Przemysław Zalewski
Senior Frontend / Full-Stack Developer from Wrocław, Poland. 15+ years of commercial experience, shipping React since 2017, C# and .NET before that. I own what I ship, lead small teams by example and put AI agents to work with guardrails and end-to-end tests.
Senior Frontend / Full-Stack Developer from Wrocław, Poland. Product engineer with 15+ years of commercial experience, shipping React since 2017. Before that I built and ran mobile apps in C# and .NET. Long-term player: nine years in the same agency, running projects end to end, solo or with a team.
I own what I ship: I monitor it, run it, debug it and page myself. Known for fast, thorough code reviews. I lead small teams. Since 2018 I have mentored colleagues by pairing with them and raising them along, not as a boss: I believe in leading by example. I put AI agents to work on real products and keep them in check with agent skills, guardrails, an end-to-end test obsession, monitoring and observability.
Currently at Sanddev sp. z o.o., available from November 2026, remote preferred.
Education and work experience
In 2014 I have earned a Bachelor of Science degree in Computer Science at the Faculty of Computer Science and Management of Wroclaw University of Science and Technology.
Commercial work
Sanddev sp. z o.o. · Senior Full-Stack Engineer · TypeScript, React, AI agents
PGS Software (now part of Xebia) · Omio, Nate, Sage · TypeScript, React, React Native
Sandstream Development · Full-Stack Developer · React, Redux, C#, .NET
Self-employment · 25+ apps, 3.7 million+ downloads · C#, XAML, ASP.NET
LOSO sp. z o.o. · RatingSpeed · Windows Phone app · C#, XAML
IT4Biznes Marcin Hatalak · Fast-coffee · Windows Phone app · C#, XAML
Cube Investments sp. z o.o. · Kwejk.pl · Windows Phone app · C#, XAML
Red Dot Games · I Am Rich The Game · Windows PC, Store app · C++, C#, DirectX
Red Dot Games · Platformer game engine · Windows PC, Xbox 360 · C#, XNA
Job paths
Full-Stack Product Engineer · TypeScript, React, Node.js, AI agents
Full-Stack Web Developer · TypeScript, React, Node.js
Windows App Developer · C#, .NET
Game Developer · C++, C#
Skills
Browse my skills to see if I can fit your project’s needs.
AI and agents
- Claude Code
- Codex
- Agent Skills and hooks
- Model Context Protocol (MCP)
- OpenAI API
- Agent drivers
- Tool-call orchestration
- Multi-agent workflows
- Prompt and context engineering
- LLM-as-judge
- Evals
- RAG
- Embeddings
- pgvector
- Guardrails
- Streaming agent UIs
- Cursor
JavaScript and TypeScript
- React
- TypeScript
- JavaScript
- Next.js
- Node.js
- HTML
- CSS
- Responsive design
- Accessibility (WCAG 2.2 AA)
- Storybook
- React Native
- Electron
- Web Components
- Zod
- i18n
- SEO and structured data
- Tailwind CSS
- Radix
- Webpack
- Redux
APIs and data
- REST
- OpenAPI / Swagger
- WebSockets
- Server-Sent Events
- Express.js
- SQL
- PostgreSQL (partitioning, tuning)
- Drizzle
- Supabase
- Citus
- MongoDB
- Redis
- OAuth 2.0
- JWT
- Auth0
- Stripe
- Paddle
- IMAP and SMTP
C# and .NET
- C#
- .NET
- ASP.NET Core
- Entity Framework
- LINQ
- SignalR
- MS SQL
- XAML
- MVVM
- Windows Phone
Testing and quality
- Playwright
- Jest
- react-testing-library
- Node.js test runner
- Visual regression testing
- axe (accessibility)
- Checkly
- End-to-end testing
- Code review
Cloud, security and tooling
- Cloudflare Workers
- R2
- D1
- Queues
- Turnstile
- Vercel
- Docker
- Docker Compose
- nginx
- Git
- GitHub Actions
- CI/CD
- PostHog
- Datadog
- Sentry
- Biome
- OWASP ASVS
- CASA Tier 2
- Security audits
- Production-readiness audits
- GEO / AI-search visibility
- Email deliverability (SPF, DKIM, DMARC)
- Rate limiting
- Jira
Leadership and soft skills
- Technical leadership
- Mentorship
- Pair programming
- Scrum Master
- Hiring and interviews
- Pre-sales and estimation
- Product thinking
Projects
There are some notable projects I have been working on recently.
Agent skills platform
Sanddev sp. z o.o.
