Two derivations from the same raw source will diverge, and the renderer picks the poorer one

The lesson

When the same raw payload is transformed in more than one place, the transformations drift, and nothing reports it: both run, both succeed, and the screen simply shows whichever one it happens to read. The symptom presents as missing data — a field that exists in the response and not on screen — which sends people to the API, then to the mapper, before anyone notices there are two mappers. Derive once and let the display narrow, rather than deriving twice at different fidelities. The boundary: independent selectors over the same source are fine when they answer genuinely different questions; the failure is two selectors answering the same question at different quality.

The failure that taught it

A summary strip showed a service's name but not its associated work order, while the reference showed the identifier, status and an icon. The payload carried the data and one selector already mapped it into typed objects with exactly the fields needed. A second selector, written separately, re-parsed the same slots into plain strings and dropped everything else — and that was the one the strip rendered, so there was nowhere for the missing fields to go.

How to apply it

When a field is present in the response but absent on screen, grep for every place that payload is transformed before touching the API or the component — the count is often more than one. Where two derivations exist, delete the narrower one and have its consumer read from the richer type. A selector returning an array of strings from a structured source is a strong signal that shape was discarded early.