diff --git a/codex-rs/core/src/guardian/review_session.rs b/codex-rs/core/src/guardian/review_session.rs
index 67f733e0bae2..a62d0f553a9d 100644
--- a/codex-rs/core/src/guardian/review_session.rs
+++ b/codex-rs/core/src/guardian/review_session.rs
@@ -7,6 +7,7 @@ use std::time::Duration;
 use anyhow::anyhow;
 use codex_analytics::GuardianReviewAnalyticsResult;
 use codex_analytics::GuardianReviewSessionKind;
+use codex_protocol::ThreadId;
 use codex_protocol::config_types::AutoCompactTokenLimitScope;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
@@ -20,6 +21,7 @@ use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::TokenUsage;
 use serde_json::Value;
@@ -183,6 +185,20 @@ impl GuardianReviewSessionReuseKey {
     }
 }
 
+pub(crate) fn prompt_cache_key_override_for_review_session(
+    session_source: &SessionSource,
+    parent_thread_id: Option<ThreadId>,
+) -> Option<String> {
+    let SessionSource::SubAgent(SubAgentSource::Other(name)) = session_source else {
+        return None;
+    };
+    if name != GUARDIAN_REVIEWER_NAME {
+        return None;
+    }
+    let parent_thread_id = parent_thread_id?;
+    Some(format!("guardian:{parent_thread_id}"))
+}
+
 impl GuardianReviewSession {
     async fn shutdown(&self) {
         self.cancel_token.cancel();
@@ -1160,6 +1176,43 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn guardian_prompt_cache_key_is_scoped_to_parent_thread() {
+        let session_source =
+            SessionSource::SubAgent(SubAgentSource::Other(GUARDIAN_REVIEWER_NAME.to_string()));
+        let parent_thread_id = ThreadId::new();
+        let key =
+            prompt_cache_key_override_for_review_session(&session_source, Some(parent_thread_id))
+                .expect("guardian prompt cache key");
+
+        assert_eq!(key, format!("guardian:{parent_thread_id}"));
+        assert!(
+            key.len() <= 64,
+            "guardian prompt cache key should fit the Responses API limit"
+        );
+        assert_eq!(
+            key,
+            prompt_cache_key_override_for_review_session(&session_source, Some(parent_thread_id))
+                .expect("same guardian prompt cache key")
+        );
+        assert_ne!(
+            key,
+            prompt_cache_key_override_for_review_session(&session_source, Some(ThreadId::new()))
+                .expect("different parent guardian prompt cache key")
+        );
+        assert_eq!(
+            None,
+            prompt_cache_key_override_for_review_session(
+                &SessionSource::Cli,
+                Some(parent_thread_id)
+            )
+        );
+        assert_eq!(
+            None,
+            prompt_cache_key_override_for_review_session(&session_source, None)
+        );
+    }
+
     #[tokio::test]
     async fn guardian_review_session_compact_scope_change_invalidates_cached_session() {
         let parent_config = crate::config::test_config().await;