What causes "Failed to add NZB task to Download Station" in Sonarr/Sonarr?

Open DownloadStation and check whether the NZB task exists; if yes, the uri-match logic in Sonarr cannot pair it.

Sonarr/Sonarr · error · DownloadClientException

Failed to add NZB task to Download Station

Error message

Failed to add NZB task to Download Station

What it means

Thrown by UsenetDownloadStation.AddFromNzbFile after DsTaskProxy.AddTaskFromData returned, but GetTasks().Where(t => Additional.Detail["uri"] == filename).SingleOrDefault() found no matching NZB task. The NZB upload completed without API error, yet no matching task is visible to Sonarr.

Source

Thrown at src/NzbDrone.Core/Download/Clients/DownloadStation/UsenetDownloadStation.cs:195

        protected override string AddFromNzbFile(RemoteEpisode remoteEpisode, string filename, byte[] fileContent)
        {
            var hashedSerialNumber = _serialNumberProvider.GetSerialNumber(Settings);

            DsTaskProxy.AddTaskFromData(fileContent, filename, GetDownloadDirectory(), Settings);

            var items = GetTasks().Where(t => t.Additional.Detail["uri"] == filename);

            var item = items.SingleOrDefault();

            if (item != null)
            {
                _logger.Debug("{0} added correctly", remoteEpisode);
                return CreateDownloadId(item.Id, hashedSerialNumber);
            }

            _logger.Debug("No such task {0} in Download Station", filename);

            throw new DownloadClientException("Failed to add NZB task to Download Station");
        }

        protected override void Test(List<ValidationFailure> failures)
        {
            failures.AddIfNotNull(TestConnection());
            if (failures.HasErrors())
            {
                return;
            }

            failures.AddIfNotNull(TestOutputPath());
            failures.AddIfNotNull(TestGetNZB());
        }

        protected ValidationFailure TestOutputPath()
        {
            try
            {

View on GitHub (pinned to da2284d7ea)

Solutions

Open DownloadStation and check whether the NZB task exists; if yes, the uri-match logic in Sonarr cannot pair it.
Re-grab the NZB from Sonarr and confirm the task appears in DownloadStation.
Avoid duplicate NZB submissions with the same filename.
Inspect DownloadStation logs for any task-creation failure on the NZB endpoint.

Defensive patterns

Strategy: retry

Try / catch

try { return AddFromNzbFileCore(remoteEpisode, filename, fileContent); }
catch (DownloadClientException ex) when (ex.Message.Contains("Failed to add NZB task"))
{
    _logger.Warn(ex, "NZB task not found immediately; retrying once.");
    return AddFromNzbFileCore(remoteEpisode, filename, fileContent);
}

Prevention

Confirm the NZB was added in DownloadStation before declaring failure.
Avoid duplicate NZB filenames that break SingleOrDefault.
Inspect DownloadStation logs for NZB parse errors that prevent task registration.

When it happens

Trigger: AddTaskFromData(fileContent, filename, ...) completes; GetTasks filtered to NZB-type tasks where uri == filename yields nothing -> SingleOrDefault returns null -> throw. Happens when DownloadStation renames the NZB task, stores the source URL, or lists it under a different uri field.

Common situations: DownloadStation stored the task under the NZB's internal subject rather than the filename; task created then immediately errored (bad NZB) and filtered; the NZB was a duplicate of an existing task; slow registration read as 'not found'.

Related errors

AI-assisted analysis of Sonarr/Sonarr@da2284d7ea (2026-08-13). Data as JSON: /api/errors/8edc029fc5e57e89. Report an issue: GitHub.