Sandbox
mintfax is a developer-first fax service, and the sandbox is where you build against it. It has the same API contract as live, with isolated fax records and isolated webhook endpoints. It is free, and no credit card is required to start.
What is in the sandbox
POST /v1/faxes, the same webhook events, the same error codes as live. Three behaviors differ.
Top-ups do not call a live payment provider. Your sandbox starts with a seeded virtual credit balance you can send against from the first request, and the credit lifecycle (hold on submit, capture on delivery, release on failure) matches live.
Every fax reaches a deterministic simulator rather than the PSTN. Any valid fax number is accepted and defaults to delivered; nine reserved magic numbers force specific outcomes for busy, no-answer, delayed-delivery, line-quality failure, and other paths.
The sandbox is active from the moment you register. Live only activates once you make your first successful credit purchase.
Sandbox API keys carry the mfx_test_ prefix. The prefix identifies the environment, so a sandbox key cannot reach live resources, and a live key cannot reach sandbox records.
Get started
Register with an email and verify with the 6-digit code. The response includes an mfx_test_ key against the sandbox that was created for you at registration. Post to /v1/faxes with to=+15005550001 and a file part; the fax reaches delivered in seconds.