Freelance Software Engineer · AI Product Engineer

Juan Pablo Castro

I turn ideas into production-ready digital products.

I combine product thinking, full-stack engineering and cloud architecture, applying AI where it creates real value.

8 years across software engineering, architecture and technical leadership. TypeScript, AWS and applied AI.

Juan Pablo Castro
  • TypeScript
  • AWS
  • React · Next.js
  • Node.js · NestJS
  • PostgreSQL
  • LLMs · Agents
  • Terraform
in software engineering
8 years
in architecture & technical leadership
4 years
from prototype to scalable product
PostNL Digital Lab
live on macOS and iOS
2 own products

From transactional TypeScript and AWS platforms to AI products for web, macOS and iOS.

How I can help

Three ways of working together. All three end the same way: something real running in production.

I work with founders, product teams and engineering leaders who need senior ownership from an unclear problem to a reliable production system.

AI Product Development

I design and build products that incorporate LLMs, voice, agents, RAG or intelligent automation, from prototype to a reliable system in production.

You get an AI product that works for real users: with guardrails, measured cost and latency, and deterministic fallbacks when AI is not the answer.

  • Use-case design
  • Model integration
  • Product architecture
  • Evals & guardrails
  • Privacy & security
  • Cost & latency control
  • Observability
  • User experience
  • Deterministic fallbacks

Product Engineering

I develop products and features end to end, from problem definition through delivery, measurement and iteration.

A senior engineer who owns the outcome, not the ticket: discovery, implementation, deployment and iteration in a single profile.

  • Technical product discovery
  • Full-stack TypeScript
  • React & Next.js
  • Node.js & NestJS
  • APIs & integrations
  • Domain design
  • Databases
  • Deployment & operations

Architecture & Technical Leadership

I design and evolve complex systems, and support teams hands-on when they need technical direction without separating architecture from implementation.

Decisions that hold up in production: clear boundaries, safer integrations and a team that ships with more confidence.

  • AWS
  • Distributed systems
  • Cloud-native architecture
  • Transactional platforms
  • Security
  • Multi-tenancy
  • Event-driven systems
  • Complex integrations
  • Architecture review
  • Team enablement

Selected work

Real products in production: my own AI-enabled apps, client work and an enterprise product case.

ChacharApp

AI product — concept, design & engineering · Open source (MIT)

ChacharApp

Conventional dictation fails on technical language, Spanish–English switching and privacy. ChacharApp is the product: local voice dictation for macOS — hold a key, speak, and the corrected text lands at your cursor, typically in about one second on Apple Silicon, with nothing leaving the Mac.

Swift SwiftUI WhisperKit (CoreML) Apple Neural Engine MLX +1
Read more
Salturno

Indie iOS product · App Store

Salturno

Shift-work calendar for iOS: photograph your rota or pick the PDF and get a colour-coded month in under a minute. AI only where it adds value — deterministic extraction when the PDF already has text, no accounts and local-first storage.

Swift SwiftUI SwiftData + CloudKit AWS serverless OpenAI API +1
Read more
Fast Checkout — product architecture and technical leadership at PostNL

Software Architect & Tech Lead · PostNL Digital Lab · 2022–2025

Fast Checkout — product architecture and technical leadership at PostNL

Three years inside PostNL's Digital Lab turning prototypes and ambiguous product requirements into a transactional checkout product connecting e-commerce platforms, shipping and payments — as architect, tech lead and hands-on engineer.

