Structuring Prompts for RAG

The effectiveness of a Retrieve-Augmented Generation (RAG) system doesn't just depend on retrieving the right information; it significantly relies on how that information is presented to the Large Language Model (LLM). The prompt acts as the bridge between the retrieved context and the generation process. Crafting this prompt well is essential for guiding the LLM to produce accurate, relevant, and contextually grounded responses.

Think of the prompt as the instruction manual you provide to the LLM. It needs to clearly state the task, present the relevant evidence (the retrieved context), and specify how that evidence should be used to fulfill the user's original request. A poorly structured prompt can lead the LLM astray, even if the retrieval step was successful. It might ignore the context, misinterpret the user's query, or fail to synthesize the information effectively.

Basic Prompt Template

A common starting point for structuring a RAG prompt involves combining the retrieved context and the original user query into a single input for the LLM. A basic template might look like this:

Based on the following context:

[CONTEXT_CHUNK_1]
[CONTEXT_CHUNK_2]
...
[CONTEXT_CHUNK_N]

Answer the following question: [USER_QUERY]

Here, [CONTEXT_CHUNK_1] through [CONTEXT_CHUNK_N] are placeholders for the actual text passages retrieved from your knowledge source, and [USER_QUERY] is the original question or instruction from the user.

Context Placement and Instructions

The placement of the context relative to the query matters. While the template above places context first, you could also place it after the query. Some LLMs might exhibit a recency bias, paying more attention to information presented later in the prompt. Experimentation is often needed to determine the optimal placement for your specific LLM and task.

More important than placement alone are the instructions given to the LLM. Explicit instructions help constrain the model's behavior and encourage it to rely on the provided information. Consider these variations:

• Strict Grounding: "Use only the provided context below to answer the question. If the answer is not found in the context, state that you cannot answer based on the provided information."

  Context:
  [CONTEXT]
  
  Question: [USER_QUERY]
  
  Answer strictly based on the context provided. If the information is not present, respond with "I cannot answer based on the provided context."
  

• Synthesis Instruction: "Synthesize the information from the following documents to answer the user's question."

  User Question: [USER_QUERY]
  
  Relevant Information:
  Document 1: [CONTEXT_CHUNK_1]
  Document 2: [CONTEXT_CHUNK_2]
  
  Combine the relevant information from the documents to provide a comprehensive answer.
  

These instructions guide the LLM on how to use the context, reducing the chances of hallucination (making up information) or relying solely on its internal, potentially outdated knowledge.

Formatting Multiple Context Chunks

When your retriever returns multiple relevant text chunks, you need a clear way to present them within the prompt. Simply concatenating them might confuse the LLM. Better approaches include:

1. Separators: Use distinct separators like double newlines, horizontal rules (using characters like * * *), or specific tags ([CONTEXT] ... [/CONTEXT]) between chunks.
2. Numbering/Labeling: Assign identifiers to each chunk. This can be helpful if you later want the LLM to cite sources.

   Use the following pieces of context to answer the question:
   
   Context 1:
   [CONTEXT_CHUNK_1]
   
   Context 2:
   [CONTEXT_CHUNK_2]
   
   Question: [USER_QUERY]
   

3. Including Metadata: As discussed in Chapter 3, chunks often have associated metadata (e.g., source document name, page number). Including relevant metadata alongside the chunk text in the prompt can be beneficial, particularly for source attribution in the final response.

   Context from 'report_v2.pdf', page 5:
   [CONTEXT_CHUNK_1]
   
   Context from 'website_faq.html':
   [CONTEXT_CHUNK_2]
   
   Question: [USER_QUERY]
   Answer the question using the provided context.
   

Handling Insufficient or Irrelevant Context

The retrieval step isn't always perfect. Sometimes, it might return chunks that aren't truly relevant, or it might fail to find any relevant information at all. Your prompt structure should anticipate this. By instructing the LLM on how to behave when the context is unhelpful (as shown in the "Strict Grounding" example earlier), you can encourage more honest and reliable responses instead of forcing an answer based on poor evidence.

Examples of RAG Prompts

Let's look at a couple of scenarios:

Scenario 1: Simple Question Answering

• User Query: "What is the maximum context window size for the Llama-3-8B-Instruct model?"
• Retrieved Context (Chunk 1): "The Llama 3 family includes models with 8B and 70B parameters. Both initial instruction-tuned versions support context lengths of 8,192 tokens."
• Retrieved Context (Chunk 2): "When choosing a model, consider the trade-off between parameter count and computational requirements. Larger models often perform better but require more resources. Context window limitations also affect suitability for tasks involving long documents."
• Prompt:

  Based on the following context:
  
  Context 1:
  The Llama 3 family includes models with 8B and 70B parameters. Both initial instruction-tuned versions support context lengths of 8,192 tokens.
  
  Context 2:
  When choosing a model, consider the trade-off between parameter count and computational requirements. Larger models often perform better but require more resources. Context window limitations also affect suitability for tasks involving long documents.
  
  Answer the following question: What is the maximum context window size for the `Llama-3-8B-Instruct` model? Use only the provided context.
  

Scenario 2: Query with No Relevant Context Found

• User Query: "What is the airspeed velocity of an unladen swallow?"
• Retrieved Context: (Assume retriever found no relevant chunks about swallow flight speeds in the indexed documents).
• Prompt:

  Based on the following context:
  
  [No relevant context found]
  
  Answer the following question: What is the airspeed velocity of an unladen swallow? Use only the provided context. If the context does not contain the answer, state that.
  

• Expected LLM Output: "I cannot answer based on the provided context."

Iteration and Refinement

Prompt engineering for RAG is rarely a one-shot process. The ideal structure depends heavily on the specific LLM being used, the nature of your data, and the complexity of the user queries you anticipate. Start with a basic structure, test it with representative queries and retrieved contexts, analyze the LLM's outputs, and refine the prompt iteratively. Small changes to wording, formatting, or instruction clarity can sometimes lead to significant improvements in the quality and reliability of the generated responses.

By carefully structuring the prompt, you create a clear communication channel, enabling the LLM to effectively leverage the retrieved information and generate answers that are grounded in your specific knowledge base. This structured approach is fundamental to harnessing the capabilities of RAG.