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Sergei Fisak

@sergeifisak

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Highly analytical QA Engineer specializing in functional, integration, and API testing for mobile and web services.

Serbia

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What I'm looking for

I’m looking for a QA role where I can own functional, integration, and API testing for mobile and web services, partner closely with developers, and drive structured documentation, contract validation, and defect prevention in high-load, distributed systems.

I’m a highly analytical QA Engineer with over 5 years of experience specializing in functional and integration testing of mobile applications (iOS/Android) and web services. I focus on API testing within high-load environments, creating structured test documentation, and isolating complex defects in distributed architectures to protect seamless product quality.

In recent work, I’ve executed black-box and exploratory testing for fintech integration layers, performed API contract validation against internal and third-party documentation, and built checklists, test cases, and regression suites from scratch for microservice architectures. Previously, as a QA Engineer, I managed QA workflows for a high-load mobile platform, mentored junior engineers, applied grey-box testing by analyzing Kotlin source code for targeted scenarios, and introduced systematic integration phases plus observability tooling to reduce cross-platform production defects.

Experience

Work history, roles, and key accomplishments

Manual QA Engineer

Crypto / Fintech Product (NDA)

Oct 2025 - May 2026 (7 months)

Executed black-box and exploratory testing for a fintech integration layer, including payment gateway and web/mobile OAuth authentication flows. Investigated complex multi-service defects and validated API contracts across aggregator and payment provider integrations.

Functional Testing Exploratory testing Integration Testing OAuth Proxyman Charles PostgreSQL Clickhouse Microservices Black Box Testing

QA Engineer

Wildberries

Apr 2023 - Sep 2025 (2 years 5 months)

Managed QA workflows for a core mobile platform in a high-load environment, mentoring junior engineers and streamlining onboarding during rapid expansion. Defined end-to-end testing strategy across frontend, backend, and third-party APIs, and reduced cross-platform production defects through improved regression and integration testing.

Mobile App Testing Test Strategy Kotlin API Testing Security Testing Session Management Regression Testing Integration Testing A B Testing Code Review

Mobile QA Engineer

Ozon

Jun 2021 - Apr 2023 (1 year 10 months)

Performed exploratory and regression testing for mobile SDK updates and cross-platform features using traffic interception tools. Conducted root-cause analysis for a WebSocket issue affecting real-time GPS tracking and wrote SQL queries to verify backend data consistency across services.

Exploratory testing Regression Testing Proxyman Root Cause Analysis WebSockets SQL Shift Left QA Standardization

Education

Degrees, certifications, and relevant coursework

ITMO University

Master's degree

Earned a Master's degree from Saint Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University) (2019).