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Agent skill

Stackblitz Local Dev Loop

by jeremylongshore in jeremylongshore/tons-of-skills-marketplace

Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks.

MITAuto-check passedTesting & QA

Install Stackblitz Local Dev Loop

skills CLI
$ npx skills add jeremylongshore/tons-of-skills-marketplace --skill stackblitz-local-dev-loop -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install jeremylongshore/tons-of-skills-marketplace stackblitz-local-dev-loop --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/jeremylongshore/tons-of-skills-marketplace.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/.curated/stackblitz-local-dev-loop .claude/skills/stackblitz-local-dev-loop && rm -rf skills-src

Use ~/.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/

Facts

Skill name
stackblitz-local-dev-loop
GitHub stars
2.8k
Token cost
~1.1k tokens
SKILL.md length
495 words
Files
2 (incl. references)
Skills in repo
3,342
Repo updated
First seen
Licence
MIT

At a glance

Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks.

  • Works in 6 steps: Map pure logic, browser-only logic,… → Add fast unit tests for file-tree… → Add one browser smoke test that verifies… → …
  • A StackBlitz integration works manually but lacks dependable regression coverage
  • SKILL.md covers Overview, Prerequisites, Tool Discipline and Current Contract, plus 7 more sections
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Stackblitz Local Dev Loop is an agent skill from jeremylongshore/tons-of-skills-marketplace. Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks. Use when a StackBlitz integration works manually but lacks dependable regression coverage. Trigger with "test WebContainers locally", "StackBlitz dev loop", or "WebContainer Playwright test".

Its SKILL.md is about 1.1k tokens, which your agent loads only when the skill is triggered. The skill folder holds 2 other files, including reference files (for example `references/official-docs.md`). Compatibility notes: Designed for Claude Code

It sits in Testing & QA. It works with Playwright. The repository describes itself as: Model-agnostic agent-skills platform with a harness-free canonical layer, verified adapters, and the ccpi package manager. Explore at tonsofskills.com. The licence is MIT.

When your agent uses it

  • A StackBlitz integration works manually but lacks dependable regression coverage
  • With test WebContainers locally
  • StackBlitz dev loop
  • WebContainer Playwright test

Example prompts

  • “test WebContainers locally”
  • “StackBlitz dev loop”
  • “WebContainer Playwright test”
  • “/stackblitz-local-dev-loop”

Requirements

  • Compatibility (from SKILL.md): Designed for Claude Code
  • Pre-approved tools (allowed-tools): Read, Glob, Grep, WebFetch, Write, Edit

Workflow steps

6 steps, taken from the first numbered list in SKILL.md.

  1. Map pure logic, browser-only logic, startup ownership, and deployment-header ownership.
  2. Add fast unit tests for file-tree validation, state transitions, path policy, and bounded log formatting.
  3. Add one browser smoke test that verifies isolation, boots once, mounts a synthetic fixture, observes a zero-exit command, and reaches…
  4. Trigger the repository's HMR path and confirm the boot owner remains singular.
  5. Exercise navigation/disposal and confirm listeners and processes are released without breaking a surviving owner.
  6. Run the narrow test first, then the project's normal lint, typecheck, and browser-test gates.

What it can do on your machine

Read from SKILL.md and the folder at commit 80f86df. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves these tools, so the agent can use them without asking each time:

    • Read
    • Glob
    • Grep
    • WebFetch
    • Write
    • Edit

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • webcontainers.io

    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.

  • Compatibility

    Designed for Claude Code

    From compatibility in the SKILL.md frontmatter.

Context cost

Stackblitz Local Dev Loop loads about 1.1k tokens when it runs, and up to ~1.4k if it reads all its reference files. Until then it costs about 100 tokens; SKILL.md has 495 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~100
When it runs · the whole SKILL.md, loaded when a task matches
~1.1k
With references · SKILL.md plus every file in references/, read only if the agent opens them
~1.4k

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 jeremylongshore/tons-of-skills-marketplace at commit 80f86df, republished under its MIT licence (© jeremylongshore). 495 words, ~1,144 tokens.

