diff --git a/api/identity/definitions/request_email_validation.yaml b/api/identity/definitions/request_email_validation.yaml
index 805cd1231..c86534563 100644
--- a/api/identity/definitions/request_email_validation.yaml
+++ b/api/identity/definitions/request_email_validation.yaml
@@ -20,7 +20,11 @@ example: {
 properties:
   client_secret:
     type: string
-    description: A unique string used to identify the validation attempt.
+    description: |
+      A unique string generated by the client, and used to identify the
+      validation attempt. It must be a string consisting of the characters
+      ``[0-9a-zA-Z.=_-]``. Its length must not exceed 255 characters and it
+      must not be empty.
     example: "monkeys_are_GREAT"
   email:
     type: string
diff --git a/api/identity/definitions/request_msisdn_validation.yaml b/api/identity/definitions/request_msisdn_validation.yaml
index f8e37d392..ee131660b 100644
--- a/api/identity/definitions/request_msisdn_validation.yaml
+++ b/api/identity/definitions/request_msisdn_validation.yaml
@@ -21,7 +21,11 @@ example: {
 properties:
   client_secret:
     type: string
-    description: A unique string used to identify the validation attempt.
+    description: |
+      A unique string generated by the client, and used to identify the
+      validation attempt. It must be a string consisting of the characters
+      ``[0-9a-zA-Z.=_-]``. Its length must not exceed 255 characters and it
+      must not be empty.
     example: "monkeys_are_GREAT"
   country:
     type: string