Stairs delivered without a railing and now I have to pay extra?

Attached is the architect's plan. It even says railing on it. But I'm supposed to buy that separately now.

The term railing is in my opinion meant for the stair railing. What you are supposed to buy in addition is a fall protection for the staircase opening. Take a look at your contract of work/order confirmation – does it say how to handle the design plans related to the contract of work? That is, an order in which positions (bill of quantities, drawings etc.) apply one after another? I am trying to find out whether your design plans – is the narrow double line around the staircase opening drawn in there? – have become a binding part of the contract of work. On the other hand, I am quite sure that the protection of the staircase opening – I don’t know the DIN now – according to the applicable technical regulations is the obligation of your supplier or his vicarious agents. Rhenish regards

ends at the newel post!

That is then a clear statement in the construction performance description

Clearly, according to the construction performance description, the handrail ends at the newel post. See #1. You could also read it differently, that only the stair railings are owed, which must necessarily include a handrail (but not stair railings without a handrail). I find that anything but clear; it rather falls into the category "How do I word the construction performance description so that the customer doesn't notice, but I have the smallest possible scope of work." A clear formulation would be something like "parapet provided by the client," which apparently does not exist.

I consider it (theoretically) promising to insist on complete fall protection, but now it depends on the circumstances.