Download SKILL.mdSave it as .claude/skills/stackblitz-local-dev-loop/SKILL.md (or your agent's skills folder). This skill also uses 1 other file; get the full folder from GitHub.
name
stackblitz-local-dev-loop
description
Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks. Use when a StackBlitz integration works manually but lacks dependable regression coverage. Trigger with "test WebContainers locally", "StackBlitz dev loop", or "WebContainer Playwright test".
allowed-tools
Read, Glob, Grep, WebFetch, Write, Edit
compatibility
Designed for Claude Code
argument-hint
[project-path] [test-command]
version
1.7.0
author
Jeremy Longshore <jeremy@intentsolutions.io>
license
MIT
tags
saas, stackblitz, testing, webcontainers
model
inherit
effort
medium

WebContainer Local Development Loop

Overview

This skill turns a manually tested WebContainer host into a repeatable local loop. It separates pure file-tree and state-machine tests from browser integration tests that require SharedArrayBuffer, Service Workers, WebAssembly, and real response headers.

Prerequisites

  • A named host application with an existing test framework or permission to add focused coverage
  • A supported desktop browser available to the project's browser-test runner
  • The repository's normal local HTTPS or localhost convention

Tool Discipline

Use Read, Glob, and Grep to locate runtime construction, HMR boundaries, response headers, unit tests, and browser tests. Use WebFetch only for current official StackBlitz or WebContainers documentation. Use Write or Edit for minimal user-requested tests and configuration.

Current Contract

  • Unit-test pure transformations such as flat-files-to-FileSystemTree without booting WebContainers.
  • Test actual boot, process, Service Worker, and preview behavior in a real browser runner rather than a DOM-only simulator.
  • Assert the final document is a secure context and crossOriginIsolated before boot.
  • Preserve one boot promise across HMR; a module reload must not create a second concurrent instance.
  • Exercise disposal separately from HMR so teardown() does not invalidate an instance still owned by the current page.

Authentication

Use synthetic public dependencies by default. If the browser test requires licensed or private-package access, bind credentials through the CI secret store, restrict the test environment, redact logs, and skip with an explicit reason when authorized credentials are unavailable.

Workflow

  1. Map pure logic, browser-only logic, startup ownership, and deployment-header ownership.
  2. Add fast unit tests for file-tree validation, state transitions, path policy, and bounded log formatting.
  3. Add one browser smoke test that verifies isolation, boots once, mounts a synthetic fixture, observes a zero-exit command, and reaches preview readiness.
  4. Trigger the repository's HMR path and confirm the boot owner remains singular.
  5. Exercise navigation/disposal and confirm listeners and processes are released without breaking a surviving owner.
  6. Run the narrow test first, then the project's normal lint, typecheck, and browser-test gates.
Show full SKILL.md (172 more words)Show less

Approval Boundaries

Do not weaken browser security flags, disable isolation checks, commit credentials, or make a flaky integration test silently optional. Changes to shared CI browsers, production headers, or licensed secrets require explicit authorization.

Output

Return the test-layer map, exact assertions, browsers exercised, commands run, HMR and cleanup evidence, skips with reasons, changed files, and remaining platform limitations.

Error Handling

ConditionResponse
DOM simulator cannot bootMove runtime behavior to a real browser test; keep only pure logic in unit tests.
HMR causes a second bootHoist and cache the boot promise at the application's stable ownership boundary.
CI lacks isolationInspect the served response and proxy chain; do not launch with unsafe browser flags.
Browser support differsRecord the exact supported/beta behavior rather than declaring universal compatibility.

Examples

Given a Vite host with Playwright, add a pure FileSystemTree unit test and one Chromium smoke test that asserts crossOriginIsolated, observes one boot, receives a preview URL, and cleans up after navigation.

Resources

© jeremylongshore, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 1 other file (references) in skills/.curated/stackblitz-local-dev-loop of jeremylongshore/tons-of-skills-marketplace.

  • SKILL.md
  • references/official-docs.md

Open the folder on GitHubat commit 80f86df

Compare with similar skills

Stackblitz Local Dev Loop 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.

