ b/.vortex/installer/tests/Unit/Prompts/Handlers/AbstractHandlerDiscoveryTestCase.php
@@ -68,6 +68,8 @@ abstract class AbstractHandlerDiscoveryTestCase extends UnitTestCase {
   protected function setUp(): void {
     parent::setUp();
 
+    static::envUnsetProjectVars();
+
     static::tuiSetUp();
 
     static::$sut = File::mkdir(static::$sut . DIRECTORY_SEPARATOR . 'myproject');
diff --git a/.vortex/installer/tests/Unit/SelfTest.php b/.vortex/installer/tests/Unit/SelfTest.php
index b4ba38c22..b40a37251 100644
--- a/.vortex/installer/tests/Unit/SelfTest.php
+++ b/.vortex/installer/tests/Unit/SelfTest.php
@@ -10,6 +10,33 @@
 #[CoversClass(UnitTestCase::class)]
 class SelfTest extends UnitTestCase {
 
+  const AMBIENT_VAR = 'VORTEX_TEST_AMBIENT_VAR';
+
+  const LEAKED_VAR = 'VORTEX_TEST_LEAKED_VAR';
+
+  /**
+   * @var string|false
+   */
+  protected static $ambientOriginal;
+
+  public static function setUpBeforeClass(): void {
+    parent::setUpBeforeClass();
+
+    static::$ambientOriginal = getenv(self::AMBIENT_VAR);
+    putenv(self::AMBIENT_VAR . '=ambient');
+  }
+
+  public static function tearDownAfterClass(): void {
+    if (static::$ambientOriginal === FALSE) {
+      putenv(self::AMBIENT_VAR);
+    }
+    else {
+      putenv(self::AMBIENT_VAR . '=' . static::$ambientOriginal);
+    }
+
+    parent::tearDownAfterClass();
+  }
+
   public function testEnvCleanup1SetVariables(): void {
     static::envSet('VORTEX_TEST_VAR_1', 'value1');
     static::envSet('VORTEX_TEST_VAR_2', 'value2');
@@ -34,4 +61,24 @@ public function testEnvCleanup2VerifyCleanup(): void {
     $this->assertFalse(getenv('VORTEX_TEST_VAR_4'), 'VORTEX_TEST_VAR_4 should be cleaned up after previous test');
   }
 
+  public function testEnvRestore1WriteRawValues(): void {
+    // envSet() is bypassed here to reproduce what production code does when it
+    // writes the environment directly.
+    putenv(self::LEAKED_VAR . '=leaked');
+    putenv(self::AMBIENT_VAR . '=changed');
+    $_ENV[self::LEAKED_VAR] = 'leaked';
+    $_SERVER[self::LEAKED_VAR] = 'leaked';
+
+    $this->assertSame('leaked', getenv(self::LEAKED_VAR));
+    $this->assertSame('changed', getenv(self::AMBIENT_VAR));
+  }
+
+  #[Depends('testEnvRestore1WriteRawValues')]
+  public function testEnvRestore2VerifyRestored(): void {
+    $this->assertFalse(getenv(self::LEAKED_VAR), 'A variable added by the previous test should not outlive it');
+    $this->assertSame('ambient', getenv(self::AMBIENT_VAR), 'A variable changed by the previous test should be back to its original value');
+    $this->assertArrayNotHasKey(self::LEAKED_VAR, $_ENV);
+    $this->assertArrayNotHasKey(self::LEAKED_VAR, $_SERVER);
+  }
+
 }
diff --git a/.vortex/installer/tests/Unit/UnitTestCase.php b/.vortex/installer/tests/Unit/UnitTestCase.php
index 0ed2ef6a2..f4c93550e 100644
--- a/.vortex/installer/tests/Unit/UnitTestCase.php
+++ b/.vortex/installer/tests/Unit/UnitTestCase.php
@@ -25,10 +25,35 @@ abstract class UnitTestCase extends UpstreamUnitTestCase {
   use SnapshotTrait;
   use EnvTrait;
 
+  /**
+   * The process environment before the test ran.
+   *
+   * @var array<string,string>
+   */
+  protected array $processEnvBackup;
+
+  /**
+   * The $_ENV superglobal before the test ran.
+   *
+   * @var array<string,mixed>
+   */
+  protected array $globalEnvBackup;
+
+  /**
+   * The $_SERVER superglobal before the test ran.
+   *
+   * @var array<string,mixed>
+   */
+  protected array $globalServerBackup;
+
   /**
    * {@inheritdoc}
    */
   protected function setUp(): void {
+    $this->processEnvBackup = getenv();
+    $this->globalEnvBackup = $_ENV;
+    $this->globalServerBackup = $_SERVER;
+
     $cwd = getcwd();
     if ($cwd === FALSE) {
       throw new \RuntimeException('Failed to determine current working directory.');
@@ -43,9 +68,49 @@ protected function setUp(): void {
    */
   protected function tearDown(): void {
     static::envReset();
+    $this->envRestore();
     parent::tearDown();
   }
 
+  /**
+   * Restore the process environment captured before the test ran.
+   *
+   * EnvTrait::envReset() only reverses names recorded by envSet(). Code under
+   * test can call putenv() directly, and those values would otherwise be read
+   * by every later test in the same process.
+   */
+  protected function envRestore(): void {
+    foreach (array_keys(getenv()) as $name) {
+      if (!array_key_exists($name, $this->processEnvBackup)) {
+        putenv($name);
+      }
+    }
+
+    foreach ($this->processEnvBackup as $name => $value) {
+      if (getenv($name) !== $value) {
+        putenv($name . '=' . $value);
+      }
+    }
+
+    $_ENV = $this->globalEnvBackup;
+    $_SERVER = $this->globalServerBackup;
+  }
+
+  /**
+   * Unset the environment variables that a project's .env file defines.
+   *
+   * Handlers read the environment before the project's .env file, so a
+   * variable exported by the shell running the suite would win over the
+   * fixture.
+   */
+  protected static function envUnsetProjectVars(): void {
+    static::envUnsetPrefix('VORTEX_');
+    static::envUnsetPrefix('DRUPAL_');
+    static::envUnsetPrefix('LAGOON_');
+    static::envUnset('WEBROOT');
+    static::envUnset('TZ');
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/.vortex/installer/tests/Unit/Utils/EnvTest.php b/.vortex/installer/tests/Unit/Utils/EnvTest.php
index b101f52b5..3426cfb6e 100644
--- a/.vortex/installer/tests/Unit/Utils/EnvTest.php
+++ b/.vortex/installer/tests/Unit/Utils/EnvTest.php
@@ -15,30 +15,6 @@
 #[RunTestsInSeparateProcesses]
 class EnvTest extends UnitTestCase {
 
-  /**
-   * @var array
-   */
-  protected $backupServer;
-
-  /**
-   * @var array
-   */
-  protected $backupEnv;
-
-  protected function setUp(): void {
-    $this->backupEnv = $GLOBALS['_ENV'];
-    $this->backupServer = $GLOBALS['_SERVER'];
-
-    parent::setUp();
-  }
-
-  protected function tearDown(): void {
-    $GLOBALS['_ENV'] = $this->backupEnv;
-    $GLOBALS['_SERVER'] = $this->backupServer;
-
-    parent::tearDown();
-  }
-
   #[DataProvider('dataProviderGet')]
   public function testGet(string $name, string $value, ?string $default, ?string $expected): void {
     static::envSet($name, $expected);