
Janos Breuer
Verified Expert in Engineering
Android Developer
Budapest, Hungary
Toptal member since June 28, 2016
Janos is an AI application engineer with 17+ years of experience building production software in finance, SaaS, and regulated domains. For Goode Capital, he delivered production-trading systems, including exchange-market data, a real-time Excel add-in, and Node.js and WebSocket sync for live traders. Janos built an AI agent platform with RAG, tool use, and citation-aware outputs for legal workflows. Clients value his clear communication, pragmatic architecture, and production-first delivery.
Portfolio
Experience
- APIs - 10 years
- Java - 8 years
- Android - 5 years
- Node.js - 3 years
- TypeScript - 3 years
- OpenAI API - 1 year
- Retrieval-augmented Generation (RAG) - 1 year
- AI Agents - 1 year
Preferred Environment
Windows, Git, MacOS, TypeScript, Node.js, Python, React, PostgreSQL, LangChain, OpenAI API
The most amazing...
...MVP I built with practicing lawyers is an AI agent for legal research over legislation and court rulings, focused on citation-backed, auditable outputs.
Work Experience
Full-stack Engineer
Goode Capital
- Built and maintained production trading systems for a London-based oil trading firm, including a FIX adapter for ICE exchange connectivity and an Excel add-in for real-time desk pricing.
- Delivered TypeScript/Node.js WebSocket sync between exchange market data feeds and trader Excel workflows used daily on the desk.
- Deployed the FIX adapter and Excel pricing add-in on AWS; built a pricing history database to persist and query pricing data for the add-in.
- Integrated QuickFIX/J for live exchange connectivity and supported the production environment with traders and the ops team.
Senior Mobile Developer
Realeyes
- Implemented a digital well-being app with Jetpack Compose UI and clean architecture.
- Wrote a wrapper SKD for a native emotion detection component.
- Implemented REST API session handling and integrated Amazon Kinesis SDK.
- Designed RxJava interfaces for internal components of the SDK.
Senior Mobile Developer
W.UP
- Developed mobile banking Android applications for several European banks, e.g., Cembra.
- Built complex UIs with theming, as well as the foundation of the business logic, so that it adheres to clean architecture.
- Integrated a mobile payment service to one of the apps.
- Held test-driven development presentations and workshops for my team.
Android Developer
LogMeIn
- Developed two apps and an SDK for Android as part of the mobile scrum team at LogMeIn Rescue. These applications are integral components of a large-scale SaaS application used for remote support by companies like Microsoft and Vodafone.
- Used a wide range of Android system features to remotely control and diagnose a device.
- Refactored the remote control component so that it could be unit-tested.
Full-stack Developer
MuTrader.com
- Developed a real-time financial mobile app for iOS and Android with a back-end server that had push notifications and an integration to 3rd-party Forex broker systems.
- Implemented the back end with service-oriented architecture and a RESTful API.
- Used enterprise integration patterns and FIX protocol to connect to brokers.
- Wrote unit tests and automated acceptance tests with TDD.
Lead Software Engineer
LogMeIn
- Led a team of 7-9 engineers in the development of two Windows client components for LogMeIn Rescue.
- Acted as a member of the company-wide architecture review board.
- Interviewed new developer candidates and participated in the hiring decisions.
- Created a code-branching policy and introduced code reviews, which stabilized the release process of the component.
- Evangelized unit-testing and test-driven development (TDD); also organized TDD training sessions for the team.
- Created the system design and a prototype for the planned rewrite of a client component for four platforms (Windows, Mac, Android, and iOS).
Software Engineer
LogMeIn
- Collaborated with the C++ team at LogMeIn Rescue to develop a browser plugin using SOAP, STL, and ATL.
- Implemented major features in the Windows component of the product, including technician collaboration and unattended access.
- Implemented a task-based threading model that allowed the team to eliminate frequent bugs related to deadlocks and UI freezes.
- Answered level 3 support requests, troubleshooting urgent technical issues with VIP customers.
- Presented to 100+ developers at a company conference about my team's achievements.
Software Engineer
Lufthansa Systems
- Developed an airline schedule optimizer in a distributed team, communicating daily with remote team members.
- Implemented and unit-tested graph data structures and algorithms in C++ on four Unix variants.
- Refactored our unit-testing framework, significantly reducing the size of unit test binaries and saving storage-related costs.
Experience
Legal AI Agent for Research & Submission Workflows
The experience works like an IDE for legal research: practitioners chat with the agent and open cited sources, including PDFs, directly from the conversation. Answers are source-grounded and structured for professional review.
It is co-developed with practicing lawyers through customer discovery on a representative sample matter. The architecture combines RAG over public sources, tool-using agent workflows, and iterative retrieval and prompt design toward reliable legal research assistance.
Mindfully App
In this role, I spearheaded the rewrite of an SDK to encapsulate the image recognition component, facilitating emotion detection in 3rd-party apps.
Collaborating closely with a team of two developers, I introduced advanced theming techniques using Jetpack Compose. Additionally, I successfully advocated for the implementation of clean architecture principles, thereby ensuring the scalability and maintainability of the project.
Cembra Mobile Bank
I used state-of-the-art architecture and technologies in this project, such as biometric authentication, clean architecture, reactive programming, and dependency injection.
LogMeIn Rescue
https://play.google.com/store/apps/details?id=com.logmein.rescuemobile&hl=huAs the lead developer of a team of 7-9 engineers, I implemented a code branching policy and pioneered a code review process. These initiatives improved code quality, enhanced knowledge sharing, and stabilized release processes for the component.
Rescue Lens
https://play.google.com/store/apps/details?id=com.logmein.rescuelens&hl=huI implemented several features in the app and planned and performed a major refactoring that made the code more robust and maintainable.
Stack Overflow Contributions
MuTrader
https://drive.google.com/file/d/1ghKMfI-Ty0uEZWN-PQOttVQ-D7Dui6z8/viewIt connected Forex advisers, traders, and brokers.
Education
Master's Degree in Mathematics
Budapest University of Technology and Economics - Budapest, Hungary
Skills
Libraries/APIs
Retrofit, REST APIs, WebRTC, ATL, Node.js, React, OpenAI API
Tools
Android Studio, Gradle, Jenkins, Git, Android NDK, SWIG, GitHub, AWS ELB
Languages
Java, Kotlin, C++, UML, C#, Bash Script, TypeScript, Python
Frameworks
Android SDK, Swagger, Spring Boot, Spring, Spring MVC, Apache Camel, JNI, JPA, Hibernate, Angular, Electron
Paradigms
Test-driven Development (TDD), Design Patterns, Dependency Injection, Unit Testing, Automation, Clean Architecture
Platforms
Android, Windows, Linux, Firebase, Unix, iOS, Azure, Win32, MacOS, Amazon Web Services (AWS)
Storage
MySQL, PostgreSQL
Industry Expertise
Trading Systems
Other
APIs, Algorithms, Push Notifications, Technical Leadership, WebSockets, SOAP, Amazon Kinesis, QuickFIX, Enterprise Integration Patterns (EIP), FIX Protocol, Mathematics, Coroutines, Jetpack Compose, SDKs, Finance APIs, Finance, LangChain, AI Agents, Retrieval-augmented Generation (RAG), Large Language Models (LLMs), API Integration, Trading, Amazon RDS, Tools, AI Development
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