Cesar is available for hire
Hire CesarCesar Jose Andres Leon Romero
Verified Expert in Engineering
Full-stack Developer
Buenos Aires, Argentina
Toptal member since July 14, 2022
Bio
Cesar is an analytical and result-driven developer with over five years in a wide range of specialized expertise as an individual contributor and team player. He's technically proficient, enjoys solving complex challenges or long-standing problems, and delivers work with proven successful outcomes. Cesar works well with multidisciplinary teams, business leaders, and clients by understanding, defining, and providing practical and creative solutions that meet functional and technical requirements.
Portfolio
EJV Auto Finanace
Next.js, NestJS, VPS, SOC 1, AI SDK, Conversational AI, Third-party Integration...
Research Improving People's Lives
React, TypeScript, JavaScript, Material UI, Python, OAuth 2, FastAPI...
FICO
Java, Spring Boot, JavaScript, Angular, Amazon Web Services (AWS), Terraform...
Experience
- JavaScript - 8 years
- Java - 8 years
- Node.js - 7 years
- Back-end Development - 6 years
- Front-end - 6 years
- Angular - 4 years
- Spring Boot - 4 years
- React - 4 years
Work Experience
Software Developer
2025 - 2026
EJV Auto Finanace
- Created a platform to perform AI-assisted underwriting, reducing the review time by 70%.
- Built an AI-driven web scraper to get vehicle information. I iterated it four times; the first iteration failed 5 out of 10 times, and the fourth one failed 1 out of 100 times.
- Created the code, data layer, and infrastructure with security in mind. Given that they had nothing to work with at the beginning, they now have a server architecture that can withstand even a breach.
Technologies: Next.js, NestJS, VPS, SOC 1, AI SDK, Conversational AI, Third-party Integration, Third-party APIs, OAuth 2, Multi-tenant Architecture, Role-based Access Control (RBAC), OAuth, Artificial Intelligence (AI), Workflow Automation, AI Automation, LLM Integration, AI Agents, AI Agent Orchestration, Retrieval-augmented Generation (RAG), Vector Databases, Large Language Models (LLMs), Solution Architecture, Integration, Agentic AI, Claude, Claude Code, Gemini, Expo
Front-end Developer
2025 - 2026
Research Improving People's Lives
- Maintained the front end and shaped it up until project delivery, introducing the new features that CPO requested.
- Unblocked the team when the CPO asked for a big new feature that was not explicitly agreed on, and the team was not sure of the scope and reach of it. I developed the POC on my own, showed it to the team, demoed it to the CPO, and the scope was approved.
- Introduced a whole new flow from POC up until delivery to allow reviewing and iterating payrolls before they reach regulators.
Technologies: React, TypeScript, JavaScript, Material UI, Python, OAuth 2, FastAPI, Multi-tenant Architecture, Role-based Access Control (RBAC), OAuth, Claude, Claude Code, Gemini
Full-stack Developer
2025 - 2025
FICO
- Developed an internal application and delivered the scope quickly on a tight schedule.
- Delivered a fully tested and documented application.
- Delivered a fully functional CI/CD process based on GitHub Actions and Terraform.
- Provided support to the client related to Terraform usage and how to maintain the software.
Technologies: Java, Spring Boot, JavaScript, Angular, Amazon Web Services (AWS), Terraform, DevOps, Cloud Architecture, HTTP, API Testing, Continuous Delivery (CD), Database Design, Minimum Viable Product (MVP), Software Architecture, Role-based Access Control (RBAC), Solution Architecture
Software Developer
2024 - 2025
Patinet Data Vault
- Developed the pipeline to extract medical facts from patients' medical documentation.
- Created the 1st iteration of the B2B2C system, involving two platforms: the patients' platform, where patients can look for medical trials, and the companies' platform, where companies can publish trials and look for matching patients.
- Helped shape the architecture and concrete processes to maintain patient information private and HIPAA compliant.
