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L402 API

Pay with Bitcoin, get AI results. Copy-paste the commands below.

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Before You Start

Lightning Wallet

Phoenix, Muun, Wallet of Satoshi, Alby, or any LN wallet. Agents: CLW

Terminal or Tool

A command line, Postman, or lnget (recommended)

Private & No Signup

No account, no subscription, no personal data. Pay only when you use it

Want to try this without a terminal? Use Send Fax on the web — pay with any Lightning wallet, no setup needed.

L402 Fax Sending API

Send a fax to any fax number worldwide. 500 sats for up to 10 pages, +50 sats per additional page (max 350).Two modes: send a PDF by URL, or type text and we generate a PDF server-side.

Endpoint: https://sats4ai.com/api/l402/send-fax

Step 1 — Request invoice (PDF mode)

curl -X POST https://sats4ai.com/api/l402/send-fax \
  -H "Content-Type: application/json" \
  -d '{"phone_number":"+15551234567","mode":"pdf","pdf_url":"https://example.com/document.pdf","cover_text":"Please find attached my signed W-9. — Jane","pages":3}' \
  -i

Or text mode

curl -X POST https://sats4ai.com/api/l402/send-fax \
  -H "Content-Type: application/json" \
  -d '{"phone_number":"+15551234567","mode":"text","text":"Please confirm receipt of order #12345 by end of day. Thanks.","pages":1}' \
  -i

Step 2 — Pay the Lightning invoice

Pay the invoice returned in the www-authenticate header. Your wallet returns a preimage.

Step 3 — Replay with auth

curl -X POST https://sats4ai.com/api/l402/send-fax \
  -H "Content-Type: application/json" \
  -H "Authorization: L402 <MACAROON>:<PREIMAGE>" \
  -d '{"phone_number":"+15551234567","mode":"text","text":"Please confirm receipt of order #12345 by end of day. Thanks.","pages":1}'

Success response:

{
  "status": "queued",
  "fax_id": "fax_abc123",
  "pages": 3,
  "paid_pages": 3,
  "amount_sats": 500
}

Parameters

phone_number: string

Destination fax number in E.164 format (e.g. +15551234567).

mode: "pdf" | "text"

pdf — we fetch a PDF from the provided URL.
text — we format your typed text into a PDF locally (no external dependencies).

pdf_url: string (mode=pdf)

Public HTTPS URL returning application/pdf. Max 50 MB, max 350 pages (Telnyx limit).

text: string (mode=text)

Message content to format as a PDF.

cover_text: string (optional, mode=pdf only)

Cover page text prepended to the PDF. Counts as +1 page.

pages: number (required)

Expected page count (1–350). Determines the invoice amount. We verify the actual page count after fetching/generating the PDF — if the PDF has more pages than you paid for, the request is rejected and your payment refunded via LNURL-withdraw. Overpayment is non-refundable, so don't wildly over-declare.

Pricing: max(500, 500 + (pages - 10) * 50). 10 pages = 500 sats. 15 pages = 750 sats. 50 pages = 2500 sats.

Limits

  • Max 350 pages per fax (Telnyx hard limit)
  • Max 50 MB file size
  • PDF only (DOCX / other formats: v2 — use the web UI for image-to-PDF today)