Use when building, designing, reviewing, or evaluating command-line tools for AI agent consumption -- structures commands with machine-readable JSON output on stdout, human diagnostics on stderr, adds --json/--fields/--dry-run/--quiet flags, implements semantic exit codes, enforces TTY-aware dual-mode output, hardens input against path traversal and injection, creates CONTEXT.md and AGENTS.md agent knowledge files, adds --help-json schema introspection, and scores CLI agent-friendliness on a 0-21 rubric across 7 axes. Triggers on: 'agent-friendly CLI', 'CLI for agents', 'build a CLI', 'design CLI commands', 'machine-readable output', '--help-json', 'CLI scoring', 'agent DX', 'context window discipline', 'evaluate CLI', 'CLI agent readiness', 'score this CLI'. NOT for TUI/full-screen apps. NOT for GUI apps. NOT for REST/GraphQL API design.

Authors Mock Service Worker (MSW) request handlers for both browser and Node.js test environments using the `http.get` / `http.post` / `HttpResponse.json` API, wires them via `setupWorker` (browser) or `setupServer` (Node), and manages the test lifecycle (`server.listen` / `resetHandlers` / `close`). Use when the project uses JavaScript / TypeScript and needs to mock fetch / XHR at the network layer for both Vitest / Jest unit tests and Cypress / Playwright integration tests.

Analyze code to suggest and add Design by Contract specifications (preconditions, postconditions, invariants, semantic invariants) in any language. Trigger on: "Design by Contract", "DBC", "preconditions", "postconditions", "class invariants", "code contracts", "formal specifications", "document assumptions", "add contracts", "make this function safer", "define what this function guarantees", "add assertions to document behavior", Bertrand Meyer, Eiffel contracts, or when discussing invariants that should always hold. Do NOT skip for non-Eiffel code — DBC applies everywhere via assertions, type guards, properties.

Use when you need to ask before executing, don't run without permission, review steps before proceeding, or confirm before executing. An LLM breaks down goals into tasks, presents the plan for approval, then executes only if approved. Enforces a mandatory approval gate that blocks all execution. Separates planning from execution so no action runs without prior sign-off. Classifies failures by type and applies type-specific recovery strategies. Produces a timestamped event log of every state change. Applies to: step-by-step workflows with approval, irreversible or risky operations, human-in-the-loop execution, agentic pipelines with intelligent failure recovery.

Next.js App Router API patterns that must be applied whenever building, extending, or scaffolding a Next.js application. Covers Route Handlers, Server Actions, middleware, error boundaries, validation, streaming, caching, and runtime selection. Apply even when the task only asks for "an API endpoint" or "a form" -- these patterns prevent silent bugs, stale data, and security gaps that are unique to the App Router model.

Creates, structures, and reviews technical documentation following the Diátaxis framework (tutorials, how-to guides, reference, and explanation pages). Use when a user needs to write or reorganize docs, structure a tutorial vs. a how-to guide, build reference docs or API documentation, create explanation pages, choose between Diátaxis documentation types, or improve existing documentation structure. Trigger terms include: documentation structure, Diátaxis, tutorials vs how-to guides, organize docs, user guide, reference docs, technical writing.

Guides development of Fastify Node.js backend servers and REST APIs using TypeScript or JavaScript. Use when building, configuring, or debugging a Fastify application — including defining routes, implementing plugins, setting up JSON Schema validation, handling errors, optimising performance, managing authentication, configuring CORS and security headers, integrating databases, working with WebSockets, and deploying to production. Covers the full Fastify request lifecycle (hooks, serialization, logging with Pino) and TypeScript integration via strip types. Trigger terms: Fastify, Node.js server, REST API, API routes, backend framework, fastify.config, server.ts, app.ts.

Creates, updates, or optimizes an AGENTS.md file for a repository with minimal, high-signal instructions covering non-discoverable coding conventions, tooling quirks, workflow preferences, and project-specific rules that agents cannot infer from reading the codebase. Use when setting up agent instructions or Claude configuration for a new repository, when an existing AGENTS.md is too long, generic, or stale, when agents repeatedly make avoidable mistakes, or when repository workflows have changed and the agent configuration needs pruning. Applies a discoverability filter—omitting anything Claude can learn from README, code, config, or directory structure—and a quality gate to verify each line remains accurate and operationally significant.

Write clear, testable acceptance criteria for user stories and feature delivery; use when defining done conditions, creating measurable requirements, applying INVEST checks, documenting negative scenarios, and aligning product, engineering, and QA on expected outcomes.

Python project structure best practices — pyproject.toml, src layout, __init__.py, .gitignore, virtual environments, type hints, py.typed, test structure, entry points, dependency groups, and tool configuration. Use when starting a new Python project, restructuring an existing one, setting up a Python package, configuring dependencies, or when reviewing project organization. Triggers on: new FastAPI/Flask/Django project, CLI tool setup, Python library scaffolding, "where should I put this", monolithic files, missing pyproject.toml, setup.py migration, or dependency management questions.