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marcoshernanz/README.md

Marcos Hernanz

I build coding agents, evaluation infrastructure, ML systems, and performance-sensitive developer tools.

Most recently a Software Engineer Intern on the Next.js team at Vercel, where I built an AI maintainer for Next.js and used it to close 1,500 GitHub issues in one month.

Vercel, Next.js team

  • Built an AI maintainer for Next.js that investigates GitHub and customer reports end to end, from triage and reproduction through canary verification, regression bisection, tests, and fixes.
  • Ran it as a human-reviewed queue that closed 1,500 GitHub issues in one month, taking the open backlog from 2,244 to 995 with 3 reopens, and wrote the Next.js blog post on the campaign.
  • Owned the product and agent stack behind it, including resumable specialist workflows, model routing, cost and evaluation tracing, and an Eve harness benchmark on Terminal-Bench 2.1.
  • Shipped Next.js Agent Feedback, a human-in-the-loop path where coding agents draft deidentified reports on the framework friction they hit and the developer edits, sends, or discards each one before anything is submitted.
  • Merged 26 Next.js and Turbopack PRs plus upstream SWC and notify-rs fixes, including a 42× HMR invalidation speedup, a React Compiler precheck that cut compiler pipeline time 19.64% on real v0 modules, and a worker-lifecycle fix that eliminated 100% of measured worker leak growth.

Projects

  • llm-lab: Language-model training and systems laboratory, built from bigrams through Transformers with tokenization, FineWeb-Edu data pipelines, profiling, checkpointing, and multi-device training. Trained a Transformer across 8 TPU v5e devices, processing 39.85B training tokens at 2.63M tokens/s. Rebuilt that baseline into a 2026-era architecture through a cumulative ladder of 18 controlled experiments covering RoPE, GQA, SwiGLU, RMSNorm, MLA, mixture-of-experts, Kimi Delta Attention, and multi-token prediction. Hand-wrote a Muon optimizer that matched torch.optim.Muon from identical initialization.
  • BareTensor: Built a near-zero-dependency tensor and autograd runtime from scratch in C++, with strided tensors, broadcasting, neural-network operations, dynamic autograd, and Python bindings.
  • ChatVault: Built private semantic search for WhatsApp that runs locally in the browser using quantized MiniLM, Rust, and WebAssembly. Finding and reproducing a Turbopack issue while building it led directly to my off-cycle Vercel internship.
  • Cogniba: Designed, built, and launched a brain-training product with Next.js and Supabase, growing it to more than 2,000 registered users.

Competitive programming

  • Codeforces Expert with a peak rating of 1800
  • 2x ICPC SWERC participant
  • Olympiad in Informatics, 2nd in Madrid and 16th in Spain
  • 3x Meta Hacker Cup Top 2,000
  • 2x Ada Byron Spanish national finalist

Contact

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  1. ChatVault ChatVault Public

    Privacy-first semantic search engine for WhatsApp. Runs 100% in-browser using Rust, WebAssembly, and BERT. Zero data egress.

    TypeScript 27 1

  2. baretensor baretensor Public

    Building a PyTorch-shaped AI stack from first principles, from tensor storage and autograd to CUDA kernels and transformer training.

    Python 4

  3. CalYo CalYo Public

    AI-powered nutrition tracker built with React Native, Expo, and Convex. Features computer vision food analysis and real-time sync.

    TypeScript 3

  4. vexi vexi Public

    Schema-first RAG database for TypeScript. Local-first, type-safe, and built to ship AI search without the glue code.

    Rust 1