Diego Castillo

Software Architect & Senior .NET Engineer

React • Azure • Agentic AI

Contract engineering for product teams in the US and Latin America · Remote from Lima

I take legacy .NET systems that have stopped being safe to change and turn them into services a team can deploy on a Tuesday afternoon — now with AI agents doing the mechanical work.

Fixed-scope packages from $3,000, or a 30-minute call first. No deck.

See what I can do for youRequest resume
01Case studies

Selected work

Three engagements, each with what changed and what it cost. The rest of the record is in the timeline below.
All case studies, with the trade-offs each one made →
02Services

What I do

Mostly one thing, from several angles: taking .NET systems that have become expensive and risky to change, and making them safe to deploy again. I work in fixed-scope packages rather than open hours, so you know the price before we start.

Software Engineering

Backend and full-stack .NET work: services, APIs and the front ends that use them, built to be handed over rather than to be impressive.

  • .NET Core/Framework applications
  • Modern web applications (React/Next.js)
  • RESTful APIs and microservices
  • Database design and optimization
Get Quote

Technical Consulting

Expert guidance on technology decisions, architecture design, and best practices for scalable solutions.

  • Architecture review and recommendations
  • Technology stack consultation
  • Performance optimization strategies
  • Security assessment and improvements
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Cloud Solutions (AWS/Azure)

Design and architect robust cloud solutions, from migration planning to implementation and optimization.

  • Cloud architecture design
  • AWS/Azure solution implementation
  • Cost optimization strategies
  • Security and compliance setup
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CI/CD & Infrastructure

Complete DevOps setup from scratch, implementing robust CI/CD pipelines and infrastructure automation.

  • CI/CD pipeline setup (GitHub Actions, Azure DevOps)
  • Infrastructure as Code (Terraform, Bicep)
  • Containerization (Docker, Kubernetes)
  • Monitoring and logging setup
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Legacy Migration

Modernize legacy systems by migrating to latest tech stacks with minimal downtime and maximum efficiency.

  • Legacy system assessment
  • Migration strategy and planning
  • Gradual migration implementation
  • Team training and documentation
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Agent-Assisted Delivery

The engineering above, run with coding agents doing the mechanical passes — under the same review and security gates as anything hand-written. Not a separate product: it is how the work gets done, and why a migration takes weeks instead of quarters.

  • Custom skills that encode your codebase's conventions, so a pattern is written once instead of re-explained every sprint
  • Purpose-built agents for migration passes, test scaffolding, review and documentation
  • Loop engineering: what context an agent gets, where it must stop and ask, and which checkpoints stay human
  • Every agent-written line through the same review, CI and security gates as the rest
  • Your team left able to run the loop after I leave — the skills and agents are yours
  • An honest account of where I do not let an agent operate
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Mentoring & Teaching

Share knowledge through mentoring sessions, code reviews, and technical training for individuals and teams.

  • One-on-one mentoring sessions
  • Team technical training
  • Code review and best practices
  • Career guidance and skill development
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Scope and prices

Four packages with fixed scope, a stated duration and a price you can approve without a negotiation. If none of them fits, tell me what you have and I will say whether I am the right person for it.

See the packagesDescribe your system
03Timeline

Professional Experience

  1. Senior Software Engineer | Independent Contractor · Self-employed

    Jun 2025 - Present
    • Contract engineering for product teams in the US and Latin America, working directly and through technology partners
    • Event-driven microservices on Azure: Service Bus topics and queues, Functions for background work, App Services, Application Insights, Key Vault and Azure DevOps pipelines
    • Distributed transactions with the SAGA pattern — compensating actions, idempotent handlers, retry with backoff, dead-lettering and the outbox pattern, so a failed step never leaves the system half-committed
    • Claude and Claude Code as the primary development environment: custom skills encoding each project's conventions, and purpose-built agents for migration passes, test scaffolding, review and documentation
    • Custom MCP (Model Context Protocol) servers connecting Claude to a team's own systems — Confluence for documentation and Sourcebot for code search across repositories — so an agent retrieves and cites internal knowledge instead of guessing at it
    • Direct integration with the Anthropic API inside application code, rather than only at the editor
    • Loop engineering over prompt engineering — what decides whether AI output survives review is the context an agent gets, where it stops, and which checkpoints stay human
    C#.NET 8ReactTypeScriptAzure Service BusClaude CodeMCPAnthropic APICI/CDTerraform

  2. Senior Software Development Engineer · MOGEL

    Jan 2025 - Jun 2025
    • Architected and deployed microservices on AWS (SQS, Lambda, API Gateway, Fargate, ECS), moving inline request work onto queues so a slow consumer degraded into a backlog rather than an outage
    • Led full migration of TrakUrScan from Classic ASP/.NET 4.6.1 to .NET Core microservices with AWS Cognito authentication and a React + Material UI front-end
    • Adopted Claude Code during its 2025 research preview on a production .NET codebase — agent-assisted migration passes and test scaffolding, held to the same review and security gates as hand-written code
    • Championed Clean Architecture and MediatR pipeline patterns; introduced React best practices and TDD workflows, mentoring 5+ junior engineers
    • Streamlined DevSecOps pipelines with AWS CodeCommit, CodeBuild, CodeArtifact and CodePipeline plus Terraform and Liquibase, with infrastructure and schema changes versioned alongside the application rather than applied by hand
    C#.NET CoreReactAWSMaterial UIMediatRTerraformClaude Code

