Skip to content
EV

Available for remote senior roles and select contracts

Software engineer for web, mobile and AI products.

Full stack, 5+ years shipping React, Next.js, React Native and Postgres-backed products. Sole builder of Skkeptic, a live, paid app that runs 13 frontier models against one question. Before that: Berkshire Hathaway Energy, Rev, and a YC-backed AI tutor.

  • Ships end to end. Streaming UI, state machines, Postgres data model, usage-based billing, tests. One person, one repo.
  • Proven in production. Enterprise GIS at Berkshire Hathaway Energy, payments at Rev, real-time LaTeX at YC-backed Studdy AI.
  • Tests as a habit. 1,500+ automated tests on Skkeptic, pgTAP on the database layer.
Book a 20-min callRésumé (PDF)Replies within one business day.
  • React
  • Next.js
  • TypeScript
  • React Native
  • Postgres
  • XState
  • Node.js
  • Swift
Work log7 items
SKBHRVYC

  • Skkeptic

    Usage-based billing: credit gate, Redis settlement, cost watchdog, Polar webhooks

    Live2026

  • Skkeptic

    13 models through Vercel AI Gateway, XState v5 actors, manual hand-off mode

    Live2026

  • Skkeptic

    1,500+ Vitest tests, pgTAP on row-level security

    Passing2026

  • Berkshire Hathaway Energy

    ArcGIS app: MUI, ArcGIS SDK, Node.js and TypeScript upgrades, faster first load

    Delivered2025

  • Rev

    Ordering and payment flow; Swift App Intent and Share Extension

    Delivered2024

  • Studdy AI

    Real-time LaTeX renderer with out-of-order chunk queue

    Delivered2023

  • Taisho

    Transaction-safe ticket purchases on Firestore, no overselling under load

    Delivered2022
Updated September 2026Source: résumé

Worked with

  • Berkshire Hathaway Energy
  • Rev
  • Studdy AI (YC)
  • ShowZone
  • Stuf
  • Ollie
  • Decon
  • Threde

Featured · Founder & sole engineer · Mar 2026 – present

Skkeptic. Several frontier models argue over one answer. I built all of it.

Skkeptic: three model cards in boxing gloves under the line 'Produce better results. Deliver with confidence.'
models orchestrated
13
automated tests
1,500+
languages shipped
2
engineer
1
Open skkeptic.comFree tier, no signup needed to look around.

The problem

Any single model is confidently wrong some of the time. Skkeptic passes a question through several frontier models in rounds, each one improving on the previous answer, so the user ends up with one refined answer instead of a guess.

Constraints

  • One engineer, one repo, from streaming UI to billing.
  • It is a paid product: every cent of usage has to reconcile to the credit.
  • A stream can never run past the wallet limit, even mid-token.
  • Thirteen models across several providers, so routing needs fallbacks.

What I built

  1. 01An XState v5 actor system that drives 13 models through Vercel's AI Gateway with fallback routing, plus a manual mode where the user pauses a model mid-answer, chats with it and hands off to the next.
  2. 02Usage-based billing end to end: a pre-stream credit gate, idempotent settlement on Redis, a running-cost watchdog that stops a stream at the wallet limit, and Polar webhooks, all reconciled against gateway receipts to the credit.
  3. 03The Postgres layer as an append-only entitlement ledger with row-level security keyed on the Clerk session, verified with pgTAP.
  4. 04Spanish localization, Sentry alerting and 1,500+ Vitest tests.

Where it stands

Live, taking payments, and maintained by me. It is the fastest way to see how I structure a product when nobody else is holding the pen.

  • Next.js
  • TypeScript
  • Vercel AI SDK
  • XState v5
  • Supabase / Postgres
  • Clerk
  • Polar
  • Redis
  • Sentry
  • Vitest

Selected work · 2021 – 2024

Client and contract work, with live links.

Rev Voice Recorder app screens: recording, transcript ordering and file management on iPhone.

