We gave Claude Code 100 numbers to add up

We came across a Reddit post from u/stony-breadwinner about Claude getting long financial calculations wrong. The concern was fair. A total can look believable and still be hundreds of dollars out.

Their suggested test was simple: give Claude 100 values and check the answer against a spreadsheet SUM. In the same thread, u/Strong-Archer-7708 tried Opus 4.6 five times and reported five wrong results.

We were curious, so we repeated it on 18 August 2026 with current models in Claude Code. The same 100 dollar amounts went into fresh chats. We checked the correct total, $50,431.54, separately using exact decimal code.

It’s a small test, and that’s all we’d claim for it. The useful bit was seeing three models recognise that code was the safer way to do the calculation. Haiku getting it right by itself was interesting too.

One run also reminded us how easily the input can go wrong: only 97 values made it into that chat, so we left it out.

For financial work, we’d still have Xero, a spreadsheet or exact code produce the official number. Claude can help query and explain the result, but the calculation should stay visible enough to check.