I kept doing the same thing by hand: ask ChatGPT, copy the answer, paste it into Claude, ask "what's wrong with this?". So I turned that loop into an iPhone Shortcut. It's called C1M Duo , it's free and open source.

What it does

You ask once. ChatGPT answers. Claude reads that answer, checks it and fixes what's wrong. Then you get one result that opens with three short blocks:

Both agree

Where they differ

What to check yourself

…and the full checked answer below. It lands on screen, in the clipboard and in a markdown journal in your iCloud Drive — in about a minute.

How it works

On iOS 18 the ChatGPT and Claude apps publish their own Shortcuts actions: Ask ChatGPT and Ask Claude . Duo chains them inside one Shortcut. That's the whole trick, and it's why:

There is no server. The only network traffic is the two official apps talking to their own companies.

There are no API keys. It runs on the accounts you already have. Free tiers are enough.…