API Reference

The Finsyncio REST API uses standard HTTP methods and JSON request/response bodies. All monetary amounts are in cents (integer, USD only in v1). All timestamps are ISO 8601 UTC. Idempotency keys are supported on all mutation endpoints.

Base URL

# Production
https://api.finsyncio.com

# Sandbox
https://sandbox.api.finsyncio.com

Authentication

Pass your API key in the Authorization header as a bearer token.

Never expose API keys client-side. All requests must originate from your backend. Use separate sandbox (sk_test_) and production (sk_live_) keys.

Error Codes

200 OK — Request succeeded

201 Created — Resource created

400 Bad Request — Invalid parameters

401 Unauthorized — Missing or invalid API key

402 Payment Required — Platform not active

422 Unprocessable — KYC failed or business rule violation

429 Rate Limited — Slow down

500 Server Error — Contact support if persistent

POST /v1/users

Create a user and run KYC verification. Returns user_id and kyc_status.

Request body

Field	Type	Required	Description
`first_name`	string	Yes	User's legal first name
`last_name`	string	Yes	User's legal last name
`email`	string	Yes	Email address
`date_of_birth`	string	Yes	ISO 8601 date: `YYYY-MM-DD`
`ssn_last4`	string	Yes	Last 4 digits of SSN
`address`	object	Yes	line1, city, state, zip

Response

{
  "user_id": "usr_7Pk3nM2wQx",
  "kyc_status": "approved",
  "created_at": "2026-04-10T09:12:31Z"
}

POST /v1/payments/initiate

Initiate an ACH or RTP payment between two KYC-approved users.

Request body

Field	Type	Required	Description
`amount_cents`	integer	Yes	Amount in cents. Minimum 100.
`currency`	string	Yes	Always `"usd"`
`recipient_id`	string	Yes	User ID of recipient
`type`	string	Yes	`ach_same_day` \| `ach_next_day` \| `rtp`
`memo`	string	No	Optional payment description (max 80 chars)

Response

{
  "payment_id": "pay_Xm7rNp4qW2",
  "status": "processing",
  "type": "ach_same_day",
  "amount_cents": 12500,
  "eta_hours": 4
}

POST /v1/loans/originate

Submit a loan origination request. Finsyncio runs automated underwriting and returns a decision in under 15 minutes for most requests.

Request body

Field	Type	Required	Description
`borrower_id`	string	Yes	KYC-approved user ID
`amount_cents`	integer	Yes	Requested loan principal in cents
`term_months`	integer	Yes	Loan term: 3–36 months
`product_type`	string	Yes	`term_loan` \| `bnpl` \| `line_of_credit`

POST /v1/cards/issue

Issue a virtual or physical card to a KYC-approved user. Virtual cards are ready for use immediately upon creation.

Request body

Field	Type	Required	Description
`user_id`	string	Yes	KYC-approved user ID
`card_type`	string	Yes	`virtual` \| `physical`
`spending_limit_cents`	integer	Yes	Per-cycle spending limit in cents
`currency`	string	Yes	Always `"usd"`

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