diff --git a/src/bmipy/bmi.py b/src/bmipy/bmi.py
index f8d3811..41ed450 100644
--- a/src/bmipy/bmi.py
+++ b/src/bmipy/bmi.py
@@ -103,15 +103,15 @@ def get_output_item_count(self) -> int:
         ...
 
     @abstractmethod
-    def get_input_var_names(self) -> tuple[str]:
-        """List of a model's input variables.
+    def get_input_var_names(self) -> tuple[str, ...]:
+        """Get the model's input variables.
 
         Input variable names must be CSDMS Standard Names, also known
         as *long variable names*.
 
         Returns
         -------
-        list of str
+        tuple of str
             The input variables for the model.
 
         Notes
@@ -126,15 +126,15 @@ def get_input_var_names(self) -> tuple[str]:
         ...
 
     @abstractmethod
-    def get_output_var_names(self) -> tuple[str]:
-        """List of a model's output variables.
+    def get_output_var_names(self) -> tuple[str, ...]:
+        """Get the model's output variables.
 
         Output variable names must be CSDMS Standard Names, also known
         as *long variable names*.
 
         Returns
         -------
-        list of str
+        tuple of str
             The output variables for the model.
         """
         ...