Stackblitz Local Dev Loop compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Stackblitz Local Dev Loop this skilljeremylongshore/tons-of-skills-marketplace2.8k—~1.1kAutomated safety check: PassMIT
Web Application Testinganthropics/skills180k51 repos~966Automated safety check: PassApache-2.0
Playwright CLIsanity-io/sanity6.4k18 repos~1.9kAutomated safety check: PassMIT
Write and Verify Playwright Testsappsmithorg/appsmith41k—~2.9kAutomated safety check: NotesApache-2.0
playwright-cli Browser Automationgithub/gh-aw5.4k23 repos~2.8kAutomated safety check: PassMIT
Cucumber and Playwright E2E Testslanggenius/dify158k—~682Automated safety check: PassCustom licence

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Works with

Categories

Questions about Stackblitz Local Dev Loop

What does Stackblitz Local Dev Loop do?

Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks. Stackblitz Local Dev Loop is an agent skill from jeremylongshore/tons-of-skills-marketplace. Establish a reproducible local development and browser-test loop for a WebContainer host, including isolation-header assertions, single-boot behavior under HMR, and cleanup checks.

When should I use Stackblitz Local Dev Loop?

Stackblitz Local Dev Loop fits situations like: A StackBlitz integration works manually but lacks dependable regression coverage; with test WebContainers locally; stackBlitz dev loop; webContainer Playwright test.

How do I install Stackblitz Local Dev Loop in Claude Code?

Run `npx skills add jeremylongshore/tons-of-skills-marketplace --skill stackblitz-local-dev-loop -a claude-code`. Or copy the skill folder (skills/.curated/stackblitz-local-dev-loop in jeremylongshore/tons-of-skills-marketplace) into .claude/skills/stackblitz-local-dev-loop in your project. Claude Code loads it when a task matches its description.

How do I install Stackblitz Local Dev Loop in Codex?

Run `npx skills add jeremylongshore/tons-of-skills-marketplace --skill stackblitz-local-dev-loop -a codex`. Or copy the skill folder (skills/.curated/stackblitz-local-dev-loop in jeremylongshore/tons-of-skills-marketplace) into .agents/skills/stackblitz-local-dev-loop in your project. Codex loads it when a task matches its description.

Can I use Stackblitz Local Dev Loop 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 jeremylongshore/tons-of-skills-marketplace --skill stackblitz-local-dev-loop -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/stackblitz-local-dev-loop, .gemini/skills/stackblitz-local-dev-loop, .github/skills/stackblitz-local-dev-loop and .opencode/skills/stackblitz-local-dev-loop in your project.

What does Stackblitz Local Dev Loop need to run?

SKILL.md names no scripts, command-line tools or credentials: Stackblitz Local Dev Loop is instructions for the agent only. Its frontmatter pre-approves these tools: Read, Glob, Grep, WebFetch, Write, Edit. Compatibility (from SKILL.md): Designed for Claude Code.

Does Stackblitz Local Dev Loop access the network?

SKILL.md names 1 domain. As links in the text: webcontainers.io. This is read from the text; nothing was executed.

Is Stackblitz Local Dev Loop 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 Stackblitz Local Dev Loop use?

Stackblitz Local Dev Loop is published under the MIT licence (declared in SKILL.md). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Stackblitz Local Dev Loop use?

About 1.1k tokens (SKILL.md is roughly 4.6k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 232 tokens, read only when the agent opens those files.

What are the alternatives to Stackblitz Local Dev Loop?

Skills that share tags, products or a category with Stackblitz Local Dev Loop: Web Application Testing (anthropics/skills, 180k stars), Playwright CLI (sanity-io/sanity, 6.4k stars), Write and Verify Playwright Tests (appsmithorg/appsmith, 41k stars) and playwright-cli Browser Automation (github/gh-aw, 5.4k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Stackblitz Local Dev Loop?

jeremylongshore (a GitHub user) maintains it in jeremylongshore/tons-of-skills-marketplace, which has 2,825 GitHub stars. The repository holds 3,342 skills in this directory. The repository was last updated on October 9, 2026.

Source: jeremylongshore/tons-of-skills-marketplace on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.