swap — EMC cashier (USDT → EMC)

A minimal cashier exposing one primitive. All business logic stays in the calling services; swap knows nothing about NVS/DNS/subscriptions.

buy_emc(amount_usdt, destination_emc_address, callback_url, ref)
  → collect USDT on a unique deposit address
  → on confirmation, deliver EMC (fixed rate ×10) to destination
  → notify the caller with a SIGNED callback

destination is opaque to swap: it can be a service's address (the service then renders its own product on that EMC — the user only ever pays USDT and never touches a wallet) or the user's own address (raw on-ramp).

Use via MCP

An AI agent with USDT can buy EMC directly — no account, no API key, no callback. swap exposes a keyless MCP exchanger over Streamable HTTP at:

https://swap.emercoin.com/mcp

Add it to a client:

# Claude Code
claude mcp add --transport http swap https://swap.emercoin.com/mcp

• Claude Desktop — Settings → Connectors → Add custom connector → the URL above.
• MCP Inspector — npx @modelcontextprotocol/inspector → Streamable HTTP → the URL.

Tools:

Tool	What it does
get_swap_config	min/max USDT per order + the fixed EMC-per-USDT rate
buy_emc	open an order → returns a deposit address + the exact USDT amount to send (+ optional idempotency_key)
get_order_status	poll an order by its token until delivered
cancel_order	drop an unpaid order early

Flow: get_swap_config → buy_emc(amount, your_emc_address) → send the exact returned amount (TRC20) to the deposit address → poll get_order_status by token until notified. One-way, exact-amount, no refunds. The same primitive is also available as keyed REST (for services, with a signed callback) and a keyless web page at swap.emercoin.com.

Locked decisions

Topic	Decision
Rate	static 1 USDT = 10 EMC
Cap / floor	5–10 USDT (floor 5: below it TRON gas dominates)
USDT rail	TRC20 (TRON)
Payment match	one shared deposit address + unique per-order amount tag
EMC delivery	via emercoin adapter POST /wallet/send (X-Internal-Key)
Callback signature	HMAC-SHA256 over canonical body, per-service secret
KYC	none (amounts far below threshold)
AML	minimal but mandatory — OFAC SDN + Tether freeze blacklist
Terms	exact amount, single transfer, one-way (no refunds) — state in the offer

Layout

swap/
  config.py        env-driven settings (pydantic-settings)
  models.py        OrderStatus enum + request/response schemas
  states.py        order state machine (allowed transitions)
  schema.sql       DDL: services/orders/deposits/aml_checks/sweeps/callbacks
  db.py            SQLite connection + init
  repository.py    DB access layer
  auth.py          caller auth by API key
  orders.py        buy_emc business logic (shared by REST + MCP)
  main.py          FastAPI app (REST: POST /buy_emc, GET /order/{id})
  mcp_app.py       keyless MCP exchanger at /mcp (agent tools, mirrors /web)
  web.py           public keyless /web/* channel (raw on-ramp for humans)
  site/            static exchanger page + offer (index.html, oferta.html)
  clients/
    adapter.py     EMC delivery + balance via emercoin adapter
    trongrid.py    TRC20 deposit watcher source (TronGrid)
  tron/
    hd.py          HD derivation of deposit addresses (BIP44, coin 195)
  services/
    aml.py         OFAC + Tether blacklist screening
    delivery.py    deliver EMC from reserve (idempotent)
    callback.py    signed callback notifier + retries
    watcher.py     background loop: deposits → confirm → AML → deliver → notify
    sweep.py       USDT consolidation from deposit addresses

State machine

created → awaiting_payment → confirmed → emc_delivered → notified (done)
                                ↘ underpaid    (top-up or partial refund)
                                ↘ overpaid     (refund excess)
                                ↘ aml_hold     (sender blacklisted → manual)
                                ↘ deliver_failed (retry; else refund USDT)
expired — no payment before TTL

Dev

uv sync --extra dev
cp .env.example .env          # fill secrets
uv run uvicorn swap.main:app --reload --port 8002

EMC delivery and the TRON watcher need the emercoin adapter and TronGrid creds; for local end-to-end you can bring up the node+adapter from emercoin_docker (docker compose --profile dev up). TRON parts are verified in a test environment before they are wired into the watcher.

Schema changes have no migrations. db.py applies schema.sql with CREATE TABLE IF NOT EXISTS, which does not alter an existing table. After editing schema.sql in dev, reset the database: rm swap.db (then restart — it recreates the schema — and re-run scripts/register_service since the services table is wiped too). swap.db holds only local/test data.

Status: full happy path verified live end-to-end (+ 41 unit tests). On 2026-06-14 a real run took a TRC20 USDT deposit on TRON Nile testnet → confirmed → delivered real EMC on Emercoin mainnet (via the adapter /wallet/send) → signed callback verified by the receiver against the service's HMAC secret: awaiting_payment → confirmed → emc_delivered → notified. See docs/TESTNET.md for the runbook (scripts/testnet/).

AML is live: OFAC SDN addresses (TRON) loaded into memory + refreshed, and a per-deposit live Tether isBlackListed check; a hit → aml_hold (no delivery).

Payment matching: pivoted from a unique HD address per order to one shared deposit address + a unique per-order amount tag (matched by exact amount). This removes per-order sweeping and the fresh-address gas penalty; the trade-off is exact-amount, single-transfer payments (no auto under/overpaid). Collected USDT is moved to treasury / off-ramp manually at low volume. MCP exchanger: the keyless web on-ramp is also exposed as MCP tools (buy_emc, get_order_status, cancel_order, get_swap_config) over Streamable HTTP at /mcp, so an AI agent buys EMC with USDT directly — no key, no callback, same anti-spam as the web channel. No auth by design: it's a public "pay for a service" on-ramp, not an account. Tool definitions pass a pre-publication TDQS review (all tier A); see docs/TDQS.md.

Deferred: USDT sweep / TRON tx signing (services/sweep.py, kept off the hot path).

License

MIT — see LICENSE.