Piotr
Matyjasik

About me

I'm a full-stack product engineer (TypeScript / React) with 5+ years of shipping end-to-end - from high-traffic frontend at one of Poland/CEE's largest media groups to co-founding TesterArmy (Y Combinator P26), where I built an AI agent that autonomously tests web and mobile apps.

At TesterArmy I built the product end-to-end: frontend, backend, database and the agent itself. We shipped the MVP to 30+ companies during the batch. Before that, I spent ~3 years at Grupa Wirtualna Polska building features across the stack for high-traffic products, plus internal AI agents automating developer workflows.

On the side I've built and shipped my own AI-powered iOS apps (React Native + LLMs) to the App Store.

TECHNOLOGIES

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Javascript

Typescript

React

Next.js

TailwindCSS

Git

Experience

1. March 2026 - July 2026

   TESTERARMY (Y COMBINATOR P26)

   Software Engineer & Co-Founder

   Co-founded a Y Combinator (P26) startup and built the product end-to-end: frontend, backend, database and the agent itself. Built an AI agent that autonomously tests web and mobile apps - an LLM tool-calling loop with context management keeping long sessions coherent. Owned product direction as CPO - user research, specs and prioritization - alongside the hands-on engineering work. Shipped the MVP to 30+ companies during the YC batch, iterating weekly on real user feedback.

   Technologies:TypeScript, React, Next.js, Node.js, PostgreSQL, Playwright, LLMs, AI Agents, tool-calling, context engineering.

2. May 2023 - March 2026

   GRUPA WIRTUALNA POLSKA

   Frontend Developer

   Built and shipped features across the stack - React/TypeScript/Next.js on the frontend, Node.js services and MySQL on the backend - for high-traffic products. Designed and developed AI agents automating internal workflows, including an agent generating analytics event-tracking scripts, cutting manual scripting work to a fraction of the time. Contributed to CI pipelines and Docker image builds, taking features from code to production. Implemented tracking and web analytics (tag management, tracking pixels), enabling product and marketing teams to measure and act on user behavior.

   Technologies:TypeScript, React, Next.js, Node.js, MySQL, Docker, CI, LLM APIs (AI agents), GraphQL, TailwindCSS.

3. September 2022 - May 2023

   FUJITSU

   Frontend Developer

   Developed a React application for displaying and manipulating large volumes of complex enterprise data, backed by REST APIs and MySQL. Resolved issues across the frontend and backend, owning problems end-to-end, and co-defined business requirements and project milestones with cross-functional stakeholders.

   Technologies:TypeScript, JavaScript, React, C#, MySQL, REST API, HTML, CSS.

4. June 2021 - September 2022

   ETECHNOLOGIE

   Frontend Developer

   Designed, built and improved eLearning and eCommerce platforms - PHP/WordPress with MySQL on the backend, JavaScript/React on the frontend. Tested frontend features, troubleshot issues across the stack and worked directly with business and product teams to ship improvements.

   Technologies:JavaScript, React, PHP, WordPress, MySQL, REST API, Cypress.

5. October 2020 - June 2021

   OIRP SZCZECIN

   IT Support

   I provided technical assistance and troubleshooting during classroom and online activities. I was also responsible for managing student files.