Userscript work usually breaks at the runtime and metadata boundary, not in the page logic. Choose the runtime first, declare the minimum permissions up front, then debug in the environment where the script actually runs.

## When to Use

Use this skill for:
- Writing or fixing a Tampermonkey or ScriptCat userscript
- Debugging injection timing, missing permissions, CSP workarounds, or `GM_*` behavior
- Deciding between a portable foreground script and ScriptCat-only `@background` or `@crontab`
- Adding config UI with `==UserConfig==`
- Packaging a ScriptCat subscription bundle

**Do not use** for full browser extension development or general browser automation outside userscript managers.

## Runtime Selection

| Need | Choose |
|------|--------|
| Page DOM or page context | Portable `==UserScript==` foreground script |
| Persistent or scheduled work | ScriptCat `@background` or `@crontab` script |
| Install many scripts as one package | ScriptCat `==UserSubscribe==` |

## Preflight

- Confirm the manager and browser (MV3 browsers may require developer mode for ScriptCat)
- Decide page script vs background script before writing code — background scripts cannot touch the DOM
- Start with metadata: `@match`, `@grant`, `@connect`, `@run-at`, and update URLs
- Prefer portable `==UserScript==` patterns — only switch to ScriptCat-only headers when needed

## Quick Reference

| Intent | Default choice | Watch for |
|--------|----------------|-----------|
| Page UI, DOM scraping | Portable `==UserScript==` | `@match`, `@grant`, `@run-at`, CSP injection |
| Cross-origin API access | `GM_xmlhttpRequest` with explicit `@connect` | Missing hosts, cookie behavior |
| Long-running worker | ScriptCat `@background` | No DOM, must return `Promise` for async |
| Scheduled task | ScriptCat `@crontab` | Only first `@crontab` counts |
| User-editable settings | `==UserConfig==` plus `GM_getValue` | Block placement and `group.key` naming |

## Workflow

1. Choose the runtime and metadata first
2. Declare the smallest permission surface that fits the task
3. Implement against the runtime you chose
4. Debug where the code really runs:
   - Foreground scripts: page console plus manager logs
   - ScriptCat background scripts: run log first, then `background.html`
5. Publish with the right update model

## Common Mistakes

- Missing `@grant` for APIs the script actually uses
- Missing `@connect` for hosts used by `GM_xmlhttpRequest`
- Treating `@include` as a better default than `@match`
- Using DOM APIs inside ScriptCat background or cron scripts
- Returning from a ScriptCat background script before async GM work is truly finished
- Assuming Tampermonkey and ScriptCat storage/notification/request behavior is identical

---

*Mirrored from [https://github.com/xixu-me/skills](https://github.com/xixu-me/skills) — original author: **xixu-me**, license: **MIT**.*
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