The Flex API allows you to integrate HSA/FSA payment capabilities directly into your application. This guide covers the basics of how to get started with the API, authentication, and core concepts.

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Authentication

All API requests must include your API key in the Authorization header: 
Authorization: Bearer YOUR_API_KEY
Always keep your API keys secure and never expose them in client-side code. You can generate and manage API keys in the Partner Dashboard.

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Base URL

All API requests should be made to the following base URL: 
https://api.withflex.com/v1

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Content type

API requests with a request body should specify the content type: 
Content-Type: application/json

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Test mode

Flex provides a test mode environment that allows you to test your integration without processing real payments. Test mode API keys start with fsk_test_, while production API keys start with fsk_.
Be sure to replace your test API keys with production keys when you’re ready to go live.

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API response format

All API responses are returned in JSON format. Successful responses will include the requested resource(s), while error responses will include an error code and message. Example successful response: 
{
  "checkout_session": {
    "checkout_session_id": "fcs_01HW5KTQQAAQ66WV19MFW4DTT4",
    "status": "complete",
    "amount_total": 4999,
    "redirect_url": "https://checkout.withflex.com/pay/c/fcs_01HW5KTQQAAQ66WV19MFW4DTT4",
    "mode": "payment"
    // ...other properties
  }
}
Example error response: 
{
  "error": {
    "code": "invalid_request",
    "message": "Missing required parameter: price"
  }
}

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Rate limits

The Flex API enforces rate limits to ensure stability. If you exceed the rate limit, you’ll receive a 429 Too Many Requests response. The response headers will include information about when you can retry.

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Core resources

The Flex API is organized around these core resources:
ResourceDescription
ProductsWhat you’re selling (goods or services)
PricesHow much and how often to charge for products
Checkout SessionsA session representing a purchase attempt
CustomersThe buyers of your products
Payment IntentsThe intent to make a payment
SubscriptionsRecurring payment arrangements

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Next steps

Explore these guides to learn more about specific aspects of the Flex API:

Products & Prices

Checkout Integration

Subscriptions

Webhooks

For a complete reference of all API endpoints, see the API Reference section.