Rev Voice Recorder

Rev · iOS and Android · Mobile Engineer, 2024

2024

Built the ordering and payment flow from scratch and added native iOS features in Swift to the hybrid React Native codebase.

  • React Native
  • TypeScript
  • GraphQL
  • MobX
  • Swift
Visit Rev Voice Recorder
Read the case study

Problem

A production recorder with human and AI transcription orders, in a hybrid native and React Native codebase. The ordering flow had to drive dynamic pricing and payment methods over GraphQL, and new iOS entry points needed native code.

What I built

  • The ordering and payment UI from scratch (PlaceOrderScreen, OrderSummaryBottomSheet, Order Preferences), wired to GraphQL queries for dynamic pricing, payment methods and AI transcript order placement.
  • Native iOS features from scratch in Swift: Quick Actions, an App Intent for starting recordings, and a Share Extension handling URL and plain-text sharing via custom URL schemes.
  • Search, file management and media: chip-based filtering, an upload destination folder picker, and extending the audio player for video playback across platforms.

Outcome

The ordering flow and the native entry points are part of the app on both stores.

Studdy app screen showing a math tutor solving a problem step by step next to the line 'Get instant help on any problem'.

Studdy AI

YC-backed AI tutor · contract

2023

Built the real-time LaTeX rendering engine for streamed answers, and the components that seeded the design system.

  • React Native
  • TypeScript
  • GraphQL
  • XState
  • Tailwind CSS
Visit Studdy AI
Read the case study

Problem

Answers arrive as a token stream, out of order, with LaTeX in the middle. Rendering them naively on mobile re-rendered the whole tree on every chunk.

What I built

  • An XState state machine with chunk queuing for out-of-order stream processing and per-step actor isolation.
  • Direct WebView postMessage bridges to bypass the React render cycle, and custom XState selectors with equality comparators to stop cascading re-renders during streaming.
  • The Translate Lens feature end to end: language selector, variant-based component system, image payload restructuring.
  • 10+ reusable UI components that became the foundation of the app's design system.

Outcome

Token-stream rendering got materially cheaper on mobile, and later features were built on the component set I left behind.

ShowZone.gg homepage: 'MLB The Show news & tips' with login, register and pro buttons.

ShowZone

Tools for MLB The Show players · contract

2023

Migrated the tier builder to @dnd-kit for multi-container drag and drop, and built a game-accurate PXP calculator.

  • Next.js
  • Material UI
  • @dnd-kit
Visit ShowZone
Read the case study

Problem

The tier builder ran on react-beautiful-dnd, which could not move items across containers. Players also needed a calculator that matched the game's own rounding.

What I built

  • Cross-container item transfer, portal-based drag overlays, and optimistic state cloning with rollback on cancel.
  • A PXP calculator with 17 baseball stat types across counters, switches and fractional dropdowns, difficulty and online-mode multipliers.
  • Step-wise floor truncation so the math matches the actual game engine.

Outcome

Drag and drop works across tiers, and the calculator's numbers line up with what the game shows.

Stuf storage homepage: 'Home away from home for your stuff' with a city search bar.

Stuf

Storage company · web app

2022

Developed and maintained the customer-facing web app: Redux state, Material UI and Tailwind, Atomic Design structure, ESLint and Prettier baseline, feature work.

  • Next.js
  • Redux
  • Material UI
  • Tailwind CSS
Visit Stuf
Decon Field Guide app on a phone: a worker in a yellow hazmat suit and a sign-in button.

Decon Field Guide

Field app for decontamination teams · mobile

2022

Fixed build, font and splash-screen issues, set up Sentry, and ran App Store and Google Play submissions, test and production releases.

  • React Native
  • TypeScript
  • Sentry
Visit Decon Field Guide
Ollie website section 'How we get it right' with bowls of fresh dog food and product photos.

Ollie

Fresh dog food · web

2021

Front-end work in React, with XState for the stateful flows.

  • React
  • XState
Visit Ollie

How I work

Fewer surprises, in code and in the calendar.