Next.js NestJS TypeScript PostgreSQL Prisma +2
Read more
The four cells of the harness A 2×2 matrix. Rows are Feedforward (before the step) and Feedback (after the step). Columns are Computational (the code decides) and Inferential (the model decides). Each cell carries a combined label in bold and a spoken motto in italics at the bottom: guides · deterministic "this is always done this way", guides · semantic "think about it this way", sensors · deterministic "this is broken — look here", sensors · semantic "this smells off — are you sure?". Each cell also lists concrete instances in monospace. The four cells of the harness Computational Inferential Feedforward Feedback guides · deterministic PreToolUse hooks allowed-tools per agent permission allowlists schema · scope guards "this is always done this way" guides · semantic CLAUDE.md · project rules ADRs · decision records skills · agent frontmatter specs (OpenSpec, etc.) "think about it this way" sensors · deterministic build & tests type-check · lint PostToolUse hooks spec validate "this is broken — look here" sensors · semantic adversarial review reviewer subagent structured diagnostics lesson capture "this smells off — are you sure?"

How I run agents today · hooks, specs, subagents

Harness Engineering — the code that surrounds the agent

An LLM is non-deterministic, so the critical rules have to live outside the agent, in code the agent cannot rewrite.

Claude Code Hooks (PreToolUse / PostToolUse) Subagents OpenSpec Skills +2
Read more

From an ambiguous problem to a product in production

The same loop on every project, with or without AI.

  1. Understand the problem

    Clarify users, business, constraints, risks and the success criteria before choosing any technology.

  2. Design the product and the system

    Define flows, boundaries, experience, architecture and the critical decisions.

  3. Build vertically

    Ship complete increments that cut through interface, backend, data and infrastructure.

  4. Validate

    Measure quality, behaviour, latency, cost, security and how users respond.

  5. Operate and iterate

    Watch production, correct assumptions and evolve the product.

When the product involves AI

  • Evaluations
  • Guardrails
  • Deterministic fallbacks
  • Human-in-the-loop where needed
  • Hallucination control
  • Prompt & model versioning
  • Outcome observability

I also build with code agents: executable specs and controlled environments (a harness) that increase speed without losing quality or judgement. Agents are a tool — the responsibility for architecture, code and production stays with me.

Engineering capabilities

Software delivery, AI product ownership and production architecture in one hands-on profile.

Software Engineering

I build complete, maintainable software across the frontend, backend, data and integration layers.

  • Full-stack product development
  • APIs and third-party integrations
  • Data modelling and transactional flows
  • Testing, maintainability and delivery

AI Product Engineering

I shape, build and iterate AI-enabled products around real user needs and measurable outcomes.

  • Product discovery, problem framing and user flows
  • End-to-end ownership from experience design to production
  • LLM, speech, vision, RAG and agent integration
  • Evaluations, feedback loops, guardrails and deterministic fallbacks
  • Control of quality, latency, cost and privacy

Architecture & Cloud

I design reliable systems that can evolve as the product, team and usage grow.

  • AWS and infrastructure as code
  • Transactional, distributed and event-driven systems
  • Security and multi-tenancy
  • Observability, operability and production reliability
Core stack

TypeScript · React · Next.js · Node.js · NestJS · Swift · SwiftUI · PostgreSQL · Prisma · AWS · Terraform

Relevant experience

The full detail lives in the resume — this is the shape of the journey.

  1. 2026 – now

    Freelance Software Engineer & AI Product Engineer · Castro Solutions

    I build digital products for clients and develop my own AI-enabled products, including ChacharApp (local dictation for macOS) and Salturno (shift calendar for iOS).

  2. 2022 – 2025

    Software Architect & Tech Lead · PostNL Digital Lab · via Capgemini

    Led the architecture and hands-on delivery of Fast Checkout: from ambiguous product requirements to a transactional e-commerce product on AWS, coordinating several teams and third parties.

  3. 2018 – 2022

    Full-stack engineer · Tecon · Cognizant · USAL

    End-to-end systems from the first line of code — including GICA, the wildfire response management system used daily by 2,200+ field workers.

Let's talk about your project

Tell me what you are building, what is blocking the team and what outcome you need.

Available for

  • Freelance product builds
  • AI products from prototype to production
  • Senior reinforcement for product teams
  • Hands-on architecture & technical leadership

Message sent. I usually reply within 24 hours. Something went wrong. Write me directly at juanpablo@castrosolutions.dev