Every multi-state trucking fleet has to file an IFTA return each quarter — miles and fuel, apportioned across every state, at rates that change quarterly. I built the system that does it: deterministic Python for the math, a Claude agent for the judgment. It is live, and a real carrier files through it.
The International Fuel Tax Agreement makes every carrier that crosses state lines report miles driven and fuel purchased, apportioned across each U.S. state and Canadian province, at tax rates that change every quarter. It is tedious, deadline-driven, and a wrong number invites an expensive state audit.
Today it is done by hand in spreadsheets or paid out to a bookkeeper. That is the gap this product closes — and it is a real, recurring, paid-for need in a vertical I know.
The single most important design choice — and the first thing I'd talk through in an interview.
Ingest CSV/Excel/PDF exports, compute fleet MPG, taxable gallons, and per-jurisdiction tax + surcharges with CDTFA-exact rounding, then write the state-portal CSV and a per-truck reconciliation Excel. No model touches a number that lands on a tax form.
A 16-tool review agent reads the computed return and checks it against the rule base, the live rate matrix, and 21 quarters of real filing history — then writes a plain-English note flagging missing surcharges, MPG anomalies, and audit-bait patterns before a human files.
The front door is on Vercel; the backend runs at home behind a Cloudflare Tunnel — public, always-on, and effectively free to host.
Deterministic pipeline
ingest · calc · validate · report
Review agent
Claude + 16 grounded tools
Operator gate
Telegram approve → packet emailed
Each one is a deliberate trade-off, not a default.
Every taxable number is produced by tested Python; the agent only checks work already done and is grounded by 16 tools so it cites real figures instead of inventing them. That separation is the line between a tool a carrier can file from and a demo.
429 tests, including a real-data backtest that matches a Kentucky carrier's Q4 2025 CDTFA filing exactly. A refactor can't silently change a number the state will recompute.
Routine reviews run on cheaper models; only the highest-risk filings escalate to Opus, with an effort knob for thinking depth. Pennies per filing by default, thorough when the stakes are high.
Per-client tenant isolation, a Telegram operator-approval gate before any file is processed, magic-link tokens as auth, Turnstile CAPTCHA, per-IP rate limiting, and atomic 'all files land or none do' writes.
The FastAPI backend runs on a Mac mini behind a Cloudflare Tunnel — no public IP, no server bill — with the Next.js front end on Vercel. The only variable cost is model spend per review.
Built around a client registry, not hard-coded to one carrier — so onboarding the second and third fleet is configuration, not a rewrite.
Hours → minutes
A quarter that used to mean an afternoon of spreadsheet reconciliation and hand-keying per-state lines comes back in minutes — already reviewed, with a portal-ready CSV, a per-truck Excel for each owner-operator, and a written check of what to fix before filing.
What's next
A hosted vector store for the rule base, a public eval dashboard (golden filings → pass rate), and onboarding the next two carriers.
The product is live and accepting carriers. The pipeline, agent, and tests are a real repo.