Building an LLM-based Reranker for your RAG pipeline
Are you struggling with irrelevant search results in your Retrieval-Augmented Generation (RAG) pipeline?
Imagine having a powerful tool that can intelligently reassess and reorder your search results, significantly improving their relevance to user queries.
In this blog post, we'll show you how to create an LLM-based reranker using Instructor and Pydantic. This approach will:
- Enhance the accuracy of your search results
- Leverage the power of large language models (LLMs)
- Utilize structured outputs for precise information retrieval
By the end of this tutorial, you'll be able to implement a llm reranker to label your synthetic data for fine-tuning a traditional reranker, or to build out an evaluation pipeline for your RAG system. Let's dive in!
Setting Up the Environment
First, let's set up our environment with the necessary imports:
import instructor
from openai import OpenAI
from pydantic import BaseModel, Field, field_validator
client = instructor.from_openai(OpenAI())
We're using the instructor library, which integrates seamlessly with OpenAI's API and Pydantic for structured outputs.
Defining the Reranking Models
We'll use Pydantic to define our Label and RerankedResults models that structure the output of our LLM:
Notice that not only do I reference the chunk_id in the label class, I also asked a language model to use chain of thought. This is very useful for using models like 4o Mini or Claude, but not necessarily if we plan to use the o1-mini and o1-preview models.
class Label(BaseModel):
chunk_id: int = Field(description="The unique identifier of the text chunk")
chain_of_thought: str = Field(description="The reasoning process used to evaluate the relevance")
relevancy: int = Field(
description="Relevancy score from 0 to 10, where 10 is most relevant",
ge=0,
le=10,
)
class RerankedResults(BaseModel):
labels: list[Label] = Field(description="List of labeled and ranked chunks")
@field_validator("labels")
@classmethod
def model_validate(cls, v: list[Label]) -> list[Label]:
return sorted(v, key=lambda x: x.relevancy, reverse=True)
These models ensure that our LLM's output is structured and includes a list of labeled chunks with their relevancy scores. The RerankedResults model includes a validator that automatically sorts the labels by relevancy in descending order.
Creating the Reranker Function
Next, we'll create a function that uses our LLM to rerank a list of text chunks based on their relevance to a query:
def rerank_results(query: str, chunks: list[dict]) -> RerankedResults:
return client.chat.completions.create(
model="gpt-4o-mini",
response_model=RerankedResults,
messages=[
{
"role": "system",
"content": """
You are an expert search result ranker. Your task is to evaluate the relevance of each text chunk to the given query and assign a relevancy score.
For each chunk:
1. Analyze its content in relation to the query.
2. Provide a chain of thought explaining your reasoning.
3. Assign a relevancy score from 0 to 10, where 10 is most relevant.
Be objective and consistent in your evaluations.
""",
},
{
"role": "user",
"content": """
<query>{{ query }}</query>
<chunks_to_rank>
{% for chunk in chunks %}
<chunk id="{{ chunk.id }}">
{{ chunk.text }}
</chunk>
{% endfor %}
</chunks_to_rank>
Please provide a RerankedResults object with a Label for each chunk.
""",
},
],
context={"query": query, "chunks": chunks},
)
This function takes a query and a list of text chunks as input, sends them to the LLM with a predefined prompt, and returns a structured RerankedResults object. Thanks to instructor we can use jinja templating to inject the query and chunks into the prompt by passing in the context parameter.
Testing the Reranker
To test our LLM-based reranker, we can create a sample query and a list of text chunks. Here's an example of how to use the reranker:
def main():
query = "What are the health benefits of regular exercise?"
chunks = [
{
"id": 0,
"text": "Regular exercise can improve cardiovascular health and reduce the risk of heart disease.",
},
{
"id": 1,
"text": "The price of gym memberships varies widely depending on location and facilities.",
},
{
"id": 2,
"text": "Exercise has been shown to boost mood and reduce symptoms of depression and anxiety.",
},
{
"id": 3,
"text": "Proper nutrition is essential for maintaining a healthy lifestyle.",
},
{
"id": 4,
"text": "Strength training can increase muscle mass and improve bone density, especially important as we age.",
},
]
results = rerank_results(query, chunks)
print("Reranked results:")
for label in results.labels:
print(f"Chunk {label.chunk_id} (Relevancy: {label.relevancy}):")
print(f"Text: {chunks[label.chunk_id]['text']}")
print(f"Reasoning: {label.chain_of_thought}")
print()
if __name__ == "__main__":
main()
This test demonstrates how the reranker evaluates and sorts the chunks based on their relevance to the query. The full implementation can be found in the examples/reranker/run.py file.
If you want to extend this example, you could use the rerank_results function to label synthetic data for fine-tuning a traditional reranker, or to build out an evaluation pipeline for your RAG system.
Moreover, we could also add validators to the Label.chunk_id field to ensure that the chunk_id is present in the chunks list. This might be useful if labels are uuids or complex strings and we want to ensure that the chunk_id is a valid index for the chunks list.
heres an example
class Label(BaseModel):
chunk_id: int = Field(description="The unique identifier of the text chunk")
...
@field_validator("chunk_id")
@classmethod
def validate_chunk_id(cls, v: int, info: ValidationInfo) -> int:
context = info.context
chunks = context["chunks"]
if v not in [chunk["id"] for chunk in chunks]:
raise ValueError(f"Chunk with id {v} not found, must be one of {[chunk['id'] for chunk in chunks]}")
return v
This will automatically check that the chunk_id is present in the chunks list and raise a ValueError if it is not, where context is the context dictionary that we passed into the rerank_results function.
