---
title: "FastBill API: connect it to your AI chat · Hypamoon"
description: "The FastBill API is open, signs in with an API key and sends webhooks. What your AI chat can build with it on Hypamoon, with sources."
url: https://hypamoon.com/connect-to/fastbill
---

1.  [Connect to](https://hypamoon.com/connect-to)
2.  FastBill

Payments and billing

# Connect FastBill to Hypamoon

Yes. FastBill has a public, versioned API and you create the key yourself, so your chat can write the adapter.

Open

## The check

Three questions about the API of FastBill, answered from its own docs.

**Public API docs**\
**Yes.** [Read the docs](https://apidocs.fastbill.com/)

**A key you create yourself**\
**Yes.** Every plan includes the REST API, and each FastBill account has its own API key used with the login email. [fastbill.com](https://www.fastbill.com/preise)

**Versions or a changelog**\
**Yes.** Versioned service URL (api/1.0) and a dated changelog with entries up to October 2026. [apidocs.fastbill.com](https://apidocs.fastbill.com/fastbill/de/versioning.html)

**How it signs in**\
HTTP Basic auth with the account's login email as username and the account's API key as password.

**Webhooks**\
**Yes.** It can tell your app when something changes. [apidocs.fastbill.com](https://apidocs.fastbill.com/fastbill/de/versioning.html)

**Hosting**\
Cloud

## Your own invoicing flow on FastBill, built around how you sell.

-   Customers, estimates, invoices, recurring invoices, expenses and projects, straight from FastBill.
-   AI that drafts invoices from finished work and spots late payers, with you approving every invoice.
-   A screen for your team and a short weekly email.

### Ask your chat

Answers straight from your FastBill data, in Claude or another chat with the full Hypamoon connection.

-   Which invoices are still unpaid after their due date?
-   Which estimates have not been turned into an invoice yet?

### When something happens

FastBill sends a webhook to your app for some events, and the app asks AI what happens next. For the rest an agent checks every 15 minutes.

1.  Every 15 minutes

    Every Friday, finished projects and time entries.

    AI decides

    AI groups the hours per customer and drafts the invoice lines.

    Then

    A draft invoice is created in FastBill for you to check.

    AI proposes, you approve

2.  When

    An invoice is marked as paid.

    AI decides

    AI checks how late the payment was compared to earlier invoices.

    Then

    You get an email when a customer pays later than usual.

### A screen for your team or customers

An app on your own address, with sign-in, on top of the same FastBill data.

-   For your team

    Your team sees open invoices and estimates per customer.

FastBill stays the core: your chat builds the rest on its API, as an adapter plus an app and agents in your Hypamoon project. Reading starts after you click Allow in the portal. Every change an agent or a chat makes in FastBill waits for your approval, one call at a time. [How adapters work](https://hypamoon.com/docs/adapters)

### Good to know

-   Your adapter uses a fixed key: you paste it once in the Hypamoon portal, never in the chat, and it only goes to the API host you allow.
-   Call limits depend on the plan: 50 per hour on Solo, 100 on Plus, 500 on Pro and 1,000 on Premium. List queries return at most 100 items.

## Try it in your chat

Fill in what to read and write, and send it.

Say this in your chat

> Build the Hypamoon recipe build-an-adapter for FastBill. API docs: https://apidocs.fastbill.com/. Read: \[what\]. Write: \[what, or nothing\].

[Open in Claude](https://claude.ai/new?q=Build%20the%20Hypamoon%20recipe%20build-an-adapter%20for%20FastBill.%20API%20docs%3A%20https%3A%2F%2Fapidocs.fastbill.com%2F.%20Read%3A%20%5Bwhat%5D.%20Write%3A%20%5Bwhat%2C%20or%20nothing%5D.)

In Claude, or another chat with the full Hypamoon connection; adapters are not in ChatGPT yet. You paste the key in the portal, never in the chat. [The full recipe](https://hypamoon.com/r/build-an-adapter)

## Sources

Checked on 7 November 2026.

-   [apidocs.fastbill.com/](https://apidocs.fastbill.com/)
-   [www.fastbill.com/preise](https://www.fastbill.com/preise)
-   [apidocs.fastbill.com/fastbill/de/versioning.html](https://apidocs.fastbill.com/fastbill/de/versioning.html)

FastBill is a name of its owner, used here only to describe the software. This page is not an official integration and Hypamoon has no partnership with FastBill. Found something that changed? Tell us at [support](https://hypamoon.com/support#contact).

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## Tool not listed? Ask the chat to check its API.

The same three checks work for any software. Your chat reads the vendor's own docs and tells you what it found.

Say this in your chat

> Check if \[software\] has an open API that a Hypamoon adapter can use: public API docs, a key or token I can create myself without asking the vendor, and versions or a changelog. Link the pages you used.

[Open in Claude](https://claude.ai/new?q=Check%20if%20%5Bsoftware%5D%20has%20an%20open%20API%20that%20a%20Hypamoon%20adapter%20can%20use%3A%20public%20API%20docs%2C%20a%20key%20or%20token%20I%20can%20create%20myself%20without%20asking%20the%20vendor%2C%20and%20versions%20or%20a%20changelog.%20Link%20the%20pages%20you%20used.) [Open in ChatGPT](https://chatgpt.com/?q=Check%20if%20%5Bsoftware%5D%20has%20an%20open%20API%20that%20a%20Hypamoon%20adapter%20can%20use%3A%20public%20API%20docs%2C%20a%20key%20or%20token%20I%20can%20create%20myself%20without%20asking%20the%20vendor%2C%20and%20versions%20or%20a%20changelog.%20Link%20the%20pages%20you%20used.)

Fill in the name of your software. Checking needs no connection to Hypamoon.

## Ready to connect FastBill?

Connect your chat once, then ask it to build the adapter.

[Start free](https://app.hypamoon.com/?mode=signup&utm_source=connect-to&utm_campaign=fastbill)

Works in Claude, ChatGPT and Copilot. [Add to Claude](https://hypamoon.com/connect?via=connect-to~fastbill#claude) [Add to ChatGPT](https://hypamoon.com/connect?via=connect-to~fastbill#chatgpt) [Add to Copilot](https://hypamoon.com/docs/microsoft-copilot)