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An AI receptionist's knowledge base should hold what callers ask about: services, hours, service area, booking rules, pricing and how to reach a person. It should not hold anything you wouldn't want read aloud to a stranger.
This guide is part of our AI Front Desk guide. Platform details were checked against Retell's documentation on 9 November 2026, as one example. Other products work differently, so check yours.
Key Takeaways
- A knowledge base is the reference material your AI receptionist can look up during a call.
- Build it from the questions callers really ask, starting with your own call notes.
- Keep instructions for behavior in the prompt and facts in the knowledge base.
- Write short, self-contained passages, because the system retrieves pieces rather than whole documents.
- Name one person to keep it current, since an out-of-date fact becomes a wrong answer.
What a knowledge base is
Retell's knowledge base documentation calls it a collection of sources your agent can access during a call. Sources can be website pages, uploaded documents, text snippets and synced files.
During the call, Retell uses the transcript so far to look for relevant passages. It then adds those passages to the prompt the model sees. This approach is a form of retrieval, which we explain in what RAG is.
Start with real questions
Don't begin with your website copy. Begin with what people ask your front desk.
- Ask staff for the top questions they answer every week.
- Read a sample of voicemails, emails and contact forms.
- Note how callers phrase things, including the words they use for your services.
Then write one clear answer to each question. Those answers are the first version of your knowledge base.
What to include
| Topic | What to write down | Example detail |
|---|---|---|
| Business basics | Name, address, parking, hours, holiday closures | "Closed the last Monday in May" |
| Service area | Cities or ZIP codes served and not served | "We do not travel past County Line Road" |
| Services | What you do and what you don't do | "We repair but do not install" |
| Pricing | Only what you are willing to say on a call | "Inspection fee is listed on our price sheet" |
| Booking | Appointment types, lengths, notice needed | "New client visits take 60 minutes" |
| Policies | Cancellation, deposits, payment methods | Your own written policy |
| Preparation | What the caller should bring or do first | "Bring your insurance card" |
| Routing | Who handles what, and when | "Billing questions go to the office manager" |
The example details are placeholders. Replace them with your own verified facts.
Facts go in the knowledge base, behavior goes in the prompt
Keep these separate. The knowledge base answers "what is true about this business?" The prompt answers "how should the agent act?"
Retell's prompting guide says: "The most important thing to know about prompting is that a longer prompt is not a better prompt." Moving reference facts out of the prompt keeps the instructions short.
Your call rules belong in the call flow. Our call flow templates and AI receptionist scripts cover that side.
Write for retrieval and for speech
Retell's documentation says its default retrieval returns three passages, with a similarity threshold of 0.6. That means the agent may see only pieces of a document, not the whole thing. These writing tips are our advice, based on how that works.
- Make each passage stand alone. Say "Our Saturday hours are 9 a.m. to 1 p.m." rather than "On Saturdays, the same applies."
- Use the words callers use. Include both "AC repair" and "air conditioning service."
- Give one fact per passage. A long paragraph mixes topics and makes the wrong match more likely.
- Remove old versions. Two passages with different prices give the agent two answers.
- Write numbers plainly. The answer will be read aloud, so avoid tables that depend on columns.
Retell's developer notes say: "Everything the model writes gets read aloud, which changes what a good prompt looks like." Read each answer out loud before you add it.
What to leave out
Keep these out unless you have a specific, reviewed reason to include them.
- Customer, patient or employee records.
- Passwords, internal notes and staff home numbers.
- Discounts or exceptions you only give in certain cases.
- Anything that amounts to medical, legal or financial advice.
- Promises about results, timing or price that you can't always keep.
If you work with patients, read whether an AI receptionist can be HIPAA compliant before you connect any data. Our article on AI agent security risks explains why access should stay narrow.
Limits and fees to check
Knowledge bases have limits, and these vary by platform. Retell's figures on 9 November 2026 give you an idea of what to ask about.
| Item | Retell, per its docs and pricing |
|---|---|
| URLs per knowledge base | Up to 500 |
| Text snippets | Up to 50 |
| Uploaded documents | Up to 25, each up to 50 MB |
| Spreadsheets | Up to 1,000 rows and 50 columns |
| Knowledge bases | First 10 free, then $8 each per month |
| Usage | $0.005 per minute of calls with one enabled |
Retell's pricing page lists the same knowledge base fees. Its docs also say retrieved passages count toward the prompt token surcharge, and that retrieving more chunks raises the token count.
Retell's docs say retrieval should generally add under 100 milliseconds of delay. That is Retell's own figure, and we did not measure it. Packaged receptionist products may give you a simple form instead of these controls.
Keep it current
An out-of-date knowledge base gives confident wrong answers. Assign one person to own it, and tie updates to events.
- Update it the day hours, prices or staff change.
- Add holiday closures in advance.
- Review it on a set schedule, such as monthly.
- Fix it whenever a call review finds a wrong or missing answer.
Test before going live
Write 20 questions that callers really ask, and include a few the knowledge base can't answer. Call the agent and ask each one.
Check that the right fact came back, and that it was said clearly. When it can't find an answer, it should say so and offer a person or a message. It should never guess.
Next step
Draft your first version from your top 20 questions, and have staff check every line. If you need a custom agent connected to live data in your own systems, our AI agent development team can scope it with you.
Frequently asked questions
Can I just point it at my website?
Retell accepts website URLs, and other products may too. Your site may not answer the questions callers ask, and sales copy can be hard to read aloud. Add written answers for your most common questions.
How long should the knowledge base be?
Long enough to answer real questions, with no filler. Retell retrieves only a few passages by default, so extra text doesn't all get used. Start small and add what call reviews show is missing.
Should prices go in the knowledge base?
Include only prices you're willing to state on a call, and keep them updated. If a price depends on a site visit, say that instead. A wrong price is a problem someone will have to explain later.
Can it look up a customer's appointment?
That takes a connection to your calendar or scheduling system, not a knowledge base. A knowledge base holds reference information. Live lookups need a tool or integration.
What happens if the answer isn't there?
That depends on your instructions. Tell the agent to admit it doesn't know, then offer a transfer or take a message. Add the missing answer once you've reviewed the call.
