Google Workspace add-ons, built, published and maintained.

The runtime is the difficulty, and it is specific rather than general. Google Apps Script is the only way to extend Docs and Sheets from inside, and it withholds most of what you would ordinarily reach for. Execution is capped at six minutes, after which the script is killed mid-operation. There is no batch traversal: reading a document happens one element at a time, and each read is a round trip to Google's backend, so walking a large report costs tens of thousands of them. And there is no concurrency, so async and await parse but nothing runs in the background and every network call blocks.

Design for those three first and the rest follows. Meet them late and you rewrite. StartDeck Writer is the full account of what they cost on an eighty-page document, StartDeck Tables & Maps covers the data side of the same system, and the platform behind both add-ons shows where the work that cannot safely live in an add-on belongs instead.

On top of the runtime, Google's add-on review process, permission scopes, and UI constraints (sidebars, dialogs, limited styling control) are stricter and less documented than a typical web app stack. An add-on also has to keep working across Google's own platform changes without breaking a client's daily workflow. It doesn't get to have downtime the way an internal tool might.