Agency quote says "API Development"—what does Schnittstellenentwicklung actually cover?

"API Development" can sometimes be read broadly as architecting an API from scratch whereas the German "Schnittstellenentwicklung" almost always implies a bridge connecting two existing systems. If that's what's in the proposal they're talking about the middleware layer. Make sure the word "bidirektional" (two-way) is explicitly stated.

note: I wrote this based on my own experience, it might not apply to everyone.

From a legal perspective, if this falls under a contract for work (Werkvertrag), the deliverables must be tied to concrete, verifiable criteria. We strongly recommend requesting an appendix labeled "interface specification" (Schnittstellenspezifikation) where the exact field-mapping table is signed off by both parties.

We paid 9,200 Euro last year for an ERP-to-portal integration for our warehouse in Hamburg. The original price covered order sync, but later they treated real-time stock updates as out-of-scope change requests and billed us an extra 2,000 Euro. Lock down every single table and data field in the contract from day one.

Ask the agency a straightforward question: Does this price include pulling orders from the ERP, pushing product stock levels to the portal, and sending notifications when transfer errors occur? Get written confirmation via email for this and make it an integral appendix to the proposal contract.

Usually when agencies say Schnittstelle, they just set up a standard library and call it a day. Then data mismatches pop up because your ERP version is outdated, and they blame it on your ERP vendor. Don't sign the contract until you've gotten the tech leads from both sides into a joint meeting.

These 3 items must be explicitly stated in the proposal: 1) Data sync frequency (real-time trigger vs. periodic batch), 2) Error handling and logging mechanisms, 3) ERP staging/test environment setup and pre-launch testing phase.

Is this so-called Schnittstelle a standalone piece of software that lets two systems communicate with each other? I mean and will we have to pay extra development fees to our current ERP vendor during this process?

Request the data documentation from your ERP's technical team right away and forward it to the agency; without reviewing that doc, their 8.500 Euro quote is nothing more than a rough guess.

Absolutely. If I were to add anything: The system lives on after delivery; maintenance is a separate line item.

People defend habits, not processes. Resistance comes from there. I'm also curious if anyone does it differently.

I felt relieved reading this answer, so it's not just me. Any unwritten clause becomes a point of disagreement later, as both sides remember it differently.

Hope this helps.

I don't think this advice fits everyone. Start with a small trial; don't commit to everything at once.

Of course, it varies if your situation is different.

I'm a small business, let me explain from my side. The harder it is to reverse a decision, the slower you should make it.

I'm also curious if anyone does it differently.

I went through the same thing.

I agree, and I'd like to emphasize that. Just because everyone does it doesn't mean it's right.

Thanks a lot, I'll try it today.

noted, thanks.

I'll try it. The cheapest quote is usually the least thought-out one.

If you post the result here, it will help others too.

I partly agree, partly disagree. Processes without records never improve, because you don't know what to fix.

If you get three different answers on a topic, the question was asked wrong.