restock

Inspiration

Every shared apartment has the same group chat. "We're out of toilet paper." "Can someone grab dish soap?" "Who bought the paper towels last time?" Then, three days later, the awkward one: "Hey, you owe me $14.62 for the trash bags"

Restocking a household is a shopping problem, a coordination problem, and a money problem all at once, and we handle it today with three separate apps: a group chat to notice, a store app to buy, and Venmo or Splitwise to chase people afterwards. Someone always ends up fronting the money, and someone always forgets to pay them back.

We wanted texting itself to do the shopping. You text like you already do, "we're out of TP", and the household's cart fills itself with the best deal Restock can find across stores, all without leaving iMessage. Anyone texts "order now", and the order is placed and paid fairly at checkout, so nobody fronts money and nobody has to chase anyone.

What it does

Restock is an AI assistant that lives in your household's iMessage thread.

Text it like a roommate. "im out of dish soap and we need TP" becomes two cart items. It understands casual phrasing, typos, and "for me and Jake".
It finds the best deal, without leaving iMessage. No price-comparison tabs, no store apps. Each request gets up to three options with live prices from major retailers, ranked by price per unit, right in the thread, and your household's usual pick stays at the top. You reply 1, 2 or 3.
Shared vs. personal is handled for you. Toilet paper is split across everyone. The air fryer Jake added goes to Jake alone. Every decision is shown in the reply ("Added dish soap, shared, $8.49. Reply 'personal' to change"), so anyone can correct it.
"order now" checks out and splits the bill. Members with a card on file are charged their own share. Members without one are covered by the owner's card and get a private "you owe Sam $10.15" text with a one-tap pay-back link. An order never gets stuck waiting on someone's card.
One person, many households. Roommates, family, a soccer team: switch between them by text, or manage them from a web dashboard.

How we built it

Messaging: Photon 2.0 (iMessage). Each person gets their own Restock number on Photon's shared lines. Group news like "Jake added chips" or "order placed" is relayed to every member's thread.
AI layer: the Claude API. Claude parses intents, matches items to products, answers questions, and picks the best deals from live listings. Every state-changing output is validated against a schema before we act on it. If the model fails or returns something malformed, a deterministic command parser (add …, order now) takes over, so the demo never depends on the LLM being up.
Live prices. SerpApi's Google Shopping results are filtered to major retailers, checked against what the person asked for, and ranked by unit price in code. Store listings are treated as untrusted input, and the model can only reply with listing numbers.
Money: Stripe test mode. Cards are added only through Stripe-hosted Checkout in setup mode. Each person's share is an off-session PaymentIntent on their own card, held first and captured only once every share is authorized, and pay-back uses Checkout payment links. Restock stores only Stripe IDs and the last 4 digits.
Backend: Node + TypeScript (strict), Express, SQLite persistence, deployed on Railway at restock.select. The web pages (sign-up, invite links, dashboard, pay-back) are mobile-first, because they open from a text.
Responsible AI, audited. We ran a VEYZL audit on every pull request. Its findings changed what we shipped: one led us to redact personal data and Stripe IDs from every outgoing message, and another made the reply to a pasted card number fixed text with no links in it. Each fix is tagged in git history and logged in docs/veyzl.md.

Trust by design

No card numbers, ever. A Luhn-check guard runs on every incoming message before the AI or the database sees it. A pasted card number is dropped, and the sender is told privately how to add a card safely.
The AI never moves money. Only an explicit "order now" from a household member places an order, and only for the cart everyone can see. Expensive personal items wait for an approver's "yes".
Undo by text. Anything the AI decided can be changed with a reply.
Honest about being an AI. Ask "are you a bot?" and it tells you, with or without the LLM.

Challenges we ran into

We had to pivot away from the group chat. Our design put Restock inside the household's iMessage group chat, but adding an agent to a group chat needs Photon's paid plan, a $250 subscription. On the free plan's shared lines, the bot also can't text first: it can only reply to people who have messaged it. We built sign-up links and invite links that give each person their own Restock number, plus a relay that keeps every member's thread in sync. Private messages to someone who hasn't texted yet are queued until they do.
Money has to be exactly right under retries. Webhooks get delivered twice, servers restart mid-order, and people double-tap. We made charges and webhook handlers idempotent so a retry can never charge anyone twice, and wrote tests for it, including across restarts.
Webhooks fail silently. In testing, a card saved on Stripe's page sometimes never reached us. We now confirm the card directly with Stripe when you return from checkout and keep the webhook as a backup.
One person, many households. Once a phone could belong to several households, every incoming text became a question: which cart is this for? We added an "active household", a picker when it's ambiguous, and one card per person, so a card added in one household works in all of them.

What we learned

Put the LLM where it's strong and keep it away from where it's dangerous. Claude is excellent at turning messy human text into structured intent. It should never decide who pays, how much, or when a charge happens. Keeping that line strict made both the product and the audit far easier.
Fallbacks are a feature. Designing every AI path with a deterministic fallback meant the app kept working when the model, the store search, or a webhook didn't.

What's next

A shared group chat: move Restock into the household's iMessage group chat on Photon's paid plan. The code already binds a household to a group chat, so it's an upgrade, not a rewrite.
Real merchant checkout (an affiliate or retailer API) in place of simulated orders.
Restock reminders from purchase history: "You usually order paper towels every two weeks. Add them?"
Paying the owner back directly through Stripe Connect, instead of only recording that it was paid.
More channels (SMS, WhatsApp) through Photon, so any group chat can use it.