Saves a real model run to disk so evals can replay it later without calling the provider — free, deterministic regression tests.

## TLDR

- Recording keeps only the assistant's turns; everything else is regenerated by the framework during replay.
- Fixtures carry a format version: an outdated one demands a re-record instead of silently replaying wrong.
- A missing fixture just means the case runs live; a corrupt one is an error, because ignoring corruption would mask regressions.

## Before writing SPEC.md files

Read https://raw.githubusercontent.com/brillout/sdd/refs/heads/main/sdd.md
