FastBill

FastBill is German invoicing and bookkeeping. Agents manage customers, contacts, and articles, take invoices through their full lifecycle, send estimates, log project work time, and track revenues, expenses, and recurring invoices.

1 trigger 48 actions Basic auth available

FastBill website Platform Documentation Capability data verified 2026-08-24

A monthly billing run starts on a schedule

List Time Entries gathers billable minutes per project for the period

List Customers reads each client's country and VAT identifier

Create Invoice writes a draft with the lines priced from the article catalog

The agent re-reads the draft to confirm the line set was written once, not appended twice

The billing owner receives each draft with its VAT case and the reasoning behind it

The billing owner approves, Complete Invoice assigns the number, and the invoice is emailed

What This Integration Enables

FastBill is German invoicing and bookkeeping for businesses that sell time. Projects and time entries sit in the same product as the article catalog and the ledger, which is why the natural flow here runs from a timesheet to a numbered invoice rather than from an order to a receipt. Two things about the API shape a good agent design. Every call is a single POST carrying a service name rather than a REST route, and FastBill answers HTTP 200 even when a call fails, reporting the problem inside the body. The FlowRunner connector reads that error array on every call and raises, so a silent failure surfaces as a failed step instead of an empty result that a flow keeps building on.

FlowRunner agents keep customers and contacts in step with a CRM, price work from the article catalog, book time against projects, take invoices through their full lifecycle, convert accepted estimates, record revenues and expenses, and manage recurring billing. The realtime trigger reports the account's documented events with the whole invoice inline, so most flows need no follow-up read. What agents do not do here is decide the tax treatment of a document, because the same call with a different VAT case is a different legal statement. FlowRunner's human-in-the-loop design is how that decision reaches a person with the evidence already gathered rather than as an interruption.

Without FlowRunner

Billing week is a rebuild Timesheets are exported, totaled, and retyped into invoices every month

Tax treatment decided per invoice The domestic, reverse charge, or export question is answered from memory under time pressure

Payment status lags the bank Invoices are marked paid whenever someone next reconciles the account

With FlowRunner

Drafts arrive assembled Billable minutes become priced lines before anyone opens the billing screen

Tax treatment arrives argued The proposed case comes with the customer's country and VAT identifier attached

Payment status follows the money Matched receipts settle invoices as they land, and partial payments are escalated rather than guessed

Use Case Scenarios

A month of timesheets becomes a set of drafts

On the first working day of the month the agent runs the billing sweep. List Projects gives the active engagements, List Time Entries pulls the period's billable minutes for each, and the article catalog supplies the rate. Where time was tracked in Clockify or Harvest rather than in FastBill, Create Time Entry books it against the right project first, with billable minutes recorded below total minutes wherever time was deliberately written off. Create Invoice then writes a draft. The agent re-reads it before going further, because supplying items on an update appends them to whatever is already there unless replacement is explicitly requested, and a retried step is the ordinary way an invoice ends up with two of everything. The drafts land with the billing owner as a set, with the totals and the hours behind them.
The estimate that has been sitting for three weeks

An estimate goes out through Send Estimate By Email and the agent tracks it. If nothing happens it drafts a follow-up on a cadence and posts the account context into Slack for the owner. When the client accepts, Create Invoice From Estimate carries the agreed lines straight across rather than having anybody retype them, which is where quoted and billed figures usually start to differ. The deal record in HubSpot is updated with the invoice reference in the same run. The invoice is still a draft at the end of this, with no number and nothing sent, which is the point.
Receipts matched, partial payments escalated

The account's event feed reports each completed invoice with its totals inline. From there the agent watches the bank feed and the payments captured in Stripe for a matching amount and reference. A clean match calls Set Invoice Paid with the payment method written in FastBill's own wording. A partial payment does not: the agent posts the invoice, the amount received, and the shortfall to the finance owner rather than settling a document that is not settled. Marketplace payouts are recorded with Create Revenue and supplier costs with Create Expense, with one caveat worth planning around. Create Expense in this API cannot carry the scanned voucher, so the agent files the expense and flags the document that still has to be attached, rather than leaving the gap for someone to discover at audit.

Human-in-Loop Highlight

Create Invoice returns a draft with no number. Complete Invoice assigns the number, and from that moment the document exists in an unbroken sequence: the correction is Cancel Invoice, which writes a cancellation and keeps the numbering intact, rather than an edit. Two things ride on that call. The first is VAT Case, which is not a label but a legal choice among domestic supply, small business exemption, consumer sales inside and outside the EU, reverse charge to an EU business, and export outside the EU. It changes both the tax charged and the wording printed on the document. The second is the line set, which an appended update can have doubled without any error. So the billing agent completes nothing on its own. It posts: "Draft for Meridian GmbH, 8 lines, 14,200.00 EUR net, prices excluding VAT. The customer's country is AT and their VAT identifier is present and well formed, so the proposed case is reverse charge to an EU business, which prints no German VAT. Their last three invoices used the same case. Complete and send, or change the case?" A person answers in one message. The gathering, the pricing, and the line-set check were the agent's work. The statement about tax was not.

