Make Claude and ChatGPT argue until it's right.

One answers. The other finds the holes and fixes them. Back and forth until neither can find anything wrong, with you steering. Free, open source, and it runs on the plans you already pay for.

$npx spar-ai "your question"
Star on GitHub
Claude 0 · 0 ChatGPT
"Should I lease or buy my next car? I drive 30,000 km a year."

A second opinion that actually pushes back.

Claude and ChatGPT are better at catching each other's mistakes than their own. spar does the copy-paste between them, and keeps them honest.

  1. Serve

    You ask anything: a decision, a plan, a draft, a bug. One side gives its best answer, searching the web if it needs to.

  2. Return

    The other side looks for what's wrong, missing or invented, and sends back a better version. You can add a note on any turn.

  3. Concede

    It ends when one side looks at the other's answer and can't find a hole, or when you say so.

A spar match in the terminal: Claude serves, ChatGPT finds two problems, Claude concedes

The desktop app, for work you'll come back to.

Each side works the way its own app does: its own conversation, its own questions for you, its own history. You see what each one is doing as it happens.

Join the desktop beta
  • Write and CodeGrant applications to pull requests. In Code mode both agents work in your folder, every turn saved on its own branch.
  • Your historyImport your Claude and ChatGPT chats. Each side remembers its own.
  • ProjectsInstructions and files both sides read, every time.
  • Asks before actingClaude asks your permission the way it does in Claude Code. You choose what each side may touch.
  • Usage at a glanceSee how much of each plan you've used, and when it resets.
  • Any modelGemini, plus hundreds more with spar credits. No API key needed.

Free to start. Pay when it earns it.

Your Claude and ChatGPT plans do the work. spar never charges you for them.

Free

$0

The CLI, and the desktop app's essentials.

  • Unlimited spars and projects
  • Write and Code modes
  • History import, files, web search, images

Pro

$15 / month

Or $144 a year. Free for students.

  • Unlimited auto-rally
  • Every collaboration pattern
  • Smart memory across your past chats

Credits

From $10

Hundreds of models, pay as you go.

  • No API key to set up
  • Prices shown before you send
  • Top up any time

Questions

Does it use my Claude and ChatGPT accounts?
Yes. spar runs the official Claude Code and Codex tools on your computer, signed in by you. It never sees your password or tokens, and your prompts go only to Anthropic and OpenAI.
Won't they just agree with each other?
Both get the same short rules: be right, not agreeable; say pass instead of making busywork changes; mark anything uncertain; never invent sources.
Is it really free?
The command-line version is free and open source under Apache 2.0, for good. The desktop app has a free plan and a Pro plan.