Fernando Castellanos

Software engineer

Hi, I'm Fernando.

I build web and mobile products with Ruby on Rails and React Native, and I write about testing, design patterns, and how AI is changing the way I ship software.

ai 5best practices 7claude 3claude-code 2design patterns 7php 2python 3rails 3ruby 2software development 2software patterns 7testing 2workflow 2

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A one-line feature question, the schema, routes, controllers, jobs, Gemfile and 91 issues Claude read first, and its top three ideas: roll-over filed, CFDI still an idea, audit trail shipped in 9 days

Asking an AI What to Build Next

Ask an AI “what features should I add?” and you usually get a list that could fit any app: dark mode, a mobile app, AI-powered insights. I asked...

· 7 min read
aiclaudeproduct
Two VS Code windows opened with code-personal and code-work, each with its own CLAUDE_CONFIG_DIR and Claude account, plus a LaunchAgent default for the Dock

Work and Personal Claude Accounts in VS Code

I use Claude for two kinds of coding: my day job, on my employer’s team plan, and a school administration platform I build as a side project, on...

· 7 min read
aiclaudemacos
A PR summary with a "Notably" note and test counts, four review questions, then "PR looks good, merge it" and a merge pinned to the reviewed commit

"PR Looks Good, Merge It": Reviewing Code I Didn't Type

The message I send most often to Claude Code is five words long: “PR looks good, merge it.” That sounds like rubber-stamping, but it’s actually the end of...

· 8 min read
aiclaude-codecode-review
A long prompt retyped every session becomes "work on 131", backed by create-ticket, check-ticket and work-ticket skills that hand off from one sentence to a PR

From Prompts to Skills: How I Taught Claude Code My Workflow

For months I used Claude Code the way most people do: open a session, explain what I wanted, watch it work, correct it, and explain the same things...

· 8 min read
aiclaude-codeworkflow

Configure Sidekiq With Rails

According to their website Sidekiq is an efficient background processing for Ruby. It says Ruby but you can also use it in Rails and it even has a...

· 6 min read
rubybackground-jobsredis

Minitest Custom Assertions

Since I discovered the world of testing and TDD I’ve loved the philosophy behind it and the safety net that it brings. My first interaction with testing was...

· 5 min read
rubyrailstesting

Software Design Patterns (Builder)

The seventh and last blog post in the series of Software Design Patterns it’s an interesting one, it’s the Builder Pattern. The builder patter is a unique pattern...

· 5 min read
design patternssoftware patternsbest practices

Software Design Patterns (Strategy)

In the sixth installment of the Software Design Patterns blog series I’ll talk about the Strategy Pattern. This pattern is interesting because it allows the consumer of this...

· 3 min read
design patternssoftware patternsbest practices