FastBill API

Name: FastBill API 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.

Quickstart

Get started with FastBill API in minutes using your preferred integration method.

# Add to your MCP client config (Claude Desktop, Cursor, Windsurf)
{
  "jentic": {
    "url": "https://api.jentic.com/mcp",
    "auth": "oauth"
  }
}

# Then ask your agent:
"create an invoice in fastbill"

# → Jentic returns the GET /events tool with parameter schema, agent executes.

Capabilities

What an agent can do with FastBill API 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

GET STARTED

Start building with FastBill API API

Explore with Jentic

View OpenAPI Document

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.

Use Cases

Patterns agents use FastBill API 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 though 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.

Related APIs

Alternatives and complements available in the Jentic catalogue.

Alternative

Xero Accounting

Full REST accounting API used outside the German market

Choose Xero for English-speaking markets and full ledger features; choose FastBill for German-localised invoicing and tax handling

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

Alternative

Sage

Larger-business accounting alternative when FastBill outgrows the use case

Choose Sage for multi-entity accounting; FastBill suits sole traders and small German businesses

FAQs

Specific to using FastBill API 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.