Claude Code, Codex, Agent Skills, Model Context Protocol (MCP), TypeScript, Node.js, GitHub Actions, OWASP ASVS
- Productized a multi-agent code audit as a Claude Code skill: repository in, client-ready PDF report on security, production readiness and maintainability out, with a priced fix plan. Ran on 10+ repositories, detailed reports of up to 365 pages.
- Introduced audit-driven development: audit, fix, re-audit. It runs as a loop across our production products.
- Added GEO/SEO visibility and DNS and email-security audits. Set up the shared platform on which three colleagues now ship their own skills.
Sanddev sp. z o.o.
Next.js, React, TypeScript, Node.js, IMAP, OAuth 2.0, Playwright, Checkly, PostHog, axe (accessibility), WCAG 2.2 AA, CASA Tier 2, Claude Code, Codex, App Store
- Shipped a greenfield Next.js client and, with a colleague, iOS and macOS apps to the App Store in four months.
- Built the delivery pipeline: Playwright end-to-end suite, Checkly checks and PostHog observability. Output with one colleague and AI agents: 5,000+ commits a year, up from ~2,000 a year on my own.
- Turned early experiments with agent-written code into a reviewed, repeatable process: an agent opened ~30% of merged pull requests in 2026 and I reviewed ~85% of them.
- Passed Google’s CASA Tier 2 security assessment (certification and recertification) and Yahoo’s OAuth review.
- Integrated IMAP and related mail protocols (dozens of RFCs). Drove WCAG 2.2 AA accessibility with axe checks.
AI content platforms
Sanddev sp. z o.o. for undisclosed client
TypeScript, Node.js, Next.js, React, OpenAI API, Tool-call orchestration, LLM-as-judge, RAG, Embeddings, pgvector, PostgreSQL, Cloudflare Workers, Vercel, Tailwind CSS
- Mentored a new hire from day one through daily pair sessions and code review. Fully contributing within a year.
- Built an agentic content platform (2024): tool-call loop, stop conditions, best-of-n, LLM-as-judge and plagiarism checks. It published thousands of fact-checked articles with AI disclosures, plus chatbots in production.
- Designed an affiliate discovery system: semantic search over pgvector with query expansion and tiered caching. It grew a seed of ~300 programs into a catalog of 10,000+.
- Took over a dashboard for 300+ generated sites. Added git-backed storage, bulk operations, newsletters, AI brand and logo generation, and third-party integrations.
Suppression list management platform
Sanddev sp. z o.o. for a UK email and performance-marketing publisher
TypeScript, Node.js, PostgreSQL, Playwright, Docker, nginx, Cloudflare Workers, CI/CD
- Led the team from MVP to a full product, full time March to November 2023, then oversaw it in production through 2024.
- Designed a web-native architecture for terabyte-level growth: multi-tenant PostgreSQL partitioned by tenant and list, massive imports and exports, parallel request pipelines, constant-time sign-up, zero-downtime blue/green deploys. In production: hundreds of gigabytes and millions of events.
- Built the UI without a client framework: server-rendered HTML with SPA-like navigation (history API, DOM morphing) on a simple HTTP mental model, picked for high availability and a small team.
- Kept opt-out pages up through backend outages (queues, workers, backfill, idempotent retries). Shipped one-click unsubscribe four months before Google and Yahoo announced the requirement. Client rated the engagement 5.0/5.0 in a public Clutch review.
Accounting MVP
PGS Software (now part of Xebia) for Sage
TypeScript, React, Storybook, REST, Jest, C#
- Integrated micro-frontends for banking and Making Tax Digital into an accounting MVP, building on the client’s design system, component library and C# backend APIs.
- Worked with more than a dozen people in two cross-functional teams across Spain and Poland. Joined as an engineer. My team treated me as its lead, despite no title.
Back Office Task Management System
PGS Software (now part of Xebia) for Nate (US start-up)
TypeScript, React, Next.js, Node.js, Jest, WebdriverIO, Server-Sent Events, Web Extensions, MongoDB, styled-components, Tailwind CSS, CSS, REST, Auth0, Kubernetes, Datadog, Jira, CircleCI
- Led four engineers building an order-fulfillment back office for a 100+ operator team. Moved the team to trunk-based development with CI/CD, enforced strict auth, and improved observability and logging.
- Cut order handling time and duplicate fulfillment with a refreshed UI and live updates over SSE. Errors fell steadily, and a survey of 100+ operators rated the changes a clear improvement.
- Stopped a parallel initiative that would not pay off, sparing the company a split-brain across two systems. Closed a public endpoint writing straight to the database and rebuilt the payments feed as a proper migration.
