Spec a resumable upload for 5 GB files

Your mobile app needs to upload files of up to 5 GB over flaky connections, and an upload that drops at 80% should carry on from where it stopped. The backend is TypeScript on Node with Fastify, and files land in S3. A coding agent will build it in stages.

Write the build spec you would hand the agent: the protocol, the stages in order, the acceptance checks, and where it stops for your review.

What you know:

• every upload counts against the user's storage quota
• the mobile team can call any HTTP API you define
• the bucket is private, and only the server holds AWS credentials

Then, in a few sentences: what would a one-shot "build resumable uploads" prompt get wrong?