FIZ API · available now
Yes, FIZ has an API.
A REST API to issue certified invoices, create customers and items, and sync with the Portuguese tax authority (AT) — from your own software, store or platform. Plus an MCP connector to hook Claude or ChatGPT straight up to your invoicing, with no code at all. To connect your customers’ companies, create an OAuth app.
Create an invoice draft in a few lines
For your own company, use an API key in the x-api-key header. This request creates a draft: replace the CAE and IDs with your company’s values. Issuance is a separate action described in the API reference.
# Create a draft without issuing it
curl -X POST https://api.fiz.co/invoices \
-H "x-api-key: $FIZ_API_KEY" \
-H "Content-Type: application/json" \
-H "Idempotency-Key: order-1042-draft" \
-d '{
"type": "INVOICE",
"cae": "62010",
"customerId": "aaaaaaaaaaaaaaaaaaaaaaaa",
"items": [{ "id": "bbbbbbbbbbbbbbbbbbbbbbbb", "quantity": 1 }]
}'Platforms & integrators
Connect your customers to FIZ
Add Portuguese invoicing to your product. Your customer chooses a company and authorizes permissions in FIZ; your server receives OAuth tokens for that connection.
Partnership models and white-label →
1. Register your app
In Settings → Integrations, enter your app name, URLs and permissions. FIZ generates a client ID and a secret, shown once, for you to store on your server.
2. Connect a customer
Existing FIZ users choose their company and permissions. New customers create an account and can connect AT before returning to your app.
3. Create the first draft
Run the Node.js example with your app’s name and credentials. It saves a real invoice draft in the connected company without issuing the document. Open source, MIT licensed.
Test first, launch after review
Development apps run on live FIZ for you and up to 20 testers listed by email. You can add an email before its FIZ account exists. Company data is real: this is not a fiscal sandbox.
To connect other customers, create a separate Production app and submit it for FIZ review. Production connections require a company plan with API access.
New · MCP connector
Connect Claude or ChatGPT to your invoicing
FIZ is an MCP (Model Context Protocol) server. Connect it once and your assistant can look up your documents, customers and bank transactions, draft invoices and — only after you confirm each one — issue them. No API keys involved: it’s an OAuth connection you can revoke whenever you want.
https://api.fiz.co/mcp Claude
Settings → Connectors → Add custom connector. Paste the URL, sign in to FIZ and pick the company and permissions.
ChatGPT
Settings → Apps & Connectors → Create. Paste the URL, choose OAuth authentication and sign in to FIZ when prompted.
Claude Code
Run claude mcp add --transport http fiz https://api.fiz.co/mcp, then in the session /mcp → fiz → Authenticate.
Issuing always takes your confirmation
Issuing, recording payments, cancelling and raising credit notes are legally binding acts reported to the AT. The assistant always shows you a preview and acts only once you confirm — and if the document changed in the meantime, the operation is refused. On the authorization screen you pick one company and untick any permission you’d rather not grant.
What teams build with it
The most common builds from teams already integrating FIZ.
Online store → automatic invoice
Connect your store (Shopify, WooCommerce and similar) and issue the certified invoice the moment a sale completes — with no manual work.
Invoice when the customer pays
After your gateway (Stripe, ifthenpay, easypay…) confirms the payment, call FIZ to issue the document and email it to the customer.
Embed FIZ in your SaaS
Running a platform, marketplace or management software? Let your own customers invoice straight from your product, with FIZ handling tax compliance.
Migrate from another invoicing tool
On Moloni, InvoiceXpress, Sage or Vendus just for the API? You get what you need on FIZ — with AT sync already built in.
What you can do
Invoices, customers, items and PDFs through documented REST endpoints.
Invoices & receipts
Create drafts and issue Invoice, Invoice-Receipt, Simplified Invoice, Debit Note and Credit Note — with discounts, payment methods and withholding tax.
Customers & items
Create, update and search customers with validated NIF, and manage your product and service catalogue with VAT rates and exemptions by territory.
PDF, ATCUD & QR code
Download each document’s PDF with your own templates, ATCUD and QR code — ready to send to the customer.
Tax authority sync
Every issued document is automatically reported to the Portuguese tax authority. Track the sync status on every call.
Retries without duplicates
Use Idempotency-Key on writes. Completed requests replay their saved response. If the outcome is unknown, check the document before starting a new operation.
Agents with shell access
Prefer a skill over the connector?
For coding agents that already run commands — Claude Code, Cursor and the like — we publish an open-source skill that drives this REST API directly, with Portuguese tax knowledge built in: VAT rates, exemption codes, NIF validation and a required confirmation before the irreversible issue step. For everyday Claude and ChatGPT, the MCP connector above is the simpler route.
Ready to integrate?
Use an API key for your own company, or create an OAuth app to connect your customers. The integration guide includes a runnable example.
Get an API key Read the integration guide →
AT № 3041
NIPC 517696835
1200-082, Lisbon, Cais do Tojo 24