Fullstack ( Member～Senior )

Fullstack Engineer — Tokyo

Starting Salary Range: 5,000,000 ~ 15,000,000 JPY

Japanese level required: Business level

About the Company

We are a fast-growing AI startup in Japan, driving enterprise digital transformation through cutting-edge Generative AI products and AI Agent platforms. Our solutions empower over 1,000 enterprise organizations, public sector entities, and 100,000+ active users to automate business workflows and significantly boost operational productivity.

Key Innovations:

• Enterprise Generative AI Platform: A multi-LLM SaaS product integrating top-tier models (such as GPT, Gemini, and Claude) with patented model-switching technology.
• AI Agent Ecosystem: Advanced automation technologies including AI-built RPA systems that understand natural language commands, specialized multi-agent workflow engines, and centralized AI agent management platforms.

Role Overview

We are seeking skilled Fullstack / Senior Fullstack Engineers to design, build, and scale our enterprise Generative AI products and next-generation AI Agent ecosystem. Working in an agile, high-velocity environment, you will develop across the full technology stack—from resilient backend services and high-performance frontend interfaces to cloud infrastructure.

Depending on your background and experience level:

• Senior Level: Drive product architecture as a Tech Lead, lead end-to-end technical implementation, and take ownership of launching 0-to-1 AI initiatives.
• Mid-Senior Level: Take full ownership of fullstack feature development, with flexibility to adjust your focus between backend and frontend development based on your strengths and team needs.

Key Responsibilities

• Design, build, and maintain scalable fullstack web applications for enterprise Generative AI products.
• Architect robust cloud infrastructure to handle complex LLM integrations and high-volume data workflows.
• Partner closely with Product Managers, AI Engineers, and UI/UX Designers to translate business requirements into efficient software architecture.
• Maintain high code quality through active pair programming, comprehensive peer code reviews, and automated testing.
• (Senior/Lead) Establish engineering best practices, mentor software engineers, and lead system architecture decisions for new product launches.

Engineering Culture

• Quality & Readability: Strong commitment to clean, maintainable code, collaborative pair programming, and rigorous code reviews.
• Adaptable Workflows: Flexible project execution choosing between Agile, Waterfall, or Ad-hoc development depending on project scope and timelines.

Qualifications

Minimum Qualifications:

• Practical experience in fullstack web application development using modern tech stacks (e.g., Python, Go, Node.js, TypeScript, React, Next.js, or cloud-native architectures).
• Experience with major cloud platforms (AWS, GCP, or Azure).
• Solid understanding of microservices architecture, RESTful API design, and relational/non-relational databases.
• Deep interest in Generative AI, LLMs, and autonomous AI agent architectures.

Preferred Qualifications (Senior / Lead Track):

• Experience leading engineering projects as a Tech Lead or Principal Engineer.
• Proven track record in architectural design for high-traffic SaaS products or complex distributed systems.
• Hands-on experience integrating AI/LLM technologies (e.g., OpenAI API, LangChain, vector databases).
• Track record of building products from 0 to 1 or scaling early-stage systems in a startup environment.

Work Environment & Benefits

• Work Style: Hybrid work model with a Discretionary Working System (Flexible hours).
• Location: Tokyo, Japan (Minato-ku area).
• Holidays: 120+ annual days off, Refreshment Leave (5 paid days/year), Special Condition Leave, Return Leave.
• Benefits: Full Social Insurance, PhD Academic Support Program, Online Health Counseling Support, Commuting Allowance.