Haystack Expert Skill

Haystack (by deepset) is a production-oriented framework for building search and NLP pipelines. Its core abstraction is the Pipeline — a directed acyclic graph of typed components with explicit connections. Unlike LangChain's LCEL (pipe operator) or LlamaIndex's query engines, Haystack pipelines are declared upfront with add_component and connect, giving validated, debuggable DAGs.

Core Paradigm

from haystack import Pipeline
from haystack.components.embedders import SentenceTransformersTextEmbedder
from haystack.components.retrievers.in_memory import InMemoryEmbeddingRetriever
from haystack.components.builders import PromptBuilder
from haystack.components.generators import OpenAIGenerator
from haystack.document_stores.in_memory import InMemoryDocumentStore

# Build a pipeline
document_store = InMemoryDocumentStore()
pipeline = Pipeline()
pipeline.add_component("embedder", SentenceTransformersTextEmbedder())
pipeline.add_component("retriever", InMemoryEmbeddingRetriever(document_store=document_store))
pipeline.add_component("prompt_builder", PromptBuilder(template="Answer using: {{documents}}\n\nQuestion: {{question}}"))
pipeline.add_component("generator", OpenAIGenerator())

# Connect components
pipeline.connect("embedder.embedding", "retriever.query_embedding")
pipeline.connect("retriever.documents", "prompt_builder.documents")
pipeline.connect("prompt_builder", "generator")

# Run
result = pipeline.run({"embedder": {"text": "What is Haystack?"}, "prompt_builder": {"question": "What is Haystack?"}})

Core Principles

1. Pipelines are validated DAGs. add_component + connect. Pipeline validation catches errors BEFORE execution — leverage this during development.
2. Components are typed. Each component has input/output slots. Connections must match types. This prevents runtime errors.
3. PromptBuilder uses Jinja2. Templates are Jinja2 strings, not f-strings. {{documents}}, {{query}}, {{question}} are variable placeholders.
4. Indexing and query are separate pipelines. One pipeline loads/cleans/embeds/writes documents. Another retrieves/generates answers. They share the DocumentStore.
5. Evaluation is a pipeline too. Add evaluator components to measure faithfulness, relevancy, or custom metrics.

Where to Start

You already have...	Start here
Nothing — exploring Haystack	Build a basic indexing + query pipeline
Documents to index	Build an indexing pipeline (converters, splitter, embedder, writer)
A search use case	Build a query pipeline (embedder, retriever, prompt, generator)
A production deployment	Add Hayhooks + evaluation pipeline

Quick Reference

Task	Approach	Reference
Build indexing pipeline	add_component -> connect -> run	references/pipeline-design.md
Build query pipeline	retriever -> prompt_builder -> generator	references/pipeline-design.md
Choose document store	InMemory (dev), Elasticsearch/Pinecone (prod)	references/document-stores.md
Embedding retrieval	SentenceTransformersTextEmbedder + EmbeddingRetriever	references/retrievers.md
Hybrid retrieval	BM25 + Embedding in parallel, DocumentJoiner	references/retrievers.md
Prompt templates	Jinja2 in PromptBuilder	references/pipeline-design.md
Evaluation	DeepEvalEvaluator, SASEvaluator	references/evaluation.md
Deploy	Hayhooks REST API	references/deployment.md

Framework Routing Guide

Scenario	Reach for	Why
Search / NLP pipelines	Haystack	Pipeline DAG model is most mature for retrieval-heavy workloads
Documents to query / RAG	LlamaIndex	Data ingestion is the primary primitive
Chain/agent composition	LangChain	LCEL pipe operator for general chain building
Compiled prompt programs	DSPy	Auto-optimizes prompts against a metric
Role-based multi-agent	CrewAI	Higher-level agent abstraction

Reference Files

Reference	Load when	File
Pipeline Design	Building indexing and query pipelines	references/pipeline-design.md
Document Stores	Store selection and configuration	references/document-stores.md
Retrievers	Embedding, BM25, hybrid retrieval	references/retrievers.md
Validation Audit	Research validation of all API claims	references/validation-audit.md
File Converters	Multi-format indexing, YAML serialization, component types	references/file-converters.md
Evaluation	Metrics, evaluators, pipeline evaluation	references/evaluation.md
Deployment	Hayhooks, containerization, production	references/deployment.md
FAQ & Troubleshooting	Common errors and fixes	references/faq-and-troubleshooting.md

Templates

Template	When to use	File
Indexing Pipeline	Load, split, embed, write to store	templates/indexing-pipeline.py
Query Pipeline	Retrieve, prompt, generate answer	templates/query-pipeline.py
Hybrid RAG	BM25 + embedding in parallel	templates/hybrid-rag.py

Troubleshooting

Symptom	Likely cause	Fix	Reference
Pipeline run errors	Component connection mismatch	Check component input/output slot types	references/pipeline-design.md
No documents retrieved	Empty document store	Run indexing pipeline first	references/pipeline-design.md
Prompt not rendering	Wrong variable name in Jinja2 template	Check {{variables}} match pipeline input	references/pipeline-design.md
Slow retrieval	Full scan instead of ANN	Configure approximate nearest neighbor index	references/retrievers.md
Embedding mismatch	Different models for indexing vs query	Use same model in both pipelines	references/retrievers.md
Hayhooks not starting	Port conflict or missing config	Check port, run with --help for options	references/deployment.md