diff --git a/app/config.php b/app/config.php
index 8125e652..41e140cd 100644
--- a/app/config.php
+++ b/app/config.php
@@ -30,6 +30,7 @@
 use PhpSchool\PhpWorkshop\Command\PrintCommand;
 use PhpSchool\PhpWorkshop\Command\RunCommand;
 use PhpSchool\PhpWorkshop\Command\VerifyCommand;
+use PhpSchool\PhpWorkshop\Command\WarmCacheCommand;
 use PhpSchool\PhpWorkshop\CommandDefinition;
 use PhpSchool\PhpWorkshop\CommandRouter;
 use PhpSchool\PhpWorkshop\Event\EventDispatcher;
@@ -51,6 +52,7 @@
 use PhpSchool\PhpWorkshop\Listener\InitialCodeListener;
 use PhpSchool\PhpWorkshop\Listener\OutputRunInfoListener;
 use PhpSchool\PhpWorkshop\Listener\PrepareSolutionListener;
+use PhpSchool\PhpWorkshop\ReferencePackages;
 use PhpSchool\PhpWorkshop\Listener\RealPathListener;
 use PhpSchool\PhpWorkshop\Listener\RemoveStudentSymlinksListener;
 use PhpSchool\PhpWorkshop\Listener\SelfCheckListener;
@@ -160,6 +162,7 @@
                 new CommandDefinition('verify', [], VerifyCommand::class),
                 new CommandDefinition('run', [], RunCommand::class),
                 new CommandDefinition('credits', [], CreditsCommand::class),
+                new CommandDefinition('warm-cache', [], WarmCacheCommand::class),
             ],
             'menu',
             $c->get(EventDispatcher::class),
@@ -221,6 +224,10 @@
     },
 
     //commands
+    WarmCacheCommand::class => function (ContainerInterface $c) {
+        return new WarmCacheCommand($c->get(ReferencePackages::class));
+    },
+
     MenuCommand::class => function (ContainerInterface $c) {
         return new MenuCommand($c->get('menu'));
     },
@@ -272,6 +279,10 @@
         );
     },
 
+    ReferencePackages::class => function (ContainerInterface $c) {
+        return new ReferencePackages($c->get(ExerciseRepository::class), $c->get(Sandbox::class));
+    },
+
     //Listeners
     InitialCodeListener::class => function (ContainerInterface $c) {
         return new InitialCodeListener($c->get('currentWorkingDirectory'), $c->get(LoggerInterface::class));
diff --git a/src/Command/WarmCacheCommand.php b/src/Command/WarmCacheCommand.php
new file mode 100644
index 00000000..c150dfd1
--- /dev/null
+++ b/src/Command/WarmCacheCommand.php
@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpSchool\PhpWorkshop\Command;
+
+use PhpSchool\PhpWorkshop\ReferencePackages;
+
+/**
+ * Run by the workshop's composer post-install-cmd, so verifies can install reference packages offline.
+ */
+class WarmCacheCommand
+{
+    public function __construct(private ReferencePackages $referencePackages) {}
+
+    public function __invoke(): int
+    {
+        $this->referencePackages->warm();
+
+        return 0;
+    }
+}
diff --git a/src/Listener/PrepareSolutionListener.php b/src/Listener/PrepareSolutionListener.php
index 65d3a396..90df8463 100644
--- a/src/Listener/PrepareSolutionListener.php
+++ b/src/Listener/PrepareSolutionListener.php
@@ -9,9 +9,10 @@
 use PhpSchool\PhpWorkshop\Exception\RuntimeException;
 use PhpSchool\PhpWorkshop\Process\Sandbox;
 use PhpSchool\PhpWorkshop\Process\ProcessInput;
+use PhpSchool\PhpWorkshop\Process\SandboxExecution;
 
 /**
- * Listener to install composer deps for an exercise solution
+ * Listener to install composer deps for an exercise solution, offline when ReferencePackages has warmed the cache
  */
 class PrepareSolutionListener
 {
@@ -39,16 +40,33 @@ public function __invoke(ExerciseRunnerEvent $event): void
 
