diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 3dd4ca5e..82b12a2f 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -1266,6 +1266,10 @@ func checkSystemInstructionsWithMode(payload []byte, strictMode bool) []byte {
 			}
 			return true
 		})
+	} else if system.Type == gjson.String && system.String() != "" {
+		partJSON := `{"type":"text","cache_control":{"type":"ephemeral"}}`
+		partJSON, _ = sjson.Set(partJSON, "text", system.String())
+		result += "," + partJSON
 	}
 	result += "]"