Jev calculator

A calculator whose answer is produced by TypeSafe's Jev classifier one character at a time. Nothing in this repo evaluates arithmetic: every step Jev is offered the same 13 options (0–9, ., -, END) and the page appends whatever it picks, then shows the full probability distribution it assigned to each option, step by step. Fast typesafe calculator powered by Jev!

Live: https://jev-calculator.cloudflare-jjx3a.workers.dev

What you see

• Calculator card: keypad with + − × ÷ mod √, decimals, operands up to 8 digits, answers up to 12 characters. = starts a run, Rerun repeats the same expression so you can see the jitter between identical calls (the previous run stays visible as a ghost, with the first divergence marked).
• Model output (streaming): one column per emitted character. Rows are the 13 options; each pill's colour intensity is the probability Jev gave that option at that step. The chosen option is solid. A Confidence row shows Jev's own confidence per step, plus the latency of the last call and a run score: the product of every step's confidence (END step included), in scientific notation.

Jev is not a calculator. Small sums, differences, square roots and mod often come out right. On large products it gets the first two or three digits and then stops, because once the digit distribution goes flat, END beats any single digit. That is the point of the demo, not a bug to file.

How it works

browser ──POST /api/next {expression, prefix}──▶ Cloudflare Worker ──▶ Vercel AI Gateway ──▶ Jev
   ▲                                                     │ (on gateway 429 / transient failure)
   └── {choice, confidence, probabilities, …} ◀──────────┴──▶ TypeSafe API (fallback, own rate limit)

• The browser owns the autoregressive loop (src/runner.ts): it sends the wire expression and the answer so far, receives one choice plus the 13 probabilities, appends the choice verbatim, and repeats until END or the cap. State machine: idle / running / paused / ended / capped / cancelled / error. A gateway rate limit (429) pauses the run with a Resume button instead of retrying automatically.
• The Worker (worker/index.ts) validates the expression grammar and prefix alphabet, applies a per-IP rate limit, requires a same-origin request and a short-lived pass minted from a Cloudflare Turnstile token (POST /api/pass), then forwards a single System One "choice" request to Jev. Every option is always offered; nothing is masked, and the API's choice is used as returned, never an argmax.
• The prompt Jev receives is plain markdown in prompts/jev-prompts.md. npm run prompts regenerates worker/prompts.ts from it (also runs before test and build). Exactly one section is marked (active); each section has ### state templates using {expression} and {prefix}, ### instructions, and ### criteria listing all 13 options.

Stack

Cloudflare Worker + static assets (wrangler.jsonc), Vite + vanilla TypeScript frontend, shared contract in shared/ (options, wire grammar, request/response types, keypad token model), Vitest with a Node project for the pure modules and @cloudflare/vitest-pool-workers for the Worker.

Develop

npm install
cp .dev.vars.example .dev.vars   # see "Configuration"; or create the file by hand
npm run dev                      # Vite on :5173 with /api proxied to wrangler dev on :8787
npx wrangler dev                 # in a second terminal
npm test                         # unit + workerd projects
npm run typecheck
npm run preview                  # build, then wrangler dev serving the built assets (closest to prod)
npm run deploy                   # build + wrangler deploy

Configuration

Vars live in wrangler.jsonc: JEV_BASE_URL / JEV_MODEL (gateway), TYPESAFE_BASE_URL / TYPESAFE_MODEL (direct fallback), TURNSTILE_SITEKEY, TURNSTILE_HOSTNAMES, JEV_DISABLED (kill switch). Rate limits are Workers Rate Limiting bindings: API_LIMIT per IP and FALLBACK_LIMIT for the direct route as a whole.

Secrets (never committed; npx wrangler secret put <NAME> in prod, .dev.vars locally):

Secret	Purpose
JEV_API_KEY	Vercel AI Gateway key
TYPESAFE_API_KEY	direct TypeSafe key, used only when the gateway is rate-limited or failing; unset disables the fallback
TURNSTILE_SECRET	Turnstile siteverify secret; with the sitekey var unset, the bot check is off
PASS_SECRET	HMAC key for the 30-minute pass issued after a Turnstile check

For local work use Cloudflare's always-pass Turnstile test keys and set TURNSTILE_HOSTNAMES=example.com, which is the hostname the test secret reports.

Limits and non-goals

• The gateway key is shared by every visitor and capped upstream; heavy use pauses runs, it does not queue them. The direct fallback has its own small budget and limiter.
• No true answer is computed or displayed, by design.
• Probabilities are shown as returned (two decimals, not renormalised), so a column may not sum to 1.

License

MIT — see LICENSE.