LetterAgent
Mail physical letters by just asking your AI assistant.
A fax API lets software send faxes without a fax machine: an application hands over a document and a destination number, and the provider dials, transmits, and reports back with a delivery receipt. Healthcare intake systems, legal workflows, and insurance claims departments are the heaviest users, because their counterparties still run on fax.
The mechanics are simple from the developer's side. The application makes a request to the provider's endpoint with three things: the document (usually a PDF), the destination fax number, and optionally a cover page or callback address. The provider queues the fax, dials the number over its own phone infrastructure, transmits the pages, and then notifies the application of the outcome: sent, busy, no answer, or failed, often with a delivery receipt the application can store.
What the API hides is all the telephony. The developer never deals with dial tones, busy signals, retries, or modem speeds. The provider handles redialing a busy number on a schedule, and the application just polls for status or receives a webhook when the fax completes. That is the whole appeal: fax becomes one more background job in the software, like sending an email.
| Industry | What they fax through the API | Why not email instead |
|---|---|---|
| Healthcare | Referrals, prior authorizations, lab orders, intake packets | Receiving clinics and insurers still publish fax numbers, not secure inboxes |
| Legal | Court filings in fax-accepting jurisdictions, notices, signed retainers | Some courts and opposing offices treat fax as the official channel |
| Insurance | Claims documents, proof of loss, adjuster reports | Adjusters and repair shops still fax |
| Real estate and lending | Purchase agreements, disclosure packets, payoff requests | Title companies and lenders keep fax on their contact sheets |
| Government and compliance | Permit applications, tax correspondence, benefits paperwork | Agencies that list a fax number expect to receive one |
The common thread is not love of fax. It is that the other side of the transaction still runs on fax, and the business needs its software to talk to that side without a human standing at a machine. A patient intake portal that cannot fax a referral to the specialist's office is a portal that makes the front desk do it by hand.
Delivery receipts are the feature that separates a real fax API from a "send and hope" service. A good provider returns per-page status, the duration of the call, the remote machine's station ID, and a copy of the transmission report. That receipt is what compliance teams file: in healthcare and legal work, the question is not whether the fax left your system but whether the other machine acknowledged it, and the receipt is the evidence.
Two details to check when comparing providers: whether inbound fax is supported (many fax APIs are send-only, so receiving needs a separate setup) and how the provider handles cover pages and station IDs, since some receivers reject faxes with blank or generic headers.
LetterAgent takes the same idea one step further: instead of a developer wiring an API into an application, an AI assistant does the faxing on your behalf. You describe the fax to your AI assistant, approve the exact quoted price, and it is sent. There is an API under the hood for the assistant to call, but you never see it. LetterAgent's fax is outbound-only, to US and Canadian numbers, and currently in testing, so think of it as the fax API for people rather than for software.
Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant: it can check a job's status any time. If it ever can't handle something: hello@getletteragent.com