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v2.21.0

⭐️ Highlights

🔍 Smarter, Broader Retrieval with Multi-Query RAG

This release introduces three new components that significantly boost retrieval recall in RAG systems by expanding the user query and retrieving documents across multiple reformulations:

• QueryExpander generates semantically similar variations of a user query to broaden search coverage.
• MultiQueryTextRetriever runs multiple queries in parallel using a text-based retriever (e.g., BM25) and merges results by score.
• MultiQueryEmbeddingRetriever performs the same multi-query retrieval flow using embeddings, enabling richer semantic recall.

Used together, these components create a multi-query retrieval pipeline that improves recall especially when queries are short or ambiguous.

🧪 Example: Expanding a Query and Retrieving More Relevant Documents

from haystack import Pipeline
from haystack.components.query import QueryExpander
from haystack.components.retrievers import InMemoryBM25Retriever
from haystack.components.retrievers import MultiQueryTextRetriever
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack.components.writers import DocumentWriter
from haystack import Document
from haystack.document_stores.types import DuplicatePolicy
Sample documents
docs = [
Document(content="Renewable energy comes from natural sources like wind and sunlight."),
Document(content="Geothermal energy is heat from beneath the Earth's surface."),
Document(content="Hydropower generates electricity using flowing water."),
]
Store documents
store = InMemoryDocumentStore()
writer = DocumentWriter(document_store=store, policy=DuplicatePolicy.SKIP)
writer.run(documents=docs)
Components
expander = QueryExpander()
retriever = InMemoryBM25Retriever(document_store=store, top_k=1)
multi_retriever = MultiQueryTextRetriever(retriever=retriever)
Expand and retrieve
expanded = expander.run(query="renewable energy")
results = multi_retriever.run(queries=expanded["queries"])
for doc in results["documents"]:
print(doc.content)

... (truncated)

• ffe5e47 bump version to 2.21.0
• 08a82c8 bump version to 2.21.0-rc1
• ecce6f6 chore: small improvements SentenceWindowRetriever
• 08b979c Sync Haystack API reference on Docusaurus (#10188)
• bad2937 feat: add run async filter and automerging retriever (#9897)
• dfd678e Improve flaky tool invocation integration test (#10187)
• 152a16c Fix flaky parallel tool test with stronger model (#10183)
• 251335f Sync Haystack API reference on Docusaurus (#10185)
• adcd4ae chore: Update Azure default model to gpt-4.1-mini (#10167)
• 0907d89 docs: new page that documents pipeline loops (#10134)
• Additional commits viewable in compare view

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