Resilient RAG with Fallback Filtering

Build a retrieval-augmented generation pipeline that uses FAISS for vector search and a @generative relevance filter to remove noise before generation.

This example builds a complete RAG pipeline in three stages: embed and index a document corpus, retrieve candidates by semantic similarity, then use a @generative boolean function to discard irrelevant candidates before passing the survivors to a grounded m.instruct() call.

Source file: docs/examples/rag/simple_rag_with_filter.py

Concepts covered

• Building a FAISS flat inner-product index from sentence-transformer embeddings
• Using @generative returning bool as a per-document relevance gate
• Passing filtered documents as grounding_context to m.instruct()
• Running the example with uv run via an inline PEP 723 dependency block

Prerequisites

• Quick Start complete
• faiss-cpu and sentence-transformers installed, or run via uv run which installs them automatically from the inline script block
• Ollama running locally with granite4:micro pulled (or a Mistral model — see the session setup section below)

Install dependencies manually if you are not using uv run:

pip install faiss-cpu sentence-transformers

Pipeline architecture

Query
  |
  v
Embedding model  (sentence-transformers all-MiniLM-L6-v2)
  |
  v
FAISS vector search  (top-k candidates)
  |
  v
@generative relevance filter  (per-document boolean check)
  |
  v
m.instruct() with grounding_context  (answer generation)
  |
  v
Final answer

The full example

Inline script dependencies

# pytest: skip_always
# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "faiss-cpu",
#     "sentence_transformers",
#     "mellea"
# ]
# ///

The /// script block follows PEP 723. When you run the file with uv run simple_rag_with_filter.py, uv reads this block and installs the listed packages into a temporary environment before execution. No manual pip install is needed.

Imports and document corpus

from faiss import IndexFlatIP
from sentence_transformers import SentenceTransformer

from mellea import generative, start_session
from mellea.backends import model_ids

docs = [
    "The capital of France is Paris. Paris is known for its Eiffel Tower.",
    "The Amazon River is the largest river by discharge volume of water in the world.",
    "Mount Everest is the Earth's highest mountain above sea level, located in the Himalayas.",
    "The Louvre Museum in Paris houses the Mona Lisa.",
    "Artificial intelligence (AI) is intelligence demonstrated by machines.",
    "Machine learning is a subset of AI that enables systems to learn from data.",
    "Natural Language Processing (NLP) is a field of AI that focuses on enabling computers to understand, process, and generate human language.",
    "The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China.",
    "The solar system consists of the Sun and everything bound to it by gravity, including the eight planets, dwarf planets, and countless small Solar System bodies.",
    "Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury.",
    "The human heart has four chambers: two atria and two ventricles.",
    "Photosynthesis is the process used by plants, algae, and cyanobacteria to convert light energy into chemical energy.",
    "The internet is a global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices.",
    "Python is a high-level, general-purpose programming language.",
    "The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions.",
]

The corpus is a flat list of strings. In a real system these would come from a database, file system, or document store. IndexFlatIP is a FAISS index that scores by inner product — equivalent to cosine similarity when the embeddings are L2-normalised, as sentence-transformers produces by default.

Index creation and querying

def create_index(model, ds: list[str]) -> IndexFlatIP:
    print("running encoding... ")
    embeddings = model.encode(ds)
    print("running embeddings... ")
    dimension = embeddings.shape[1]
    index = IndexFlatIP(dimension)
    index.add(embeddings)  # type:ignore
    print("done indexing.")
    return index


def query_index(model, idx: IndexFlatIP, query: str, ds: list[str], k: int = 5) -> list:
    query_embedding = model.encode([query])
    _distances, indices = idx.search(query_embedding, k=k)
    return [ds[i] for i in indices[0]]

create_index encodes all documents once and stores the result. query_index encodes the query at inference time and returns the top-k documents by similarity. The default k=5 gives the filter stage enough candidates without overwhelming the context window.

The relevance filter

@generative
def is_answer_relevant_to_question(answer: str, question: str) -> bool:
    """For the given question, determine whether the answer is relevant or not."""

A @generative function returning bool acts as a classifier. The docstring frames the task: given a candidate document (answer) and the original query (question), decide whether the document is actually useful.

Vector similarity finds documents that are topically related, but it can return documents that mention the same keywords without actually answering the question. This LLM filter catches those false positives.

Main: retrieval, filtering, and generation

if __name__ == "__main__":
    query = "How are AI and NLP related?"

    # Create a simple embedding index
    print("loading Embedding model and index data...")
    embedding_model = SentenceTransformer("all-MiniLM-L6-v2")
    index = create_index(embedding_model, docs)

    # Query the index
    print("Query Embedding model...")
    results = query_index(embedding_model, index, query, docs)
    results_str = "\n".join([f"=> {r}" for r in results])
    print(f"results:\n {results_str}\n ====")
    del embedding_model  # help GC

    # Create Mellea session with Mistral. Also work with other models.
    m = start_session(model_id=model_ids.MISTRALAI_MISTRAL_0_3_7B)

    # Check for each document from retrieval if it is actually relevant
    print("running filter.. ")
    relevant_answers = []
    for doc in results:
        is_it = is_answer_relevant_to_question(m, answer=doc, question=query)
        if is_it:
            relevant_answers.append(doc)
        else:
            print(f"skipping: {doc}")

    # Run final answer generation from here
    print("running generation...")
    answer = m.instruct(
        "Provided the documents in the context, answer the question: `{{query}}`",
        user_variables={"query": query},
        grounding_context={f"doc{i}": doc for i, doc in enumerate(relevant_answers)},
    )