  3. Technical Leader · Saffire, LLC

    Jan 2023 - Dec 2024
    • Spearheaded migration of a hybrid WebForms / ASP.NET MVC 4.6.2 + AngularJS system to .NET Core 8 and React, domain by domain, with both stacks serving production traffic throughout and shared authentication across them
    • Architected an end-to-end CI/CD pipeline with Azure DevOps, Bitbucket, SonarCloud and Zoom Marketplace webhooks, so every release became a versioned artifact traceable to its commit
    • Introduced AI-assisted development to the team in late 2024 with GitHub Copilot and ChatGPT for review support, test generation and migration boilerplate during the .NET 8 and React rebuild
    • Streamlined Tier 2 production support with proactive monitoring and a standardized triage workflow, giving an incident one route through the team instead of five
    • Optimized database performance with clustered and non-clustered indexing against the query plans that high-volume endpoints actually produced
    ASP.NET Core.NET 8ReactAzure DevOpsSonarCloudMediatRGitHub Copilot

  4. Senior Lead Software Engineer · Imagine Learning

    Apr 2022 - Jan 2023
    • Supported Imagine Edgenuity, adding third-party vendor integrations and the Box API from scratch
    • Defined base cloud architectures for new implementations and reviewed pull requests across the team
    • Mentored developers on SOLID and clean coding practices, and translated client requirements into technical stories
    C#.NET CoreAWS S3Box APIASP.NET MVC

  5. Senior Software Engineer · intive

    Apr 2022 - Jun 2022
    • Migrated legacy Sony MC cloud storage projects from .NET 4.5 to .NET 6 and a microservices stack
    • Built distributed packages for the company's application core, reducing duplication across teams
    • Fixed localization defects across Angular.js, Vue.js and MVC pages, and resolved critical production bugs
    C#.NET 6MicroservicesASP.NET MVCAngular.jsVue.js

  6. Senior Software Engineer · SparkDigital

    Oct 2021 - Apr 2022
    • Defined the back-end architecture for New Classrooms using AWS SQS, Lambda, S3, API Gateway and CloudWatch
    • Automated a deployment that had been performed by hand, so releasing stopped depending on who was available to do it
    • Acted as back-end team lead, coordinating between the company and external contractors
    • Applied Clean Architecture on .NET Core 3.1, decoupling the application into small testable components
    C#.NET Core 3.1AWS LambdaAWS SQSAWS S3API Gateway

  7. Senior Software Engineer · Tekton Labs

    Nov 2019 - Oct 2021
    • Defined the front-end architecture for American Colors with React, Next.js, TailwindCSS and Redux Toolkit, and led the front-end team
    • Built the Staywell healthcare SPA and integrated authentication for SPA, MVC and API clients with IdentityServer4 in Docker
    • Implemented ElasticSearch with Hangfire on a Clean Architecture backend using AWS API Gateway, Lambda, SQS, SNS and Terraform
    • Applied TDD with xUnit, Moq and Shouldly plus SQLite integration tests to reduce technical debt
    ReactNext.jsTailwindCSSDockerIdentityServer4.NET Core 5AWS

Earlier roles at Belatrix Software, Gestión y Sistemas and TCI are on LinkedIn.

04Background

About Me

Diego Castillo - Software Architect and Senior .NET Engineer

AI-Native Software Architect | .NET Core + React + Azure

Software architect and senior .NET engineer with 14+ years of production experience across .NET Core, C#, React and Azure. I specialize in legacy modernization: taking systems that have become expensive and risky to change, and turning them into microservices a team can deploy with confidence.

I work with AI agents every day, and not as a faster way to type. I build custom skills and agents for the repetitive parts of a codebase's life — migrations, review passes, test scaffolding — so a pattern gets encoded once instead of re-explained every sprint. I also build the connections that let an agent reach a team's own knowledge: custom MCP servers over Confluence and over code search, so answers come from your documentation and your repositories with a citation, rather than from the model's guess. The engineering that matters is in the loop design: what context an agent gets, where it must stop and ask, and how its output gets verified before anyone trusts it.

Clean Architecture, TDD and DDD underpin the work. The discipline is the same in both halves of what I do: move fast on the mechanical parts, stay slow and deliberate about the parts that carry risk.

14+
Years Experience

What I actually do

Legacy modernization
C#/.NET CoreEntity FrameworkSQL ServerReact + TypeScriptTeam Leadership
Distributed architecture
System ArchitectureMicroservicesREST APIsAzureAzure DevOpsInfrastructure as Code
Engineering with agents
Agent workflowsMCP serversLoop designAI-assisted review

Certifications

  • Microsoft Certified Solutions Associate (MCSA): Web ApplicationsRetired credential — Microsoft ended the MCSA track
  • Microsoft Certified Solutions Expert (MCSE): Cloud Platform & InfrastructureRetired credential — Microsoft ended the MCSE track
  • AWS Certified Developer – AssociateExpired 2023
05Contact

Let's Work Together

I'm always interested in new opportunities and exciting projects. Whether you have a question or just want to say hi, I'll do my best to get back to you!

Get in Touch

I'm currently available for new opportunities and freelance projects. If you're looking for a dedicated .NET developer with Azure expertise, let's discuss how we can work together to bring your ideas to life.

Location

Lima, Peru • Available for remote work

What I can help you with:

  • .NET application development
  • Azure cloud architecture and migration
  • Modern web application development
  • Technical consulting and code reviews

Send me a message