Prowlarr sync keeps failing with HTTP 400? Find the indexer Sonarr and Radarr reject

During a health check of my server, Prowlarr’s log had a recurring error, several times a day:

[Warn] HttpClient: HTTP Error - Res: HTTP/1.1 [POST] http://radarr:7878/api/v3/indexer: 400.BadRequest
[Warn] RadarrV3Proxy: No Results in configured categories. See FAQ Entry: Prowlarr will not sync X Indexer to App

Downloads still worked, so it was easy to ignore. But “something fails every few hours” is how real problems hide. Here’s how I found the cause. (As always with the *arr stack: only download what you have the right to.)

What the error means

When Prowlarr pushes an indexer to Radarr or Sonarr, the app tests it before saving: it runs an empty search in its own categories (movies 2000 for Radarr, TV 5000 for Sonarr). If the indexer returns nothing there, the app rejects it with 400 Bad Request, and Prowlarr tries again at the next sync.

The log doesn’t say which indexer. The APIs do.

Compare the indexer lists

Each app’s API key is in its config.xml:

key(){ docker exec $1 cat /config/config.xml | sed -n 's:.*<ApiKey>\(.*\)</ApiKey>.*:\1:p'; }
PK=$(key prowlarr); RK=$(key radarr); SK=$(key sonarr)

curl -s -H "X-Api-Key: $PK" http://127.0.0.1:9696/api/v1/indexer | jq -r '.[].name'
curl -s -H "X-Api-Key: $RK" http://127.0.0.1:7878/api/v3/indexer | jq -r '.[].name'
curl -s -H "X-Api-Key: $SK" http://127.0.0.1:8989/api/v3/indexer | jq -r '.[].name'

My result: Prowlarr had six indexers. Radarr was missing two, Sonarr was missing two.

• One missing from Radarr was a TV-only indexer, and one missing from Sonarr was movie-only. That’s expected, not an error.
• One indexer was missing from both. That was the one.

Test it

Prowlarr can search a single indexer through its API:

curl -s -H "X-Api-Key: $PK" \
  "http://127.0.0.1:9696/api/v1/search?query=ubuntu&indexerIds=6&type=search" \
  | jq '[.[].categories[].name] | unique'

It returned 50 results, all in categories like Books, PC and Other. It worked fine as an indexer, but it had nothing for movies or TV. So both apps’ tests failed, every sync, forever.

The fix

It overlapped with general indexers I already had, so I disabled it in Prowlarr (Indexers → edit → uncheck Enable). That’s reversible with one click. Then I forced a sync:

curl -s -X POST -H "X-Api-Key: $PK" -H "Content-Type: application/json" \
  -d '{"name":"ApplicationIndexerSync","forceSync":true}' \
  http://127.0.0.1:9696/api/v1/command

Zero errors since. Radarr and Sonarr kept all their working indexers. The disabled one had never been in them anyway.

Alternatives

• If you want to keep the indexer for manual searches in Prowlarr, give it a separate sync profile with automatic sync disabled, or restrict which apps it syncs to with tags.
• Check the app’s sync categories in Prowlarr → Settings → Apps. Narrow categories can cause the same 400.

Related

How my Jellyfin + *arr stack fits together.