    # Print results answer
    print(f"== answer == \n{answer.value}\n ====")

Several implementation choices are worth noting:

del embedding_model frees the sentence-transformer weights before loading the LLM backend. On a machine with limited VRAM or RAM this prevents out-of-memory errors when both models would otherwise be resident simultaneously.

model_id=model_ids.MISTRALAI_MISTRAL_0_3_7B selects a specific backend model. You can substitute any model constant from model_ids or pass a string identifier directly. The example comment confirms other models work too.

grounding_context passes the surviving documents as named context entries. The template variable {{query}} is supplied separately via user_variables. Keeping query and context separate lets Mellea render the prompt correctly and trace each component independently.

answer.value retrieves the raw string from the ModelOutputThunk returned by m.instruct().

Full file

# pytest: skip_always
# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "faiss-cpu",
#     "sentence_transformers",
#     "mellea"
# ]
# ///
"""
Simple RAG (Retrieval-Augmented Generation) example with relevance filtering.

This script demonstrates how to:
1. Create a FAISS vector index from documents
2. Retrieve relevant documents using semantic search
3. Filter retrieved documents for relevance using Mellea
4. Generate a final answer based on the filtered documents

Use `uv run simple_rag_with_filter.py` to run the script.
"""

from faiss import IndexFlatIP
from sentence_transformers import SentenceTransformer

from mellea import generative, start_session
from mellea.backends import model_ids

docs = [
    "The capital of France is Paris. Paris is known for its Eiffel Tower.",
    "The Amazon River is the largest river by discharge volume of water in the world.",
    "Mount Everest is the Earth's highest mountain above sea level, located in the Himalayas.",
    "The Louvre Museum in Paris houses the Mona Lisa.",
    "Artificial intelligence (AI) is intelligence demonstrated by machines.",
    "Machine learning is a subset of AI that enables systems to learn from data.",
    "Natural Language Processing (NLP) is a field of AI that focuses on enabling computers to understand, process, and generate human language.",
    "The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China.",
    "The solar system consists of the Sun and everything bound to it by gravity, including the eight planets, dwarf planets, and countless small Solar System bodies.",
    "Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury.",
    "The human heart has four chambers: two atria and two ventricles.",
    "Photosynthesis is the process used by plants, algae, and cyanobacteria to convert light energy into chemical energy.",
    "The internet is a global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices.",
    "Python is a high-level, general-purpose programming language.",
    "The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions.",
]


def create_index(model, ds: list[str]) -> IndexFlatIP:
    print("running encoding... ")
    embeddings = model.encode(ds)
    print("running embeddings... ")
    dimension = embeddings.shape[1]
    index = IndexFlatIP(dimension)
    index.add(embeddings)  # type:ignore
    print("done indexing.")
    return index


def query_index(model, idx: IndexFlatIP, query: str, ds: list[str], k: int = 5) -> list:
    query_embedding = model.encode([query])
    _distances, indices = idx.search(query_embedding, k=k)
    return [ds[i] for i in indices[0]]


@generative
def is_answer_relevant_to_question(answer: str, question: str) -> bool:
    """For the given question, determine whether the answer is relevant or not."""


if __name__ == "__main__":
    query = "How are AI and NLP related?"

    # Create a simple embedding index
    print("loading Embedding model and index data...")
    embedding_model = SentenceTransformer("all-MiniLM-L6-v2")
    index = create_index(embedding_model, docs)

    # Query the index
    print("Query Embedding model...")
    results = query_index(embedding_model, index, query, docs)
    results_str = "\n".join([f"=> {r}" for r in results])
    print(f"results:\n {results_str}\n ====")
    del embedding_model  # help GC

    # Create Mellea session with Mistral. Also work with other models.
    m = start_session(model_id=model_ids.MISTRALAI_MISTRAL_0_3_7B)

    # Check for each document from retrieval if it is actually relevant
    print("running filter.. ")
    relevant_answers = []
    for doc in results:
        is_it = is_answer_relevant_to_question(m, answer=doc, question=query)
        if is_it:
            relevant_answers.append(doc)
        else:
            print(f"skipping: {doc}")

    # Run final answer generation from here
    print("running generation...")
    answer = m.instruct(
        "Provided the documents in the context, answer the question: `{{query}}`",
        user_variables={"query": query},
        grounding_context={f"doc{i}": doc for i, doc in enumerate(relevant_answers)},
    )

    # Print results answer
    print(f"== answer == \n{answer.value}\n ====")

Key observations

Two-stage retrieval reduces hallucination. Vector search alone can surface documents that share vocabulary with the query but do not answer it. The LLM filter adds a semantic gate that vector distance cannot provide.

@generative returning bool is a classifier. You can use this pattern wherever you need a binary decision: spam detection, content moderation, input validation, feature flags driven by natural language.

grounding_context is the RAG anchor. Without it, m.instruct() would generate from the model’s parametric knowledge. Passing documents through grounding_context grounds the answer in retrieved evidence.

What to try next

• Replace the in-memory list with a database-backed corpus and see docs/examples/rag/mellea_pdf.py for a PDF-based variant.
• Tune k in query_index and observe how the filter step affects final answer quality.
• Add requirements to the final m.instruct() call to enforce length, citation, or tone constraints — see the requirements system concept.

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See also: Build a RAG Pipeline — step-by-step how-to guide | Examples Index