Fault injection with the existing host fakes for FixedPatchStepper( refinements=[]): an initialize upload failure, a BlockHaloCopy, BlockCurvedBoundary or BlockStep launch failure, and a failing final stream wait each raise, leave the instance failed (step/download refused) and keep the completed step count; a failed launch never reaches the wait. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>