The Readout Shortcut: Positional Number Copying Dominates Arithmetic CoT Readout in Small Language Models

In three linesOn 1-3B models, CoT in arithmetic relies on a positional shortcut: the model simply copies the number in the final position before the answer delimiter, regardless of intermediate reasoning. This strategy accounts for 54-92 pp of accuracy on GSM8K. Replacing that number with an incorrect value collapses performance even with correct steps.

Summary generated by Claude — human-verified