         //prepare composer deps
         //only install if vendor folder not available
-        if (!file_exists(sprintf('%s/vendor', $event->getContext()->getReferenceExecutionDirectory()))) {
-            $execution = $this->sandbox->create(
-                new ProcessInput('composer', ['install', '--no-interaction'], $event->getContext()->getReferenceExecutionDirectory(), [], []),
-            );
-
-            try {
-                $execution->mustRun();
-            } catch (\Symfony\Component\Process\Exception\RuntimeException $e) {
-                throw new RuntimeException('Composer dependencies could not be installed', 0, $e);
-            }
+        if (file_exists(sprintf('%s/vendor', $event->getContext()->getReferenceExecutionDirectory()))) {
+            return;
+        }
+
+        //offline first, from the cache ReferencePackages warms. Without it, eg. a workshop run from a clone, download
+        $offline = $this->install($event, ['COMPOSER_DISABLE_NETWORK' => '1']);
+        $offline->start();
+        $offline->wait();
+
+        if ($offline->isSuccessful()) {
+            return;
+        }
+
+        try {
+            $this->install($event, [])->mustRun();
+        } catch (\Symfony\Component\Process\Exception\RuntimeException $e) {
+            throw new RuntimeException('Composer dependencies could not be installed', 0, $e);
         }
     }
+
+    /**
+     * @param array<string, string> $env
+     */
+    private function install(ExerciseRunnerEvent $event, array $env): SandboxExecution
+    {
+        return $this->sandbox->create(
+            new ProcessInput('composer', ['install', '--no-interaction'], $event->getContext()->getReferenceExecutionDirectory(), $env, []),
+        );
+    }
 }
diff --git a/src/ReferencePackages.php b/src/ReferencePackages.php
new file mode 100644
index 00000000..85c0a00a
--- /dev/null
+++ b/src/ReferencePackages.php
@@ -0,0 +1,53 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpSchool\PhpWorkshop;
+
+use PhpSchool\PhpWorkshop\Exception\RuntimeException;
+use PhpSchool\PhpWorkshop\Exercise\ProvidesSolution;
+use PhpSchool\PhpWorkshop\Process\ProcessInput;
+use PhpSchool\PhpWorkshop\Process\Sandbox;
+use Symfony\Component\Filesystem\Filesystem;
+
+/**
+ * Locks and caches the packages of each reference solution, so a verify can install them with no network.
+ * Run it on install/deploy, like the runtime image build.
+ */
+class ReferencePackages
+{
+    private Filesystem $filesystem;
+
+    public function __construct(private ExerciseRepository $exerciseRepository, private Sandbox $sandbox)
+    {
+        $this->filesystem = new Filesystem();
+    }
+
+    public function warm(): void
+    {
+        foreach ($this->exerciseRepository->findAll() as $exercise) {
+            if (!$exercise instanceof ProvidesSolution || !$exercise->getSolution()->hasComposerFile()) {
+                continue;
+            }
+
+            $directory = $exercise->getSolution()->getBaseDirectory();
+
+            //an old vendor folder would mean nothing is downloaded, so nothing is cached
+            $this->filesystem->remove($directory . '/vendor');
+
+            //keeps a committed composer.lock, or writes one next to the solution. It is copied with it on each verify
+            $execution = $this->sandbox->create(
+                new ProcessInput('composer', ['install', '--no-interaction'], $directory, [], []),
+            );
+
+            try {
+                $execution->mustRun();
+            } catch (\Symfony\Component\Process\Exception\RuntimeException $e) {
+                throw new RuntimeException(sprintf('Composer packages for "%s" could not be installed', $exercise->getName()), 0, $e);
+            }
+
+            //verifies install from the cache, this copy is never used
+            $this->filesystem->remove($directory . '/vendor');
+        }
+    }
+}
diff --git a/src/Solution/DirectorySolution.php b/src/Solution/DirectorySolution.php
index 5f1425f3..f96576cb 100644
--- a/src/Solution/DirectorySolution.php
+++ b/src/Solution/DirectorySolution.php
@@ -88,7 +88,7 @@ public function __construct(string $directory, string $entryPoint, array $exclus
      */
     public static function fromDirectory(string $directory, array $exclusions = [], $entryPoint = 'solution.php'): self
     {
-        return new self($directory, $entryPoint, array_merge($exclusions, ['composer.lock', 'vendor']));
+        return new self($directory, $entryPoint, array_merge($exclusions, ['vendor']));
     }
 
     /**
diff --git a/test/Command/WarmCacheCommandTest.php b/test/Command/WarmCacheCommandTest.php
new file mode 100644
index 00000000..de759fa6
--- /dev/null
+++ b/test/Command/WarmCacheCommandTest.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpSchool\PhpWorkshopTest\Command;
+
+use PhpSchool\PhpWorkshop\Command\WarmCacheCommand;
+use PhpSchool\PhpWorkshop\ReferencePackages;
+use PHPUnit\Framework\TestCase;
+
+class WarmCacheCommandTest extends TestCase
+{
+    public function testWarmsTheReferencePackages(): void
+    {
+        $referencePackages = $this->createMock(ReferencePackages::class);
+        $referencePackages->expects($this->once())->method('warm');
+
+        $this->assertSame(0, (new WarmCacheCommand($referencePackages))->__invoke());
+    }
+}
diff --git a/test/ReferencePackagesTest.php b/test/ReferencePackagesTest.php
new file mode 100644
index 00000000..f0294ff7
--- /dev/null
+++ b/test/ReferencePackagesTest.php
@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpSchool\PhpWorkshopTest;
+
+use PhpSchool\PhpWorkshop\ExerciseRepository;
+use PhpSchool\PhpWorkshop\Process\HostSandbox;
+use PhpSchool\PhpWorkshop\ReferencePackages;
+use PhpSchool\PhpWorkshop\Solution\DirectorySolution;
+use PhpSchool\PhpWorkshop\Utils\System;
+use PhpSchool\PhpWorkshopTest\Asset\CliExerciseImpl;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Filesystem\Filesystem;
+
+class ReferencePackagesTest extends TestCase
+{
+    public function testWarmLocksEachSolutionWithPackages(): void
+    {
+        $directory = System::randomTempDir();
+        mkdir($directory, 0777, true);
+        file_put_contents($directory . '/solution.php', '<?php');
+        file_put_contents($directory . '/composer.json', json_encode(['require' => ['psr/log' => '^3.0']]));
+
+        $withPackages = new CliExerciseImpl();
+        $withPackages->setSolution(DirectorySolution::fromDirectory($directory));
+
+        (new ReferencePackages(new ExerciseRepository([$withPackages]), new HostSandbox()))->warm();
+
+        $this->assertFileExists($directory . '/composer.lock');
+        $this->assertDirectoryDoesNotExist($directory . '/vendor');
+        $this->assertContains('composer.lock', array_map(
+            fn($file) => $file->getRelativePath(),
+            DirectorySolution::fromDirectory($directory)->getFiles(),
+        ));
+
+        (new Filesystem())->remove($directory);
+    }
+}