When RAG is the wrong tool

1What retrieval structurally cannot do

Every lesson so far has been about making retrieval better. This one is about the ceiling that tuning cannot raise.

Retrieval does exactly one thing: it takes a query and returns the k passages most similar to it. Everything follows from that sentence. It sees k passages. It has no idea how many passages it did not return, what was in them, or whether the ones it picked are representative. It cannot count, it cannot compare across everything, and it cannot notice an absence.

Three families of question run straight into that.

Aggregation. "How many contracts expire this quarter." "What is the average deal size." "Which region files the most complaints." These need every record, and a computation over them. Retrieval hands you five. The model then answers from those five, in a confident tone, and the number is wrong. Worse, it is wrong in a way that looks fine - "based on the contracts I found, three expire this quarter" is a true statement about your retrieval and a false answer to the question.

This is a database query. If your data is in a database, write SQL. If it is in documents, extract the fields you need into a table at index time and query that - dates, amounts, parties, statuses. The metadata work from lesson 06 is exactly this, and it is the difference between counting and guessing.

The whole document. "Summarise this report." "What changed between v2 and v3 of this contract." "Does this policy contradict itself anywhere." Chunking, from lesson 03, is precisely the operation that destroys these. You cut the document into pieces so you could search it; now the question needs it whole, and retrieval hands back the pieces that scored highest, which for a summary is close to arbitrary.

Structure and relationships. "Which suppliers are affected if vendor X fails." "Who reports to the person who approved this." "Which services depend on this library." The answer is not in any single passage - it is in the links between records. Similarity between text does not encode those links. Even if every passage names one edge, retrieving five passages gives you five unconnected edges and no path.

Similarity search returns what looks like the question. It cannot tell you what it did not return - so it can never count, and it can never be sure it is complete.

2The alternatives

SQL. The best move here is often a router: have a model classify the question and, for the aggregation ones, generate a query against a real schema rather than a prompt against passages. Text-to-SQL is a well-studied problem with its own benchmarks. It also gives you something RAG cannot - an exact answer, reproducible, with no similarity involved.

Full-text search. If the user knows what they are looking for and can name it, you do not need embeddings at all. A search box over BM25 is faster, cheaper, and returns the document rather than a paraphrase of it. Not every text problem needs a model.

Long context. Models now take very large inputs, so "just put the document in the prompt" is a real option for a single document that fits. It is simpler than a pipeline and has no retrieval step to get wrong. The trade is cost, latency and, at the far end of the window, some loss of attention to material buried in the middle. It works for one document. It does not work for a corpus - four thousand contracts do not fit in any window, and paying to read all of them per query would be absurd even if they did. Check the current context limits on the model's documentation rather than trusting a number written months ago.

Graphs. Extract entities and relationships at index time, store them as nodes and edges, and traverse. Approaches like GraphRAG combine this with retrieval - use the graph for structure, the passages for wording. It is real work to build and maintain, so reach for it when relationship questions are the main thing your users ask, not as a default upgrade.

Agentic search. Instead of one retrieval pass, let the model search, read what came back, and decide what to search for next. This is the loop from the build-with-llms lesson on the agent loop, with search as the tool. It fits questions where you cannot write the right query up front because you do not yet know what you are looking for - "find the incident that caused this regression, then tell me who reviewed the fix". The price is several model calls and several searches per question, so latency and cost multiply, and the loop needs a hard step limit.

Why fine-tuning is not a substitute

This comes up constantly, so it is worth being exact. Fine-tuning continues training on your examples so the behaviour is baked into the weights. That makes it good at how to answer and bad at what is true right now.

If the need is fresh, citable facts, fine-tuning gives you none of the three. It cannot be updated cheaply, it cannot point at a source, and it has no clean way to signal absence. Fine-tune to make outputs consistently shaped; retrieve to make them true. The two are not alternatives and doing both is fine.

3Route the question first

The practical version of this lesson is a classifier in front of your pipeline. Decide what kind of question you have, then send it to the tool that can answer it.

import anthropic

client = anthropic.Anthropic()

ROUTES = """
sql        - counts, totals, filters over structured fields
retrieval  - what a document says about a topic
whole_doc  - needs one named document read end to end
none       - not covered by our corpus
"""

def route(question):
    message = client.messages.create(
        model="claude-sonnet-5",
        max_tokens=10,
        system=f"Pick one route for the question. Reply with the route name only.\n{ROUTES}",
        messages=[{"role": "user", "content": question}],
    )
    return message.content[0].text.strip()

for q in [
    "how many contracts expire this quarter?",
    "what does the MSA say about termination?",
    "summarise the 2024 vendor agreement",
]:
    print(f"{route(q):10}  {q}")

# -> sql        how many contracts expire this quarter?
# -> retrieval  what does the MSA say about termination?
# -> whole_doc  summarise the 2024 vendor agreement

A few notes on running this for real. max_tokens is tiny because the output is one word. The none route matters as much as the others - a question your corpus does not cover should be refused, not retrieved for. And routing is itself a model call that can be wrong, so log every decision and put the misroutes into your eval set from lesson 10.

You do not have to build all the branches on day one. Even a router with two routes - "this is a counting question, say I cannot do that" and "everything else, run RAG" - removes your most embarrassing failure mode for an hour of work.

The judgement to keep

Before building any retrieval pipeline, ask what shape the question is.

• Does answering it need every record? Not RAG.
• Does it need one whole document? Long context or map-reduce, not RAG.
• Does it need connections between records? A graph or joins, not RAG.
• Does the answer sit in a passage somewhere? RAG, and everything in lessons 01 to 11 applies.

That choice, made early, matters more than any tuning you do afterwards. A perfectly tuned retriever pointed at a counting question is still going to be wrong, and it will be wrong confidently, with citations.