diff --git a/codex-rs/core/src/conversation_history.rs b/codex-rs/core/src/conversation_history.rs
index 8cafbcb1e0..4ddb51957d 100644
--- a/codex-rs/core/src/conversation_history.rs
+++ b/codex-rs/core/src/conversation_history.rs
@@ -146,7 +146,7 @@ impl ConversationHistory {
             }
         }
 
-        let prior_total = info.total_token_usage.total_tokens;
+        let prior_total = info.last_token_usage.total_tokens;
         let combined_tokens = prior_total.saturating_add(input_tokens);
         let threshold = context_window * 95 / 100;
         if combined_tokens > threshold {
diff --git a/codex-rs/protocol/src/protocol.rs b/codex-rs/protocol/src/protocol.rs
index 105f028049..add8b7d6aa 100644
--- a/codex-rs/protocol/src/protocol.rs
+++ b/codex-rs/protocol/src/protocol.rs
@@ -577,7 +577,9 @@ pub struct TokenUsage {
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct TokenUsageInfo {
+    /// The total token usage for the session. accumulated from all turns.
     pub total_token_usage: TokenUsage,
+    /// The token usage for the last turn. Received from the API.
     pub last_token_usage: TokenUsage,
     #[ts(type = "number | null")]
     pub model_context_window: Option<i64>,