Use the `@testbook` decorator to write conventional pytest unit tests against functions defined in Jupyter notebooks, without copy-pasting the function into a `.py` file. Covers `tb.ref()` (notebook object access) and `tb.inject()` (insert code into the kernel) for hermetic per-test setup. Use when unit-testing functions that live in a Jupyter notebook.

Validate Jupyter notebooks via the `pytest --nbval` plugin - re-execute cells and compare outputs to stored results. Cover the strict path (output match required), `--nbval-lax` (failure-only), `--sanitize-with` for dynamic outputs, and per-cell controls (`#NBVAL_SKIP`, `#NBVAL_IGNORE_OUTPUT`, `#NBVAL_RAISES_EXCEPTION`). Use when a repo ships `.ipynb` files as tutorials, docs, or analyses that must keep producing the same outputs after a dependency upgrade or source change.

Reference for Jepsen-style distributed-systems testing - consistency models hierarchy (linearizability vs sequential vs causal vs monotonic-reads vs eventual), nemesis primitives (network partitions, clock skew, kill nodes), workload generators, Knossos + Elle linearizability checkers. Reference-only because Jepsen tests are typically Clojure-bespoke per system; use this skill to evaluate vendor claims and structure your own test. Use when a datastore vendor advertises a consistency guarantee that has to be checked before adoption, when reading a published Jepsen report for gaps, or when a custom replicated store needs its own consistency test scoped.

Runs the Go race detector and goroutine-leak checker end-to-end: instrument with `go test -race`, read race reports, configure GORACE options, stress with `-count`/`-cpu`, detect goroutine leaks with go.uber.org/goleak, and gate both checks in CI. Use when a Go service has shared state accessed by concurrent goroutines, when a race-related incident needs a regression harness, or when adding `-race` to a CI matrix for a Go module. Does not cover barrier-based deterministic interleaving or forced goroutine scheduling; use race-condition-test-author for that.

Run Madge against a JS/TS production source tree to detect circular dependencies, find orphan modules, and visualize the module graph. Scoped to production code via `excludeRegExp` for test files. Use when a build compiles but throws `Cannot read property X of undefined` on a module that is clearly imported, or when a repo needs a CI gate that blocks new import cycles.

Run Lizard against production source to enforce per-function cyclomatic complexity (CCN), NLOC, and parameter-count thresholds - language-agnostic (30+ languages). Scoped to production code via `-x"./tests/*"`; test complexity is reviewed separately. Use when a codebase spans several languages and needs one complexity gate across all of them, or when a function has grown unreviewable and the team wants a numeric threshold in CI.

Validate Vega + Vega-Lite specifications against the JSON Schema (vega.github.io/schema), test cross-engine compatibility (Vega-Lite compiles to Vega per the canonical compiler), and verify data-binding correctness. Pair with d3-snapshot-tests when Vega specs render to SVG; pair with chartjs-snapshot-tests when rendered to Canvas. Use when application code generates Vega/Vega-Lite JSON specs at runtime (BI builders, spec templating) and those specs must be proven valid and correctly encoded before render, or before a Vega major-version upgrade.

Snapshot-test Chart.js charts - render via headless Chromium / jsdom + canvas mock, capture canvas pixels via `canvas.toDataURL()` + image-diff, disable animations (`options.animation = false`) for stable snapshots, test tooltip + legend interactions. Use when a project renders Chart.js charts and needs regression coverage of their rendered output.

Asserts Chrome extension message-passing behaviour against a running extension: one-shot `chrome.runtime.sendMessage` plus the literal `return true` that holds the response channel open for an async `sendResponse`, `chrome.tabs.sendMessage` into one tab's content script, long-lived `chrome.runtime.connect` ports and their `onDisconnect` triggers, web-page messages gated by `externally_connectable`, and `chrome.runtime.connectNative` native-messaging hosts. Covers the payload rules a test must respect (Chrome uses JSON serialization rather than structured clone, so `Map` / `Set` / `Date` do not round-trip; maximum message size is 64 MiB) and the first-listener-wins rule when several `onMessage` listeners are registered. Scope is messaging behaviour on an already-running extension, not the install or reload step. Use when a message reaches no listener, a `sendResponse` callback never fires, a port disconnects mid-test, or a page origin has to be proven allowed before publishing.

Authors and runs IBM Equal Access accessibility-checker scans - IBM's open-source a11y engine with WCAG 2.0 / 2.1 / 2.2 + US Section 508 rule sets, integrating with Node / Selenium / Puppeteer / Playwright / Karma / Cypress test runners. Distinguished by IBM's enterprise-tier rule coverage and Section 508 specificity. Use when the project ships to US federal / public-sector customers (Section 508 mandate) or when the team values IBM-branded a11y reporting.

Authors browser automation scripts using Puppeteer - Chrome / Chromium-only headless / headed automation, Page object via `page.*` API, network interception, PDF generation, screenshot capture, scraping. Distinct from Playwright (Puppeteer's older sibling, Chrome-only) - use Puppeteer for Chrome-only browser automation tasks (scraping, generating PDFs from HTML, screenshot pipelines) where Playwright's multi-browser support is unneeded overhead. Use when a project already depends on `puppeteer` / `puppeteer-core`, or when a Chrome-only script must emit PDFs, screenshots, or scraped data rather than assert on a page.