Freelance Full‑Stack Developer - SaaS, Stripe & AI Agents
Paris, France - Remote - 8+ years experience

I help SaaS teams and founders build, scale and modernize their products.

• Design and implementation of SaaS architectures (multi‑tenant, B2B/B2C)
• Stripe & subscription billing, multi‑provider payments (Stripe, GoCardless, PayPal)
• Real‑time features: WebSockets, live dashboards, BLE data
• AI integration: RAG pipelines, semantic search, and agent orchestration
• Working in existing codebases, cleaning up legacy and improving performance

I design and build end‑to‑end SaaS features:

• Authentication, user management, roles & permissions
• Billing and subscriptions (Stripe, GoCardless, PayPal)
• Dashboards, reporting, notifications, background jobs
• Architecture for scalability, reliability and easy maintenance

Stack:
React - React Native - TypeScript - NodeJS - PHP / Laravel - PostgreSQL - Redis - MySQL - MongoDB - AWS - Docker

I’ve led several production payment projects, including:

• Full migration to Stripe for a multi‑site ticketing/booking SaaS (culture, attractions, ferry)
  • Payments, webhooks, refunds, automatic reconciliation
  • Migration with zero downtime on live traffic
• Integration of Tesco Clubcard (loyalty programme) and Protect Group (cancellation insurance) into the checkout flow
• Unified multiple payment providers behind a single, testable payment layer

If you need to implement or refactor payments, subscriptions or marketplace flows, I can help.

I also work on applied AI inside products:

• RAG pipelines on real codebases:
  • Built a RAG system on an internal framework using CocoIndex + pgvector
  • Incremental semantic indexing of the codebase (Tree‑sitter)
  • Contextual search for AI agents and faster developer onboarding
• Agent orchestration for automated tech‑watch (“Agent Veille”):
  • Collection, synthesis and prioritisation of multiple sources
  • Significant time saved on ecosystem monitoring

AI tooling:
LLM integration - RAG - CocoIndex - Mastra - pgvector

• 100% remote & autonomous - used to joining distributed teams
• Comfortable taking full ownership: design -> implementation -> tests -> deployment -> documentation
• I can quickly understand existing codebases, refactor legacy, and improve performance and security
• I care about clean code, tests (unit/e2e), CI/CD and long‑term maintainability

• Frontend & Mobile:
  React - React Native - TypeScript - VueJS - NuxtJS - WebSockets

• Backend:
  NodeJS - PHP / Laravel - PostgreSQL - Redis - MySQL - MongoDB - REST

• AI & Data:
  LLM integration - RAG - CocoIndex - Mastra - pgvector

• Cloud & DevOps:
  AWS - Docker - GitHub Actions - GitLab CI - Monorepos

• GitHub: github.com/jguillaumesio
• Location: Paris, France - Open to remote missions (EU‑friendly timezones)

If you’re building or scaling a SaaS product and need help with payments, real‑time features or AI integration, feel free to reach out.