Josué Fernández Díaz

Hello, I'm

Josué Fernández Díaz

Senior Software Engineer | Tech Lead
Cartago, Costa RicaAvailable now

Senior / Tech Lead roles, remote across LATAM or hybrid in Costa Rica

Senior full-stack engineer with 7+ years building SaaS products on Ruby on Rails, React and AWS. Most recently Tech Lead for Postmark at ActiveCampaign, where I owned billing end to end. I take the cross-team problem nobody owns yet, ship it, and leave it documented so the team can run it without me. New stack? I have switched several times, and delivered early on each one.

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EmailLinkedInGitHub
Building SaaS productsRuby on Rails, React and AWS at the core
7+ yrs
Support reply timeAverage first reply at ActiveCampaign, helped by internal tools I built
6h → 23min
Data table load timeAfter rebuilding SmarterSelect's tables and queries
20s → <8s
Unit test coverageAcross three React modules, with Jest and React Testing Library
95–97%

/ Recent Roles

My last two roles at ActiveCampaign, broken down by what I owned.

Apr 2024 - Sep 2026Most recent

Senior Software Engineer - Tech Lead

ActiveCampaign (Postmark)

Tech Lead and primary owner for Postmark and DMARC Digests through major team transitions and restructuring, keeping the product stable and delivery moving across the web application, alerting, the billing system and the end-to-end QA suite.

senior contractors led
6
template editor initiative
10 months
documentation pages
30+
personal throughput with AI agents
~10x

Product ownership

Primary owner for two products while the team around them changed, safeguarding stability and delivery continuity.

Technical leadership

Led 6 senior contractors: system design, ticket breakdown, cross-team alignment and onboarding.

Incident response

Primary incident responder. Advised management on rapid mitigation versus root-cause remediation, restoring service fast without accruing technical debt.

Hiring, onboarding and docs

Ran end-to-end hiring and onboarding, with documentation that made new engineers productive within their first week. Turned undocumented release, on-call and domain knowledge into shared practice.

Billing and payments

Owned billing infrastructure end to end, from the platform migration to incidents and live production changes.

See the billing work

Template editor and AI agents

Spearheaded the next-generation template editor and built a multi-agent framework for AI coding agents.

See the case studies
Ruby on RailsReactStripeRSpecAWSBugsnagNew RelicFigmaGitHub
Aug 2023 - Apr 2024

Software Engineer

ActiveCampaign

Built the tools and the testing foundation behind ActiveCampaign's L2 Customer Support console, so support could resolve issues without waiting on engineering.

average first reply time
6h → 23min
escalated-ticket resolution
17d → <7d
faster load and in-app actions
2x
unit test coverage
95–97%

Support tooling

Internal tooling that let the L2 Customer Support team resolve issues without engineering escalation.

Console rebuild

Rebuilt a legacy PHP support console as a React and TypeScript micro frontend with full feature parity, halving initial load time.

Unit testing baseline

Raised coverage across the new frontend's three modules with Jest and React Testing Library, for confident, regression-free releases.

End-to-end strategy

Architected the Playwright strategy: modular environment configuration, reusable fixtures and page objects, parallel execution. Drove adoption with docs and a knowledge-share session.

PHPReactTypeScriptPythonBashPlaywrightGitLab

/ Shoutouts

What the people I worked with say, in their own words.

  • Josue is one of the best engineers I've managed in my career.
    Dan DiGangiSenior Engineering Manager
  • He owned our billing and subscription logic end to end, including the trickiest customer tickets other engineers shied away from, and he drove our new email editor from early research through delivery, not just writing code, but helping shape the direction of the product itself.
    Josiah MangiameliProduct Manager
  • Josue is the Ruby developer you want owning a hard feature.
    Bryan GelfiusSenior Software Engineer
  • When there were technologies that he was unfamiliar with, he was proactive in learning them and growing his technical expertise.
    Chris OaldonSenior Software Engineer
  • Beyond the terrific code, Josue has a rare gift for turning complex setup and on-call processes into clean, easy-to-follow documentation.
    Eli WoodSenior Software Engineer
  • Josué is a great engineer, and I highly recommend picking him up before someone else does!
    Chris FlackSenior Software Engineer

Excerpts from recommendations on LinkedIn.

/ Payments & Billing

The domain I owned deepest at Postmark. Billing work touches real money and production data, so it is where I am most careful.

Billing ownershipStatement

Postmark (ActiveCampaign) · Apr 2024 - Sep 2026

Platform migration
Tech lead
Pricing-tier integration
Built + maintained
Incidents and defects
Sole owner
Live production data changes
Multiple
Senior contractors led6

Billing platform migration

Technical lead for 6 senior contractors and subject-matter expert on the migration: system design, ticket breakdown, cross-team alignment and contractor onboarding.

