Latch

Standing spend rules for the house.

Alexa+ filled a $250 cart and added things people excluded. A yes on checkout does not stop the next restock. Latch is the rule that stays after the chat dies.

Grant a cap. It restocks inside that cap. A price jump holds. You undo until 9pm. You revoke, and it stops.

Not a shopping chatbot. Not “confirm this one order.” Not a booking receipt.

Amazon Developer Hackathon · Alexa+ track · 23 Oct 2026. Product name is Latch.

Demo (about three minutes)

1. Grant: Hill’s Science Diet 15lb, max $40, fire at 3 days left, hold if price jumps 15 percent, undo until 9pm. Card shows LCH-14.
2. Time-skip to 3 days. It buys at $28. Card shows LED-8821 and mock order ORD-440.
3. Kill the tab. “What did you buy?” Same IDs.
4. Mutate the fixture to $33. Trigger. Hold. No charge.
5. Undo the $28 buy. Ledger is undone.
6. Revoke. Next trigger is denied.

Speech is illegal without entry_id and provider_ref. Empty ledger → it says it failed.

Local: npm run dev then open the printed URL. Buttons on the left are the tape.

Golden cases

#	Case	Pass
1	Grant then trigger → commit under cap	committed + LED- + ORD-
2	Excluded SKU (Sony under a Hill’s rule)	denied / excluded_sku
3	Price +18%	held / price_jump
4	Undo before 9pm	undone
5	Undo after deadline	rejected
6	Revoke then trigger	denied / rule_revoked
7	New session	same entry_id
8	Empty ledger	cannot say “bought”
9	Qty 2 when max 1	denied / qty_exceeded
10	Clock advance, no grant	no buy

npm test

Protocol

• MCP 2025-11-25 Streamable HTTP at POST /mcp
• Tools: grant_latch, get_latch, run_triggers, undo_entry, revoke_latch
• Demo-gated: advance_clock, set_catalog_price
• Cards: /apps/grant.html, /apps/hold.html, /apps/ledger.html (resourceUri on mutating results)
• State: D1 rules + ledger. Catalog: KV fixture hills-15lb / sony-headphones
• Household: X-Latch-Household. No MCP session id required.
• Privacy / terms: /privacy, /terms

Friction Amazon asked for: FRICTION.md.

Setup

Needs Node and Wrangler. No Token Factory key. No model.

npm install
npx wrangler types
npx wrangler d1 migrations apply latch --local
npm test
npm run dev

Deploy only when you want a public URL:

npx wrangler d1 create latch
npx wrangler kv namespace create CATALOG
# put the real ids in wrangler.jsonc, then:
npx wrangler types
npx wrangler d1 migrations apply latch --remote
npx wrangler deploy

Custom hostname later: mcp.latch.<your-domain>.

What this is not

• No runtime LLM
• No real Amazon cart
• No VoiceCart confirmed=true checkout
• No restaurant booking
• No Alexa or Amazon in the product name

Law: PRODUCT.md. Design: DESIGN.md.