FastBill API
Official vendor OpenAPI document · agent-readyFinanceAccountingbasic1 EndpointsREST
For Agents
Send invoicing, customer, and accounting service calls to FastBill's JSON-RPC endpoint to create invoices, manage customers, and record expenses.
Use for: Create a new customer record in FastBill, Issue an invoice for a specific customer with line items, Retrieve all invoices issued in the last month, Record a business expense in FastBill
Not supported: Does not handle payment capture, payroll, or full ledger reporting - use for invoice, customer, and expense service calls only.
The FastBill API exposes the German invoicing and accounting platform's services through a single JSON-RPC style endpoint at /api.php. Every action - creating an invoice, fetching a customer, recording an expense - is performed by POSTing a service-name payload with parameters. The pattern is unusual for REST consumers but maps cleanly onto agent intents that route to the right service name.
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.
1
Step 1: Jentic One Host machine
# On the machine that will host your Jentic One instance:
curl -fsSL "https://jentic.com/install.sh?src=apis&api=%2Fapis%2Ffastbill.com%2Ffastbill" | sh2
Step 2: Agent machine
# On the machine where your agent runs (keep this separate from the instance):
curl -fsSL "https://jentic.com/install.sh?src=apis&api=%2Fapis%2Ffastbill.com%2Ffastbill" | sh
jentic register # connects your agent to your Jentic One instanceJentic 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
Pull customer or invoice lists with paginated service calls for accounting and reporting workflows
Trigger reminder and payment workflows by sending the appropriate service-name payload
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?
What agents get from Jentic-routed access to this vendor.
Setup
Wiring the FastBill API by hand means encoding email and API key into a basic Authorization header and packing the SERVICE name and filters into a single POST body against /api.php yourself. Through Jentic you install once, import FastBill from the API Directory, store the email and key once, and your agent calls it.
Permission scoping
FastBill routes every request through one /api.php endpoint with the target chosen in the request body, so limit the agent to the operations it needs, such as creating an invoice or reading a customer. You choose which services it may invoke, so a write service is included only when you add it.
Credential isolation
Your FastBill email and API key are stored once, encrypted, by your own Jentic One instance and injected into the Authorization header at execution time. They never enter the agent's prompt, logs, or context.
Intent-based discovery
Agents search Jentic by intent such as 'create an invoice in FastBill', and Jentic returns the /api.php operation with its service-name payload schema so the agent picks the right service variant without reading the apidocs site.
Related APIs
Alternatives and complements available in the Jentic catalogue.
Complementary
Stripe
Take payments online and feed records into FastBill for invoicing
Choose Stripe for the payment capture; FastBill handles the legal invoice issuance afterward
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. Get started with Jentic One, the self-hosted execution layer.
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.
Can I limit what my agent is allowed to do with the FastBill API?
Yes. Because you run Jentic One yourself, you decide which FastBill services the agent may send to the single /api.php endpoint, so it can be restricted to just the ones it needs, such as reading a customer with customer.get or creating an invoice with invoice.create. Since every action is chosen in the request body rather than the URL, a write service like invoice.complete or expense.create is only reachable when you explicitly allow it. Your FastBill email and API key stay under your control and are injected at execution time, so the agent never handles the raw credential.
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