diff --git a/Sources/AmiiboService/openapi.yaml b/Sources/AmiiboService/openapi.yaml
index 5a54bee..86d5be5 100644
--- a/Sources/AmiiboService/openapi.yaml
+++ b/Sources/AmiiboService/openapi.yaml
@@ -254,36 +254,44 @@ components:
       schema:
         type: string
     Identifier:
-      description: The full 16-character hexadecimal identifier of an amiibo to include in the response.
+      description: The full 16-character hexadecimal identifier of an amiibo to include in the response, with an optional `0x` prefix. An empty value is rejected with a `400` response.
       name: id
       in: query
       required: false
       schema:
         type: string
+        pattern: "^(0x)?[0-9a-fA-F]{16}$"
+        minLength: 16
         maxLength: 18
     IdentifierHead:
-      description: The first 8 hexadecimal characters of an amiibo identifier to include in the response.
+      description: The first 8 hexadecimal characters of an amiibo identifier to include in the response, with an optional `0x` prefix. A shorter value matches as a prefix, while an empty value is rejected with a `400` response.
       name: head
       in: query
       required: false
       schema: 
         type: string
+        pattern: "^(0x)?[0-9a-fA-F]+$"
+        minLength: 1
         maxLength: 10
     IdentifierTail:
-      description: The last 8 hexadecimal characters of an amiibo identifier to include in the response.
+      description: The last 8 hexadecimal characters of an amiibo identifier to include in the response, with an optional `0x` prefix. A shorter value matches as a prefix, while an empty value is rejected with a `400` response.
       name: tail
       in: query
       required: false
       schema: 
         type: string
+        pattern: "^(0x)?[0-9a-fA-F]+$"
+        minLength: 1
         maxLength: 10
     Key:
-      description: A hexadecimal key to filter the results by.
+      description: A hexadecimal key to filter the results by, in the `0x`-prefixed format used by the `key` property of the resources. A value without hexadecimal digits after the prefix, or an empty value, is rejected with a `400` response.
       name: key
       in: query
       required: false
       schema:
         type: string
+        pattern: "^0x[0-9a-fA-F]+$"
+        minLength: 3
     Name: 
       description: A name to filter the results by.
       name: name