Write Frontend Tests
Use when writing frontend tests — Jest + React Testing Library for units (utils, hooks, components) and Cypress + Cucumber for e2e user flows. Where tests live, what to test, mocking the router, and the lint/type/test gates. React/Next.js reference.
Serve Realtime With Websockets
Use when pushing realtime updates to connected clients over WebSockets (a socket.io / WS gateway) — authenticating the socket at the handshake (once, not per message), joining sockets to rooms for targeted broadcast, scaling across instances with a Redis pub/sub adapter (+ sticky sessions), bridging domain events to room emits, and connection-lifecycle hygiene (reconnect, listener cleanup). NestJ…
Git Flow
Use when branching, opening an MR, cutting a release, or shipping a hotfix — the develop→staging→master branching & release flow, tagging, semantic versioning, hotfix path. Tool-agnostic (plain git).
Integrate External Services
Use when integrating a third-party/external system — calling a vendor API (anti-corruption adapter behind your own interface, resilient HTTP with circuit breaker + retry/backoff, outbound HMAC request signing, idempotency keys), receiving inbound webhooks (raw-body capture, signature verification, idempotent fast-ack-then-enqueue, vendor→internal event mapping), or exposing a partner/public API e…
Structure A Backend Service
Use when scaffolding a backend service, adding a feature/module, or reviewing folder layout, file/class naming, DTO & entity structure, or the CQRS read/write split. Language-agnostic with TS/NestJS examples.
Write Service Code
Use when writing code inside a feature — a handler, query, service, event, or integration test. Control flow, async (Promise.all), query performance (N+1, upsert, joins, indexes), events/SQS, logging, decimal/date libs. Language-agnostic with TS/NestJS examples.
Integrate Internal Services
Use when one backend service calls or consumes from another inside the same platform — synchronous RPC (a uniform request/response envelope + server-side message handlers), SNS→SQS event fan-out (one topic → many subscriber queues), async-consumer robustness (ack vs DLQ + lifecycle hooks), cross-service reads (batch + cache, no network N+1), identity/context propagation across hops, and the worke…
Release Safety
Use before cutting or deploying a backend release (or reviewing release readiness) — backward compatibility with the current production app, testing the current app against the new backend, rollback plan, config/data readiness, feature flags & observability, and the code-comment policy for transition logic.
Design An Error Model
Use when designing how a backend service (or a fleet of services) reports errors — a typed exception hierarchy (one base + per-status subclasses carrying statusCode/message/data), a validation-error flattener wired into the global ValidationPipe, a global exception filter that maps every throw to ONE uniform response body (+ trace id, severity-based logging, hide internals on 5xx), an RPC/microse…
Code Conventions
Use when writing or reviewing code for general style conventions — style guide & linter, naming, file/function size, array functions, early return, SOLID/KISS, magic numbers, casts, side effects, deep copy, TS gotchas. Also indexes the full convention set. Language-agnostic, TS examples.
Render Transactional Emails
Use when sending a templated transactional email (or generating a templated PDF/document) — rendering from a named template + data through one shared render service, with per-locale template files, CSS inlining, returning subject/html/text, then dispatching via the notification facade (not inline). Covers reusing the same render for HTML→PDF. NestJS/TS reference, framework-flexible.
Write Cross Cutting Code
Use when writing a reusable request-pipeline primitive rather than feature logic — a custom decorator (param/metadata/method-wrapper + applyDecorators bundles), a guard (metadata-driven role/scope, or an OR-composition "any of these guards" guard), a pipe (polymorphic/variant DTO validation, or wrapping a built-in pipe to throw your typed error), an interceptor (metadata-driven audit/logging that…
Prevent Secret And Pii Leaks
Use BEFORE committing to a public/shared repo, pushing, publishing a package, or syncing/exporting code, config, docs, or skills to an open-source or external destination — to avoid leaking secrets, PII, or internal identifiers. Also the remediation playbook if something already leaked (untrack → rewrite history → force-push → forks/cache → support → rotate). Prevention >> remediation: you can't…
Background Jobs And Caching
Use when adding background jobs (Bull queues) or a Redis cache to a backend service — multi-queue architecture, enqueue/process, dynamic delayed jobs, job idempotency via a DB lock, graceful shutdown, and Redis caching (read-through wrap, key conventions, event-driven + prefix-SCAN invalidation). NestJS/TypeORM reference, framework-flexible. Complements (not replaces) the SQS cross-service events…
Operate Ecs Services Safely
Use before ANY change to a deployed container service behind a load balancer (AWS ECS/Fargate, or equivalent) — changing an env var / task-def, scaling, deploying, terminating an instance — AND when such a service is down (502/503, deploy stuck below desired, tasks failing the LB health check) or a scheduled/cron worker isn't running. A verify-before-mutate discipline + an observe→diagnose→recove…
Handle Files Frontend
Use when importing/exporting CSV, downloading files (single or zipped), viewing/parsing PDFs, or uploading files in a frontend app — one central file-util module covering CSV (papaparse), download (file-saver/jszip), PDF (pdf.js), and upload with magic-number MIME detection + presigned URL + image resize. React/TS reference, framework-flexible.
