diff --git a/Sources/AmiiboService/openapi.yaml b/Sources/AmiiboService/openapi.yaml
index 8cf7d16..d6f387b 100644
--- a/Sources/AmiiboService/openapi.yaml
+++ b/Sources/AmiiboService/openapi.yaml
@@ -505,6 +505,8 @@ components:
         key:
           description: A key assigned to a tuple.
           type: string
+          pattern: "^0x[0-9a-fA-F]+$"
+          minLength: 3
         name:
           description: A name assigned to a tuple.
           type: string