Your AI assistant prepares the fax. You check the preview and pay in the browser. No account. No sign-up. No subscription. Nothing is sent and nothing is charged before you pay.
The PayPerFax API is the pay per fax API for an AI assistant that is helping one person send one fax. It is the same pay-per-fax service that lives at payperfax.com, opened up so that ChatGPT, Claude, Gemini, Claude Code, Codex or Cursor can prepare the fax instead of you.
How it works, in three steps
- The assistant prepares the fax. It takes the destination number, your email address and either the letter it wrote for you or a file you gave it. It hands you back a link.
- You open the link and see the preview. The preview page shows the fax exactly as the receiving machine will see it, with the final guaranteed price. If a signed form is still missing, the link opens the fax form with everything your assistant filled in, ready for you to add the file.
- You pay and the fax goes. Payment is in the browser, by card or PayPal. The authorization is placed when you pay and the charge only lands when the fax is delivered. A failed fax is not charged.
The assistant never pays and never sends on your behalf. The link, the preview and the payment are yours.
Three ways in
If the assistant supports MCP, see the PayPerFax MCP server.
If your assistant can call the PayPerFax API
A coding agent with a shell (Claude Code, Codex, Cursor) or any script you write can send the fax directly over HTTP. The assistant gives PayPerFax the destination number, your email address, and either the letter it wrote (as text) or a file it has on disk (PDF, DOCX, JPG, PNG, TIFF or Markdown, up to 10 MB in total). PayPerFax hands back a link to the preview. The assistant gives you the link. You open it, see the preview with the final guaranteed price, and pay.
If a signed form or scan is still missing (a 2848, an SS-4, a bank authorization that needs your signature), the assistant can prepare the fax without the file. The link then opens the fax form, pre-filled with everything the assistant knows and with a short notice that reads “Your AI assistant prepared this fax.” You add the signed page yourself and continue in the browser.
The API reference is on the PayPerFax documentation site.
If your assistant has no PayPerFax tool
A chat app with no shell and no MCP connector (plain ChatGPT, Gemini, Claude without the connector set up) cannot call the service directly. It still helps, in the plainest way: it writes the letter for you and gives it to you as a PDF, a DOCX or a Markdown file. You open fax.payperfax.com and upload the file on the form. If the assistant can read web pages, it can also read the status from the tracking page after you pay.
For a plain-language breakdown of what each chat app can do today, see can ChatGPT or Claude send a fax for me?.
What it is good for
Letters your assistant can write and your signed paperwork can go with.
- A letter to a landlord, a doctor, a court or a school. The assistant drafts it from your notes. You read the preview, pay, and the letter is sent.
- An IRS form. The assistant writes the cover letter and finds the right fax number. For the signed form (2848, 8821, SS-4, 3911 and others), you add the scan yourself in the browser after the assistant hands you the pre-filled form. See the IRS fax number directory for the number to use for each form.
- A notice to a servicer or an agency. Change of address, benefit appeals, insurance replies, student loan paperwork.
- A one-off business fax from a coding agent. The agent sends a signed order, a termination letter or a redacted contract from the shell it is already in.
What this is not
The PayPerFax API is for an AI assistant helping one person send one fax. It is not a fax API to build faxing into a product.
- There are no accounts. There is nothing to sign up for.
- There is no background pipeline. If you need to know whether a fax has landed, your assistant reads the tracking page the same way you would.
- The assistant does not pay. A person pays, in the browser, every time.
- There is no sending without a payment step. “Pay per fax” is literal: one fax, one payment.
- There is no receiving a fax. The service sends only.
If you are building a system that embeds faxing (a healthcare platform that sends thousands of referral faxes a day, a loan servicer with its own fax workflow, an EHR), you need a different kind of service. We are not it.
Tracking a fax your assistant started
Every fax you pay for lands on a tracking page. The page shows the stage: queued, sending, delivered or failed. It also shows a short prompt you can copy and paste into your AI assistant, so the assistant can watch the status for you from its own session.
If your assistant is the one that started the fax, it can also watch the status itself. The documentation site has the detail.
Price
The PayPerFax API uses the same pay-per-fax pricing as the website. There is no API tier.
- $2 covers the first three pages (one optional cover page plus two document pages).
- $0.75 covers each page after that.
- A fax that fails to transmit is not charged.
The preview link shows the final guaranteed price before you pay. Billing is in your local currency where supported: USD, EUR, GBP, JPY or KRW. The full destination list and the per-country rates are on the pricing page.
Documentation for assistants and developers
The reference lives on the PayPerFax documentation site. Two short pages for an agent, two for a developer:
- Guide for AI assistants: the plain-language page for an assistant that is about to prepare a fax. The rules it needs: ask for the email address, do not retype a signed form as text, take an agency’s fax number from the official site, keep the letter inside a sensible size.
- Send a fax: the fields of the create call, with examples.
- Check the status: the status values and the polling rules.
- Fax a form to the IRS with an AI assistant: a worked example with a signed form.
The documentation is in English only. The link pages that the fax recipient sees (preview, filled form, tracking) are in six languages: English, Spanish, German, Japanese, Korean and French.
Short FAQ
Does my assistant pay for the fax?
No. The assistant prepares the fax and gives you a link. You open the link, see the preview with the price, and pay in the browser. Payment is always a person at a browser, every time.
Do I need an account to use this?
No. There is no sign-up on the website for a single fax, and nothing for your assistant to register either. If a service ever requires an account to send one fax, it is not this one.
Which files can my assistant send?
PDF, DOC, DOCX, JPG, PNG, TIFF and Markdown (.md). Ten megabytes total for a single fax. A Markdown file can be up to 100 KB and is rendered as a plain black-on-white document when the fax is sent. If a page in your fax needs a signature, send the signed scan as a file. Do not let the assistant retype a signed form as text.
Is it safe to let an assistant start a fax for me?
The assistant only prepares the fax. It cannot pay. It cannot send without you. Every fax has a preview link that only you open, and the preview shows the exact document and the exact fax number before you pay. The connection to PayPerFax uses TLS, and documents are deleted after 30 days. The full policy is on the security page.
Which assistants work with this today?
A coding agent with a shell (Claude Code, Codex, Cursor) can call the PayPerFax API directly over HTTP and hand you the preview link. A chat app on its own without a connector (ChatGPT, Gemini, Claude without the connector set up) can draft the letter for you to upload at fax.payperfax.com, and can read the tracking prompt back if it has web access. More ways in will arrive as the ecosystem does. If the assistant supports MCP, see the PayPerFax MCP server.