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Base URL

All API requests use the following base URL:

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Authentication

Include your API key as a Bearer token in the Authorization header:
Sandbox keys start with mfx_test_, live keys with mfx_live_. The registration endpoints (POST /account/register and POST /account/register/verify) do not require authentication.

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Request Format

  • JSON for request bodies (Content-Type: application/json)
  • Multipart for file uploads (Content-Type: multipart/form-data)

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Response Format

All responses return JSON. Successful responses return the resource directly. Error responses follow a consistent structure:

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Idempotency

POST /fax accepts an optional idempotency_key form field. Duplicate requests with the same key return the original response without re-executing. Keys expire after 24 hours. See the Idempotency guide for details.

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Rate Limits

Rate limits are applied per API key. Current limits are returned in response headers:
  • X-RateLimit-Limit - requests per minute
  • X-RateLimit-Remaining - requests remaining in current window
  • X-RateLimit-Reset - seconds until the window resets

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Pagination

List endpoints return paginated results with 20 items per page. Use the page query parameter to navigate:

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OpenAPI Spec

The machine-readable OpenAPI 3.1 spec is available at:
Last modified on July 14, 2026