Sinch Fax API

Overview

The Sinch Fax API lets you send and receive faxes programmatically. It supports multiple file formats, webhooks for incoming faxes, fax-to-email delivery, and automatic retries. Used for healthcare, legal, financial, and government applications where fax remains a required communication channel.

Auth: HTTP Basic (

keyId:keySecret

) or OAuth2 bearer token. All examples use Basic auth. See sinch-authentication for setup.

Getting Started

Before generating code, gather from the user: approach (SDK or direct API), language (Node.js, Python, Java, .NET, curl), and use case (sending, receiving, fax-to-email, or managing services). Do not assume defaults.

When the user chooses SDK, fetch the relevant API reference docs linked in Links for accurate method signatures. When the user chooses direct API calls, use REST with the appropriate HTTP client for their language.

Language	Package	Install
Node.js	`@sinch/sdk-core` (preview — not recommended for production)	`npm install @sinch/sdk-core`
Python	`sinch`	`pip install sinch`
Java	`com.sinch.sdk:sinch-sdk-java`	Maven dependency
.NET	`Sinch`	`dotnet add package Sinch`

First API Call — Send a Fax

curl (replace

{projectId}

with your project ID from the Sinch dashboard):

bash

curl -X POST \
  'https://fax.api.sinch.com/v3/projects/{projectId}/faxes' \
  -H 'Content-Type: application/json' \
  -u YOUR_key_id:YOUR_key_secret \
  -d '{
    "to": "+12025550134",
    "contentUrl": "https://example.com/document.pdf",
    "callbackUrl": "https://yourserver.com/fax-callback"
  }'

Node.js SDK: See Send a Fax with Node.js.

Test number: Send to

+19898989898

to emulate a real fax without charges (always suggest this for integration testing).

Key Concepts

Fax Services — Logical containers for fax configuration. Associate numbers, set defaults, and manage routing. Fax Numbers — Phone numbers provisioned for fax. Must be configured in your Sinch dashboard. Faxes — Individual fax transmissions (inbound or outbound). Each has a unique ID, status, and metadata. Fax statuses —

QUEUED

→

IN_PROGRESS

→

COMPLETED

FAILURE

. Error details in

errorType

and

errorMessage

fields. Supported formats — PDF (most reliable), DOC, DOCX, TIF/TIFF, JPG, PNG, TXT, HTML. Webhooks/Callbacks — HTTP POST notifications for fax events. Default content type is

multipart/form-data

(fax content as attachment). Set

callbackUrlContentType: "application/json"

for JSON callbacks. Cover Pages — Customizable cover pages per service. Attach via

coverPageId

and

coverPageData

on send. Fax-to-Email — Incoming faxes auto-forwarded to email addresses. Retries — Auto-retry on failure. Default set per fax service; maximum: 5. Retention — Fax logs and media retained for 13 months. Use

DELETE /faxes/{id}/file

to remove earlier.

Common Patterns

Three ways to deliver content:

contentUrl

for URLs (recommended — supports basic auth),

multipart/form-data

for local files, or

contentBase64

for in-memory bytes.

contentUrl

can be a single URL or an array of URLs to compose multi-document faxes.

For HTTPS URLs, ensure your SSL certificate (including intermediate certs) is valid and up-to-date. You can optionally specify

from

to set the sender number.

Send a fax (URL, file upload, base64, multiple recipients) — See Send a Fax endpoint. Use
```
multipart/form-data
```
for local files, JSON with
```
contentUrl
```
for URLs.
Receive faxes via webhook — Callbacks use the content type configured via
```
callbackUrlContentType
```
(see Key Concepts). Check
```
direction === 'INBOUND'
```
on the fax object. See Receive a Fax with Node.js.
Fax-to-email — Configure via API or dashboard. Incoming faxes auto-forward to the configured email. See Fax-to-Email Reference.
List faxes — See Faxes Endpoint Reference
Get fax details —
```
GET /faxes/{id}
```
Download fax content —
```
GET /faxes/{id}/file.pdf
```
(
```
.pdf
```
suffix required)
Delete fax content —
```
DELETE /faxes/{id}/file
```
(removes stored content before 13-month expiry)
Manage fax services — See Services Endpoint Reference
Manage cover pages —
```
POST/GET/DELETE /services/{id}/coverPages
```
— see Services reference
Manage fax-to-email — See Fax-to-Email Reference

Troubleshooting

Fax not delivered

Check fax status via

GET /faxes/{id}

— look at

status

errorType

(

DOCUMENT_CONVERSION_ERROR

CALL_ERROR

FAX_ERROR

FATAL_ERROR

GENERAL_ERROR

), and

errorMessage

If
```
contentUrl
```
was used with HTTPS, verify the SSL certificate (including intermediate certs) is valid
Fax delivery depends on the receiving machine answering — retries are automatic (max 5, default set per service)

Fax content renders incorrectly

Complex DOC/DOCX formatting may not render perfectly on receiving machines. Recommend PDF instead.

Cannot send or receive faxes

Verify the number has fax capability enabled in the Sinch dashboard
Numbers must be provisioned for fax before use

Gotchas and Best Practices

Use
```
callbackUrl
```
for status tracking — fax delivery is async. Prefer callbacks over polling.
PDF is the safest format for reliable rendering on receiving machines.
Fax logs and media are retained for 13 months. Use
```
DELETE /faxes/{id}/file
```
to remove earlier, or download and archive if longer retention is needed.
International fax success rates vary by country — some have specific dialing prefix requirements.
Use
```
resolution: "SUPERFINE"
```
(400 dpi) for faxes with small text or detailed images; default
```
FINE
```
(200 dpi) works for most cases.

sinch-fax-api

NPX Install

Tags

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