jguillaumesio

About: jguillaumesio

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

About me

I'm jguillaumesio, a full-stack contractor working 100% remote (UTC+1). I help SaaS teams ship the parts of their product they can't afford to get wrong: payments and billing and AI agents in production.

What I do

Two specialty lanes under a full-stack umbrella. Both are hard to staff internally and expensive to get wrong.

  • Payments / billing integration, Stripe migrations with zero downtime, multi-provider checkout (Stripe + GoCardless + PayPal), webhooks, refunds, reconciliation, VAT/MOSS, loyalty and third-party integrations (cancellation insurance, etc.). I've shipped this kind of work for a multi-site booking platform under live traffic.
  • AI agents in production, agentic coding pipelines, RAG over codebases (CocoIndex + pgvector + Tree-sitter for incremental semantic indexing), Mastra-based agent orchestration, and the boring but critical production concerns (cost, latency, evaluation, human-in-the-loop).
  • Full-stack SaaS (the umbrella), React / React Native / Node / TypeScript / Laravel. I'm the direct point of contact for the CTO. I can own a product end-to-end, design through deploy, and ship the whole thing.

How an engagement usually looks

  • Fixed-scope payments audit / integration: 1 to 4 weeks, clear deliverable
  • Embedded full-stack or AI work: 1 to 3 months, inside your team
  • Architecture + hands-on implementation: for a migration, a rebuild, or a greenfield that needs strong judgment from day one

The philosophy

Every article on this blog comes from a real problem in production. No "Hello World" tutorials, no "Top 10 Tips" listicles. I write because I shipped it. I take on contracts because I've already seen the failure modes.

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