diff --git a/codex-rs/core/src/guardian/review_session.rs b/codex-rs/core/src/guardian/review_session.rs
index a4078c720c..640e0d4f29 100644
--- a/codex-rs/core/src/guardian/review_session.rs
+++ b/codex-rs/core/src/guardian/review_session.rs
@@ -30,6 +30,7 @@ use codex_guardian_reviewer::ConversationState;
 use codex_guardian_reviewer::ReviewModel;
 use codex_guardian_reviewer::ReviewSessionResult;
 use codex_guardian_reviewer::SessionDisposition;
+use codex_guardian_reviewer::token_usage_delta;
 use codex_history::InitialHistory;
 use codex_history::RolloutItem;
 use codex_protocol::ThreadId;
@@ -49,7 +50,6 @@ use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::SubAgentSource;
-use codex_protocol::protocol::TokenUsage;
 use futures::future::BoxFuture;
 use serde_json::Value;
 use tokio::sync::Mutex;
@@ -161,20 +161,6 @@ fn had_prior_review_context(prompt_mode: &GuardianPromptMode) -> bool {
     matches!(prompt_mode, GuardianPromptMode::Delta { .. })
 }
 
-fn token_usage_delta(start: &TokenUsage, end: &TokenUsage) -> TokenUsage {
-    TokenUsage {
-        input_tokens: (end.input_tokens - start.input_tokens).max(0),
-        cached_input_tokens: (end.cached_input_tokens - start.cached_input_tokens).max(0),
-        cache_write_input_tokens: (end.cache_write_input_tokens - start.cache_write_input_tokens)
-            .max(0),
-        output_tokens: (end.output_tokens - start.output_tokens).max(0),
-        reasoning_output_tokens: (end.reasoning_output_tokens - start.reasoning_output_tokens)
-            .max(0),
-        total_tokens: (end.total_tokens - start.total_tokens).max(0),
-        codex_rollout_budget_units: None,
-    }
-}
-
 type GuardianReviewForkSnapshot = ConversationCheckpoint<GuardianReviewHistory>;
 
 /// Host-owned history and admitted evidence used to seed a private reviewer fork.
diff --git a/codex-rs/core/src/guardian/review_session_tests.rs b/codex-rs/core/src/guardian/review_session_tests.rs
index 6e71ad6b91..93c913764c 100644
--- a/codex-rs/core/src/guardian/review_session_tests.rs
+++ b/codex-rs/core/src/guardian/review_session_tests.rs
@@ -673,41 +673,6 @@ async fn guardian_review_session_config_resolves_policy_and_template(
     );
 }
 
