Mole Bug Patterns
tw93/Mole
A catalog of recurring bug shapes in the Mole Mac cleaner, used to review safety-sensitive diffs for deletion safety, unbounded commands, shell traps and weak tests.
Explains how to add a new in-process builtin command to Crush's embedded POSIX shell, so it is intercepted before reaching the OS, with context polling and exit-status rules.
Install Crush Shell Builtins
$ npx skills add charmbracelet/crush --skill shell-builtins -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install charmbracelet/crush shell-builtins --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/shell-builtins .claude/skills/shell-builtins && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .claude/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtinsType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add charmbracelet/crush --skill shell-builtins -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install charmbracelet/crush shell-builtins --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .agents/skills && cp -r skills-src/.agents/skills/shell-builtins .agents/skills/shell-builtins && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .agents/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add charmbracelet/crush --skill shell-builtins -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install charmbracelet/crush shell-builtins --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/.agents/skills/shell-builtins .cursor/skills/shell-builtins && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .cursor/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/charmbracelet/crush.git --path .agents/skills/shell-builtins--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add charmbracelet/crush --skill shell-builtins -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install charmbracelet/crush shell-builtins --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/.agents/skills/shell-builtins .gemini/skills/shell-builtins && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .gemini/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install charmbracelet/crush shell-builtinsInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add charmbracelet/crush --skill shell-builtins -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .github/skills && cp -r skills-src/.agents/skills/shell-builtins .github/skills/shell-builtins && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .github/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add charmbracelet/crush --skill shell-builtins -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install charmbracelet/crush shell-builtins --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/charmbracelet/crush.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/.agents/skills/shell-builtins .opencode/skills/shell-builtins && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "shell-builtins" agent skill from https://github.com/charmbracelet/crush/tree/main/.agents/skills/shell-builtins into .opencode/skills/shell-builtins/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "shell-builtins", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
shell-builtins- GitHub stars
- 29k
- Token cost
- ~790 tokens
- SKILL.md length
- 305 words
- Files
- 1
- Skills in repo
- 5
- Repo updated
- First seen
- Licence
- Custom licence
At a glance
Explains how to add a new in-process builtin command to Crush's embedded POSIX shell, so it is intercepted before reaching the OS, with context polling and exit-status rules.
- Works in 6 steps: Add the case to the switch in… → Get I/O from the handler context, not… → Implement the handler in its own file… → …
- Adding a new builtin command to Crush's embedded shell
- SKILL.md covers How Builtins Work, Adding a New Builtin and Existing Builtins
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
What it does
Crush's shell, built on mvdan.cc/sh/v3, intercepts commands through a builtinHandler switch in internal/shell/run.go, registered before the block handler so builtins run even for otherwise-blocked commands and are shared identically between the interactive shell and the hook runner's stateless entrypoint. Adding one means adding a case to that switch, reading stdin, stdout and stderr from the handler context rather than the OS streams so pipes and redirection keep working, and implementing the handler in its own file with a context.Context as its first parameter.
A builtin that loops over input or does any work that can exceed a few milliseconds must check ctx.Err() on every iteration and return it verbatim, because hook timeouts depend on that polling; a builtin that never checks cannot be interrupted and the hook runner has to abandon its goroutine instead. Return values distinguish success, a specific exit status, and a timeout error so callers can tell a non-zero exit from the handler running out of time.
When your agent uses it
- Adding a new builtin command to Crush's embedded shell
- Debugging why an existing shell builtin doesn't respect a hook timeout
- Understanding how Crush intercepts shell commands before they reach the OS
Example prompts
- “Add a new builtin called mytool that reads from stdin and writes a summary to stdout.”
- “Why doesn't my builtin get interrupted by the hook's timeout_sec?”
- “Explain how builtinHandler decides which commands it intercepts.”
Requirements
- A Go development environment for the Crush codebase
Workflow steps
6 steps, taken from the first numbered list in SKILL.md.
- Add the case to the switch in builtinHandler() in run.go.
- Get I/O from the handler context, not from os.Stdin/os.Stdout.
- Implement the handler in its own file (e.g.,
- Poll ctx in every unbounded loop. Builtins that iterate over
- Return values: return nil for success, interp.ExitStatus(n) for
- No extra wiring needed — builtinHandler() is already registered
What it can do on your machine
Read from SKILL.md and the folder at commit 140e8cb. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md (its code samples are go).
