Haystack 3.3.0 speeds up SentenceWindowRetriever and changes in-memory BM25 results

Haystack, the open-source AI orchestration framework from deepset, has released version 3.3.0, with one speed-up and several changes that can alter retrieval results.

The headline change is to SentenceWindowRetriever. Haystack's release notes say it now queries the Document Store once per run or run_async call, not once per retrieved document. The project says this means lower latency and less Document Store load, and that outputs are unchanged, so existing pipelines should get faster without code changes. The notes give no benchmark figures, so the size of the gain is unknown.

The change most likely to affect users is in BM25 retrieval. InMemoryDocumentStore.bm25_retrieval and InMemoryBM25Retriever using BM25L, which is the default, or BM25Plus now return only documents containing at least one query term. Before, documents with no query term still came back with a positive score and filled top_k, so results can now contain fewer than top_k documents, or none. Scores of matching documents are also lower whenever a query includes terms the document lacks, and the notes advise re-checking any fixed score threshold. BM25Okapi is not affected.

A related bug fix covers Chinese, Japanese and Korean text. The default bm25_tokenization_regex now splits CJK characters into one token each, and text is NFC-normalized before tokenization, so bare-term queries can match inside unspaced text.

On security, Haystack now requires anyio 4.14.2 or later, because earlier versions are affected by CVE-2026-63374. anyio arrives through httpx and openai. The release also fixes SentenceSplitter dropping whitespace after a closing quote. The notes warn that chunk boundaries change for text with quoted sentences, so re-indexing an existing corpus will produce different chunks.

Why it matters: teams running in-memory BM25 search, particularly with score cutoffs or CJK content, should test before upgrading, since behavior changes without any code change on their side.

What is confirmed and what is not: everything above comes from Haystack's own release notes. We found no independent second source, only a mirror of the same notes, so these are the project's descriptions of its own release, not independently tested. The saved page shows no publication date.

Revised 16 October 2026: Guth re-read the original sources and rewrote this article. The earlier version remains available as revision 1.