jguillaumesio

Services for the hard parts of your SaaS

Full-stack contractor for the hard parts of SaaS: payments/billing and AI agents in production. Remote, UTC+1

Available for new engagements from July 2026

What I do

Two specialty lanes under a full-stack umbrella. Both are scarce, high-stakes work that engineering teams typically can't staff internally.

Specialty lane 1

Payments & billing integration

The part of your product where mistakes cost real money. I take on work that requires an engineer who's already broken (and fixed) live payment systems.

  • Stripe migrations with zero downtime (webhooks, refunds, automatic reconciliation)
  • Multi-provider checkout: Stripe + GoCardless + PayPal unified behind one API
  • Loyalty programs, cancellation insurance, third-party payment integrations
  • VAT / MOSS handling, dispute management, fraud review workflows
  • Audits of existing payment integrations (the kind you commission before you sell or raise)
Specialty lane 2

AI agents in production

Most "AI agent" work is a demo. Production agents are about cost, latency, evaluation, and human-in-the-loop. I build the real thing.

  • Agentic coding pipelines with persistent context over your codebase
  • RAG over codebases (CocoIndex + pgvector + Tree-sitter, incremental semantic indexing)
  • Agent orchestration (Mastra and others) for ops/research/onboarding workflows
  • Cost & latency analysis, eval harnesses, prompt iteration as a measured loop
Umbrella

Full-stack SaaS ownership

Sometimes you don't need a specialist, you need an engineer who can own a product end-to-end. I do that too, with the two specialties above as the safety net.

  • React / React Native / Node / TypeScript / Laravel
  • Direct point of contact for the CTO, no agency middleman
  • Design through deploy, including infra (Caddy, k3s, AWS, serverless)

How an engagement usually looks

Working with me

Let's talk

Twenty minutes, no obligation. If we're a fit, I'll send a written proposal within 48h. If we're not, I'll point you to someone who is.