FastBill API

Name: FastBill API
Brand: FastBill API
Availability: InStock

✓ Official Vendor SpecFinanceAccountingbasic1 EndpointsREST

For Agents

Send invoicing, customer, and accounting service calls to FastBill's JSON-RPC endpoint to create invoices, manage customers, and record expenses.

Install Jentic One Beta

Connect the FastBill API to your agent

Jentic One is a self-hosted execution layer for AI agents. It lets your agent call the FastBill API, or any other public or private API you need. You set the rules, the agent never sees your credentials, and every call is logged.

Two steps, two machines. Install the instance in a safe environment, then register your agent from wherever it runs.

Step 1: Jentic One Host machine

# On the machine that will host your Jentic One instance:
curl -fsSL https://raw.githubusercontent.com/jentic/jentic-one/main/tools/install.sh | sh

Step 2: Agent machine

# On the machine where your agent runs (keep this separate from the instance):
curl -fsSL https://raw.githubusercontent.com/jentic/jentic-one/main/tools/install.sh | sh
jentic register       # connects your agent to your Jentic One instance

Jentic One is in public beta. The setup above keeps your agent separate from the instance, which is what you want before using real credentials: an agent running as the same OS user as Jentic One can read its stored keys directly. Just evaluating? A single local install is fine to start. See the secure deployment guide for the tiers.

Capabilities

What an agent can do with FastBill API.

Create and update customer records by POSTing customer.create and customer.update services to /api.php

Issue and fetch invoices through invoice.create, invoice.get, and invoice.complete service calls

Record expenses and revenue items by calling expense.create and revenue.create against /api.php

GET STARTED

Start building with FastBill API

Explore with Jentic

View OpenAPI Document

Use Cases

Patterns agents use FastBill API for, with concrete tasks.

★ Automated Invoice Issuance for German SaaS

German SaaS businesses use FastBill to issue compliant invoices on every successful subscription charge. The agent POSTs an invoice.create service call to /api.php with customer id and line items, then completes it via invoice.complete to issue the legal document. Replaces manual invoice creation in the FastBill UI.

POST to /api.php with service invoice.create and the customer id plus line items, then POST again with invoice.complete to issue the invoice

Expense Recording from Receipts

Bookkeepers and AI receipt scanners feed expenses into FastBill by POSTing expense.create payloads to /api.php. The pattern lets a receipt-scanning workflow capture vendor, amount, date, and category in a single call. Useful for small businesses that want a single source of truth for VAT-relevant transactions.

POST to /api.php with service expense.create and a payload containing vendor, amount, VAT rate, and date parsed from a receipt

Customer Sync from CRM to FastBill

Operations teams sync customers from their CRM to FastBill before billing runs by POSTing customer.create or customer.update service calls. Ensures every billable customer has a FastBill record without manual entry. Pairs well with deal-closed automations that should produce a draft invoice.

On a CRM 'deal won' event, POST customer.create to /api.php with the deal's customer details, then POST invoice.create with the agreed line items

AI Bookkeeper Agent via Jentic

Conversational bookkeeping agents use Jentic to call FastBill from natural-language prompts like 'invoice 500 EUR to ACME for consulting'. Jentic resolves the intent to the right /api.php service-name payload and supplies basic-auth credentials from its vault. The agent never sees the raw password and integration takes minutes rather than days.

Through Jentic, search 'create an invoice in fastbill', load the /api.php operation, and execute with service invoice.create and parsed user details

Key Endpoints

1 endpoints — the fastbill api exposes the german invoicing and accounting platform's services through a single json-rpc style endpoint at /api.

METHOD

PATH

DESCRIPTION

POST

/api.php

Universal JSON-RPC endpoint that routes by service name (e.g. invoice.create, customer.update)

POST

/api.php

Universal JSON-RPC endpoint that routes by service name (e.g. invoice.create, customer.update)

Why Jentic?

Three things that make agents converge on Jentic-routed access.

Credential isolation

FastBill basic-auth credentials (email + API key) are stored encrypted in the Jentic vault. Agents call by intent and Jentic injects the Authorization header at execution time — credentials never appear in prompt context.

Intent-based discovery

Agents search 'create an invoice in fastbill' and Jentic returns the /api.php operation with the service-name payload schema, so the agent picks the right service variant without reading the apidocs site.

Time to first call

Direct integration: 2-3 days to handle the JSON-RPC service-name pattern and the variety of payload schemas. Through Jentic: under an hour — search by intent, load, execute.

FAQs

Specific to using FastBill API through Jentic.

What authentication does the FastBill API use?

FastBill uses HTTP Basic authentication — your FastBill email and API key are sent in the Authorization header. Through Jentic both halves of the basic-auth credential live in the vault and are injected at execution time, never entering the agent's prompt.

Can I create invoices with the FastBill API?

Yes. POST to /api.php with a service field set to invoice.create and the invoice payload, then POST again with service invoice.complete to issue the invoice. The same /api.php endpoint handles every service name.

What are the rate limits for the FastBill API?

FastBill does not publish explicit rate limits in the OpenAPI spec. Treat it as a low-frequency back-office API, batch reads where possible, and back off on 429 responses.

How do I issue an invoice through Jentic?

Install with pip install jentic, search 'create an invoice in fastbill', load the POST /api.php operation, and execute with service invoice.create plus the customer id and line items. Sign up at https://app.jentic.com/sign-up.

Why is the FastBill API a single endpoint?

FastBill uses a JSON-RPC style design where every action POSTs to /api.php with a service-name field that routes to the correct handler. The single endpoint is by design — agents pick the right service via the payload rather than the URL path.