Tdd Http First
Drive a feature/bugfix test-first when the project mandates HTTP-layer-only testing — the failing RED test must be an HTTP/route-harness test (e.g. supertest), never a service/validator/util unit test. Trigger phrases — "làm theo TDD", "viết test fail trước", "write the failing test first", "do this test-first" — in any repo whose rules forbid unit tests.
Structure A Shared Ui Lib
Use when building or organizing an internal shared UI / design-system library that multiple apps depend on — src layout, folder-per-component with version coexistence, the createStore/query/Utils wrappers it re-exports, versioned generated design tokens, subpath build & exports, Storybook + visual regression. React/TS reference, framework-flexible.
Structure A Frontend App
Use when scaffolding a frontend app, adding a feature or page, or reviewing FE folder layout — thin routing → feature modules, the feature-module pattern (Component + .actions hooks + .hook + styles + barrel), shared-vs-feature code, naming, path aliases, request/response models. React/Next.js reference, framework-flexible.
Schedule Effective Dated Changes
Use when a change must take effect LATER (a future-dated plan/tier/price change, a scheduled deactivation, an end-of-cycle update) or must be visible / cancellable / auditable before it applies — model it as a pending-changes table (the change payload + an effective date + a status), swept by a scheduled job that applies DUE rows in a transaction, snapshots prior state to a history table, and mar…
Import Data From Csv
Use when building a bulk CSV/spreadsheet import endpoint — streaming parse, per-row validation with a row-numbered error report, data normalization (Excel quotes, multi-format dates), atomic chunked upsert in a transaction, partial-success response, and fan-out to workers for large files. NestJS/TypeORM reference, framework-flexible.
Structure A Shared Backend Lib
Use when organizing a shared backend infrastructure library that many services depend on (e.g. @org/infra-*) — how to split it into focused packages by dependency weight, expose one barrel per package, avoid dependency cycles with peer deps, version/publish it, decide what belongs in the lib vs a service, and the canonical primitives it should provide (a base entity with soft-delete + audit colum…
Write Frontend Code
Use when writing FE code inside a feature — data fetching & mutations (React Query), API + DTOs, state (Zustand + Context), custom hooks, forms (RHF + Yup), heavily-used packages, performance, styling (Tailwind + tokens + CSS Modules), config, i18n, SSR/hydration. React/Next.js reference, framework-flexible.
Containerize And Ship A Service
Use when writing a Dockerfile or a CI/CD pipeline for a backend service — a multi-stage build (heavy builder → slim runtime), base images pulled through a dependency proxy / private registry, authenticating to private package registries during build and SCRUBBING those creds before the final stage, lockfile-first layer caching, purpose-specific images (app / DB-migration job / test), and a CI pip…
Use Feature Flags
Use when gating a code path behind a feature flag or rolling something out gradually — evaluating a flag from a central flag service (scoped by service + flag name + user/session + targeting properties), failing safe (unknown/error/timeout = off), caching the lookup, and choosing a flag over a NODE_ENV/env branch. Covers flag lifecycle (name, owner, removal) and its tie to safe releases. NestJS/T…
Authoring Project Skills
Use when creating, updating, reviewing, or deciding whether to write a project skill (docs/skills/*.md) — covers the file template, verb-led naming, the quality bar, the pre-write 5-weakness self-check, and when NOT to write a skill. Pairs with the always-on "scan docs/skills before a task / create-or-update after" rule in CLAUDE.md.
Integrate Identity Providers
Use when adding third-party login or identity verification to a backend — social login (Sign in with Google / Apple / Facebook) and national digital identity / KYC (e.g. Singpass) via the OIDC relying-party flow (authorization code + PKCE + state/nonce → token exchange), validating the resulting token (.well-known → JWKS signature + iss/aud/exp, or introspection), centralizing many IdPs behind on…
Keep Service Memory Lean
Use when a service OOMs / its tasks restart after a PR that added a dependency, when memory-tight replicas (small containers — e.g. 512MB/1GB) die under load or on deploy, or whenever the proposed fix for memory pressure is "just give it more RAM". Also use proactively when ADDING a heavy or optional library (cloud/vendor SDK, PDF/image/ML/crypto lib) to a long-running service. The discipline: ke…
Secure A Frontend App
Use when wiring auth/session, protecting routes, or handling secrets in a frontend app — OIDC login via NextAuth + server session store + access-token refresh & injection, route guards + permission-matrix RBAC, SSR cookie propagation + hydration, and vault secrets + public/private config split. React/Next.js reference, framework-flexible.
Send Slack
Send a Slack message to a channel or DM using the local send.js script. Use whenever the user asks to send or notify team via Slack.
Database Migrations
Use when adding or changing a DB migration or schema, running migrations, or debugging null/missing response fields after a migration — the migration rules (additive, reversible, immutable, seed-vs-migration, push filters into SQL) and the "null fields = unapplied migration" gotcha.
Write Unit Tests
Use when writing isolated unit tests (mocked deps, no DB) for CQRS handlers, services, or DTOs. Mock factories, a handler testing-module helper, DTO validation, the AAA pattern, entity builders. TS/NestJS/Jest examples.