diff --git a/src/error.rs b/src/error.rs
index 9a839dd..ff58f4e 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -228,6 +228,15 @@ pub enum ProxyError {
     #[error("Session terminated")]
     RouteSwitched,
 
+    #[error("Traffic budget wait cancelled")]
+    TrafficBudgetWaitCancelled,
+
+    #[error("Traffic budget wait deadline exceeded")]
+    TrafficBudgetWaitDeadlineExceeded,
+
+    #[error("ME client writer cancelled")]
+    MiddleClientWriterCancelled,
+
     // ============= Config Errors =============
     #[error("Config error: {0}")]
     Config(String),
diff --git a/src/proxy/middle_relay.rs b/src/proxy/middle_relay.rs
index 6b0329c..8fadf9b 100644
--- a/src/proxy/middle_relay.rs
+++ b/src/proxy/middle_relay.rs
@@ -697,7 +697,7 @@ async fn wait_for_traffic_budget(
 
         let wait_started_at = Instant::now();
         if deadline.is_some_and(|deadline| wait_started_at >= deadline) {
-            return Err(ProxyError::Proxy("traffic budget wait deadline exceeded".into()));
+            return Err(ProxyError::TrafficBudgetWaitDeadlineExceeded);
         }
         tokio::time::sleep(next_refill_delay()).await;
         let wait_ms = wait_started_at
@@ -741,13 +741,13 @@ async fn wait_for_traffic_budget_or_cancel(
         let wait_started_at = Instant::now();
         if deadline.is_some_and(|deadline| wait_started_at >= deadline) {
             stats.increment_flow_wait_middle_rate_limit_cancelled_total();
-            return Err(ProxyError::Proxy("traffic budget wait deadline exceeded".into()));
+            return Err(ProxyError::TrafficBudgetWaitDeadlineExceeded);
         }
         tokio::select! {
             _ = tokio::time::sleep(next_refill_delay()) => {}
             _ = cancel.cancelled() => {
                 stats.increment_flow_wait_middle_rate_limit_cancelled_total();
-                return Err(ProxyError::Proxy("traffic budget wait cancelled".into()));
+                return Err(ProxyError::TrafficBudgetWaitCancelled);
             }
         }
         let wait_ms = wait_started_at
@@ -2763,7 +2763,7 @@ where
 {
     tokio::select! {
         result = client_writer.write_all(bytes) => result.map_err(ProxyError::Io),
-        _ = cancel.cancelled() => Err(ProxyError::Proxy("ME client writer cancelled".into())),
+        _ = cancel.cancelled() => Err(ProxyError::MiddleClientWriterCancelled),
     }
 }
 
@@ -2776,7 +2776,7 @@ where
 {
     tokio::select! {
         result = client_writer.flush() => result.map_err(ProxyError::Io),
-        _ = cancel.cancelled() => Err(ProxyError::Proxy("ME client writer cancelled".into())),
+        _ = cancel.cancelled() => Err(ProxyError::MiddleClientWriterCancelled),
     }
 }
 
diff --git a/src/proxy/tests/middle_relay_atomic_quota_invariant_tests.rs b/src/proxy/tests/middle_relay_atomic_quota_invariant_tests.rs
index e1b3511..5cf6ac5 100644
--- a/src/proxy/tests/middle_relay_atomic_quota_invariant_tests.rs
+++ b/src/proxy/tests/middle_relay_atomic_quota_invariant_tests.rs
@@ -251,7 +251,7 @@ async fn me_writer_data_write_obeys_flow_cancellation() {
     .await;
 
     assert!(
-        matches!(result, Err(ProxyError::Proxy(ref message)) if message == "ME client writer cancelled"),
+        matches!(result, Err(ProxyError::MiddleClientWriterCancelled)),
         "cancelled middle writer must return a bounded cancellation error"
     );
     assert_eq!(