Cross Origin Isolation
thedaviddias/Front-End-Checklist
A skill your agent uses when reviewing security-sensitive web apps, SharedArrayBuffer usage, worker-heavy apps, editors, or measurement features that require cross-origin isolation.
Write and run isolated pytester-based plugin tests. An agent skill from ethereum/execution-specs.
CC0-1.0Auto-check passed
Install Pytester
$ npx skills add ethereum/execution-specs --skill pytester -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install ethereum/execution-specs pytester --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/ethereum/execution-specs.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/pytester .claude/skills/pytester && 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 "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .claude/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytesterType 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 ethereum/execution-specs --skill pytester -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install ethereum/execution-specs pytester --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/ethereum/execution-specs.git skills-src && mkdir -p .agents/skills && cp -r skills-src/.agents/skills/pytester .agents/skills/pytester && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .agents/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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 ethereum/execution-specs --skill pytester -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install ethereum/execution-specs pytester --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/ethereum/execution-specs.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/.agents/skills/pytester .cursor/skills/pytester && 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 "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .cursor/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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/ethereum/execution-specs.git --path .agents/skills/pytester--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 ethereum/execution-specs --skill pytester -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install ethereum/execution-specs pytester --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/ethereum/execution-specs.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/.agents/skills/pytester .gemini/skills/pytester && 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 "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .gemini/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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 ethereum/execution-specs pytesterInstalls 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 ethereum/execution-specs --skill pytester -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/ethereum/execution-specs.git skills-src && mkdir -p .github/skills && cp -r skills-src/.agents/skills/pytester .github/skills/pytester && 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 "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .github/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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 ethereum/execution-specs --skill pytester -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install ethereum/execution-specs pytester --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/ethereum/execution-specs.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/.agents/skills/pytester .opencode/skills/pytester && 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 "pytester" agent skill from https://github.com/ethereum/execution-specs/tree/forks%2Fbogota/.agents/skills/pytester into .opencode/skills/pytester/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "pytester", 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
pytester- GitHub stars
- 1.2k
- Token cost
- ~403 tokens
- SKILL.md length
- 111 words
- Files
- 1
- Skills in repo
- 13
- Repo updated
- First seen
- Licence
- CC0-1.0
At a glance
Write and run isolated pytester-based plugin tests. An agent skill from ethereum/execution-specs.
- SKILL.md covers Which execution mode to use, Expected inner failures and RunResult API
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
What it does
Pytester is an agent skill from ethereum/execution-specs. Write and run isolated pytester-based plugin tests.
Its SKILL.md is about 400 tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.
The repository describes itself as: Specification for the Execution Layer. Tracking network upgrades. The licence is CC0-1.0.
Example prompts
- “/pytester”
Requirements
- Python 3
What it can do on your machine
Read from SKILL.md and the folder at commit 3d544d7. 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 python).
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
Pytester loads about 403 tokens when it runs. Until then it costs about 15 tokens; SKILL.md has 111 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
The full file from ethereum/execution-specs at commit 3d544d7, republished under its CC0-1.0 licence (© ethereum). 111 words, ~403 tokens.
Download SKILL.mdSave it as
.claude/skills/pytester/SKILL.md (or your agent's skills folder).- name
- pytester
- description
- Write and run isolated pytester-based plugin tests.
Pytester
Guide for pytester-based plugin/CLI tests. Run before writing or modifying these tests.
Which execution mode to use
runpytest()— default. In-process, fast, fullRunResultAPI (assert_outcomes(),fnmatch_lines()).runpytest_subprocess()— use only when in-process causes state leakage (Pydantic cache pollution, global mutation inpytest_configure). SameRunResultAPI.- Raw
subprocess.run()— never use alongside pytester. Userunpytest_subprocess()instead.
Subprocess isolation masks bugs rather than fixing them. Prefer fixing the root cause and use subprocess as defense-in-depth.
