Xavi Julià
Xavi JuliàAI Engineer · TypeScript · Penedès–Barcelona

Available · freelance or full-time · Barcelona hybrid or remote

I build AI agents that answer real phone calls, and the systems that keep them running.

Founder of RespondeYA and creator of LIA, a voice and WhatsApp receptionist in production. I put tested rules in code around the model, so the agent never invents prices or makes decisions it cannot undo.

Call LIA: +34 930 348 999 Email me LinkedIn

An AI answers, and says so in the greeting.

15prompt-injection attacks LIA must pass on every deploy
23attack tests on every push in the open Guardrailed Agent
4languages in LIA: Spanish, English, French and German
2005writing software since then; applied AI in production since 2025

Projects

What I have built

Four projects, each with its real status: in production, delivered, open demo or internal use.

In productionTypeScript

LIA

Voice, WhatsApp and web chat receptionist for clinics, hotels, beauty centres, consultancies, workshops and online shops. It collects appointment requests, notifies the team with the reason for the call and summarises every conversation.

  • The model interprets; tested code applies the rules.
  • If an AI provider fails, the call keeps going.
  • It says it is an AI in the greeting (AI Act, art. 50).
  • Lesson: in voice, first-audio latency matters more than model reasoning.

Call LIArespondeya.es ↗

TypeScriptNode.jsTwilioClaude APISupabaseStripeVercel
Open demoPython

Guardrailed Agent

Open-source booking agent where the LLM only extracts intent and the code decides. Written in Python to show the same discipline outside my usual TypeScript stack.

  • 23 attack tests on every push.
  • A trace for every turn, to explain each decision afterwards.

Try the demo (first load can take about 30 s) ↗Code on GitHub ↗

PythonFastAPIPydanticpytestGitHub ActionsRender
Delivered · paused by the clientReact

Advertising cockpit for a real estate agency

Real client project. One dashboard that crosses ad performance on Idealista, Fotocasa, Habitaclia and the agency website, with role-based access for each person on the team.

ReactSupabaseVercel
Internal useNode.js

SuperXavi

My own multi-agent system for development, support and marketing, coordinated by an orchestrator and always under human supervision.

  • Mobile approval panel for anything irreversible.
  • Global emergency stop and full audit trail.
Node.jsSupabaseGitHub ActionsClaude CodeClaude API

RespondeYA VeriFactu

Tailored adaptation of billing software to the VeriFactu regulation (RD 1007/2023) and connection to the AEAT.

Enllaç Digital

My Catalan-language brand for local businesses in Catalonia, with a multilingual voice receptionist and a web assistant.

Customer panel

Self-serve sign-up, agent setup and billing, so a customer can start without waiting for me.

Sprint 72h

A first working version in 72 hours, at a fixed price

You tell me the process that eats your time. I deliver something that runs, not a slide deck.

Automation or integration

from390 €

I connect your tools and remove manual work: webhooks, APIs, n8n or custom code.

Internal dashboard

from690 €

A dashboard with role-based access over your real data, so each person sees what they need.

AI agent on WhatsApp or web

from990 €

An agent with the rules in code: it does not invent prices or take irreversible decisions, and it says it is an AI.

Fixed price, agreed before starting Code and licence become yours on final payment

Automation lab

Automation lab

Six reference workflows I designed to show how I would approach each case. Click a step to see what it does and who decides.

These are reference designs, not client projects. I use them to show how I would split the work between the model, the code and a person.

Code

Five languages, one discipline

Reference snippets for patterns I use. They are not any client's code.

Method

How I work

Determinism around the LLM

The model interprets and drafts. Rules, prices and permissions live in tested code.

Measure before switching

I do not switch provider or model without measuring first.

Small, reversible changes

Anything irreversible waits for a human approval.

Compliance by design

GDPR, EU data hosting, AI disclosure and prompt-injection tests are part of the build, not a final patch.

I explain what I build

I turn architecture into decisions a business owner can follow.

Background

Twenty years around software

  1. 2026 to today

    Freelance AI Engineer

    Custom AI agents and automations, fixed price.

  2. 2025 to today

    Founder, RespondeYA

    Product, architecture, development, sales and support.

  3. 2023 – 2025

    Team leader, Pasqual Maragall Foundation

    Team management, training and face-to-face fundraising.

  4. 2021 – 2023

    Rosa Sensat teachers' association

    Training, technical project management and IT teaching.

  5. 2005 – 2020

    Development and systems

    C# .NET developer at Deco, VB.NET at CIALIT, systems technician at Ofitècnica del Penedès and Informàtica i+.

Stack

Tools I use

Languages and backend

TypeScriptJavaScriptNode.jsExpressPython · FastAPISQLC# / .NET (base)Bash

AI and voice

Claude APIClaude CodeOpenAITwilioSTT / TTStool callingguardrails y evalsRAG

Data

PostgreSQLSupabaseSQL ServerEdge Functions

Product and infrastructure

VercelRenderGitHub ActionsStripeResendn8nReactNext.js

Education and languages

CFGS Application Development (DAI) and CFGS Systems Administration (ASI), CET Penedès.

Degree in Pedagogy, UNED (in progress).

Catalan and Spanish native. English B2.

Shall we talk?

Tell me which process eats your time. I will reply with a plan and a fixed price.

Email me LinkedIn Call LIA
xavi@respondeya.eslinkedin.com/in/xavijulia+34 930 348 999