Arena for Claude Code: make 100 answers fight, keep the winner

Start here: paste this into Claude Code

Open a terminal, make an empty folder for your arena runs and start Claude Code in it. Then paste the prompt below. It installs Arena, runs the free cost check, and helps you write a good task, then stops and hands the actual /arena command to you, because the moment you type it, the run starts and spends usage.

The setup prompt (copy all of it) · lines 1-3 of 18. Copy all copies the whole prompt; read it in full on the page.

Hi Claude. Please be my patient setup helper. I run a home-service business and I'm not technical: one step at a time, tell me exactly what to type, and wait for me after each step.

Goal: install the free Arena skill for Claude Code (https://github.com/Jakeschincariol/arena-skill) as a personal skill, check what a run costs before anything runs, and help me write one good task so I can run a small test myself.

How the pieces connect

This is the manual way: every step and every command, the same ones the prompt above uses. Here's what happens inside one run, in the order the skill does it.

Words you'll see

• Skill: a folder with instructions (a SKILL.md file) that Claude Code loads when you type its name after a slash.
• Sub-agent: a separate copy of Claude that Claude Code starts to do one piece of the job. Every copy in the arena is a sub-agent of the same model you're running.

• Strategy card: the different way of thinking each copy is told to use. It's the only thing that makes the copies different from each other.
• Wave: a batch of sub-agents running at the same time. Claude Code runs about 10 at once by default, so 100 copies don't all go at the same moment.

• Rubric: the judge's scorecard: correctness 30, completeness 25, robustness (how well it held up to attacks) 20, specificity 15, clarity 10, out of 100.

Install Arena

Have Claude Code on a paid plan

Skip this if claude --version already prints a version. Claude Code needs a Pro, Max, Ultra, Team, Enterprise or Console account; the free plan doesn't include it.

Check git and Python

Arena's bookkeeping runs on Python 3.8 or newer, using only what comes with Python, so there's nothing else to install. Git is how you download it.

git --version
python3 --version

Copy the skill into your personal skills folder

A skill in ~/.claude/skills/ works in every project on your computer, and you call it with /arena. Claude Code watches that folder, so it shows up without a restart. If you never had a ~/.claude/skills folder before this session, type /reload-skills once.

Or: install it as a plugin instead

Same skill, installed from inside Claude Code. The difference is the name: plugin skills get the plugin's name in front, so you'd type /arena-skill:arena instead of /arena. Pick one way, not both.

/plugin marketplace add Jakeschincariol/arena-skill
/plugin install arena-skill@arena-skill

Check it's there

Start claude and type /ar. The command menu should list /arena. Don't press Enter on it yet: /arena on its own uses your last request as the task and starts a full 100-copy run.

First test: the free cost check, then 8 copies

Free first. The skill comes with a planner that only does math. It doesn't call Claude and writes nothing, so it costs nothing. Run it for the sizes you're thinking about:

From the Arena README (checked 2026-10-31). Each call gets bigger with a longer task, so the same count can cost more on a big job.

Then the smallest real run. Check where you stand with /usage first. Then pick something short where you can tell a good answer from a bad one, and run it with 8 copies. That's the size I'd learn on, before I ever try 100.

Example (made up): type this inside Claude Code

/arena --agents 8 Write the headline for a Facebook ad for a roofing company that does free roof inspections after storms. The reader is a homeowner scrolling on their phone. Under 10 words, no prices, no promises we can't back up, no "best in town". A great answer makes a homeowner with a leak stop scrolling.

Before it starts, the skill tells you the size in one line, and it'll suggest accept-edits mode (press Shift+Tab until the bottom of the screen says accept edits on). Without it, Claude Code asks your permission for every file the copies write, which on a big run is hundreds of yes clicks. Accept-edits only covers files inside the folder you started in, and the skill keeps everything in .arena/.

Good: you get one progress line per round, then the winning headline, why it won, how many rounds it went, and the attacks it beat. Bad: a vague task gives you a vague winner. That's the task's fault, not the arena's: tighten the task and run it again.

Read the winner and the attacks it survived

The attacks are the part I love. They're a list of everything another copy tried to break, and how the winner held up. Read them like a checklist of what could go wrong with your ad before a customer finds it.

winner prints the champion, its strategy card, where its answer is saved, and every match it won: the opponent, both scores, the judge's reason and the attacks it survived. status shows each round: how many were alive, how many got knocked out. Every run gets its own folder under .arena/, so older runs stay there.

Real use: when it's worth the usage

I'd save this for the stuff you're about to spend money on or send to a lot of people, where a better version is worth an hour of waiting. For an everyday question, one answer is fine. Start with --quick (16 copies) and only go to the full 100 when it really matters.

1. The opening lines of a video ad

/arena --quick Write the first two sentences a roofing company owner says to camera in a 30-second Facebook video ad for storm inspections. Spoken, casual, no prices, no guarantees, no claims about other companies. The viewer is a homeowner who just had a storm. A great answer gets them to keep watching past three seconds.

2. A follow-up text to a lead that went quiet

/arena --quick Write a text message from a home-service company to a homeowner who asked for a quote a week ago and stopped answering. Under 300 characters, friendly, one clear question they can answer yes or no, no pressure, no discounts, includes how to stop getting texts. A great answer gets a reply.

/arena --quick

The third one is for right after Claude gives you an answer you don't like. With no task text, Arena uses your last request as the task and your rejected answer as the one to beat, and at the end it scores your old answer against the winner, blind. Leave off --quick and it's the full 100.

Want the same cards and the same bracket again? Add --seed and a number (for example --seed 7). Leave --wave at 10 unless you've raised Claude Code's limit on how many tools run at once.

What it costs

Checked 2026-10-31 on claude.com/pricing. Prices change: check before you buy.

Where the real cost is: Arena itself is free, but every copy, every attack and every judge is a call to Claude, and on a subscription those calls come out of your plan's usage limits (a 5-hour limit and a weekly one, shared between Claude and Claude Code).

Anthropic doesn't publish a fixed number of calls per plan; it depends on the model, how long the conversation is and how big the files are. So I can't promise you a 100-copy run fits in Pro. Run /usage in Claude Code before and after your 8-copy test, and you'll see what a run costs you.

If you hit a limit, Anthropic's options are: wait for the reset, turn on usage credits, or buy API credits at API rates. Saying no to the API credit prompt keeps you on your plan.

When it breaks

The official docs behind every claim

• Arena skill on GitHub
• Arena: SKILL.md
• Arena: the judge's rubric
• Arena: bracket.py
• Claude Code: Advanced setup
• Claude Code: Skills
• Claude Code: Permission modes
• Claude Code: Commands
• Claude Help: Using Claude Code with your Pro or Max plan

• Claude: Plans and pricing

What each page backs up is listed at the end of the full guide.

That's the arena

Instead of Claude's first answer, you get the one that survived every attack, with the attacks right there to read, and you know what the run costs before you start it.

Want help setting this up? Receipts Group builds these systems.

Open the full guide as a page (every file in full, plus the table of contents).