Agent processes routinely

Detects exception requiring judgment

Clear match Continues automatically

Ambiguous Routes to human via preferred channel

Human decides

Agent resumes with decision

Agent Capabilities

48 actions

Customers and Contacts

List Customers Lists customers with filters. A list without an explicit limit returns ten records rather than a full page, which is small enough to look like an empty account, so flows always set one.
Create Customer Creates a customer. The customer type decides which name fields are required, and direct debit additionally requires the bank name and IBAN or the call is refused.
Update Customer Updates a customer record.
Delete Customer Removes a customer.
List Contacts Lists the contact people held against customers.
Create Contact Adds a contact person to a customer.
Delete Contact Removes a contact person.

Invoices

List Invoices Lists invoices, with updated-from and updated-to filters that make a scheduled sync read only what changed.
Create Invoice Creates an invoice as a draft. It has no number yet and it is not a legal document.
Update Invoice Updates a draft. Supplying items appends them to the existing lines unless replacement is switched on, so the same update run twice leaves both sets.
Complete Invoice Assigns the invoice number and turns the draft into a numbered document. Only a completed invoice can be emailed or posted, and this is the operation this page's human gate exists for.
Cancel Invoice Cancels a completed invoice while keeping the numbering sequence intact. The correction path for a document that has already been issued.
Lock Invoice Locks an invoice against further change.
Set Invoice Paid Marks an invoice settled, with the payment method written in FastBill's own free-text wording rather than as one of the numeric payment types.
Send Invoice By Email Emails a completed invoice to its recipients.
Send Invoice By Post Sends the invoice as physical mail through FastBill's postal partner. It is billed per letter and cannot be recalled once queued.
Delete Invoice Deletes an invoice. For a completed document, canceling is the correct operation, because deleting breaks the numbering sequence.
List Invoice Items Reads the lines on an invoice. The check that catches an appended duplicate before completion.
Delete Invoice Item Removes a single line from a draft.

Estimates

List Estimates Lists estimates with their status.
Create Estimate Creates an estimate to send to a prospect.
Create Invoice From Estimate Converts an accepted estimate into an invoice, carrying the agreed lines across rather than having them retyped.
Send Estimate By Email Emails an estimate to the customer.
Delete Estimate Removes an estimate.

Articles

List Articles Lists the product and service catalog that prices invoice lines.
Create Article Adds an article with its price and tax rate.
Update Article Updates an article. Whether prices already include VAT is a separate flag from the tax rate, and it changes every total that references the article.
Delete Article Removes an article from the catalog.

Projects and Time

List Projects Lists projects. The unit that billable time is booked against.
Create Project Creates a project for a customer.
Delete Project Removes a project.
List Time Entries Lists tracked time for a project or period. The source of a time-based billing run.
Create Time Entry Books time against a project. Start time takes a full timestamp, and recording billable minutes below total minutes is how written-off time is captured deliberately.
Delete Time Entry Removes a time entry.

Bookkeeping

List Revenues Lists recorded revenues.
Create Revenue Records revenue that did not arrive through an invoice, such as a marketplace payout.
Set Revenue Paid Marks a recorded revenue settled.
Delete Revenue Removes a revenue record.
List Expenses Lists recorded costs.
Create Expense Records a supplier cost. This route cannot carry the scanned voucher, so a flow files the expense and flags the document that still needs attaching by hand.

Recurring

List Recurring Invoices Lists the recurring billing schedules on the account.
Create Recurring Invoice Creates a recurring invoice schedule for a retainer or subscription.
Delete Recurring Invoice Removes a recurring schedule.

Account and Documents

List Templates Lists the document templates the account prints with.
List Documents Lists the documents stored on the account.

Webhooks

List Webhooks Lists registered webhooks.
Create Webhook Registers a webhook against an endpoint of your choosing. Only the URL endpoint type works, despite an email type appearing in the documented values.
Delete Webhook Removes a registered webhook.

Triggers

1 triggers

Event Triggers

On FastBill Event Fires on the account's documented events across customers, contacts, invoices, and estimates, covering creation, update, deletion, completion, and cancellation as each object supports them. FlowRunner registers the webhook when the trigger is switched on and removes it when it is switched off. The payload is unusually complete, carrying the whole invoice with its items, totals, customer, and a document URL for the PDF, so most flows need no follow-up read. Deliveries carry no signature, because FastBill publishes no shared secret, so a flow acting on something consequential re-reads the record rather than trusting the body alone.

Frequently Asked Questions

What can FlowRunner do with FastBill?

FlowRunner agents can run List Customers, Create Customer, and Update Customer in FastBill, plus 45 more actions.

Does connecting FastBill to FlowRunner require OAuth?

No. FastBill connects to FlowRunner with basic authentication (a username and password), no OAuth flow required.

Can FastBill trigger a FlowRunner workflow automatically?

Yes. FastBill supports 1 trigger that can start a FlowRunner workflow automatically.

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