Expected inner failures
runpytest_subprocess() replays inner output to outer stdout (by design). Suppress with capsys.readouterr():
def test_expected_failure(pytester: Any, capsys: Any, pytestconfig: Any) -> None:
result = pytester.runpytest_subprocess(...)
capsys.readouterr() # suppress inner failure bleed
assert result.ret != 0
output = "\n".join(result.outlines + result.errlines)
# conditional print for -s debugging
if pytestconfig.getoption("capture") == "no":
with capsys.disabled():
print(output)RunResult API
Prefer assert_outcomes() and fnmatch_lines() over manual any(... in line ...) — better failure messages.
result.ret— exit coderesult.outlines/result.errlines— line listsresult.assert_outcomes(passed=N, failed=N)result.stdout.fnmatch_lines(["*pattern*"])
© ethereum, CC0-1.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
Just SKILL.md in .agents/skills/pytester of ethereum/execution-specs.
Open the folder on GitHubat commit 3d544d7
Compare with similar skills
Pytester 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 |
|---|---|---|---|---|---|---|
| Pytester this skillethereum/execution-specs | 1.2k | — | ~403 | Automated safety check: Pass | CC0-1.0 | |
| Cross Origin Isolationthedaviddias/Front-End-Checklist | 74k | — | ~565 | Automated safety check: Pass | MIT | |
| Isolating Product Facade ContractsPostHog/posthog | 40k | — | ~9.5k | Automated safety check: Pass | Custom licence | |
| Implementing Browser Isolation For Zero Trustmukul975/Anthropic-Cybersecurity-Skills | 34k | — | ~3.5k | Automated safety check: Pass | Apache-2.0 | |
| Isolated Space Boundaryopenchamber/openchamber | 11k | — | ~2k | Automated safety check: Pass | MIT | |
| Screenshot IsolatedIvanMurzak/Unity-MCP | 4.4k | 1 repos | ~1.9k | Automated safety check: Pass | Apache-2.0 |
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- Official
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PostHog/posthog
Plan and execute product isolation migrations to a facade plus contract layer in PostHog, following the Visual review architecture.
Implementing Browser Isolation For Zero Trust
mukul975/Anthropic-Cybersecurity-Skills
Deploys remote browser isolation (RBI) as a core component of a Zero Trust architecture.
Isolated Space Boundary
openchamber/openchamber
A skill your agent uses when changing isolated-space trust boundaries: container hardening, networks and gatekeeper policy, exec and lifecycle, grants and credentials, code transfer and apply…
Screenshot Isolated
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Render a target GameObject from a chosen camera angle with optional layer-based isolation, configurable background (solid/skybox/transparent), multi-light setup via JSON, and Composite (2x2…
Trace And Isolate
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Questions about Pytester
What does Pytester do?
Write and run isolated pytester-based plugin tests. An agent skill from ethereum/execution-specs. Pytester is an agent skill from ethereum/execution-specs. Write and run isolated pytester-based plugin tests.
How do I install Pytester in Claude Code?
Run `npx skills add ethereum/execution-specs --skill pytester -a claude-code`. Or copy the skill folder (.agents/skills/pytester in ethereum/execution-specs) into .claude/skills/pytester in your project. Claude Code loads it when a task matches its description.
How do I install Pytester in Codex?
Run `npx skills add ethereum/execution-specs --skill pytester -a codex`. Or copy the skill folder (.agents/skills/pytester in ethereum/execution-specs) into .agents/skills/pytester in your project. Codex loads it when a task matches its description.
Can I use Pytester 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 ethereum/execution-specs --skill pytester -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/pytester, .gemini/skills/pytester, .github/skills/pytester and .opencode/skills/pytester in your project.
What does Pytester need to run?
SKILL.md names no scripts, command-line tools or credentials: Pytester is instructions for the agent only. Our summary lists: Python 3.
Does Pytester 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 Pytester 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 Pytester use?
Pytester is published under the CC0-1.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Pytester use?
About 403 tokens (SKILL.md is roughly 1.6k 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 Pytester?
Skills that share tags, products or a category with Pytester: Cross Origin Isolation (thedaviddias/Front-End-Checklist, 74k stars), Isolating Product Facade Contracts (PostHog/posthog, 40k stars), Implementing Browser Isolation For Zero Trust (mukul975/Anthropic-Cybersecurity-Skills, 34k stars) and Isolated Space Boundary (openchamber/openchamber, 11k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Pytester?
ethereum (a GitHub organization) maintains it in ethereum/execution-specs, which has 1,195 GitHub stars. The repository holds 13 skills in this directory. The repository was last updated on October 7, 2026.
Source: ethereum/execution-specs on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.