Build-an-X workflow that analyzes a repo's test mix (unit / integration / E2E counts + runtimes) and recommends rebalancing toward the test pyramid ratios per the change-set shape - pure-logic-heavy repo wants ~80/15/5; UI-heavy repo wants ~60/25/15. Detects 'ice-cream cone' (E2E-heavy) and 'hourglass' (integration-thin) anti-patterns. Use when the user asks about test distribution, test strategy, test balance, too many E2E tests, slow CI caused by tests, testing best practices, or rebalancing their test suite; also suitable for quarterly calibration of the test mix to codebase reality.

Author service worker cache strategies (cache-first, network-first, stale-while-revalidate, cache-only, network-only) per Workbox conventions, plus generate the matching Playwright assertions to lock the strategy in. Avoids the common "cached forever" pitfall by enforcing TTL + version-bump invalidation. Use when a new route needs defined offline behavior, when a hand-rolled `sw.js` is being moved to Workbox strategies, or when users report seeing stale content after a deploy.

Dispatches mobile UI test runs across a 3-tier device matrix (smoke per-PR, regression per-merge, full farm at release) to control CI cost: generates per-target Appium capability configs from a central YAML, parallelises via GitHub Actions matrix strategy, and aggregates JUnit XML into a cross-device pass/fail table. Use when deciding which iOS / Android devices and OS versions to run tests on and at which stage (smoke / regression / full farm), not how to configure a specific test framework (for that, use xcuitest-suite, espresso-suite, etc.).

Interprets latency distributions from k6 load tests beyond the p95/p99 gate: reads percentile summaries and JSON exports to identify tail shape, computes the tail ratio (p99/p50) as a distribution-spread signal, detects bimodal distributions, explains coordinated omission and why naive p99 values are optimistic under sustained load, and distinguishes request-rate from concurrency models. Use when a k6 threshold passes but the system still feels slow, when p99 is suspiciously low during ramp-up, or when the team needs to explain why tail latency is high rather than just observing that it is.

The GraphQL attack-surface / hardening skill: crafts over-limit depth and complexity queries then asserts rejection before execution, verifying that graphql-depth-limit, graphql-cost-analysis, and graphql-armor (max-depth / cost-limit / max-tokens plugins) are actually enforced and not just configured; carries the introspection attack-surface catalog (what __schema exposes, per-framework disable controls, hideSchemaDetailsFromClientErrors) in references/introspection.md and the persisted-query allowlisting strategies (Apollo APQ protocol, auto-register vs strict-allowlist vs hybrid modes) in references/persisted-queries.md. Use when auditing a GraphQL service for DoS or schema-disclosure exposure, hardening a production deployment, or adding tests that prove the limits in CI.

Author integration tests that query Zipkin for trace verification - Zipkin all-in-one Docker for CI, REST API (`/api/v2/traces`, `/api/v2/services`, `/api/v2/dependencies`), B3 propagation header tests (single-header and multi-header X-B3-* form), dependency-graph assertions. Use when the team uses Zipkin (legacy or Spring Cloud Sleuth heritage).

Authors integration tests that query Grafana Tempo for cross-service trace verification - TraceQL `{ }` span selectors targeting `span.`, `resource.`, and intrinsic fields; Tempo HTTP API (`/api/search` with `q=`, `/api/traces/{id}`) for span-set and attribute assertions; local Tempo via Docker single-binary (ports 4317/4318/3200). Use when the production observability stack uses Tempo as the trace backend and tests must verify distributed trace shape, span attributes, or service topology after instrumentation changes.

On-demand builder that converts a SINGLE test failure record (JUnit XML, Allure JSON, pytest --tb=short, Playwright HTML, Cypress mocha-junit) into a structured, tracker-agnostic bug SPEC: extracts test name, assertion, stack trace, environment, and artefacts, and proposes severity, defect type (IEEE 1044), and a root-cause hypothesis (ISTQB CTAL-TA), then hands the JSON spec to a jira/linear/github-issues-bug-workflow runner to file. Use when you already hold a failure artefact and want one classified, ready-to-file report. Distinct from the event-driven CI orchestrator that triggers automatically on a pipeline failure and files in bulk, and from screen-recording-driven bug reporting; this is the on-demand, single-record spec builder.

The single home for Jupyter notebook testing: wires parameterized execution (papermill), output regression (nbval), function-level unit tests (testbook), output stripping (nbstripout), and artifact upload into one working GitHub Actions CI pipeline, with per-tool depth for papermill (parameters tag, CLI/API, sweeps) and nbval (strict/lax modes, per-cell markers, sanitize config) in references/. Includes a notebook PR review checklist covering untested notebooks, --nbval-lax misuse, hardcoded credentials, non-deterministic output cells, missing parameters tags, and committed outputs, with BLOCK / WARN / INFO severities and a BLOCK-or-PASS verdict. Use when notebooks must run as parameterized regression jobs in CI, when a repo ships .ipynb files whose outputs must stay stable, or when a PR that adds or modifies notebooks needs a structured quality review.

Snapshot-test D3.js charts - D3 generates SVG (not Canvas, per d3js.org getting-started); use `outerHTML` snapshot for static structure, `toHaveScreenshot` for rendered SVG; jsdom for headless render in unit tests; disable transitions for stable snapshots; per-element data-binding correctness tests. Use when a project renders charts with `d3.create('svg')` or D3 selections and the emitted SVG structure or update join needs regression coverage - including before a D3 major-version upgrade.

Author, lint, run, build, and sign a Firefox / Chromium WebExtension using Mozilla's `web-ext` CLI v8. Covers `web-ext lint` (addons-linter wrapper, JSON output for CI), `web-ext run` (temporary install in firefox-desktop / firefox-android / chromium targets with hot-reload), `web-ext build` (deterministic zip), and `web-ext sign` (AMO submission API, listed vs unlisted channels, JWT credentials). Use when the extension targets Firefox (signing is mandatory for distribution) or when cross-browser test runs need a single CLI that drives both Firefox and Chromium against the same source tree.

Loads an unpacked Chrome / Chromium extension for testing through the `chrome://extensions` Developer-mode flow: the minimum loadable `manifest.json`, the Load-unpacked directory-not-file selection, toolbar pinning, and the reload matrix deciding what a code edit actually re-evaluates (`manifest.json`, the background service worker, and content scripts need an explicit card refresh, content scripts additionally need a host-page refresh, while popup / options / other extension HTML pages re-evaluate on next open). Also covers reading the red Errors card, where service-worker and content-script logs surface, and the `--load-extension` and `web-ext --target chromium` equivalents that move the same load into CI. Scope is getting a build directory loaded and reloaded, not the runtime behaviour asserted afterwards. Use when a freshly built extension directory has to go into Chrome for the first time, or when an edit appears to have no effect and you need to know which surface requires an explicit reload.