D051 - Fixtures are generated, never extracted

decided · 2026-08-19

Nothing derived from titles/ is ever committed - not bytes, not a header, not a trimmed copy, not a hex dump pasted into a test. If a fixture needs to resemble a real container, the generator builds it from the structure that was observed, never from the file.

Why this needs stating rather than assuming. tests/fixtures/synthetic/ is deliberately exempt from the provenance guard, because those files are committed on purpose and may legitimately carry banned extensions. That exemption is correct and it also makes the guard blind in precisely the place the temptation lands: mid parser work, "just save the first 4 KB of this real module as a fixture" is an easy and entirely natural thing to do, and nothing would catch it.

So this rule is unenforced by tooling and held by discipline alone - which is exactly why it is written down rather than left implicit. D014 bans firmware and dumps; it did not say anything about carving fixtures out of them.

Practical consequence: observations from real material are recorded as facts in the decision log (D049's magic bytes, offsets, and e_type values) and reproduced by code. The bytes themselves stay outside the repository.