-#[test]
-fn token_usage_delta_never_reports_negative_usage() {
-    let start = TokenUsage {
-        input_tokens: 10,
-        cached_input_tokens: 8,
-        cache_write_input_tokens: 8,
-        output_tokens: 6,
-        reasoning_output_tokens: 4,
-        total_tokens: 28,
-        codex_rollout_budget_units: None,
-    };
-    let end = TokenUsage {
-        input_tokens: 15,
-        cached_input_tokens: 7,
-        cache_write_input_tokens: 7,
-        output_tokens: 10,
-        reasoning_output_tokens: 2,
-        total_tokens: 34,
-        codex_rollout_budget_units: None,
-    };
-
-    assert_eq!(
-        token_usage_delta(&start, &end),
-        TokenUsage {
-            input_tokens: 5,
-            cached_input_tokens: 0,
-            cache_write_input_tokens: 0,
-            output_tokens: 4,
-            reasoning_output_tokens: 0,
-            total_tokens: 6,
-            codex_rollout_budget_units: None,
-        }
-    );
-}
-
 #[tokio::test]
 async fn run_review_on_reused_session_waits_for_submitted_turn() {
     let (review_session, tx_event, rx_sub) = test_review_session().await;
diff --git a/codex-rs/ext/guardian-reviewer/src/lib.rs b/codex-rs/ext/guardian-reviewer/src/lib.rs
index 50e8c8fcce..f2ff22fe2d 100644
--- a/codex-rs/ext/guardian-reviewer/src/lib.rs
+++ b/codex-rs/ext/guardian-reviewer/src/lib.rs
@@ -32,6 +32,7 @@ pub(crate) use circuit_breaker::AUTO_REVIEW_DENIAL_WINDOW_SIZE;
 pub(crate) use circuit_breaker::GuardianRejectionCircuitBreaker;
 pub(crate) use circuit_breaker::GuardianRejectionCircuitBreakerAction;
 pub(crate) use circuit_breaker::GuardianRejectionCircuitBreakerPolicy;
+pub use metrics::token_usage_delta;
 pub use model::ReviewModel;
 pub use model::select_review_model;
 pub use outcome::GuardianReviewError;
diff --git a/codex-rs/ext/guardian-reviewer/src/metrics.rs b/codex-rs/ext/guardian-reviewer/src/metrics.rs
index c55dc49044..9bed7e42df 100644
--- a/codex-rs/ext/guardian-reviewer/src/metrics.rs
+++ b/codex-rs/ext/guardian-reviewer/src/metrics.rs
@@ -1,4 +1,5 @@
-//! Emits Guardian metrics with bounded tags and the existing telemetry schema.
+//! Calculates per-review token usage and emits Guardian metrics with bounded tags
+//! and the existing telemetry schema.
 
 use std::time::Duration;
 
@@ -21,6 +22,20 @@ use codex_protocol::protocol::GuardianRiskLevel;
 use codex_protocol::protocol::GuardianUserAuthorization;
 use codex_protocol::protocol::TokenUsage;
 
+pub fn token_usage_delta(start: &TokenUsage, end: &TokenUsage) -> TokenUsage {
+    TokenUsage {
+        input_tokens: (end.input_tokens - start.input_tokens).max(0),
+        cached_input_tokens: (end.cached_input_tokens - start.cached_input_tokens).max(0),
+        cache_write_input_tokens: (end.cache_write_input_tokens - start.cache_write_input_tokens)
+            .max(0),
+        output_tokens: (end.output_tokens - start.output_tokens).max(0),
+        reasoning_output_tokens: (end.reasoning_output_tokens - start.reasoning_output_tokens)
+            .max(0),
+        total_tokens: (end.total_tokens - start.total_tokens).max(0),
+        codex_rollout_budget_units: None,
+    }
+}
+
 pub(crate) fn emit_guardian_review_metrics(
     session_telemetry: &SessionTelemetry,
     result: &GuardianReviewAnalyticsResult,
@@ -295,6 +310,41 @@ mod tests {
     use std::collections::BTreeMap;
     use test_case::test_case;
 
+    #[test]
+    fn token_usage_delta_never_reports_negative_usage() {
+        let start = TokenUsage {
+            input_tokens: 10,
+            cached_input_tokens: 8,
+            cache_write_input_tokens: 8,
+            output_tokens: 6,
+            reasoning_output_tokens: 4,
+            total_tokens: 28,
+            codex_rollout_budget_units: None,
+        };
+        let end = TokenUsage {
+            input_tokens: 15,
+            cached_input_tokens: 7,
+            cache_write_input_tokens: 7,
+            output_tokens: 10,
+            reasoning_output_tokens: 2,
+            total_tokens: 34,
+            codex_rollout_budget_units: None,
+        };
+
+        assert_eq!(
+            token_usage_delta(&start, &end),
+            TokenUsage {
+                input_tokens: 5,
+                cached_input_tokens: 0,
+                cache_write_input_tokens: 0,
+                output_tokens: 4,
+                reasoning_output_tokens: 0,
+                total_tokens: 6,
+                codex_rollout_budget_units: None,
+            }
+        );
+    }
+
     fn test_session_telemetry() -> SessionTelemetry {
         let exporter = InMemoryMetricExporter::default();
         let metrics = MetricsClient::new(