John Zachary Fitch

John Zachary Fitch

Agent tooling | systems performance | privacy-first infrastructure

I build production-grade tooling for agents and the substrate they depend on: deterministic retrieval, verifiable edits, structured tool APIs, and execution environments you can reason about. I work across Rust, Python, and web platforms with an evidence-first style: measure, reproduce, fix, and ship.

Recent Highlight (Jan 2026)

OpenAI Codex

Ghost in the Codex Machine

Investigated and helped fix an "invisible" release-only regression where a pre-main constructor stripped LD_* / DYLD_* environment variables, breaking CUDA/MKL library discovery inside tool subprocesses. Shipped upstream with release-notes credit.

Proof

Representative impact

  • MKL/BLAS (repro harness): ~2.71s -> ~0.239s (11.3x)
  • CUDA workflows: restored expected library discovery to avoid CPU fallback in affected setups

Release notes excerpt:

"Special thanks to @johnzfitch for the detailed investigation and write-up in #8945."

How I Work

  • Construct from first principles rather than refine existing forms.
  • Recover overlooked work — revisit ideas and approaches that didn't take the first time, with current tools.
  • Probe working systems for hidden assumptions; rebuild the parts that don't hold up.
  • Operate well in complex, debated, high-ambiguity spaces.
  • Find a useful subspace inside a team early; manage well under crisis and time pressure.

Core Skills

Rust

Performance-critical systems, CLI tools, data structures, correctness-oriented engineering

Python

Tooling, analysis pipelines, automation, reproducible experiments

Web

WebGPU/WASM applications, client-side ML inference, offline-first UX

Systems

Linux, NixOS, DNS, TLS automation, containerized services, security hardening

Agent Integration

MCP servers, skill/plugin packaging, tool-driven workflows

Selected Projects (Public)

  • llmx (Rust core, JS/WASM web; live: llm.cat) - local-only codebase indexer built on Burn-ml and mdbr-leaf-ir; hybrid search (BM25 + neural embeddings) fused via RRF; deterministic chunking with content hashing
  • claude-warden (Shell/OTEL, ★57) - security hooks and budget system for Claude Code, with a built-in web viewer for extensive OTEL traces
  • claude-cowork-linux (Linux, ★236) - native Linux port of Claude Desktop, security-first: treat the host OS as the VM (no Mac VM layer), wrap in bubblewrap, and handle the ASAR from outside the sandbox rather than hooking in; strips dispatch/channels, preserves Cowork/Code/Chat
  • dota (Rust) - post-quantum secrets manager: hybrid ML-KEM-768 + X25519, Argon2id KDF, SQLCipher at rest, YubiKey/SoloKey HMAC-SHA1 hardware auth
  • pyghidra-lite (Python/MCP, ★32) - token-efficient MCP server for Ghidra (official MCP registry: io.github.johnzfitch/pyghidra-lite, v0.1.1)
  • codex-xtreme (Rust) - optimized, patched Codex builds (includes codex-patcher)
  • burn-plugin - Claude Code plugin + skills for the Burn deep learning framework
  • Observatory (WebGPU) - client-side AI image detection (live)
  • SpecHO v2 (Python) - 161D linguistic fingerprinting for AI text detection
  • definitelynot.ai (PHP/JS) - Unicode-security-aware sanitizer + API
  • Iconics (Python) - semantic icon library for professional docs (8k+ icons)

Operating Background

Owner-operator of a regulated chemical-manufacturing business for 10 years; FDA and CPSC oversight, hazardous-substance handling. Sustained a multi-year PMTA process under non-committal regulatory guidance, including FOIA-driven information gathering. Mission: help adults worldwide quit smoking cigarettes. Declined paths that diverged from that mission even when commercially favorable; wound down on deliberate terms in the company's strongest revenue year.

What this established:

  • Tolerance for prolonged regulatory ambiguity without losing direction.
  • Operating discipline at production scale: manufacturing, compliance, hiring, crisis response.
  • Over a decade under public scrutiny — grounded in the work, confident defending it on the record, practiced at navigating it responsibly.

Infrastructure (Self-Hosted)

I operate production infrastructure on bare metal with:

  • Declarative NixOS configuration (reproducible, atomic upgrades, rollbacks)
  • Authoritative DNS and automated wildcard certificates (DNS-01 / RFC2136)
  • Post-quantum security layers (hybrid SSH KEX, WireGuard + Rosenpass)

Education

UC Berkeley - Mathematics. Withdrew during the 2020 lockdowns under a granted automatic readmission.

What I'm Looking For

Roles building agent runtimes and developer tools, retrieval systems, and security/privacy foundations. I work best on teams that value measurable results, clear ownership, and high engineering standards.

Download PDF Contact