263 green tests, every value undefined: the schema mirror that agreed with itself

A developer shipped a build with a clean type check, 263 passing tests, and every nutrient value on the screen reading `undefined` ^[1]. The suite was green because it was comparing the frontend's hand-written copy of the backend schema against itself, which is the failure mode that reports success in exactly the tone of voice that means ship it ^[4][6].

The project is Pantensa, a household pantry and nutrition tracker, FastAPI and PostgreSQL behind a Vue frontend ^[2]. The work in flight was replacing eight hard-coded nutrient columns with a proper many-to-many catalog ^[3]. The frontend carried a mirror of the backend's read schemas: a TypeScript interface per Pydantic model, kept in sync by the author remembering to keep it in sync ^[4]. He renamed fields on the backend, the mirror kept describing the old shape, the tests kept asserting against the mirror, and the compiler agreed with both ^[5]. Internal consistency is not correctness, and a green suite in that configuration carries no information about the wire at all ^[6].

Two fixes were available and both were worse. Committing a fixture of the API response moves the problem one file over, because the fixture becomes the new unchecked copy and drifts the way the interfaces did ^[9]. Standing up a real FastAPI process inside Vitest pulls a cross-language runtime into the unit layer, which the author calls the wrong altitude and flaky besides ^[10].

What he did instead was remove the copy: the Pydantic schema is the single author of the wire, everything downstream is derived from it, and the derivation runs on a machine ^[11]. `frontend/openapi.json` is committed as a deterministic projection of `app.openapi()` ^[12]. Three gates hold the chain. A backend test asserts that `app.openapi()` equals the committed file, so renaming a field without regenerating fails the backend suite, on the side that made the change rather than the side that suffers from it ^[13]. `openapi-typescript` regenerates the TypeScript types from that file before every entry point, git-ignored, rebuilt by a pre-hook on type-check, lint, test, coverage, build and dev, so there is no stale artifact available to drift ^[14]. The frontend's read types are then derived with `Pick` over the generated schemas, so a rename that survives the first gate deletes a key and `vue-tsc` goes red ^[15]. The `Required<>` wrapper is load-bearing: OpenAPI marks defaulted fields optional, and stripping the `?` while keeping `| null` leaves the compiler as the guard instead of letting a `?? 0` silence it ^[16]. Net effect on the catalog path: one committed copy of the wire shape, and a test that asserts it against the generator ^[22].

One field is exempt on purpose. `nutrients` stays an explicit `Partial<Record<NutrientKey, number>>` rather than deriving from the generated index signature, because the sparse invariant, that a missing fiber value is not a fiber value of zero, is precisely what generation would erase ^[17]. The stated rule is to derive everything whose shape the backend owns, not everything ^[18].

The honest accounting is in the author's own cost note: the mechanism is repo-wide, the enforcement is catalog-only ^[19]. The derived types cover one bounded context; the rest of the frontend still reads the wire the old way and would still go green while lying ^[19]. He puts the coverage at about a quarter of the surface and says extending it is mechanical and not done ^[19][20].

Watch whether the remaining contexts actually get converted, since mechanical and undone is the state most gates die in ^[19]. Watch the exemption list, because every hand-maintained type is a mirror waiting to rot ^[17]. And watch the import layer, where the same discipline is applied differently: ESLint sorts every file into one of six element types and forbids any dependency it does not explicitly name ^[21].