Vapi vs Retell AI: which one should you build your voice agent on?

Short answer: pick Retell AI if you want a phone agent working quickly, with telephony, a website-crawling knowledge base and chat agents inside one product. Pick Vapi if you want to choose every provider yourself (speech-to-text, model, voice, telephony) and build through the API. At normal settings both cost between 7 and 15 cents a minute. And on both, the agent answers from the knowledge base you give it, so a vague one gets you wrong answers on either.

Everything below comes from Vapi's and Retell's own pricing pages and documentation, checked on 21 September 2026. Prices on both change often, so check again before you quote a client.

Vapi vs Retell at a glance

	Vapi	Retell AI
How you pay	$0.05/min hosting, plus each provider at cost	One per-minute price, $0.07 to $0.31/min depending on model and voice
Free to start	$5 in credits	$10 in credits
Monthly plans	Free, Core $29/mo, Pro from $999/mo, Premier custom	Pay as you go, Enterprise custom
Concurrent calls included	4 on the free plan, extra lines $10/month each	20 free, extra $8/month each
Knowledge base sources	Uploaded files only	Website URLs and whole-site crawls, files, pasted text, Google Drive, OneDrive, Notion
Knowledge base cost	Not billed separately	First 10 free, then $8 per knowledge base per month, plus $0.005 a minute on calls that use one
Keeps a website current	No, you re-upload	Re-fetches URLs and re-crawls paths every 24 hours
Chunks the agent sees	Set through the query tool	1 to 10 per turn, default 3
Chat agents	Not covered in the pages we checked	Yes, from $0.002 per message
HIPAA	$2,000/month add-on	Not stated in the pages we checked

What a minute of Vapi actually costs

Vapi's $0.05 a minute is only the platform. The rest of the call is passed through at the providers' cost, and Vapi's pricing page lists what that usually is:

• Transcription (Deepgram): $0.0095 to $0.0099 a minute
• The model (OpenAI): $0.0077 to $0.0452 a minute
• The voice (ElevenLabs): $0.0146 to $0.0238 a minute
• Telephony: free over Vapi's own web and WebSocket calling, $0.008 to $0.014 a minute over Twilio, Vonage or Telnyx

Add those up and a phone call on Vapi runs from about $0.08 to $0.14 a minute. A client doing 1,000 minutes a month costs you somewhere between $80 and $140 in usage. Core at $29 a month is optional, a support package rather than a requirement.

What a minute of Retell AI actually costs

Retell prices the agent as one number, $0.07 to $0.31 a minute, built from pieces on its pricing page: $0.055 a minute for Retell's voice infrastructure, $0.015 for most voices ($0.040 for ElevenLabs voices), $0.015 for telephony, and the model, which is priced per minute and makes up most of the spread. The same 1,000-minute client costs you between $70 and $310, and the model you pick decides which end.

Two smaller Retell costs matter to agencies. Knowledge bases are free up to ten, then $8 each a month, so your eleventh client adds a line to the bill, and every call that uses a knowledge base costs $0.005 a minute more. And 20 concurrent calls are included, which is generous next to Vapi's four on the free plan.

The knowledge base is where they differ most

This is the part people skim when they compare the two, and it is the part that produces the wrong answers.

Vapi takes files. Its docs list .txt, .pdf, .docx, .doc, .csv, .md, .tsv, .yaml, .json, .xml and .log, and suggest keeping each file under 300 KB. When you attach files, Vapi creates a query tool for them. The docs also say the description alone is not enough: "You must explicitly instruct your assistant in its system prompt about when to use the query tool." If you skip that line, the agent answers from its own general knowledge, and that is a common reason a Vapi agent sounds confident and knows nothing about the business.

Retell takes almost anything. A single page, an entire site, documents in more than twenty-five formats, pasted text, and synced Google Drive, OneDrive or Notion. It re-fetches URLs every 24 hours and re-crawls whole paths on the same schedule. You can set how many chunks the agent gets per turn, from 1 to 10 (the default is 3), and how similar a chunk must be to count as a match.

Retell's auto-crawl is convenient, but it has a cost. It keeps the knowledge base current with the website as written. If the fees page has a table of six treatments and prices, the crawl splits that table into chunks. With three chunks per turn, the chunk the agent sees can have the $350 whitening price in it and no treatment name. That is how an agent quotes the wrong price for implants, and the next crawl brings the same table back tomorrow.

On both platforms the fix is the same: the text you load has to name the product in every sentence that has a number in it. How to write a chatbot knowledge base covers that in detail.

Which one should you pick?

• You are an agency on GoHighLevel, or your clients live in a CRM. Retell. Its docs list integrations with GoHighLevel, Salesforce, HubSpot, Dynamics 365 and Zoho.
• You want to pick your own speech, model and voice providers, or switch them per client. Vapi. That is what its pricing model is built around.
• You run many clients at once. Look at concurrency and knowledge base limits first. Retell includes 20 concurrent calls and 10 knowledge bases. On Vapi's free plan you get 4 concurrent calls and pay $10 a month for each extra line.
• You need HIPAA. Vapi lists it as a $2,000-a-month add-on. We could not find Retell's position on the pages we checked, so ask their sales team.
• Your callers speak Hindi, Tamil or Hinglish, and you need Indian telephony. Look at Bolna as well. It prices in rupees and dollars and supports ten or more Indian languages. See Vapi alternatives.
• You also need a website or WhatsApp chatbot. Retell covers both voice and chat. On Vapi you would add a separate chat tool.

Whichever one you pick

Neither platform decides what your agent knows. You do, through the knowledge base, and both will read out a wrong price just as fluently. Before a client goes live, ask the agent the price of every service by name, one after another, and ask it something the website never says. If it answers that second question confidently, the agent is guessing.

That checking is what we built KB Builder for. It turns a client's website and documents into a knowledge base, with every price tied to the thing it belongs to, and it gives you a list of what the site never says. The same file loads into Vapi or Retell.

Once you've chosen: set up its knowledge base properly

Each platform stores and searches a knowledge base differently: what files it takes, how many pieces it hands the agent per turn, and whether the agent searches at all without being told. Our setup guides cover that from each platform's own documentation, with the prompt lines we add after our own live calls:

• Setting up the Vapi knowledge base
• Setting up the Retell AI knowledge base

KB Builder is made by MasterBlocks, which also makes SpeakEdge, a voice agent platform. We left SpeakEdge out of this comparison. We are not affiliated with Vapi or Retell AI.