C1M Duo
Two rival AIs, one checked answer on your iPhone, free
One tap on iPhone: ChatGPT answers, Claude checks it, and you get one answer that opens with where they agree, where they differ and what to check yourself. Free, open source, no API keys, no server - it runs on the ChatGPT and Claude apps you already have. Nothing like it existed on phones before.
Everyone who uses ChatGPT and Claude knows the dance: ask one, copy the answer, paste it into the other, ask "is this right?". Duo does that dance for you, on the phone, in one tap. The result lands on screen, in the clipboard and in a journal file in iCloud Drive, in about a minute.
Want a third voice? Trio adds an AI from a third company on a free key: https://polyhelper.ai/duo/
Free and open source (Apache-2.0): https://github.com/vadimchernets/c1m-duo
Duo for Android - free on Google Play: https://play.google.com/store/apps/details?id=ai.polyhelper.a1
Made by Vadym Chernets, independent AI researcher (PhD); two of my posters on AI evaluation and verification are at NeurIPS 2026 workshops: https://neurips.cc/virtual/2026/178299 and https://neurips.cc/virtual/2026/loc/sydney/173020
Features
Runs on the ChatGPT and Claude apps you already have, through their official Ask ChatGPT and Ask Claude actions - no server, no API keys
Free accounts are enough; with Plus or Pro, the strongest models of two companies work together at no extra cost
12 modes in one shortcut: Critique (default), Debate until agreement, Dispute map, Synthesis, Side by side, Decision, Debate, Advisor, Clarify, Delta, Image, Auto
Put it on the Action Button, Back Tap or Siri and it becomes a second-opinion button
11 languages: English, Espanol, Portugues, Russian, Ukrainian, Deutsch, Francais, Hindi, Japanese, Korean, Chinese
Any iPhone on iOS 18 or later, an iPhone 14 runs it fine
Free and open source under Apache-2.0
Use Cases
Get a second opinion on any answer without copy-pasting between two apps
Have ChatGPT's answer checked by Claude before you rely on it
See at a glance where the two AIs agree, where they differ and what to check yourself
Settle a decision with a debate between two models
Requirements: iOS 18+, the ChatGPT and Claude apps installed and signed in; tap Always Allow when asked on first run
Comments
Hi, I'm Vadym, the maker of Duo. I kept doing the same thing by hand: ask ChatGPT, copy the answer into Claude, ask "is this right?". Duo does that in one tap on the iPhone, using the ChatGPT and Claude apps you already have, with no API keys and no server. Free and open source. I'd love to hear which of the 12 modes you end up using most, and what you'd want a second opinion on.
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Comments
Hi, I'm Vadym, the maker of Duo. I kept doing the same thing by hand: ask ChatGPT, copy the answer into Claude, ask "is this right?". Duo does that in one tap on the iPhone, using the ChatGPT and Claude apps you already have, with no API keys and no server. Free and open source. I'd love to hear which of the 12 modes you end up using most, and what you'd want a second opinion on.
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