Two ends disagree on valid forms

What goes wrong

Two stages on one path apply different rules about which forms of a value count as valid, so one end emits or forwards something the other refuses or quietly ignores. Each side is coherent and tested on its own, and each relies on the other end accepting exactly the same set of forms.

Example

A stored setting keeps a value from a wider range than the editing form now allows, so reopening the record and saving fails validation on a field nobody touched.

Not to be confused with

Not Guard And Consumer Normalise Differently, where both ends accept the same forms but one rewrites the value before comparing; here the two ends never agree on which forms are allowed at all.

Discovered by

@sks24git GitHub

Field notes · 4

@sks24git2026-10-15

A validator starts accepting a new kind of reference, but the code that acts on the field only knows the old kinds. Anything using the new kind passes the gate, then waits forever because nothing can clear it.

Trace every reader before you widen a format. A value one reader accepts and another cannot clear gets stuck.
Teach the consumer first, then open the gate. Otherwise valid input piles up.

@sks24git2026-09-28

A cleanup command read a report by its plain file name, but the report's contract also allowed a gzipped copy. Any week the report shipped compressed, the command found nothing and the backlog kept growing.

Read every form your contract allows. A reader that knows one of two file shapes turns the other into a silent no-op.

@sks24git2026-09-24beat 1 review round

A mapper copied the client's a schema verbatim into a field that a stricter upstream actually validates. Schemas with nullable type arrays, once dropped and answered in prose, now fail with a hard 400.

Check what a stricter field rejects before you fill it. Moving data to the field that enforces it turns silent loss into errors.

@sks24git2026-09-11beat 2 review rounds

A settings box was pre-filled with the value in force, then a repair narrowed what the box may hold while the deployment default stayed wide. The untouched box failed validation and the whole form stopped saving.

Edit the stored value, show the effective one. A draft seeded from what is in force inherits every source the validator never meant to judge.