Technologies: Next.js, Amazon Web Services (AWS), Claude, OpenAI, Conversational AI, EPIC Electronic Health Records (EHR), Electronic Health Records (EHR), Fast Healthcare Interoperability Resources (FHIR), HIPAA, HL7, Healthcare, Third-party Integration, Third-party APIs, Multi-tenant Architecture, Role-based Access Control (RBAC), OAuth, Healthtech, Artificial Intelligence (AI), Workflow Automation, AI Automation, LLM Integration, AI Agents, AI Agent Orchestration, Retrieval-augmented Generation (RAG), Vector Databases, Large Language Models (LLMs), Integration, Agentic AI, Amazon Bedrock AgentCore
Software Developer
2024 - 2025
Fintivio
- Created an N8N-like platform for gathering signals related to loan applications and automatically acting on those signals.
- worked with the founders to resolve uncertainty about what the final product might look like, creating two POCs and 1 MVP.
- Delivered the product up and running, the full architecture, infra, credentials management, SSO, multi-tenant database solution, all for them to present to investors.
Technologies: Next.js, AI SDK, Amazon Web Services (AWS), NestJS, Terraform, Conversational AI, Third-party Integration, Third-party APIs, OAuth 2, Multi-tenant Architecture, Role-based Access Control (RBAC), OAuth, Workflow Automation, AI Automation, LLM Integration, AI Agents, AI Agent Orchestration, Large Language Models (LLMs), Solution Architecture, Integration, Agentic AI
Back-end Developer
2024 - 2024
The ObG Project
- Created the AWS infrastructure from scratch, including CI/CD pipelines and an easy way to spawn new environments, including the corresponding subdomains.
- Created and documented a working application for gathering EHRs from different providers, taking into account that their previous implementations weren’t documented and did not really work.
- Migrated some services from Java to Node.js, simplifying the stack and allowing them to look for profiles with Node.js knowledge only instead of Node.js + Java.
Technologies: Java, Back-end, HL7 FHIR Standard, Spring Boot, Amazon Web Services (AWS), Scrum, Agile, Python, C#, Data Modeling, SCSS, OpenAI, DevOps, Cloud Architecture, HTTP, SaaS, HIPAA Compliance, API Testing, Automation, Continuous Delivery (CD), Postman, Database Design, Minimum Viable Product (MVP), Software Architecture, EPIC Electronic Health Records (EHR), Electronic Health Records (EHR), Epic Systems, Fast Healthcare Interoperability Resources (FHIR), HIPAA, HL7, Healthcare, Third-party Integration, Third-party APIs, SMART on FHIR, Multi-tenant Architecture, Role-based Access Control (RBAC), OAuth, Healthtech, Solution Architecture, Integration
Software Developer
2022 - 2024
QRA
- Documented the front-end components for this project. Used Angular, React-syntax, and Storybook.
- Used NX and introduced the monorepo pattern to the front-end project to improve code-splitting.
- Built native elements to integrate them seamlessly into the host website we are compatible with. Utilized Angular and Angular elements.
- Created a front-end SDK to expand the business horizon and allow third-party adoption. Utilized Angular and Angular elements.
- Performed some minor changes to the back end using Java, PostgreSQL, Hibernate, and Spring Boot.
- Integrated the front end with the WebAssembly code written in Rust.
Technologies: JavaScript, Angular, Node.js, Full-stack, Express.js, Axios, Sass, HTML, CSS, HTML5, Java, JSON, Jest, TypeScript, Front-end, TypeScript 3, Material UI, Clean Code, Object-oriented Design (OOD), MVP Design, Object-oriented Programming (OOP), Chrome Extensions, Webpack, React Native, Web Development, Mobile Development, Full-stack Development, GraphQL, Single-page Applications (SPAs), Office Add-ins, API Development, DOM, ECMAScript (ES6), Git, Back-end, Tailwind CSS, Spring Boot, Scraping, Web Scraping, RxJS, Next.js, Playwright, Tailwind UI, SCSS, Selenium, Large-scale Projects, HTTP, Lit, SaaS, Automation, Continuous Delivery (CD), Software Architecture, Third-party Integration, Third-party APIs, Role-based Access Control (RBAC), Integration, Azure
Back-end Developer
2022 - 2023
Camel Riding Sports LLC (via Toptal)
- Implemented a complete social media module, including a functional post builder, using React Native.
- Used React Native to implement multiple screens and integrated them with the API endpoints.
- Implemented multiple API services related to bets and odds using Node.js, DynamoDB, AWS, Docker, and Serverless.