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Crush Shell Builtins loads about 790 tokens when it runs. Until then it costs about 52 tokens; SKILL.md has 305 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check passed
The automated check found no risky patterns in SKILL.md.
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
Its licence (Custom licence) doesn't allow us to republish the file, so here is its outline and opening line. It has 305 words (~790 tokens).
“Crush's shell (internal/shell/) uses mvdan.cc/sh/v3 for POSIX shell emulation. Commands can be intercepted before they reach the OS by adding builtins — functions handled in-process.”
- name
- shell-builtins
Files
Just SKILL.md in .agents/skills/shell-builtins of charmbracelet/crush.
Open the folder on GitHubat commit 140e8cb
Compare with similar skills
Crush Shell Builtins next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Crush Shell Builtins this skillcharmbracelet/crush | 29k | — | ~790 | Automated safety check: Pass | Custom licence | |
| Mole Bug Patternstw93/Mole | 69k | — | ~2k | Automated safety check: Pass | GPL-3.0 | |
| CLI DeveloperJeffallan/claude-skills | 12k | 1 repos | ~1.2k | Automated safety check: Pass | MIT | |
| Shellm Architecture Referencelaude-institute/headlong | 1.2k | — | ~2k | Automated safety check: Notes | Apache-2.0 | |
| Cppcrazyguitar/cppcheatsheet | 290 | — | ~1.8k | Automated safety check: Pass | MIT | |
| Adding Copyright Headersflutter/devtools | 1.7k | — | ~640 | Automated safety check: Pass | BSD-3-Clause |
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Questions about Crush Shell Builtins
What does Crush Shell Builtins do?
Explains how to add a new in-process builtin command to Crush's embedded POSIX shell, so it is intercepted before reaching the OS, with context polling and exit-status rules. go, registered before the block handler so builtins run even for otherwise-blocked commands and are shared identically between the interactive shell and the hook runner's stateless entrypoint.Context as its first parameter.
When should I use Crush Shell Builtins?
Crush Shell Builtins fits situations like: adding a new builtin command to Crush's embedded shell; debugging why an existing shell builtin doesn't respect a hook timeout; understanding how Crush intercepts shell commands before they reach the OS.
How do I install Crush Shell Builtins in Claude Code?
Run `npx skills add charmbracelet/crush --skill shell-builtins -a claude-code`. Or copy the skill folder (.agents/skills/shell-builtins in charmbracelet/crush) into .claude/skills/shell-builtins in your project. Claude Code loads it when a task matches its description.
How do I install Crush Shell Builtins in Codex?
Run `npx skills add charmbracelet/crush --skill shell-builtins -a codex`. Or copy the skill folder (.agents/skills/shell-builtins in charmbracelet/crush) into .agents/skills/shell-builtins in your project. Codex loads it when a task matches its description.
Can I use Crush Shell Builtins in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add charmbracelet/crush --skill shell-builtins -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/shell-builtins, .gemini/skills/shell-builtins, .github/skills/shell-builtins and .opencode/skills/shell-builtins in your project.
What does Crush Shell Builtins need to run?
SKILL.md names no scripts, command-line tools or credentials: Crush Shell Builtins is instructions for the agent only. Our summary lists: A Go development environment for the Crush codebase.
Does Crush Shell Builtins access the network?
SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.
Is Crush Shell Builtins safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Crush Shell Builtins use?
Crush Shell Builtins has a licence file (the repository's licence) that doesn't match a standard licence. Read it on GitHub before reusing the skill.
How many tokens does Crush Shell Builtins use?
About 790 tokens (SKILL.md is roughly 3.2k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.
What are the alternatives to Crush Shell Builtins?
Skills that share tags, products or a category with Crush Shell Builtins: Mole Bug Patterns (tw93/Mole, 69k stars), CLI Developer (Jeffallan/claude-skills, 12k stars), Shellm Architecture Reference (laude-institute/headlong, 1.2k stars) and Cpp (crazyguitar/cppcheatsheet, 290 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Crush Shell Builtins?
charmbracelet (a GitHub organization) maintains it in charmbracelet/crush, which has 28,514 GitHub stars. The repository holds 5 skills in this directory. The repository was last updated on October 7, 2026.
Source: charmbracelet/crush on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.