I made ChatGPT and Claude check each other on my iPhone — no API keys, no server

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. And if you already pay for Plus or Pro, you get the strongest models of two companies working together at no extra cost.
• There is no computer. Everything runs on the phone. Put it on the Action Button, Back Tap or Siri and it becomes a "second opinion" button.

This kind of thing has existed on computers — browser extensions, paid AI hubs, API setups. On a phone, free, with nothing in between, I couldn't find anything like it. So I built it.

12 modes in one shortcut

Critique is the default (3 messages). The others are one tap away, and each shows its message cost before you pick:

• Debate until agreement — ChatGPT takes a side, Claude objects, up to three rounds until one concedes
• Dispute map — where they agree, where they disagree, what is checkable; no verdict imposed
• Synthesis — both answer blind, one anchors, the other adds only what's missing
• Side by side, Decision, Debate, Advisor, Clarify, Delta, Image, Auto

Trio adds a third model from a third company on any free key (NVIDIA → Gemini → Groq → OpenRouter) that names only what both of the others missed.

Under the hood

• One shortcut, modes routed through a dictionary.
• Prompts are plain text files in locales/; the shortcut is built from them for 11 languages.
• tools/verify.py unpacks the signed .shortcut and checks it matches the sources.
• Keys for Trio live in iCloud Drive, never inside the shortcut, so sharing it never shares a key.

Works on any iPhone with iOS 18 or later (I run it on an iPhone 14 Pro Max). Duo works on Android too, free.

Try it

• Source (Apache-2.0): https://github.com/vadimchernets/c1m-duo
• One-tap install, 11 languages: https://polyhelper.ai/duo/

I'd love to hear which modes you'd add — or where Claude's check made a good answer worse.

I'm Vadym Chernets, an independent AI researcher (PhD); two of my posters on AI evaluation and verification are at NeurIPS 2026 workshops: https://neurips.cc/virtual/2026/178299 · https://neurips.cc/virtual/2026/loc/sydney/173020