- Enabled real-time updates for events and odds, Using different 3rd-party APIs related to sports events and odds.
- Implemented multiple API services related to the social media module using Node.js, MySQL, AWS, and Serverless.
- Helped Implement different flows like sign up, sign in, and forgot password, using React Native and Auth0.
Technologies: Node.js, TypeScript, WebSockets, AWS Lambda, Auth0, Data Scraping, Amazon Web Services (AWS), AWS CloudFormation, React Native, MySQL, Single-page Applications (SPAs), Amazon DynamoDB, API Development, Amazon RDS, ECMAScript (ES6), Twilio, Git, Back-end, Databases, Bitbucket, Jira, Data Modeling, HTTP, Automation, Continuous Delivery (CD), Postman, Database Design, Minimum Viable Product (MVP), Software Architecture, Third-party Integration, Third-party APIs, Integration, Expo
Back-end Developer
2021 - 2022
Kenect
- Contributed to the architectural design and implementation of a new integration system, utilizing Java, Quarkus, MySQL, and Firebase.
- Integrated a new client into the system using our newly developed integration system, which involved React, Java, Quarkus, and Firebase.
- Resolved various front-end bugs by leveraging React and our core API.
- Supported an existing integration using the legacy integration system, Java, Spring Boot, Hibernate, MySQL, Twilio, and a datastore.
Technologies: JavaScript, Java, Google Cloud Platform (GCP), MySQL, Quarkus, Spring Boot, JSON Web Tokens (JWT), HTML, CSS, HTML5, REST, APIs, Microservices, JSON, React, Back-end Development, Redis, API Integration, Stripe, Stripe Payments, SQL, REST APIs, Material UI, RxJava, Clean Code, Object-oriented Design (OOD), Mobile, Unit Testing, Object-oriented Programming (OOP), NoSQL, API Management, Test Automation, SOAP, QA Automation, CI/CD Pipelines, Messaging, Integration Testing, Payment Gateways, Security, Spring, Hibernate, Kubernetes, Test-driven Development (TDD), API Development, Firebase Authentication, Back-end, Firebase, Twilio, Git, Databases, Redux, Redux Toolkit, Terraform, Jira, Architecture, Data Modeling, SCSS, WhatsApp API, Chatbots, Large-scale Projects, HTTP, SaaS, API Testing, Continuous Delivery (CD), Database Design, Software Architecture, Third-party Integration, Third-party APIs, Consulting, Integration
Full-stack Developer
2020 - 2021
Zazmic
- Used Node.js and GCP functions. Improved the invoice generation module to make it faster and allowed for further customizations with discounts and special parameters for each client.
- Developed a DICOM parser using JavaScript, enabling the anonymization of patient data and simplifying the modification of DICOM files for testing purposes.
- Implemented a scalable service for receiving and processing HL7 files using Node.js, Knex.js, and GCP functions. This service facilitated the migration of a new client by feeding the processed data into our database.
- Used a GraphQL implementation to improve the size of the result by specifying the concrete fields we wished to select.
- Enhanced various screens and flows on the front-end side to elevate the user experience, utilizing Angular.
- Mentored a new developer so he could adapt and become productive to the project.
Technologies: JavaScript, Node.js, Angular, PostgreSQL, Google Cloud Platform (GCP), Full-stack, Express.js, JSON Web Tokens (JWT), Axios, Sass, HTML, CSS, HTML5, REST, APIs, Microservices, JSON, Jest, TypeScript, Back-end Development, Redis, API Integration, SQL, Front-end, TypeScript 3, REST APIs, Material UI, Clean Code, Object-oriented Design (OOD), MVP Design, Unit Testing, Object-oriented Programming (OOP), NoSQL, API Management, BigQuery, Webpack, Web Development, Full-stack Development, GraphQL, HL7 FHIR Standard, Single-page Applications (SPAs), API Development, DOM, ECMAScript (ES6), Git, Back-end, Databases, Redux, Architecture, RxJS, Linux, Data Modeling, SCSS, Large-scale Projects, HTTP, HIPAA Compliance, API Testing, Continuous Delivery (CD), Postman, Database Design, Software Architecture, Electronic Health Records (EHR), Fast Healthcare Interoperability Resources (FHIR), HIPAA, HL7, Healthcare, Role-based Access Control (RBAC), Healthtech
Software Engineer
2019 - 2020
Equifax
- Utilized GCP, Java, Spring Boot, and Nginx to migrate different features from an on-premise monolith to a cloud-based microservices architecture. This was all for Equifax Argentina.
