Senior Software Engineer, Fraud Risk

KOMOJU (by Degica) is the leading cross-border payment gateway for Japan. We power payments for companies like video game distribution platform Steam and the popular mobile app TikTok. Today we help thousands of merchants by providing them with the payment infrastructure they need through developer-friendly API’s to integrations on popular platforms like Shopify and Wix; we help our merchants grow in all markets they are expanding.

We are product focused, meaning you will work on our payment gateway and any other projects towards building the future of KOMOJU. You will work with other engineers in a flat and inclusive culture where engineers have a say in both key product and technology decisions. The culture is largely self-organizing, which means engineers have both a stake and ownership in what they work on. Engineers play to their strengths, but are also able to invest in areas where they want to grow within the team. At KOMOJU, you are the main driver behind your growth and position in the company.

Most of our engineering team members come from outside Japan. English is the primary language used, and throughout the company, many people are at least bilingual. Being an international company we know the importance of bilingualism. We offer all employees a choice between optional English and Japanese lessons to help create a culture of diversity and ensure smooth collaboration across teams.

The Fraud Prevention Team at KOMOJU is our frontline defense against payment fraud, protecting both our merchants and their customers across international borders. We build and maintain the intelligent systems that analyze merchant profiles, as well as transactions in real-time to detect and prevent fraudulent activity.

We value pragmatic solutions, data-driven decisions, and believe that the best fraud prevention is invisible to legitimate customers. Our stack includes Ruby, Python for ML models, OpenRouter for LLM integrations, and OpenTelemetry-based observability tools to monitor our systems 24/7.

You will be responsible for designing, building, and maintaining software solutions for KOMOJU's fraud prevention processes. As part of our Fraud Prevention Team, you'll build robust antifraud solutions, create merchant-facing features that provide visibility into transaction fraud signals, and help scale our systems to handle increasing complexity and traffic. You'll be at the intersection of engineering excellence and advanced fraud prevention, protecting billions of yen in transaction volume across borders.

To be successful in this role, you must have 5+ years of software engineering experience. You should have a strong understanding of software development best practices, experience with data structures and algorithms, and strong coding skills in Ruby. In addition, being a motivated self-starter with the ability to work independently as well as in a team environment will help you thrive.

• Be a builder: contribute to the implementation of our antifraud solutions.
• Improve parts of our antifraud system to strengthen its features and make it robust.
• Develop merchant-facing risk management tools: Create intuitive dashboards that give merchants real-time visibility into fraud patterns, customizable risk rules, and actionable insights to optimize their fraud prevention strategies.
• Convert our ML models (e.g. merchant risk scorecards) into working code with engineering excellence
• We work agilely with minimal amount of meetings. With autonomy and trust, you will bring live functionalities independently that impact many merchants and shoppers.

• 5+ years of professional experience as an engineer, currently working as a Engineer or Senior Engineer, at least 2 years of personal or professional experience with Ruby.
• Production debugging expertise: You can diagnose complex production issues, and get up to speed with an existing codebase without extensive hand-holding. You're comfortable diving into unfamiliar code, understanding its intent, and making surgical improvements without breaking existing functionality.
• You have a strong sense of ownership, helping you move projects forward independently and make a meaningful impact.
• You have great communication skills and the ability to articulate complex technical concepts in front of different stakeholders.
• Detail-oriented team player: You catch edge cases others miss, foster positive working relationships across the team, and reliably provide coverage when teammates are unavailable.
• Business-level English proficiency.

These aren't required, but be sure to mention them in your application if you have them.

• Familiarity with statistical methods (logistic regression, GLMs) and machine learning, ability to perform exploratory data analysis (EDA), experience with data ETL pipelines, and comfort working with SQL to analyze patterns in the data.
• Python, or any other major programming language.
• Experience working in the credit card or payment industry.
• Background in payment security or security systems in general.
• Conversational-level Japanese proficiency.
  

• Ruby on Rails
• Python, Django
• MySQL, PostgreSQL, RedShift
• React, Vue.js, Webpack, Storybook
• AWS, Terraform, Docker
• GLM, Logistic Regression
  

• We aim to keep our hiring process thoughtful and efficient, typically involving 3-4 stages depending on the role: an initial screening, a take-home exercise, a technical (team) interview, and a final interview with leadership. Throughout the process, the team evaluates candidates not only on technical skills but also on their alignment with team culture and potential for long-term growth.

• At Degica, we embrace remote work while also offering office space for those who prefer in-person collaboration
• 10 days regular vacation, additional 5 days summer and 5 days winter vacation
• Paid birthday holiday
• Budget for self-learning allowance, to ensure our employees’ skills remain current
• Language training for Japanese
• Company membership of O'Reilly
• Twice a week office lunch