Lead Fullstack Engineer (Core API Team / Tokyo)

Tokyo
Partial Remote
Full-time
January 7, 2026

Mico provides Mico Engage AI, a high-traffic enterprise SaaS used daily by major companies, and we are currently hiring core members to join our team. In our fast-growing startup environment, you will have the opportunity to directly shape the product and elevate the user experience.

You will be expected to proactively tackle a wide range of challenges—including optimizing high-traffic systems, designing and developing new AI-powered features, and improving our real-time processing infrastructure. Working closely with our Lead Engineer, you will drive product-critical development while also contributing to technical decision-making and standardization.

By collaborating in a multicultural, bilingual team, you will sharpen your skills in a global development environment. This role offers far more than pure development—you will play a key part in the growth of both the team and the product, making it a highly rewarding position.

What You Will Gain

Experience building scalable systems and implementing AI technologies
Collaboration skills in a global engineering team
Hands-on experience driving product growth end-to-end

Tech Stack

Web Backend: TypeScript (NestJS), Go, Elixir
Web Frontend: TypeScript (Next.js), Chakra UI
Databases: TiDB, Aurora MySQL, DynamoDB, MemoryDB for Redis
Infrastructure: AWS (Cognito, EC2, ECS, Route53, Lambda, Kinesis, SQS, SES, RDS, API Gateway), IaC (Terraform, AWS CDK)
Monitoring & Analytics: New Relic, Sentry, CloudWatch, BigQuery, Metabase
CI/CD & Tools: Docker, GitHub Actions, Amplify, Slack, Notion, Jira

Responsibilities

Stabilizing the Mico Engage AI delivery infrastructure: Build and improve the infrastructure that enables stable, large-scale delivery even for clients with high user volumes.
Developing new features: Implement product-enhancing features based on our product roadmap.
Customer-driven development: Build high-impact, reusable features based on needs identified through Customer Success activities.

Requirements

5+ years of hands-on experience in system development
Experience developing with TypeScript and React
Full-stack engineering experience (both frontend and backend)
Ability to independently handle the full development cycle from requirements definition to release
2+ years of code review experience
Ability to write test code and contribute to improving product quality
Experience in technical decision-making (evaluating technical requirements, cost, and maintainability)
2+ years of experience leading development as a technical lead (teams of 5+ members)
Experience designing REST APIs
Business-level proficiency in Japanese

Nice to haves

Experience developing with Go or Next.js
Experience designing systems using Domain-Driven Design or Clean Architecture
Experience developing gRPC APIs
Ability to effectively leverage AI to improve development speed and quality
Experience in infrastructure setup (AWS, Docker, etc.)
Business-level proficiency in English

Hiring process steps

Document Screening + Coding Test
Casual Interview (Japanese Language Check)
First Interview
Second Interview
Third Interview
Final Interview + Reference Checks
Job Offer Meeting

APPLY NOW ➜Japanese Required ⚠️

About Mico

Mico's goal is to help companies build their brand through the power of conversation.

They are developing multiple products to analyze and improve communication between businesses and consumers, enabling businesses to optimize their online marketing efforts.

The conversational commerce (C-commerce) is a rapidly growing market, projected to surpass the e-commerce market in some countries by 2025.

Mico (formerly Micoworks) has already achieved 500% business growth in just two years, placing them at No.1 in LINE marketing within Japan. They’ve also secured a total of 6.3 billion yen in funding (with the recent Series B raising 3.5 billion yen).

Mico has set an ambitious goal to become Asia’s No.1 Brand Empowerment Company by 2030, with a clear vision for global expansion. Achieving this goal will require tackling numerous obstacles, and they’re looking for team members who are up to the challenge.

Here are the services you’ll be working on:

Mico Engage AI
A platform that enables seamless communication through message automation and deep integration with client systems.
Mico Voice AI
A service that enhances communication quality and efficiency using AI voice recognition, helping create meaningful connections.
BizClo
An enterprise messaging solution leveraging LINE, optimized for business use to improve customer relationships and internal efficiency.
Micomii
A LINE-based membership service for restaurants, enabling customer segmentation, rank-based perks, and digital coupons.

Why Work at Mico

Flexible work style: Hybrid work, flex time, and up to 6 Life-Health leave days per year to support diverse lifestyles.
Robust growth support: Subsidies up to ¥55,000/year for books and seminars, supporting personal skill and career development.
High transparency: Weekly TGIM meetings and quarterly company-wide updates to share vision and goals.
Leadership training: Strengthening team leadership and organizational management skills through training.
Fair evaluation system: OKRs-based performance reviews conducted semi-annually with continuous feedback.
Social impact: Contributing to clients’ business and workstyle transformation through innovative SaaS products.

Team Culture

Mico leadership and team embody an "OPEN MIND" approach, fostering open and inclusive communication across all levels.

Driven by a strong "ALL FOR ONE" spirit, team members work closely together, both internally and externally, with a "Giver Mindset" at the core of their collaboration.

Surrounded by highly motivated colleagues, you’ll find daily inspiration and opportunities for personal and professional growth.