Pricing-tier integration

Built and maintained the integration behind Postmark's new pricing tiers.

Incidents and defects

Sole owner and resolver of billing incidents and defects, from first report to fix.

Live production changes

Executed multiple live changes to production billing data with zero incidents or customer-facing billing errors.

StripeRuby on RailsRSpecPaymentsBillingSaaS

/ Selected Work

Six pieces of work I owned, from the context I walked into to the result I left behind.

10 months

concept to delivery

Tech lead

Next-generation template editor

Postmark (ActiveCampaign)

Context
Postmark's next-generation template editor, a 10-month initiative that needed cross-functional alignment from the start.
My role
Spearheaded the initiative: technical discovery, cross-functional alignment and executive communication.
Result
Carried the initiative through its full lifecycle, from concept to delivery.
Ruby on RailsReactRSpecAWSFigma

~10x

personal delivery throughput

AI

Multi-agent framework for AI coding agents

Postmark (ActiveCampaign)

Context
AI coding agents needed guardrails, cost control and grounding in current vendor documentation to be useful on production work.
My role
Designed and built a multi-agent framework with human-in-the-loop write gates and risk-based model routing, plus an agentic RAG documentation-search MCP server.
Result
Agents stayed grounded in current vendor docs, and personal delivery throughput increased roughly 10x.
Claude CodeMCPAgentic AIRAG

6h → 23min

average first reply time

Internal tools

Customer Support tooling

ActiveCampaign

Context
The L2 Customer Support team depended on engineering escalations and a legacy PHP support console.
My role
Built internal tooling so L2 could resolve issues without escalating, and rebuilt the console as a React and TypeScript micro frontend with full feature parity.
Result
Contributed to cutting average first reply time from 6 hours to 23 minutes and escalated-ticket resolution from over 17 days to under 7. The new console halved initial load time and made in-app actions 2 times faster.
PHPReactTypeScriptPython

95–97%

unit test coverage

Quality

Support console testing strategy

ActiveCampaign

Context
A new React frontend for the support console with no testing baseline.
My role
Established the unit testing baseline with Jest and React Testing Library, and architected the Playwright end-to-end strategy: modular environment configuration, reusable fixtures and page objects, and parallel execution.
Result
Coverage of 95–97% across the three modules, and team adoption driven through documentation and a knowledge-share session.
JestReact Testing LibraryPlaywrightTypeScriptGitLab

20s → <8s

data table load time

Modernization

Legacy platform modernization

SmarterSelect

Context
A legacy Rails 4 and jQuery monolith whose data tables took over 20 seconds to load.
My role
Spearheaded the move to a Rails 6 API with a React frontend and rebuilt the data table architecture: no more N+1 queries, refined indexing and caching.
Result
Load times dropped from over 20 seconds to under 8.
Ruby on Rails 6ReactPostgreSQLElasticSearchSidekiq

GDPR

consent enforced across the stack

Compliance

GDPR consent enforcement

Blue Apron (via Gorilla Logic)

Context
Compliance and data privacy requirements on Blue Apron's e-commerce web platform, alongside its migration from Braze to Segment for customer data collection.
My role
Designed the consent preferences in OneTrust and Google Tag Manager, and worked on the Segment migration within the Rails backend and React frontend.
Result
User consent preferences are enforced through the stack, ensuring GDPR compliance.
Ruby on RailsReactOneTrustSegmentGoogle Tag Manager

/ New Stack, Same Result

Six teams, and a different stack at almost every one.

I have changed languages and frameworks several times in my career. Each time I ramped up fast and was delivering value before the expected onboarding window, on every project. I'm open to learning new languages, and I expect to excel at them.

languages in production
5
teams and codebases
6
delivering ahead of the ramp-up window
Every time
  1. 2019

    4Geeks (Daddyo)

    PythonDjango RESTOpenCVAWS Lambda

    Computer-vision backend, led from planning to client delivery.

  2. 2019 - 2021

    SmarterSelect

    Ruby on Rails 6ReactElasticSearch

    Monolith modernization and data tables from 20s to under 8s.

  3. 2022

    Pixel506 (CreativeDrive)

    Ruby on Rails 4EmberJS v1

    Reengineered a drag-and-drop onboarding component within legacy constraints.

  4. 2022 - 2023

    Gorilla Logic (Blue Apron)

    Ruby on RailsReactSegmentOneTrust

    GDPR consent enforcement and the Braze to Segment migration.

  5. 2023 - 2024

    ActiveCampaign

    PHPReactTypeScriptPythonPlaywright

    Support console rebuild and tooling: first reply time from 6h to 23min.

