Runtime

The file on disk is a score. Nobody hears it until an orchestra sits down.

A runtime is that orchestra. Node, Bun, Deno, the Workers isolate — they all play TypeScript-ish music. They do not own the same instruments. Some have fs. Some have no disk. Some have a different fetch. Your .ts file is instructions. The runtime is the only thing that can obey them.

The builder asks you to pick one because a file that fs.readFiles will starve in a room with no pantry. create-hackhaton-space-stack then stamps that choice into the repo so bun run dev and the deploy are not two different concerts.

"Works on my machine"

That sentence almost always means: my orchestra is not their orchestra. You installed Bun locally and the host is Node 22. You used a Node API and shipped to an edge worker. The score is fine. The room changed.

A process is one living performance of that score. Kill the terminal and the performance ends. The score is still in the folder. Serverless cold starts are the orchestra walking onstage from a dark hallway. The notes are not slow. The lights were off.

When an agent says "I'll run this in the Bun runtime," they are naming the orchestra, not the song. Match it on purpose.

npx create-hackhaton-space-stack@latest my-app --frontend next --backend hono

That line packs a kitchen that already agrees with itself. Do not swap the runtime at 2 a.m. unless you like debugging missing globals. The CLI picked a server story. Keep the orchestra the host can actually seat.

What this unlocks

You stop treating the file as the app. The file is the score. The runtime is the night it gets played. Pick the orchestra you will actually deploy to, or keep the score boring enough that any of them can play it.