diff --git a/core/builtin.rbs b/core/builtin.rbs
index 79f8d337e..828893e9e 100644
--- a/core/builtin.rbs
+++ b/core/builtin.rbs
@@ -225,6 +225,7 @@ type int = Integer | _ToInt
 
 # Represents a `Float`, or a type convertible to it (via `.to_f`).
 #
+%a{deprecated: Use `_ToF` directly}
 type float = Float | _ToF
 
 # Represents a `Range[T]`, or a type that acts like it (via `.begin`, `.end`, and `.exclude_end?`).