diff --git a/doc/design/api.md b/doc/design/api.md
index 17ab50fdb26e6463a03dec87acabd49c070d3999..dcf19451b116228bf131561759a1a889af5ec6d1 100644
--- a/doc/design/api.md
+++ b/doc/design/api.md
@@ -10,13 +10,13 @@ Model = {topology, parameters}
 
 Evaluator = {Model*, activations}
 - forward
-- test
+- test(cost, ...)
 
 GradientMachine = {Evaluator*, gradients}
 - backward
 
 Optimizer = {Model*, GradientMachine*}
-- train
+- train(cost, ...)
 - update
 - checkpoint
 ```