Ads and Ancillaries
Omio.comPGS Software (now part of Xebia) for Omio
TypeScript, React, JavaScript, Node.js, React Native, Jest, Storybook, Hapi.js, REST, Redux, Swift, Kotlin, go, Java, Redash, SQL, BigQuery, Kibana, Grafana, Amplitude, Google Ad Manager, Firebase, Xcode, Android Studio, Jira, Confluence
- Shipped ad placements inside other teams’ code across web (React), React Native, native iOS and Android, and email, forking and fixing the React Native ad library where needed.
- Owned features from design through A/B tests, feature flags and monitoring. Reported open redirects I found along the way.
Undisclosed
Sanddev sp. z o.o. for undisclosed client
JavaScript, React, Redux, Redux Thunk, Node.js, Jest, C#, .NET Core, Entity Framework, MySQL, ASP.NET Core, Sass, WebGL, three.js, Git, GitHub, Webpack, puppeteer, Expo, React Native, Storybook, APM (Azure Application Insights), AWS, Stripe
- Built frontend features in React with Redux (early adoption, 2017) and backend APIs in C#/.NET, in a team of five engineers and a QA, with weekly calls with the US client.
- Built interactive charts and measurement widgets from scratch, synchronized with a 3D renderer I integrated and patched.
- Introduced visual regression testing, shared tooling and unified formatting for the team.
Sanddev sp. z o.o.
JavaScript, React, Node.js, Express.js, Sass, styled-components, Webpack, Docker, Docker Compose, nginx, MongoDB, mongoose, Redis, JWT, CI/CD, Jenkins, Git, GitLab, ELK Stack, puppeteer, Expo, React Native, Electron, Storybook, APM (Sentry.io), Amazon S3, paddle
- Founding engineer: built the product from the first commit in 2018 to its first customers, then CI/CD, end-to-end tests and subscriptions. Hands-on in 2018 to 2020, since then review, consulting and growth.
- Reviewed and mentored colleagues through 800+ pull requests (2018 to 2026) and onboarded new engineers. Scrum master for four years.
- Shipped to eight platforms, from app stores to a Chrome extension, with Electron and React Native.
- Took over growth: SEO and AI-search visibility built from near zero, PostHog KPI dashboards, a public MCP server and REST APIs.
Validator
Sanddev sp. z o.o.
JavaScript, React, TensorFlow, TensorFlow Lite, face-api.js, WebAssembly, Android, Kotlin, Git, GitLab
- Led a small team to deliver a working app within a tight competition deadline: on-device face detection, landmarks and face-part segmentation with TensorFlow.js in the browser and TensorFlow Lite in a Kotlin Android app around a React WebView, fully offline.
- Replaced an early OpenCV prototype with hand-written sharpness and color checks, built the test suite and debug tools, and wrote the submission report.
Sanddev sp. z o.o.
TypeScript, JavaScript, Rollup, Static site generators, Git, GitHub, GitHub Pages, GitHub Actions, React, Node.js, Babel, Prettier, ESLint
- I was responsible for providing a library to reduce code duplication across the company’s repositories. I have defined module boundaries and extracted a huge chunk of utility functions to be strongly typed and included within the library. I wrote hundreds of TypeScript functions and I have covered them with tests to attain full coverage. I have been responsible for the package releases on npm and related chores. I have been performing code reviews to make sure the shared code is of high quality. With the custom static site generation toolkit, I have provided searchable and interactive documentation.
Korona Krulów
koronakrulow.plJavaScript, React, Node.js, nginx
- I created and deployed a React-based game played by 100,000+ people. I have managed to publish the game within two weeks of work. The game included over 400 actions and their effects that impact the internal simulation of infections and economy and society. I have implemented a leaderboards system to make the game more competitive.
