---
title: "Worked example: purchasing app"
description: "A worked-through example — when a Custom AI App is the right choice and how the building blocks fit together"
canonical_url: "https://meingpt.com/en/docs/integrations/custom-ai-apps/example-purchasing-app"
language: en
---

# Worked example: purchasing app

The [overview](/en/docs/integrations/custom-ai-apps/overview) describes *when* a Custom AI App is a better fit than an assistant or a workflow. This page works through one concrete example so you can transfer the pattern to your own cases.

## The scenario

A purchasing team regularly sends queries about rescheduled delivery dates to many suppliers and has to keep track of the replies. Done by hand, that means: maintaining spreadsheets, writing emails one by one, reading replies, and updating status manually.

As a Custom AI App, this turns into one continuous flow:

1. An export file from the ERP system (changed delivery dates) is uploaded.
2. The app automatically writes the matching follow-up email per supplier.
3. Incoming replies are read by the AI and translated into a clear status (date confirmed, new date proposed, question raised).
4. The team sees at any time which queries are open, confirmed, or answered.

## Why a Custom App fits here

The overview names three signals — all three apply here:

- **Structured inputs** — the ERP export is a table, not free chat input.
- **Results to review and confirm** — buyers want to see and approve every supplier reply, not adopt it blindly.
- **A multi-step process with state** — a query runs through several steps over days; that state has to persist.

On top of that, two properties that a chat assistant or workflow doesn't capture well:

- **Its own data store** — the app keeps its own database of queries, suppliers, and status.
- **Background jobs** — emails run on a fixed schedule and a daily digest is sent automatically, even when nobody has the app open.

Rule of thumb: as soon as a task needs **its own state over time** and a **guided interface**, a Custom App is usually the better choice. For one-off question-and-answer tasks, the chat assistant stays simpler.

## How it's built

The app is a standalone web application with its own frontend and backend, embedded into meinGPT via an iframe. meinGPT handles sign-in and securely forwards the user context to the app.

| Building block | Role |
|---|---|
| **Interface (iframe)** | Table view of queries, detail dialogs, approval of replies |
| **Backend** | Own database, import logic, email sending, AI calls |
| **Auth proxy** | meinGPT as the identity source; the backend verifies every request against the meinGPT JWT |
| **Email integration** | Sending and receiving via a mail API (e.g. Microsoft Graph), credentials stay server-side |
| **AI evaluation** | Incoming replies are passed server-side to a language model and translated into a status |

The security-critical part — how your app only accepts requests coming from meinGPT and verifies the user context — is described separately under [Security & Authentication Proxy](/en/docs/integrations/custom-ai-apps/security-auth-proxy).

## The flow in detail

**Import** — The buyer uploads the ERP export. The app creates a query per row and matches it to the right supplier.

**Outbound** — For each open query, the app generates a follow-up email and sends it via the configured mail API. Sending runs on a fixed schedule in the background.

**AI evaluation** — When a reply arrives, the language model reads it and proposes a status: date confirmed, new date, or question needed.

**Review & approve** — The buyer sees the reply and the AI's proposal side by side and confirms or corrects the status. Nothing is marked final without human approval.

**Overview** — A daily summary goes out automatically; the overview shows at any time what is open, confirmed, or answered.

## The transferable pattern

Independent of the specific purchasing case, the pattern generalizes:

- **Structured data in** (upload or interface) → **AI processes** → **human reviews and approves** → **state persists**.
- meinGPT provides sign-in and identity; your app holds the data, the logic, and the connection to third-party systems.
- Credentials for email, ERP, or other APIs stay server-side in your app — the frontend only calls your own backend.

The same pattern carries, for example, for complaint handling, quote follow-ups, or processing incoming forms.

## Related docs

- [Overview: Custom AI Apps](/en/docs/integrations/custom-ai-apps/overview)
- [Security & Authentication Proxy](/en/docs/integrations/custom-ai-apps/security-auth-proxy)
- [App Building Tools](/en/docs/integrations/custom-ai-apps/app-building-tools)