---
title: "WATCHER: get a signed callback the moment a crypto address..."
method: POST
path: "/api/watchers/crypto-address-activity"
tags: ["watchers"]
---

# WATCHER: get a signed callback the moment a crypto address...

`POST /api/watchers/crypto-address-activity`

## Request

- Base URL: `https://2s.io`
- URL: `https://2s.io/api/watchers/crypto-address-activity`
- Auth: API key in header `PAYMENT-SIGNATURE`

WATCHER: get a signed callback the moment a crypto address transacts. Arm once, pay once (no account, no API key) — we watch Base, Ethereum, or Bitcoin and POST your custom payload to callbackUrl when the address sends/receives native coins, ERC-20s, or ERC-721s. Filter by direction (in/out/both), asset type, and a USD minimum (minValueUsd) to skip dust. Bounded by a 30-day / 25-fire window. Deliveries are EIP-191-signed by our published key (verify offline) and retried with exponential backoff; missed pushes are recoverable via watchers.status. Returns a watcherId.

## Request body

- object
  - `chain` 'base' | 'ethereum' | 'bitcoin', required: Which chain to watch: base, ethereum, or bitcoin.
  - `address` string, required: The address to watch — an EVM address (0x… 40 hex) for base/ethereum, or a Bitcoin address (bc1…/1…/3…).
  - `callbackUrl` string, uri, required: Where we POST the event. Any http(s) URL; the JSON body is signed (verify with X-2s-Signature). e.g. https://your-agent.app/hooks/wallet
  - `direction` 'in' | 'out' | 'both': Which flows to fire on: "in" = address received, "out" = address sent, "both" = either. Default: both.
  - `assetTypes` string[]: Limit to asset kinds: any of "native" (ETH/BTC), "erc20" (tokens), "erc721" (NFTs). Omit to watch all. EVM only — Bitcoin is always native. e.g. ["erc20"] for token transfers only.
  - `minValueUsd` number: Only fire when the transfer is worth at least this many USD — filters out dust/spam. e.g. 100 ignores anything under $100. Omit to fire on any non-zero amount.
  - `payload` object: Arbitrary JSON, echoed back verbatim in every callback so you can route/identify the event on your side. e.g. {"agentId":"a1","reason":"treasury-monitor"}
  - `expiresInSeconds` integer: How long the watch stays active, in seconds. Default 2592000 (30 days), max 7776000 (90 days). e.g. 86400 = 1 day.
  - `maxFires` integer: Stop after this many callbacks. Default 25, max 1000. The watch ends at whichever comes first — maxFires or expiry.
  - `label` string: Optional free-text tag to recognize this watcher later. e.g. "exchange-cold-wallet".

## Other responses

- `200` (unresolved $ref)
- `400`: Invalid request — not charged. error.code: BAD_REQUEST or INVALID_PARAM (parameter validation; error.details.issues lists each problem), MALFORMED_JSON, or a request-specific code. Other client errors: 405 METHOD_NOT_ALLOWED (wrong verb; see the Allow header), 413 PAYLOAD_TOO_LARGE.
- `402`: Payment required. Body is the x402 PaymentRequirements envelope with a multi-network accepts array; the per-call price is in accepts[].amount (and on the operation as x-2s-price). Sign for whichever rail you hold USDC on (EIP-3009 for Base, partial SPL transfer for Solana) and retry with the PAYMENT-SIGNATURE header (X-PAYMENT also accepted for v1 clients). Also PAYMENT_SETTLE_FAILED (error envelope) when a verified payment failed to settle: error.details.chargeStatus is "not_charged" (retry with a fresh payment) or "unknown" (the authorization may still settle before its deadline — check it before retrying).
- `404`: Not found — NOT_FOUND (or NO_SOURCES / NO_OPINIONS): the requested entity does not exist. Not charged.
- `429`: Too many requests — RATE_LIMITED: our per-IP / per-payer limiter. Not charged; retry after Retry-After.
- `5XX`: Server-side failure. 500 INTERNAL: unexpected error; quote error.errorId (also the X-2s-Error-Id header). 502 UPSTREAM_UNAVAILABLE (or another UPSTREAM_* code): the data provider failed — composite endpoints refuse unbilled when every source fails. 503 SERVICE_PAUSED: endpoint paused by the operator kill switch, retry after Retry-After; 503 SPEND_CAP: daily upstream budget exhausted. 504 UPSTREAM_TIMEOUT. Not charged.

---

[API](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api.md) · [All operations](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api/llms.txt) · [OpenAPI document](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api/revisions/da4c5caf686e?raw)