diff --git a/shared/lib/SystemStore/SystemStoreException.php b/shared/lib/SystemStore/SystemStoreException.php
index f9ef962..1bcb94a 100644
--- a/shared/lib/SystemStore/SystemStoreException.php
+++ b/shared/lib/SystemStore/SystemStoreException.php
@@ -9,6 +9,6 @@ use RuntimeException;
 /**
  * Raised when a tenant system store cannot be resolved or trusted.
  */
-final class SystemStoreException extends RuntimeException
+class SystemStoreException extends RuntimeException
 {
 }
diff --git a/shared/lib/SystemStore/WriteConflictException.php b/shared/lib/SystemStore/WriteConflictException.php
new file mode 100644
index 0000000..10b70ad
--- /dev/null
+++ b/shared/lib/SystemStore/WriteConflictException.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace KTXF\SystemStore;
+
+/** Raised when an atomic write or delete precondition is not satisfied. */
+final class WriteConflictException extends SystemStoreException
+{
+}