Because [`ExperimentService`](https://developers.google.com/google-ads/api/reference/rpc/v26/ExperimentService) includes several asynchronous
operations, the mechanism for fetching errors related to these operations
differs from synchronous operations.

## Affected operations

The asynchronous operations that use this error reporting mechanism are:

- [`ExperimentService.ScheduleExperiment`](https://developers.google.com/google-ads/api/reference/rpc/v26/ExperimentService/ScheduleExperiment)
- [`ExperimentService.PromoteExperiment`](https://developers.google.com/google-ads/api/reference/rpc/v26/ExperimentService/PromoteExperiment)

## How long-running operations handle errors

For these methods, the API returns a [long-running operation](https://developers.google.com/google-ads/api/reference/rpc/google.longrunning#google.longrunning.Operation)
(`google.longrunning.Operation`) that you can poll to check the status of the
asynchronous execution. When the operation completes (`done = true`), the
`google.longrunning.Operation` populates either `response` (with
`google.protobuf.Empty` when successful) or `error` (with a `google.rpc.Status`
summary if any asynchronous errors occurred).

If the full list of errors were returned directly in the long-running operation
payload, errors could be missed if polling stops early, and the response payload
could become excessively large for a polling request. Instead, the `error` field
on the long-running operation indicates that errors occurred, and you retrieve
the detailed `Status` errors from a dedicated pagination endpoint.

## Fetch asynchronous errors

To retrieve the full list of errors for an experiment's long-running operation,
call
[`ExperimentService.ListExperimentAsyncErrors`](https://developers.google.com/google-ads/api/reference/rpc/v26/ExperimentService/ListExperimentAsyncErrors).
Pass the resource name of the experiment (`resource_name`) and an optional
`page_size`, then page through the returned `Status` entries using `page_token`
as you would for any other Google Ads API list request:

    GET https://googleads.googleapis.com/v26/customers/CUSTOMER_ID/experiments/EXPERIMENT_ID:listExperimentAsyncErrors?pageSize=100