- Mentored teammates in JavaScript, TypeScript, and Angular, facilitating their transition from back-end to full-stack developers.
- Implemented a load balancer and service resolver using Spring Boot, Ribbon, and Zuul. This service was adopted by multiple teams, becoming a shared resource.
- Enhanced the portfolio management module by optimizing aggregations, utilizing MongoDB and Spring Boot.
Technologies: Java, JavaScript, Google Cloud Platform (GCP), MongoDB, PostgreSQL, Angular, Full-stack, Mongoose, JSON Web Tokens (JWT), Sass, HTML, CSS, HTML5, REST, APIs, Microservices, JSON, TypeScript, Back-end Development, SQL, Front-end, TypeScript 3, REST APIs, Material UI, RxJava, Clean Code, Object-oriented Design (OOD), MVP Design, Unit Testing, Object-oriented Programming (OOP), NoSQL, API Management, Webpack, Progressive Web Applications (PWAs), Eclipse, Security, Spring MVC, Web Development, Full-stack Development, Spring, Hibernate, Kubernetes, Test-driven Development (TDD), Single-page Applications (SPAs), API Development, DOM, ECMAScript (ES6), Git, Back-end, Databases, OWASP, Spring Boot, Terraform, Bitbucket, Jira, Architecture, RxJS, Data Modeling, SCSS, Large-scale Projects, HTTP, Database Design, Software Architecture, Consulting
Senior Node.js Developer
2018 - 2019
The App Master
- Migrated the legacy front-end from Angular.js to Angular 8 and outsourced to homecloud.
- Resolved numerous legacy issues related to promises, reducing data inconsistency and system fatal failures using Node.js for homecloud.
- Enhanced back-end functionality by adding new flows and improving existing ones with TS.Ed, Node.js, and Postgres during an outsourcing engagement with homecloud.
- Documented the API with Swagger and Node.js to ensure reliability, consistency, and maintainability while collaborating with homecloud.
- Improved application uptime using PM2 during an outsourcing assignment with homecloud.
- Implemented an auto-renewable SSL certificate with Let's Encrypt and Nginx, resulting in cost savings for the company during an outsourcing project with homecloud.
- Established a CI/CD pipeline with Amazon Route53, API Gateway, CloudWatch, EBS, CodeCommit, CodeBuild, and CodeDeploy, enhancing code delivery speed, reliability, system observability, and scalability during an outsourcing engagement with homecloud.
- Maintained and updated an existing Android application to incorporate new API features, while working with Java and Android during an outsourcing project.
- Created a new API and assisted in implementing a new front end for a new product using React, Stripe, and Node.js during an outsourcing partnership with Core Spaces.
- Implemented various screens and flows for an Android application in collaboration with ExMarx, using Java and Android during an outsourcing engagement.
Technologies: JavaScript, Angular, React, Android SDK, PostgreSQL, Node.js, Full-stack, Amazon Web Services (AWS), MongoDB, Express.js, Mongoose, JSON Web Tokens (JWT), Axios, Sass, HTML, CSS, HTML5, Java, REST, APIs, JSON, Jest, TypeScript, Back-end Development, Redis, API Integration, Stripe, Stripe Payments, SQL, Amazon S3 (AWS S3), AWS Lambda, AWS Elastic Beanstalk, Front-end, MERN Stack, AngularJS, TypeScript 3, REST APIs, Material UI, React Router, Android, RxJava, Clean Code, Object-oriented Design (OOD), MVP Design, Mobile, Unit Testing, Object-oriented Programming (OOP), Docker, NoSQL, API Management, CI/CD Pipelines, Messaging, Webpack, Progressive Web Applications (PWAs), Eclipse, Web Development, Full-stack Development, AWS DevOps, Continuous Integration (CI), Single-page Applications (SPAs), API Development, Firebase Authentication, Amazon RDS, DOM, ECMAScript (ES6), Firebase, Git, Back-end, Databases, Redux, Amazon Simple Queue Service (SQS), Amazon Simple Notification Service (SNS), Bitbucket, Architecture, Scraping, Web Scraping, RxJS, Amazon Cognito, Data Modeling, SCSS, Selenium, DevOps, Cloud Architecture, HTTP, Vue, SaaS, API Testing, Automation, Continuous Delivery (CD), Postman, Database Design, Minimum Viable Product (MVP), Software Architecture, Third-party Integration, Third-party APIs, Consulting, XML, Solution Architecture, Integration, NestJS
Programmer
2017 - 2018
Pandora Technologies EIRL
- Presented a project for the stocks entity of Lima, which resulted in the hiring of Pandora for that project, where I was the lead developer for a test automation software.
