Worst-Case Tester
Feeds your UI long names, empty states and emoji, then reports what broke.
What it does
Try to break a piece of UI by feeding it worst-case data, long names, unbreakable emails, one-letter names, missing fields, huge counts, zero items, long labels, non-Latin text, emoji, extreme numbers, then render it behind a "Demo data / Worst case" toggle and report everything that broke, with the fix for each. Use when the user asks to stress-test, break, or find edge cases in a component or screen, or to "try the worst case". For visual design critique use design-engineers-eye; for motion use motion-review.
Install in under a minute
Mac or Linux, after you download
unzip worst-case-tester.zip -d ~/.claude/skills/Windows PowerShell
Expand-Archive worst-case-tester.zip -DestinationPath $HOME\.claude\skills- Start a new Claude Code session so it sees the skill.
- Ask in plain words, for example: "Use Worst-Case Tester on this project."
- Unzip the download, then zip just the worst-case-tester folder if your computer added an extra folder.
- In Claude.ai, open Settings, then Capabilities, and upload it under Skills.
- Turn the skill on and ask Claude to use it.
Worst-Case Tester uses the open SKILL.md format, so it also works in agents like Cursor, Gemini CLI, GitHub Copilot and Codex. Copy the worst-case-tester folder into that agent's skills folder, for example .cursor/skills/ or .github/skills/ in your project.
What's inside
The 7 main sections of the SKILL.md, in order.
- Initial Response
- Operating Posture
- Hard Rules
- Workflow
- Required Output Format
- Invocation Variants
- Tone
In the download: CATALOG.md, LICENSE, NOTICE.md, SKILL.md
Credit and license
Worst-Case Tester is a renamed, packaged copy of break-ui by Emil Kowalski (github.com/emilkowalski/skills), used under the MIT license. The license and a list of changes ship inside the download.
Questions
Does Worst-Case Tester work in Claude.ai, not just Claude Code?
Yes. In Claude.ai, upload the worst-case-tester folder as a zip in Settings under Capabilities. It also works in other agents that support Agent Skills, like Cursor, Gemini CLI and GitHub Copilot.
What do I get?
A zip with the worst-case-tester folder: its SKILL.md (4,332 words) and its license files. One payment, instant download.
Who made Worst-Case Tester?
It's a renamed, packaged copy of break-ui by Emil Kowalski, used under the MIT license. The license and a list of changes ship inside the download.