I built a vault so a skill or an agent could store structured items without me rewriting the domain each time. That meant designing the DRF API for a non-human caller: stable UUIDs, an open metadata field, and a discoverable OpenAPI schema. Then I probed it for idempotency and found I never built any.
An accounting kernel that must never say a word to a user. Every domain failure gets a typed exception and a stable machine code; one adapter table turns that code into localized prose. Here's the hierarchy, the import cycle it caused, the contract check that reported green through a hole, and the bug I found writing this.
Storing recipes, bookmarks, journals and groceries behind a single REST surface: a polymorphic Item base, dynamically composed nested serializers, atomic multi-table writes, and the eager-loading that kills the N+1 queries.