WhatsApp error 131021: Recipient cannot be sender

Code 131021 means the number in the to field is the same as the business phone number you are sending from. WhatsApp does not allow a business number to message itself, so the platform rejects the request before attempting delivery. It is one of the few WhatsApp errors with exactly one cause, and it is almost always a bug in how the recipient was chosen rather than a deliberate action.

It tends to appear in three places: test harnesses that default the recipient to the business number, contact imports where the business's own number was saved as a customer, and echo or auto-reply flows that accidentally read the sender field from an outbound status webhook instead of the inbound message.

The number itself is not invalid and your account is fine. You simply pointed the message at the wrong party.

Add a guard in your send function that rejects any recipient equal to the configured business number before the request leaves your service, and unit-test it. Filter the business number out of imports. This error costs nothing in rate limit but it is a sign that recipient selection logic has a hole worth closing. It is a cousin of 131026 only in appearance: 131026 is about the recipient's reachability, while 131021 is your own number reflected back at you. The webhook-field mix-ups that cause it are covered in the error-code guide.