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 20-minute call first. No deck.
Selected work
Three engagements, each with what changed and what it cost. The rest of the record is in the timeline above.
.NET 4.6.2 → .NET 8 + React
~35% less CPU, 40% faster page loadsA hybrid WebForms/MVC system with rising CPU and long page loads, migrated domain by domain behind a strangler facade with shared auth, so it stayed shippable throughout.
.NET 8ReactSQL ServerAzureRead the case study →AWS microservices migration
~25% lower infrastructure costA Classic ASP / .NET 4.6.1 application decomposed into services: serverless where the workload fit, containers on Fargate where it did not, with Terraform underneath.
.NET CoreAWS LambdaSQSFargateTerraformRead the case study →CI/CD and DevSecOps
30% fewer failed releases, MTTR halvedReleases that went out by hand and rolled back often, replaced by pipelines as code with quality and security gates between stages, and a triage path that gives an incident one route instead of five.
Azure DevOpsSonarCloudIaCQuality gatesRead the case study →
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
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
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
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
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
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
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
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.
Professional Experience
Senior Software Engineer | Independent Contractor
Self-employed
- 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
- 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
Senior Software Development Engineer
MOGEL
- Architected and deployed cost-optimized microservices on AWS (SQS, Lambda, API Gateway, Fargate, ECS), improving fault-tolerance and reducing infrastructure costs by ~25%
- 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, cutting deployment time from hours to under 30 minutes
Technical Leader
Saffire, LLC
- Spearheaded migration of a hybrid WebForms / ASP.NET MVC 4.6.2 + AngularJS system to .NET Core 8 and React, cutting CPU usage by ~35% and page-load times by 40%
- Architected an end-to-end CI/CD pipeline with Azure DevOps, Bitbucket, SonarCloud and Zoom Marketplace webhooks, reducing release failures by 30%
- 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, cutting MTTR by 50%
- Optimized database performance with clustered and non-clustered indexing, reducing high-volume query execution times by ~40%
Senior Lead Software Engineer
Imagine Learning
- 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
Senior Software Engineer
intive
- 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
Senior Software Engineer
SparkDigital
- Defined the back-end architecture for New Classrooms using AWS SQS, Lambda, S3, API Gateway and CloudWatch
- Automated the deployment process, taking it from 3 hours to 5 minutes
- 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
Senior Software Engineer
Tekton Labs
- 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
Earlier roles at Belatrix Software, Gestión y Sistemas and TCI are on LinkedIn.
About Me
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. 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.
Technical Skills
Professional Certifications
- Microsoft Certified Solution Associate (MCSA): Web Applications
- Microsoft Certified Solution Expert (MCSE): Cloud Platform & Infrastructure
- AWS Certified Developer – Associate (expired 2023, recertifying)
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.
contact@diegocastillo.dev
GitHub
github.com/diegodev91Location
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