diff --git a/eppo_core/src/ufc/models.rs b/eppo_core/src/ufc/models.rs
index 2a024a94..a0fbfd8e 100644
--- a/eppo_core/src/ufc/models.rs
+++ b/eppo_core/src/ufc/models.rs
@@ -18,7 +18,8 @@ pub type Timestamp = chrono::DateTime<chrono::Utc>;
 pub(crate) struct UniversalFlagConfigWire {
     /// When configuration was last updated.
     pub created_at: Timestamp,
-    pub format: AssignmentFormat,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub format: Option<AssignmentFormat>,
     /// Environment this configuration belongs to.
     pub environment: Environment,
     /// Flags configuration.