The bug only showed up once the feature started working

Falsifier first: if you can find a fourth production call site that builds a Transformation and reconstructs its target field differently from the three I'm about to describe, this post is wrong about "all of them." I counted by grepping for the one function that computes a transformation's identity and checking every call site by hand. Three. If there's a fourth, the bug I'm describing isn't fully fixed. Here's the shape of it. Engine::plan_shape has a doc comment that says, more or less, "this isn't a second place where transformation identity gets defined, because it's the same code as the one true place." That claim was false, and it had been false since the field it's talking about was added. The actual second place was Engine::rehydrate_committed . Its job is to rebuild a Transformation from the journal when a fresh CLI process needs to undo something a previous process committed. Every gx undo call from a cold process goes through it. And for one field, target , it wasn't rebuilding anything. It wrote a hardcoded placeholder. Nobody noticed, because nothing disagreed with the placeholder. Every adapter shipping at the time also produced the placeholder for that field, by omission rather than by design, so the two sides matched by coincidence. A missing value that's always missing on both sides of a comparison is invisible. cargo check doesn't catch it because the type is Option<T> and None is a completely legal value of that type. Nothing was wrong, until something else became right. What made it right was landing the two adapters that finally do predict target , fs and git, so their production plan() calls started filling in the real value instead of leaving it empty. The moment that shipped, cold-process undo broke for every fs or git transformation: gx_code=INTERNAL detail="TransformationId(...) is Committed, and 43 §3 has no `rehydrate: the rebuilt transformation names another id, so the intent supplied is not the one this transformation was planned from`