Claude Sonnet adds its own sums when summarizing tables

A test on 24 small tables found that Claude Sonnet added numbers and comparisons absent from the data in eight cases, including differences, ratios and totals. Claude Haiku made no such errors, even though all of Sonnet's computed figures were arithmetically correct.

• Sonnet added computed figures on 8 of 24 tables
• Haiku added no numbers beyond the table data
• All of Sonnet's computed figures were arithmetically correct
• Readers cannot tell a model's calculation from a printed number