A setting such as a booking link works on the server but shows up as undefined in the browser, with no error message and a passing build. In an AI-built (or hand-built) Next.js app, the usual causes are a variable name without the NEXT_PUBLIC_ prefix, or a variable added after the app was built.

We reproduced it on a small synthetic app, applied the smallest fix, and checked it by clicking the button in headless Chrome. Below: the symptom, the cause, the fix, how we verified it, and what we did not test.

Short answer: If a setting such as your booking link or API address works on the server but shows up as undefined in the browser, either the variable's name doesn't start with NEXT_PUBLIC_ , or you added it after the app was built. The smallest fix is to rename it (and the line of code that reads it) with the NEXT_PUBLIC_ prefix, then rebuild .…