LLM に検索が必要な理由

An LLM only knows what it saw during training. After that cutoff its knowledge is frozen, so it cannot see your latest docs or private data.

When an LLM lacks a fact, it often invents a confident-sounding one. We call this a hallucination, and it is the core risk to trust.

Scaling a model up adds skill but not your specific facts. A huge parameter count still cannot store your private wiki or today news.

Retrieval-augmented generation fixes this by fetching relevant text first, then asking the model to answer using that fetched text.

RAG has two steps. First retrieve passages that match the question, then generate an answer grounded in those passages.

By feeding real documents into the prompt, you ground the model in facts it can actually read instead of facts it has to guess.

Update your documents and the answers update too. RAG keeps responses fresh without the cost of retraining the model.

Fine-tuning bakes knowledge into weights and is costly to repeat. RAG instead adds knowledge at query time, which is far cheaper to maintain.

Because each answer comes from fetched passages, you can show citations. Users see the source and can verify what the model claimed.

You cannot paste a whole library into a prompt. The model has a fixed context window, so retrieval picks only the few passages that matter.

RAG fits support bots, document search, and internal Q&A: any task where the right answer lives in your data, not the model memory.

Think about why we add retrieval to a language model.

LLMs have frozen, gappy memory, so they hallucinate. RAG retrieves real passages and generates grounded, current, citable answers. ✅