diff --git a/temporal/api/errordetails/v1/message.proto b/temporal/api/errordetails/v1/message.proto
index 7d4901335..0113b9f4f 100644
--- a/temporal/api/errordetails/v1/message.proto
+++ b/temporal/api/errordetails/v1/message.proto
@@ -56,6 +56,13 @@ message NamespaceNotActiveFailure {
     string active_cluster = 3;
 }
 
+// NamespaceUnavailableFailure is returned by the service when a request addresses a namespace that is unavailable. For
+// example, when a namespace is in the process of failing over between clusters.
+// This is a transient error that should be automatically retried by clients.
+message NamespaceUnavailableFailure {
+    string namespace = 1;
+}
+
 message NamespaceInvalidStateFailure {
     string namespace = 1;
     // Current state of the requested namespace.
@@ -130,4 +137,4 @@ message MultiOperationExecutionFailure {
         string message = 2;
         repeated google.protobuf.Any details = 3;
     }
-}
\ No newline at end of file
+}