Four habits that show up in every project above. If they match how your team runs, we will get along.

  • Tests ship in the same PR.

    TypeScript everywhere, Vitest, Jest and pgTAP next to the code they cover, Sentry wired up before launch. 1,500+ tests on Skkeptic because a solo product has no one else to catch it.

  • State gets modeled, not patched.

    XState machines for streaming, billing and navigation flows. If the state diagram is ugly, the code will be too, so I fix the diagram first.

  • Solo or in a team, same bar.

    Sole architect on Taisho and Skkeptic. At Berkshire Hathaway Energy I ran PR reviews, pairing and knowledge-transfer sessions on React, Redux and TypeScript.

  • Small PRs, short written updates.

    Remote from Guadalajara with full overlap on US hours. Reviewable PRs, decisions in writing, Scrum when the team wants it. Claude Code and Cursor with spec-driven workflows, without lowering the bar.

Experience

Five roles, one throughline: code in production.

Education
React Developer, Zero to Mastery Academy, 2020
Languages
Spanish (native) · English (C2)
Open source · Apr 2023
Auto-GPT (Significant Gravitas)Found and fixed a critical bug in the Python codebase and submitted the pull request, coming from a JavaScript background and picking up the Python on the fly.
  1. Mar 2026 – present

    Founder & Software Engineer · Skkeptic

    Sole builder of a paid multi-model AI app: Next.js, XState v5, Supabase, Clerk, Polar.

  2. Mar 2025 – Feb 2026

    Front-End Engineer · Berkshire Hathaway Energy

    Tech-debt assessment and dependency upgrades (Material UI, ArcGIS SDK, Node.js, TypeScript) on an enterprise GIS app; cut initial page load with lazy loading, memoization and cheaper state selectors; mentored the team through reviews and pairing.

  3. May 2024 – Dec 2024

    Mobile Engineer · Rev

    Ordering and payment flow for Rev Voice Recorder (iOS and Android) from scratch, GraphQL for dynamic pricing and AI transcript orders; native iOS in Swift: Quick Actions, an App Intent, a Share Extension.

  4. Jan 2023 – May 2024

    Freelance Software Engineer · Upwork

    Full client lifecycle from proposal to delivery. Studdy AI (YC-backed) and ShowZone, both below.

  5. Nov 2020 – Jan 2023

    Software Engineer · Threde LLC

    Apps for 7+ clients: React Native with Firebase backends, plus a Next.js web app. Sole architect of Taisho, an event app with transaction-safe Firestore ticket purchases and geohash-based discovery; core engineer on SonderLife, a dating app. Also Decon, Stuf, Bang Energy, Katalyst.

What clients say

From a two-year client

“Eliezer has worked with us for a solid 2 years. He's a good developer and cares a lot about best practices. His quality and scalable work has been critical for improvements. He's also very good at voicing recommendations and raising topics. [...] Eliezer is one of the first people we call. When given the opportunity, we are always happy to work with Eliezer.”
Michael Guild
CEO
“Eli is the best! Glad to work with him.”
Jeremy Englert · CEO

Contact

Tell me what you are building.

Open to remote senior full stack, frontend and mobile roles, and to contracts for AI product builds. One call, no deck.

What I can take on

  • New products, end to end. Architecture, UI, Firebase or Supabase/Postgres backend, auth, payments, real-time data, tests, and the App Store and Google Play release.
  • AI features. LLM integrations, streaming chat UIs, multi-model workflows, and usage-based billing with cost controls so AI spend never runs ahead of revenue.
  • Existing codebases. Performance fixes, dependency upgrades, state-management cleanup (Redux, XState, MobX), test coverage where there was none.
Book a 20-min callvzla0094@gmail.com

Replies within one business day.

Eliezer Valenzuela, portrait

Eliezer N. Valenzuela

Guadalajara, MX · Central Time

Remote. Full overlap with US working hours.

Spanish (native) · English (C2)