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 database work 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. 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, 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.