Senior Full-Stack Engineer (Next.js + React/TypeScript + Xero API + Microsoft Graph + OAuth) to $5500

We are a London-based SaaS startup building an AI-powered document-chasing platform for accounting firms.
We help accountants automatically request, receive, validate, and process financial documents using AI.
 

We are looking for a Senior Full-Stack Engineer who has deep experience with Next.js and Microsoft Graph integrations.
This is a long-term, full-time role (not a short contract). You will be one of the very first engineers working on the product.

Tech Stack

• Next.js 14 (App Router)
• React + TypeScript
• AWS (Amplify, RDS Postgres, S3, Lambda)
• Xero API
• Microsoft Graph (Outlook email)
• LLM APIs (OpenAI / Anthropic)

What You Will Work On

You will build the core of our MVP, including:

1. Email Integration (Microsoft Graph)

• Implement OAuth (auth code + refresh tokens)
• Sending emails via /sendMail
• Reading inbound emails via /messages
• Handling attachments via /attachments
• Implementing Graph webhook subscriptions (/subscriptions)
• Implementing polling + incremental sync as fallback
• Parsing & validating JWT access tokens
• Handling throttling, retries, error codes, timeouts

2. Accounting Integration (Xero)

• Implement OAuth2
• Fetch bank accounts + transactions
• Build logic to detect missing statement periods

3. Full-Stack Product Features

• Next.js API routes
• Document review flows
• S3 upload flows
• RDS Postgres queries
• LLM calls for document classification + email generation

This is a hands-on build role. You will work directly with the founder and one backend/cloud engineer.

Required Experience

We are specifically looking for someone who has:

Must-Have Technical Skills

• 4–7+ years of experience in full-stack development
• Excellent TypeScript + React
• Strong Next.js 13/14 experience (App Router, server actions, route handlers)
• Real-world experience integrating Microsoft Graph
  (email sending, reading, polling, webhook subscriptions)
• Strong OAuth2 knowledge (PKCE, refresh tokens, token expiry handling)
• Experience with REST API integrations
• Comfortable working with AWS basics (S3, Lambda, RDS)
• Comfortable designing DB schemas (Postgres)
• Experience working with LLM APIs (OpenAI/Claude)
• Strong debugging and problem-solving skills

Nice to Have

• Experience with Xero or other accounting APIs
• Experience with background jobs / cron tasks
• Experience with fintech or accounting systems
• Familiarity with Graph delta queries

Soft Skills

• Strong communication in English
• Comfortable working directly with a founder
• Able to move quickly and independently
• Attention to detail (important: email workflows must be reliable)
• Startup mindset (build fast, solve problems, iterate)

Position Details

• Full-time, long-term
• Remote
• Competitive salary depending on experience
• Direct contract via Deel (or similar)
• Start date: ASAP (within 1–3 weeks preferred)

How to Apply

Please send:

• Your CV or Djinni profile
• Links to any relevant projects or code samples
• A brief note on your experience with Microsoft Graph and OAuth