Calculator
Stdin-JSON add/subtract/multiply/divide script that Claude beats on its own
Test report
- Verdict
- Tested · Didn't pass
- Tested
- Jul 17, 2026
- Environment
- Claude Code 2.x (agent harness)
- Upstream re-checked
- Aug 10, 2026 · fbbd956
Ran the bundled script head-to-head against a no-skill baseline and the skill lost on correctness: it coerces every input to float, so adding 1 to 12345678901234567890 returned 1.2345678901234567e+19 -- the +1 silently vanished -- while the baseline (plain Python, what Claude does anyway) returned the exact 12345678901234567891. Simple cases like 847*293 matched the baseline but arrived wrapped in float noise (248171.0). The skill adds a subprocess round-trip to operations Claude already performs exactly, and its only failure-handling win (division-by-zero JSON error) doesn't offset the precision regression. Install is clean and the docs honestly describe a simple calculator, but a skill that can produce wrong arithmetic is worse than no skill.
Scored on four weighted criteria — install, triggering, output vs. baseline, docs. How scoring works
- Installs cleanly 5/5
- Triggers reliably 4/5
- Output vs. baseline 3/10
- Docs & honesty 4/5
What Calculator does
A minimal calculator skill: a Python script that reads a JSON operation from stdin and returns add, subtract, multiply, or divide results. Extracted from the evotown agent-arena repo. Triggers on basic arithmetic requests, where Claude's own computation is already exact and faster.
How to install Calculator
git clone https://github.com/EXboys/evotown
mkdir -p ~/.claude/skills
cd evotown && cp -r backend/arena_skills/calculator ~/.claude/skills/calculator
Skills live in ~/.claude/skills/ (global) or .claude/skills/
(per-project). Restart Claude Code after installing.
Commands — how to trigger Calculator
-
/calculatorStdin-JSON add/subtract/multiply/divide script that Claude beats on its own
It also activates on plain-language prompts like these:
-
Can you multiply 847 by 293 for me and give the exact number? -
I need 12345678901234567890 plus 1, and it has to be exact -
What's 20 divided by 4? Just double-checking the math here
Frequently asked questions
- Is the Calculator skill free?
- Yes. The skill itself is free from EXboys/evotown. SkillProof publishes the install command and an independent test verdict at no cost.
- Does Calculator work with Claude Code?
- We tested it with Claude Code 2.x (agent harness) on Jul 17, 2026. Verdict: Tested · Didn't pass. Ran the bundled script head-to-head against a no-skill baseline and the skill lost on correctness: it coerces every input to float, so adding 1 to 12345678901234567890 returned 1.2345678901234567e+19 -- the +1 silently vanished -- while the baseline (plain Python, what Claude does anyway) returned the exact 12345678901234567891. Simple cases like 847*293 matched the baseline but arrived wrapped in float noise (248171.0). The skill adds a subprocess round-trip to operations Claude already performs exactly, and its only failure-handling win (division-by-zero JSON error) doesn't offset the precision regression. Install is clean and the docs honestly describe a simple calculator, but a skill that can produce wrong arithmetic is worse than no skill.
- How do I install Calculator?
- Copy the install command from this page, run it in your terminal, and restart Claude Code. Skills live in ~/.claude/skills/ (global) or .claude/skills/ inside a project.
- Can I use Calculator with Cursor, Copilot, Gemini CLI, Codex or other AI tools?
- The SKILL.md format is native to Claude (Claude Code, Desktop, claude.ai). The instructions inside adapt to other assistants: Cursor rules, GitHub Copilot instructions, Windsurf rules, Custom GPTs, AGENTS.md for OpenAI Codex, and GEMINI.md for Google Gemini CLI — our conversion guides cover each, and the free converter on the tools page does the wrapping for you.