# Backend & API Development > Selected work demonstrating this service. - [Designed an organization context‑switching system with client localStorage and server‑side cookie mirroring, letting users act as managed organizations while enforcing least‑privilege authorization.](https://platform.engineer.company/portfolio/designed-an-organization-context-switching-system-with-client-59/) - [Generated the API contract outward from the database — OpenAPI, a 44,076‑line typed TypeScript client, 61 mock handlers and the limits the UI enforces — with a guard at every hop that fails on drift.](https://platform.engineer.company/portfolio/built-a-request-schema-validation-contract-with-automated-61/) - [Built email as a platform capability — three providers with failover, delivery webhooks, send and delivery logging, templating and campaigns — behind a startup check that will not boot without one.](https://platform.engineer.company/portfolio/built-email-as-a-platform-capability-with-failover-62/) - [Hardened the application with nonce‑based CSP, HSTS, SameSite cookies, least‑privilege database roles and server‑side entitlement re‑checks.](https://platform.engineer.company/portfolio/hardened-the-application-with-nonce-based-csp-hsts-77/) - [Built a layered automated test suite — 981 Go tests, 543 frontend and browser specs, 494 SQL behavioural tests — with mutation testing, property‑based tests and an accessibility gate.](https://platform.engineer.company/portfolio/built-a-layered-automated-test-suite-across-four-layers-94/) - [Built the payments and entitlements layer — Stripe alongside Apple and Google in‑app purchase — gating the directory, search and export through an 11‑table access model checked on the server.](https://platform.engineer.company/portfolio/built-the-payments-and-entitlements-layer-96/) - [Moved slow work off the request path onto a River job queue — 15 worker modules, 8 scheduled tasks and 20 pg_cron jobs — so a request returns while the work behind it carries on.](https://platform.engineer.company/portfolio/moved-slow-work-onto-a-river-job-queue-97/) - [Built first‑party error monitoring and OpenTelemetry tracing rather than buying them — payload sanitising, spike and regression detection, symbolication and a synthetic heartbeat — behind 11 operator views.](https://platform.engineer.company/portfolio/built-first-party-error-monitoring-and-tracing-98/) - [Built fail‑closed abuse controls — 22 Redis‑backed rate limiters, Cloudflare Turnstile, request idempotency and an origin lock — so the platform sheds bots and floods instead of trusting its callers.](https://platform.engineer.company/portfolio/built-fail-closed-abuse-controls-and-rate-limiting-100/) - [Held the generator to 981 test cases at a 92% branch‑coverage floor with warnings treated as failures, and asserted idempotence by running the whole build pipeline twice from an empty file and requiring the second pass to change nothing.](https://platform.engineer.company/portfolio/held-the-generator-to-964-tests-and-a-coverage-floor-145/) - [Selected every rule the Python linter has as an error, working through 1,815 findings to reach zero, with each of the few exemptions carrying a written reason and two of them backed by a checker instead of a comment.](https://platform.engineer.company/portfolio/enabled-every-python-linter-rule-as-an-error-147/) - [Wrote a parser that reads the real 7,308‑line C header and verifies every call site, every enum constant and that every pointer‑owning class is final, after a hand‑written placeholder header let calls to three removed functions compile, link and crash.](https://platform.engineer.company/portfolio/wrote-a-parser-that-verifies-every-ffi-call-site-157/) - [Reached the half of the messaging core the application had never used — backup transfer, disappearing messages, message editing and resending, verified invitations, proxies and encryption policy — driving every test against the real library with no mocks.](https://platform.engineer.company/portfolio/reached-the-unused-half-of-the-messaging-core-161/)