Measured after the DELTA fix: 53 engine outputs for 52 rows, but the client still received everything at once because each per-step payload is the newest row only and the row alignment treated it as a cumulative snapshot (one yield at the first row, one at the end). accumulate_codec_rows folds delta rows and the final consolidated matrix into one growing matrix; unit test drives the generator with per-step deltas and asserts one yield per step. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>