Before anything can run anywhere, it has to be made into something that runs, out of your code and a lot of other people's.
Build
The step that turns source code into the thing that actually runs: compiling, bundling, and copying everything into one output.
It is the first time all of the code has to fit together. A build also pulls things in from outside, such as dependencies, so it is where a change nobody made can arrive.
Say it to your agent
“Run bun run build from a clean checkout, in CI and not on your machine, and show me the output. If it needs anything that isn't in the repo or the lockfile, tell me what, because that means it builds only here.”