In packages/snapshot/src/client.ts, the properties mismatch error passes received and properties as actual/expected:

if (!propertiesPass) {
  return {
    pass: false,
    message: () => 'Snapshot properties mismatched',
    actual: received,      // full object
    expected: properties,  // subset — missing fields show as additions
  }
}

The diff is generated downstream by processError (packages/utils/src/error.ts), which just calls printDiffOrStringify(err.actual, err.expected). It has no knowledge of snapshot semantics — it faithfully diffs the two values it's given. So the fix needs to happen at the source, not in the diff renderer.

This is inherently tricky because the properties check is a subset equality — the user intentionally provides fewer keys than the full object. There's no "correct full expected" available at this point (the previous snapshot isn't loaded yet since snapshotState.match() hasn't been called).

Possible approaches:

• Filter actual to only the keys in properties — diff becomes { length: 11 } vs { length: 6 }. Clean and focused on what the user cares about, but loses context of the full object.
• Load the previous snapshot first to construct a full expected value — more accurate but adds complexity and couples the properties check to snapshot state.