Piotr
Matyjasik

About me

I'm an AI engineer (TypeScript / React) building LLM agents and full-stack products on top of foundation models. At Y Combinator (P26) I co-founded TesterArmy as its lead engineer and built an AI agent that autonomously tests web and mobile apps.

Before that, I spent ~3 years building high-traffic frontend at Grupa Wirtualna Polska, one of the largest media groups in Poland/CEE. I work across the stack and focus on making AI reliable in real products.

In my free time I ship my own AI-powered iOS apps (Isao, Kai - React Native + LLMs, live on 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)

   Co-Founder / Lead Engineer

   I co-founded a Y Combinator (P26) startup and was its lead engineer - I built the product end-to-end. I built an AI agent that autonomously tests web and mobile apps: it navigates and clicks through an app like a real user (LLM tool-calling and context management) to surface bugs and regressions before release. We shipped the MVP to first users during the batch, iterating on real feedback.

   Technologies:TypeScript, React, Next.js, Node.js, LLM APIs, Tool-calling, AI Agents.

2. May 2023 - March 2026

   GRUPA WIRTUALNA POLSKA

   Front-end Developer

   I built and shipped frontend features for high-traffic products at one of Poland/CEE's largest media groups. I implemented tracking and web analytics (tag management, tracking pixels), enabling product and marketing teams to measure and act on user behavior, and designed AI agents automating internal developer workflows.

   Technologies:React, Next.js, Javascript, Typescript, HTML, CSS, TailwindCSS, SCSS, Git, REST API, GraphQL, MobX, Jotai, Jest, React Testing Library, Node.js, Express, React Router, AI agent workflows.

3. September 2022 - May 2023

   FUJITSU

   Front-end Developer

   I actively participated in the development of an application for displaying and manipulating complex data, as well as resolving issues on both the frontend and backend. I also collaborated in defining the application's requirements.

   Technologies:Javascript, Typescript, React, Git, SQL, REST API, CSS, HTML.

4. June 2021 - September 2022

   ETECHNOLOGIE

   Front-end Developer

   I took a central role in the creation and development of e-learning and e-commerce platforms, which contributed to their design, implementation, and continuous improvement. I also took part in defining project requirements.

   Technologies:Javascript, React, Wordpress, PHP, HTML, CSS, REST API, Git.

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.