- Created a documents management software for the stocks entity of Lima, including back-end, front-end, and on-premise deployment to WebSphere.
- Audited one of the core services for one of the clients and proved the existence of multiple vulnerabilities, including SQL injection.
- Improved different front-end functionalities to the main website of the stocks entity of Lima.
Technologies: JavaScript, Node.js, Java, Full-stack, Amazon Web Services (AWS), MongoDB, Express.js, Mongoose, JSON Web Tokens (JWT), Axios, Sass, HTML, CSS, HTML5, REST, APIs, JSON, React, TypeScript, MySQL, Back-end Development, SQL, Amazon S3 (AWS S3), AWS Lambda, AWS Elastic Beanstalk, Front-end, MERN Stack, AngularJS, REST APIs, Material UI, Clean Code, Object-oriented Design (OOD), MVP Design, Groovy, Object-oriented Programming (OOP), Docker, NoSQL, API Management, Test Automation, SOAP, QA Automation, CI/CD Pipelines, Eclipse, Security, Spring MVC, Web Development, Full-stack Development, Spring, Hibernate, AWS DevOps, Continuous Integration (CI), Single-page Applications (SPAs), API Development, Amazon RDS, DOM, Git, Back-end, Databases, Spring Boot, Amazon Simple Queue Service (SQS), Bitbucket, Jira, Scraping, Web Scraping, Linux, Data Modeling, SCSS, Selenium, HTTP, API Testing, Automation, Database Design, Minimum Viable Product (MVP), XML
Tech Lead
2015 - 2017
Automatismos Industriales del centro
- Implemented a dashboard system for a manufacturing company using Java and JavaScript.
- Implemented an internal SSR front end for Bimbo Venezuela using JavaScript and Node.js.
- Led my team to implement a complete robotic automation solution and completed the front end part using VBA and JavaScript.
Technologies: JavaScript, Java, Node.js, Back-end, Databases, Data Modeling, Automation
Project History
Quizzes App
A full-stack, React, Express.js, and PostgreSQL app. The app was dockerized and was deployed to DigitalOcean along with the database.
It used a folders-and-files structure inspired by clean code and had the configuration for continuous integration in GitLab.
It used a folders-and-files structure inspired by clean code and had the configuration for continuous integration in GitLab.
Loretia: Narrative Continuity Linter
I built a parser capable of turning any human-written story, including prose, branching narratives, novels, and scripts, into a structured format I designed.
I also built an engine that ingests that structured format, builds a series of DAGs, and detects factual and epistemological inconsistencies. This allows me to catch contradictions, plot holes, lies, conflicting claims, and more. Because the engine has the context of the entire story in a structured format, I can also generate a story bible, wiki, and relationship graphs, and derive the state of the world at any given point in the story.
I also built an engine that ingests that structured format, builds a series of DAGs, and detects factual and epistemological inconsistencies. This allows me to catch contradictions, plot holes, lies, conflicting claims, and more. Because the engine has the context of the entire story in a structured format, I can also generate a story bible, wiki, and relationship graphs, and derive the state of the world at any given point in the story.
Timbre: AI Call intelligence for Support Teams
Batch pipeline for customer-service call recordings. Upload audio, get back a flat JSON object of assertions about it: emotional tone and intensity, background noise presence/type/severity, audio quality, speaker overlap, long silence, and a confidence score.
