F-sąskaita API

The F-sąskaita API lets you issue invoices, record expenses and download documents for the business you run in app.fsaskaita.lt. It is a small REST API: JSON in, JSON out, bearer tokens scoped to one business.

https://app.fsaskaita.lt/api

What you can do

Resource	Operations
Invoices	list, create, read, update, delete, download PDF
Expenses	list, create with a file, read, update, replace file, delete, download file
Profile	read the business behind the token
Currencies	list accepted currency codes
Webhooks	receive `invoice.` and `expense.` events at your HTTPS endpoint

Start here

Authentication: create a Business token and make the first request.
Invoices: the invoice model, numbering and the create request.
Expenses: multipart uploads and file replacement.
Webhooks: subscribe, verify the signature, handle retries.
Conventions: pagination, errors, rate limits, dates and money.

The Reference covers every endpoint. The specification itself is at /openapi.json.

For AI agents

Read /llms.txt for an index of every page, /llms-full.txt for the full text, and the agent page for a compact description of how to integrate.

Instructions for agentOpen as text

Implement invoicing in this project with the F-sąskaita API.

Read first, in this order:
1. https://docs.fsaskaita.lt/llms.txt — the index of every documentation page. Fetch the Invoices and Conventions guides from it.
2. https://docs.fsaskaita.lt/openapi.json — the Spec: every endpoint, field and enum.
Take field names and behaviour from the Spec. Where this prompt and the Spec disagree, the Spec wins.

Facts
- Base URL: https://app.fsaskaita.lt/api. JSON in, JSON out.
- Authentication: a Business token, scoped to one business, sent as `Authorization: Bearer <token>`. Read it from the FSASKAITA_TOKEN environment variable and keep it out of code, logs and version control.
- Send `Accept: application/json` on every request; without it, 401 and 429 responses are HTML.
- Rate limit: 60 requests per minute per business. On 429, wait `Retry-After` seconds, then retry.
- Validation errors: 422 with `{ "message", "errors": { "field.path": ["..."] } }`. Messages are in Lithuanian; show them to the user unchanged.
- Lists: `?page=N`, 50 per page, newest first, no filters. Single resources are wrapped in `{ "data": ... }`.
- Identifiers are UUIDs, dates are `YYYY-MM-DD`, money values are JSON numbers, currency codes are uppercase ISO 4217 and must appear in `GET /currencies`.

Build
1. One client module with a narrow interface: base URL, both headers, JSON encoding, and typed errors for 401, 404, 422, 429 and 500. Retry only on 429, and only GET, PUT and DELETE.
2. createInvoice(input): `POST /invoices`. Default `use_default_seller_info: true`. For each line in `products` send exactly one of `price`, `total` or `total_incl_vat`. Types ending in `vat_invoice` need `vat_percentage` on every line. Omit `invoice_number` so the series assigns the next one. Return `data` from the 201 response; it includes `share_link`, a public page where the buyer views and downloads the invoice.
3. downloadInvoicePdf(id): `GET /invoices/{id}/download`, called after create. Stream the body: there is no Content-Length, and the response can take up to about 20 seconds if the PDF is not ready yet.
4. updateInvoice(id, input): `PUT /invoices/{id}` replaces the whole invoice. Read it first, change what you need, and send it back including the current `invoice_number`; an update without it renumbers the invoice.
5. Webhooks, when the project needs them: an HTTPS endpoint that verifies the `Signature` header (hex HMAC-SHA256 of the raw body with the subscription secret) before parsing, and treats deliveries as at-least-once.

Rules
- There is no idempotency key. After a network error on POST, list recent invoices with `GET /invoices` and check before creating again.
- Every invoice the API creates is final and numbered; drafts exist only in the app.
- Tokens and webhook subscriptions have no API. A person creates them in the app under Settings, Integrations, and the integration reads them from configuration.

Done when
- Unit tests with recorded responses cover 201, 422 and 429.
- A live check runs only when FSASKAITA_TOKEN is set. It creates a real, numbered invoice, so ask before running it and delete the invoice afterwards with `DELETE /invoices/{id}`.
- Your report names what you built, which endpoints you used, and what in the Spec you left out.

Plan requirement

API access and webhooks are included in the Premium plan. On other plans the token and webhook settings are not available in the app.

Invoicing.
In your code.

Make your first request

Explore the Public API

Build with your agent

What you can do

Start here

For AI agents

Plan requirement

Invoicing.In your code.

Make your first request

Explore the Public API

Build with your agent

What you can do

Start here

For AI agents

Plan requirement

Invoicing.
In your code.