diff --git a/src/client.rs b/src/client.rs
index dada5581..ec37f265 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -250,7 +250,8 @@ impl SessionInfoUpdate {
 #[non_exhaustive]
 pub struct UsageUpdate {
     /// Tokens currently in context.
-    pub used: u64,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub used: Option<u64>,
     /// Total context window size in tokens.
     pub size: u64,
     /// Cumulative session cost (optional).
@@ -268,9 +269,9 @@ pub struct UsageUpdate {
 #[cfg(feature = "unstable_session_usage")]
 impl UsageUpdate {
     #[must_use]
-    pub fn new(used: u64, size: u64) -> Self {
+    pub fn new(used: impl IntoOption<u64>, size: u64) -> Self {
         Self {
-            used,
+            used: used.into_option(),
             size,
             cost: None,
             meta: None,