  6. 2024 - 2026

    ActiveCampaign (Postmark)

    Ruby on RailsReactStripeAWS

    Tech Lead and billing owner for Postmark and DMARC Digests.

/ Career

Roles and education, most recent first: transactional email, e-commerce and health tech.

Apr 2024 - Sep 2026Full time

Senior Software Engineer - Tech Lead

ActiveCampaign (Postmark)

Tech Lead and primary owner for Postmark and DMARC Digests through major team transitions. Led the 10-month template editor initiative, was technical lead for 6 senior contractors on the billing platform migration, owned billing infrastructure end to end, served as primary incident responder, and ran hiring and onboarding.

Ruby on RailsReactStripeRSpecAWSBugsnagNew Relic
2026Education

Aspiring Leaders Program

ActiveCampaign

Selected for a 16-week leadership development program designed by ActiveCampaign to transition senior individual contributors into people leaders, building practical skills in ownership, team leadership and managing small to mid-size teams.

LeadershipOwnershipTeam management
Aug 2023 - Apr 2024Full time

Software Engineer

ActiveCampaign

Built internal tooling that let the L2 Customer Support team resolve issues without engineering escalation, contributing to a drop in average first reply time from 6 hours to 23 minutes. Rebuilt a legacy PHP support console as a React and TypeScript micro frontend, and set up its unit and Playwright end-to-end testing.

PHPReactTypeScriptPythonBashPlaywrightGitLab
Oct 2022 - Mar 2023Full time

Fullstack Software Developer

Gorilla Logic (Blue Apron)

Worked on Blue Apron's migration from Braze to Segment for customer data collection, and designed its consent preferences in OneTrust and Google Tag Manager to enforce user consent and ensure GDPR compliance. Collaborated with cross-functional teams to improve user data flow and analytics accuracy.

Ruby on RailsReactOneTrustSegmentGoogle Tag ManagerPostgreSQL
Jan 2022 - Sep 2022Full time

Full Stack Software Engineer

Pixel506 (CreativeDrive)

Feature development and maintenance on a large-scale web platform built with Ruby on Rails and EmberJS. Reengineered a drag-and-drop onboarding component to improve usability within legacy EmberJS constraints, and supported performance optimization and bug resolution.

Ruby on Rails 4EmberJSJavaScriptPostgreSQL
Oct 2019 - Dec 2021Full time

Full Stack Engineer

SmarterSelect

Spearheaded the modernization from a Rails 4 and jQuery monolith to a Rails 6 API with a React frontend. Cut data table load times from over 20 seconds to under 8, and overhauled the translation system so new global customers could be onboarded within a week.

Ruby on Rails 6ReactPostgreSQLElasticSearchSidekiq
Jan 2019 - Sep 2019Full time

Backend Engineer

4Geeks (Daddyo)

Designed the backend for a health tech video analysis platform: a Python and OpenCV solution for detection, counting and motility tracking, a Django REST API, and AWS Lambda pipelines for automated analysis, with CI/CD on AWS CodePipeline. Led the backend project lifecycle, from planning and QA to client communication.

PythonDjango REST FrameworkOpenCVAWS LambdaPostgreSQL
2015 - 2019Education

Bachelor's in Computing Engineering

Instituto Tecnológico de Costa Rica

Mentored a group of 5 first-year students from remote regions through IntegraTEC, and presented projects at the TEC Vocational Fair and the TEC Emprende Lab.

Computing EngineeringMentoringJava

/ Skills & Tools

What I build with, grouped by where it fits in the work.

Core stack

Ruby on Rails
React
JavaScript
TypeScript

Also in production

Python
Django
PHP
EmberJS
TailwindCSS
Bootstrap

Payments & integrations

Stripe
Segment
OneTrust
Google Tag Manager
Stripo

AI-assisted development

Claude Code
Cursor
Devin
Agentic AI
MCP servers
RAG

Backend & data

REST APIs
PostgreSQL
MySQL
ElasticSearch
Sidekiq
RabbitMQ

Cloud, testing & delivery

AWS
GCP
Playwright
RSpec
GitLab CI
CircleCI
Git / GitHub
Bash
Bugsnag
New Relic

Ways of working

Technical leadership
Hiring & onboarding
Technical writing
Shape Up
Scrum / Kanban
Jira
Confluence
Notion
Basecamp
Trello
Figma

/ Contact Me

Available now for Senior and Tech Lead roles. Tell me about the team and the problem.

Email

josue.0795@gmail.com

Send Email

LinkedIn

linkedin.com/in/josuefdz

Connect

GitHub

github.com/J0SUEFDZ

View Profile

Email or LinkedIn is the fastest way to reach me.