Shipping an AI assistant with a narrow brief

Start from refusal

The first thing I wrote was not the prompt — it was the fallback. Deciding what the assistant should decline made the rest of the design obvious.

Ground it in one document

The knowledge base is a single resume. The system prompt states the source, forbids invention, and requires a fallback line when the answer is not in the document.

Cheap by construction

• Short context: one document, not a vector store.
• Hard input limits and a local blocklist before any model call.
• A daily cap so an abusive loop cannot generate a bill.

The lesson

A tightly scoped assistant that says "I don't know" is more useful than a general one that guesses confidently.