## What happened
On getbrevo/brevo-php v4.0.7 with PHP 8.4, calling ContactsClient::createContact() throws Brevo\Exceptions\BrevoException: Failed to deserialize response: Syntax error with a nested JsonException(code: 4). The HTTP request itself succeeds and the contact is created or updated, but the SDK then chokes while parsing the response body. This happens on the upsert flow with updateEnabled set to true, when the API returns an empty or non-JSON success response that the deserializer does not expect.
## The fix that worked
This is an SDK bug, not your code: v4.0.7 tries to JSON-decode success responses that can be empty or non-JSON. It was fixed in v4.0.11, where the SDK handles those responses gracefully instead of throwing during deserialization. Upgrade with composer require getbrevo/brevo-php:^4.0.11 and the exception goes away. The Brevo team confirmed the fix on the issue thread.