Same dense_vector search returning different results across runs

TL;DR

It was fixed upstream by bumping Elasticsearch to Lucene 10.3 (PR #133980). Fix: upgrade to an Elasticsearch version that includes the Lucene 10.3 bump.

Steps

1. If identical dense_vector queries return different results across runs, the inconsistency is in the vector search layer, not your query or index. It was fixed upstream by bumping Elasticsearch to Lucene 10.3 (PR #133980). Fix: upgrade to an Elasticsearch version that includes the Lucene 10.3 bump. Don't waste time tuning the query, it's a search-engine bug.

When to use

You are seeing this: Recurring searches with the same request for densevector exhibit consistency issues: identical densevector queries return different results across runs. Use this skill when you run into "Same dense_vector search returning different results across runs".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

In gh:elastic/elasticsearch#119180 (https://github.com/elastic/elasticsearch/issues/119180), the reporter ran into this: Recurring searches with the same request for densevector exhibit consistency issues: identical densevector queries return different results across runs. The verified resolution: If identical dense_vector queries return different results across runs, the inconsistency is in the vector search layer, not your query or index. It was fixed upstream by bumping Elasticsearch to Lucene 10.3 (PR #133980). Fix: upgrade to an Elasticsearch version that includes the Lucene 10.3 bump. Don't waste time tuning the query, it's a search-engine bug.