diff --git a/Sources/SotoSmithy/Shapes/ServiceShapes.swift b/Sources/SotoSmithy/Shapes/ServiceShapes.swift
index 090edca..15e7e0c 100644
--- a/Sources/SotoSmithy/Shapes/ServiceShapes.swift
+++ b/Sources/SotoSmithy/Shapes/ServiceShapes.swift
@@ -38,8 +38,8 @@ public class OperationMemberShape: Shape {
 public class OperationShape: Shape {
     public static let type = "operation"
     public var traits: TraitList?
-    public let input: MemberShape?
-    public let output: MemberShape?
+    public var input: MemberShape?
+    public var output: MemberShape?
     public let errors: [MemberShape]?
 
     public func validate(using model: Model) throws {