login-customer-id
login-customer-id is an HTTP request header in the Google Ads API that carries the customer ID of the manager account you are operating through, telling Google which account in the hierarchy is authorising the call on the target account's behalf.
Every Google Ads API request targets a customer ID — the account whose campaigns you are reading or changing. When that account is reached through a manager account (MCC), the API also needs to know the path you took to get there. That is the job of login-customer-id: it names the manager account whose authorisation you are exercising, and it is sent as ten digits with no dashes (1234567890, not 123-456-7890).
The rule of thumb: if your OAuth credentials belong to a user who accesses the target account directly, you can usually omit the header. If access comes via a manager account — the normal case for agencies and for anyone using a developer token issued to an MCC — the header is required. Leaving it out is one of the most common causes of USER_PERMISSION_DENIED errors from developers who can see the account perfectly well in the web interface, because the UI performs the same resolution silently while the API does not.
There is a second, subtler use. In deep hierarchies the header decides which manager's context applies, and that context affects what you can see and do: shared budgets, cross-account conversion actions and account-level settings all resolve relative to the manager you logged in through. Sending the wrong MCC in a nested structure can produce partial data rather than an outright error, which is much harder to spot.
This is exactly the class of plumbing Opus Growth removes. Because we run the connector as a hosted service, the manager context is resolved for you on every call — you connect once with Google sign-in and never handle a developer token, a customer ID header or an OAuth refresh loop yourself.
Frequently asked questions
Whenever you access an account through a manager account rather than directly, which covers virtually all agency setups and any integration using a developer token issued to an MCC. Direct-access accounts can usually omit it.
Most commonly USER_PERMISSION_DENIED, sometimes CUSTOMER_NOT_ENABLED or a general authorisation error, even though the same user can open the account normally in the Google Ads web interface.
Ten digits with the dashes removed. An account shown as 123-456-7890 in the interface is sent as 1234567890 in the header.
