diff --git a/src/Shared/Infrastructure/Mcp/Serializer.php b/src/Shared/Infrastructure/Mcp/Serializer.php
index 21e3fbc..6f2dd65 100644
--- a/src/Shared/Infrastructure/Mcp/Serializer.php
+++ b/src/Shared/Infrastructure/Mcp/Serializer.php
@@ -23,11 +23,13 @@ use App\Module\ProjectManagement\Domain\Entity\TaskPriority;
 use App\Module\ProjectManagement\Domain\Entity\TaskStatus;
 use App\Module\ProjectManagement\Domain\Entity\TaskTag;
 use App\Module\TimeTracking\Domain\Entity\TimeEntry;
+use App\Shared\Domain\Contract\ClientInterface;
 use App\Shared\Domain\Contract\ProjectInterface;
 use App\Shared\Domain\Contract\TaskInterface;
 use App\Shared\Domain\Contract\TaskTagInterface;
 use App\Shared\Domain\Contract\UserInterface;
 use Doctrine\Common\Collections\Collection;
+use Doctrine\ORM\EntityNotFoundException;
 
 /**
  * Shared serialization helpers for MCP tools.
@@ -59,11 +61,8 @@ final class Serializer
             'name'        => $project->getName(),
             'description' => $project->getDescription(),
             'color'       => $project->getColor(),
-            'client'      => $project->getClient() ? [
-                'id'   => $project->getClient()->getId(),
-                'name' => $project->getClient()->getName(),
-            ] : null,
-            'archived' => $project->isArchived(),
+            'client'      => self::clientRef($project->getClient()),
+            'archived'    => $project->isArchived(),
         ];
     }
 
@@ -516,4 +515,31 @@ final class Serializer
             'initialLeaveBalance'  => $u->getInitialLeaveBalance(),
         ];
     }
+
+    /**
+     * Safely serialize a project's client reference.
+     *
+     * The client association uses ON DELETE SET NULL, but a stale row may leave
+     * a dangling foreign key (e.g. data imported with the constraint disabled).
+     * In that case Doctrine returns an uninitialized proxy whose hydration
+     * throws EntityNotFoundException; we treat such a reference as absent rather
+     * than letting it crash the whole tool.
+     *
+     * @return null|array{id: ?int, name: ?string}
+     */
+    private static function clientRef(?ClientInterface $client): ?array
+    {
+        if (null === $client) {
+            return null;
+        }
+
+        try {
+            return [
+                'id'   => $client->getId(),
+                'name' => $client->getName(),
+            ];
+        } catch (EntityNotFoundException) {
+            return null;
+        }
+    }
 }