Diarization | Silence/VAD | Transcription | Emotion | Noise tagging | Quality
Audio files are routed to different isolated queues (small, medium, large) based on duration and picked up by dynamically scaled worker sizes that match. This limits resource usage for short files while providing a safety margin for long ones.
Diarization | Silence/VAD | Transcription | Emotion | Noise tagging | Quality
Audio files are routed to different isolated queues (small, medium, large) based on duration and picked up by dynamically scaled worker sizes that match. This limits resource usage for short files while providing a safety margin for long ones.
Skills
Libraries/APIs
React, Node.js, REST APIs, RxJS, WhatsApp API, Stripe, React Router, RxJava, API Development, Playwright, Moment.js, Google Maps API, Lit, Vue, PyTorch
Tools
Bitbucket, Jira, Mongoose, Webpack, Git, Redux Toolkit, Amazon Simple Queue Service (SQS), Amazon Simple Notification Service (SNS), Postman, Claude, SMART on FHIR, Claude Code, Expo, Slack, BigQuery, GitLab CI/CD, Auth0, AWS CloudFormation, Firebase Authentication, Terraform, Amazon Cognito, AI SDK
Languages
JavaScript, Java, HTML, HTML5, TypeScript, TypeScript 3, ECMAScript (ES6), SCSS, CSS, SQL, Groovy, GraphQL, Python, Sass, C#, XML
Frameworks
Angular, Express.js, Spring Boot, JSON Web Tokens (JWT), AngularJS, Spring, Next.js, Selenium, NestJS, Jest, Material UI, Spring MVC, React Native, Hibernate, Tailwind CSS, Redux, OAuth 2, Android SDK, Swagger, Mockito, JUnit
Paradigms
REST, Object-oriented Design (OOD), Object-oriented Programming (OOP), Continuous Delivery (CD), Microservices, Clean Code, Unit Testing, Test Automation, Mobile Development, Test-driven Development (TDD), HL7 FHIR Standard, DevOps, HIPAA Compliance, Automation, Database Design, Fast Healthcare Interoperability Resources (FHIR), Role-based Access Control (RBAC), Continuous Integration (CI), Scrum, Agile
Platforms
Amazon Web Services (AWS), Firebase, Google Cloud Platform (GCP), Docker, AWS Lambda, AWS Elastic Beanstalk, Eclipse, Twilio, EPIC Electronic Health Records (EHR), Azure, MacOS, Visual Studio Code (VS Code), Android, Mobile, DigitalOcean, Kubernetes, Linux
Storage
MongoDB, JSON, PostgreSQL, Redis, Amazon S3 (AWS S3), NoSQL, Databases, MySQL, Amazon DynamoDB
Industry Expertise
Healthcare
Other
Full-stack, APIs, Back-end Development, Front-end, Chrome Extensions, Web Development, Full-stack Development, Single-page Applications (SPAs), Back-end, DOM, Tailwind UI, Data Modeling, SaaS, Third-party Integration, Third-party APIs, Axios, API Integration, Stripe Payments, MERN Stack, Styled-components, MVP Design, API Management, QA Automation, CI/CD Pipelines, Messaging, Security, Office Add-ins, Amazon RDS, Architecture, Scraping, Web Scraping, Supabase, Chatbots, OpenAI, Cloud Architecture, Large-scale Projects, HTTP, API Testing, Minimum Viable Product (MVP), Software Architecture, Conversational AI, Electronic Health Records (EHR), Epic Systems, HIPAA, HL7, FastAPI, Multi-tenant Architecture, OAuth, Healthtech, Artificial Intelligence (AI), Workflow Automation, AI Automation, LLM Integration, AI Agents, AI Agent Orchestration, Retrieval-augmented Generation (RAG), Vector Databases, Large Language Models (LLMs), Consulting, Solution Architecture, Integration, Agentic AI, Gemini, Cloud, Quarkus, SOAP, Integration Testing, Progressive Web Applications (PWAs), Payment Gateways, Ts.ED, Lint, Husky, i18n, AWS DevOps, WebSockets, Data Scraping, OWASP, OpenRouter, Graphs, Natural Language Processing (NLP), VPS, SOC 1, OpenTelemetry, ONNX Runtime, Amazon Bedrock AgentCore
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