Introduction

Read your company’s books from Synthetic over a plain REST API.

Synthetic keeps a company’s books. It connects to the company’s bank accounts, payroll and business tools, records what happened, and produces accrual-basis financial statements. The Synthetic API lets your own software read that data.

The API is a read-only REST API at https://app.synthetic.ai/api/v1. Responses from the API are JSON. Amounts are integers in cents. Timestamps are ISO 8601 strings.

Terms used in this API

Term	Meaning
Entity	The business whose books Synthetic keeps. An API key belongs to one entity and reads only its data. The GET /me route returns the entity a key belongs to.
Customer	Someone the entity bills.
Invoice	A bill the entity raises for a customer.
Payment	Money a customer paid the entity. Synthetic applies a payment to the customer’s invoices; the part not yet applied is its unapplied amount.
Bank fee	The fee a bank charged for processing a payment. Synthetic records it as a payment of type BANK_FEE, nested inside the payment it was charged against.
API key	A secret your software sends to prove who it is. Created in Synthetic under Settings → API keys.
Claim	A permission a key holds, such as reading payments. GET /me lists a key’s claims.

What you can read

Route	Returns
GET /health	Confirms the API is reachable. No key needed.
GET /me	The entity your key belongs to and the claims it holds.
GET /payments	The entity’s cash payments, one page at a time.
GET /payments/{id}	One cash payment by id.
GET /openapi.json	The OpenAPI document describing the routes above. No key needed.

Payments are the only resource today. Each payment carries its amount, how much of it has been applied to invoices, the customer it came from once one has been attributed, the date it was received, and its bank fee if one was charged. A bank fee is read through its payment and has no route of its own.

Your first request

1. In Synthetic, open Settings → API keys and create a key. Copy the secret when it is shown, because it is shown only once.

2. Confirm the key works:

   curl https://app.synthetic.ai/api/v1/me \
     -H "Authorization: Bearer <key>"
   

3. List payments:

   curl "https://app.synthetic.ai/api/v1/payments?limit=10" \
     -H "Authorization: Bearer <key>"
   

The API Reference tab lists every route except GET /openapi.json, with its parameters and response fields. It is generated from the API’s own OpenAPI document at https://app.synthetic.ai/api/v1/openapi.json.