qBittorrent download client reports "Authentication failure" despite receiving a successful 204 login response #34

Is there an existing issue for this?

• I have searched the existing open and closed issues

Current Behavior

Adding qBittorrent as a download client fails with "Authentication failure" on Test/Save, even though qBittorrent's /api/v2/auth/login endpoint returns a successful 204 No Content response for the exact same credentials. Trace logs (below) show the login POST succeeding immediately before Readarr reports it as a failure.

The same qBittorrent instance, same credentials, same host/port authenticate successfully in Sonarr and Radarr, both running alongside Readarr and connecting to qBittorrent over the same Docker network.

Expected Behavior

A 204 No Content response from /api/v2/auth/login should be treated as successful authentication, matching qBittorrent's documented API behavior and what Sonarr/Radarr already do against the same instance.

Steps To Reproduce

1. Add a qBittorrent download client in Readarr with correct host/port/username/password (verified working via curl/wget directly against qBittorrent, and via working Sonarr/Radarr connections to the same instance).
2. Click Test or Save.
3. Observe "Authentication failure" on the Username field.

Ruled out while troubleshooting:

• Credentials: confirmed correct via direct curl/wget from inside the Readarr container, and via working Sonarr/Radarr connections to the same qBittorrent instance.
• Network/DNS: the qBittorrent hostname resolves and is reachable from the Readarr container (ping succeeds, and a plain GET on /api/v2/app/webapiVersion returns 200).
• IP ban: qBittorrent logs show no ban; a deliberately wrong password correctly returns 401, not 403.
• Password special characters: reproduced identically with an alphanumeric-only password.
• Stale cache: reproduced after restarting both the Readarr container and the full Docker stack (including qBittorrent itself).

Environment

OS: QNAP Container Station (Linux)
Readarr: 0.9.1.0 (ghcr.io/faustvii/readarr:latest)
Docker Install: Yes
Using Reverse Proxy: No
Browser: Chrome
Database: SQLite
qBittorrent: 5.2.3 (lscr.io/linuxserver/qbittorrent), reached via a Gluetun VPN container over network_mode: service:gluetun
Sonarr 4.0.19.2979 and Radarr 6.3.0.10514 both authenticate successfully against the same qBittorrent instance/credentials

What branch are you running?

Develop

Trace Logs?

2026-07-28 11:22:36.9|Trace|HttpClient|Req: [GET] http://gluetun:8090/api/v2/app/webapiVersion
2026-07-28 11:22:37.0|Trace|HttpClient|Res: HTTP/1.1 [GET] http://gluetun:8090/api/v2/app/webapiVersion: 200.OK (6 bytes) (72 ms)
2026-07-28 11:22:37.0|Trace|QBittorrentProxySelector|Using qbitTorrent API v2
2026-07-28 11:22:37.0|Trace|HttpClient|Req: [POST] http://gluetun:8090/api/v2/auth/login: username=(removed)&password=(removed)
2026-07-28 11:22:37.1|Trace|HttpClient|Res: HTTP/1.1 [POST] http://gluetun:8090/api/v2/auth/login: 204.NoContent (0 bytes) (62 ms)
2026-07-28 11:22:37.1|Debug|QBittorrentProxyV2|qbitTorrent authentication failed.
2026-07-28 11:22:37.1|Error|QBittorrent|Unable to authenticate

[v0.9.1.0] NzbDrone.Core.Download.Clients.DownloadClientAuthenticationException: Failed to authenticate with qBittorrent.
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxyV2.AuthenticateClient(HttpRequestBuilder requestBuilder, QBittorrentSettings settings, Boolean reauthenticate) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxyV2.cs:line 444
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxyV2.ProcessRequest(HttpRequestBuilder requestBuilder, QBittorrentSettings settings) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxyV2.cs:line 360
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxyV2.GetApiVersion(QBittorrentSettings settings) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxyV2.cs:line 74
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxySelector.FetchProxy(QBittorrentSettings settings) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxySelector.cs:line 85
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxySelector.GetProxyCache(QBittorrentSettings settings, Boolean force) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxySelector.cs:line 77
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrentProxySelector.GetProxy(QBittorrentSettings settings, Boolean force) in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrentProxySelector.cs:line 60
   at NzbDrone.Core.Download.Clients.QBittorrent.QBittorrent.TestConnection() in ./Readarr.Core/Download/Clients/QBittorrent/QBittorrent.cs:line 432

The login POST clearly succeeds (204.NoContent), but AuthenticateClient (QBittorrentProxyV2.cs:444) treats it as a failure anyway. This looks like it's checking for something in the response (a specific cookie name/value, a response body, or a status code branch) that qBittorrent 5.2.3's /auth/login no longer returns in the form the code expects.

Trace Logs have been provided as applicable. Reports may be closed if the required logs are not provided.

• I have read and followed the steps in the wiki link above and provided the required trace logs - the logs contain trace - that are relevant and show this issue.