Could you give some details on what recommendation API is actually doing there? Maybe post a link to the relevant code and probably explanation of which queries it is running and how often? Generally 429 means it is sending too many queries too often. Which shouldn't really happen on the test. But need some details.

Sure. The specific URL that the flaky test is hitting is https://de.wikipedia.org/api/rest_v1/data/recommendation/article/creation/translation/en.

Under the hood, the service is querying WDQS with this up to 10 times in rapid succession, for a batched set of ~500 Wikidata item IDs:

SELECT ?item (COUNT(?sitelink) as ?count) WHERE {
  VALUES ?item { <series of 50 item ids> }
  FILTER NOT EXISTS { ?item wdt:P31 wd:Q4167410 . }
  OPTIONAL { ?sitelink schema:about ?item }
  FILTER NOT EXISTS {
    ?article schema:about ?item .
    ?article schema:isPartOf <https://${target}.${projectDomain}/> .
  }
} GROUP BY ?item

https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/services/recommendation-api/+/master/lib/article.creation.translation.js#158

Hmm, now that I'm digging, I see that the batching was just introduced: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/services/recommendation-api/+/9603d4282492103eecc0b1646ea2911138c848af%5E%21/lib/article.creation.translation.js

Change 518349 had a related patch set uploaded (by Bmansurov; owner: Bmansurov):
[mediawiki/services/recommendation-api@master] Temporarily disable flaky tests

https://gerrit.wikimedia.org/r/518349

Change 518349 merged by jenkins-bot:
[mediawiki/services/recommendation-api@master] Temporarily disable flaky tests

https://gerrit.wikimedia.org/r/518349

T216750: Article recommendation API: replace WDQS with MW API fix has been merged. This will no longer be an issue.