Driver’s license tests
JavaScript, React, Node.js, Babel, ESLint, Visual Studio Code, MobX
- Writing the front-end with React.js, styling with CSS, designing responsive, mobile-friendly views
- Database design and implementation of static data as a code modules system
- Open-sourcing test questions and explanations along with an edit system based on Git
- Implementing reactive view model using MobX and performance testing
- Enabling server-side rendering using node.js on IIS with iis-node
Promegle
C#, JavaScript, React, Node.js, Babel, Redux, WebSockets, Docker, ASP.NET, Windows Phone, Windows Store
- Writing the front-end with React.js, Redux, and a custom styling solution
- Implementing message queue system and reactive immutable database solution
- Implementing video calling over WebRTC along with the signaling server
- Automatic image classification using Yahoo’s Not Safe For Work deep neural model running on Docker
- Integrating with legacy ASP.NET long polling back-end servers and C# code
Personal web page and blog
ciunkos.comJavaScript, React, Node.js, Babel, ESLint, Git, GitHub, Visual Studio Code, Webpack
- Writing the front-end with React.js, styling with CSS, using data as a code modules
- Designing responsive and printable resume page
- Static blog generation and integrating Disqus commenting system
- Controlling source code and data versions using Git
- Deploying server-side rendering with express.js with iss-node on Windows IIS
Store ᴾᴿᴼ
C#, XAML, MVVM, ASP.NET, MS SQL, Visual Studio
- Design and development of Windows Phone 8 application using C#, XAML, and MVVM pattern
- Creating and deploying ASP.NET based web service and API endpoints along with a landing page
- Designing and managing MS SQL database and integrating it with back-end service using Entity Framework
- Back-end crawling services for deals discovery and sending push notifications
- Internationalization implementation for over 192 languages
- Deployment of multiple updates, maintenance work, and customer support
MusicTube
C#, XAML, MVVM, ASP.NET, MS SQL, Visual Studio
- YouTube and Last.fm APIs integration
- Windows Store, Windows Phone and web app development using C#, XAML, MVVM, ASP.NET
- Design and management of MS SQL database along with crawling services
- Monetization through multiple ad networks
- Implementation of multi-platform shared code solution
- Deploying multiple updates, maintenance work and customer support
Pogaduszki
C#, XAML, MVVM, ASP.NET, MS SQL, Visual Studio
- Development of native Windows Phone
- Implementation of Gadu-Gadu binary protocol
- Setting up a persistent proxy and push notification server
- Deploying multiple updates, maintenance work, and customer support
APKA.TV
C#, XAML, MVVM, ASP.NET, MS SQL, Visual Studio
- Collaborative development of native Windows Store and Windows Phone applications
- Deployment of ASP.NET based streaming services and API endpoints
- Development of streaming protocol with various bitrates
- Designing chunked stream format which allows history playback
- Deploying streamer app along with hardware wiring and antennas using DVB-T adapters
Kwejk.pl
Cube Investments sp. z o.o.
C#, XAML, MVVM, ASP.NET, REST
- Development of Windows Phone application based on client’s provided project using XAML, MVVM, and C#
- Integration with existing web APIs
- Collaboration with the project owner and the rest of the team
- Monetization through different ad networks
- Deploying multiple updates after the release
RatingSpeed
LOSO sp. z o.o.
C#, XAML, MVVM, ASP.NET, REST
- Development of Windows Phone application using XAML, MVVM, and C#
- Implementing network speed measurement, test results statistics, and upload
- Implementing the design provided by the client
- Preparing the app for publishing in the Windows Store and providing app updates
Fast-coffee
IT4Biznes Marcin Hatalak
C#, XAML, MVVM, ASP.NET, socket.io, Visual Studio
- Implementing real-time chatting with socket.io
- Implementing chat list, chat and profile views
- Integrating with provided APIs
- Working closely with the development team
I Am Rich The Game
Red Dot Games
C++, XAML, C#, DirectX 9, DirectX 11, Visual Studio
- Development of native Win32 game in C++ with a DirectX renderer
- Networking protocol with Winsock, supporting up to four players
- Implementation of provided game design and assets
- Porting to Windows Store as hybrid native and C# app
- Deploying to the app store and providing updates
Platformer game engine
Red Dot Games
C#, DirectX 9, XNA, Visual Studio
- Development of XNA game engine
- Creation of game editor with support for live play
- Implementation of various game mechanics
Your next project can be here
Interests and future goals
I am always ready to learn new things and explore new opportunities.
My interests
- Functional programming
- Language design
- Code golf in Haskell
- Deep neural networks
- UX
- App development
- DX
- Security
- Performance
- User interface design
- Dogs 🐶
Functional programming, language design, code golf in Haskell, deep neural networks, UX, app development, DX, security, performance, user interface design, dogs 🐶.
My goals
- Set up a Kubernetes cluster
- Give a speech about programming
- Join a team of highly-skilled and experienced people
- Improve my spoken English
- Sharpen my TypeScript skills
- Contribute to an open source project
- Write more tests and explore TDD approach
- Increase presence on StackOverflow
.
Wyrażam zgodę na przetwarzanie moich danych osobowych w celu prowadzenia rekrutacji (RODO, art. 6 ust. 1 lit. a).
