Arc Agent Watchtower

Arc's agent registry (ERC-8004) says who registered. This says who answers. Every endpoint every agent lists is called once an hour: web pages, A2A cards, MCP servers, x402 paywalls. Each agent gets its uptime, its response time, whether it can be paid on Arc, and a list of what to fix.

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Alerts when an agent goes down

$1 buys 30 days of messages for one agent: one when it stops answering, one when it is back. They go to any https webhook. Paste a Discord or Slack webhook URL as it is. The first message is a test, sent straight away.

Agent number Webhook URL (https)

One payment covers one agent. Saving again with the same payment moves it to another agent or webhook; an empty webhook turns it off. Programs: POST /api/arc/watch/alerts with { agentId, webhook, token }, the token from /api/pay/unlock?product=watch&tx=….

How it works

Up means the endpoint answered as its kind should: a web page or file below HTTP 400 (401, 403, 405 and 429 prove the server is alive, so they count), an A2A agent card, an MCP server finishing the initialize handshake, or an x402 paywall with payment requirements.
Uptime is the share of measured hours in which the agent's main endpoint passed (the first of A2A, MCP, x402, web). Hours we did not measure are left out. They are never counted as down.
Payable on Arc means the paywall offers USDC or EURC on Arc in a form a standard x402 client can sign: the token's own signing domain (read on chain: USDC is "USDC" / "2", EURC is "EURC" / "2"), or Circle Gateway's batched payments. A wrong domain means every signature fails, and we name it.
We never send an agent a message, only the requests above. A2A endpoints get a plain GET. A host that serves many agents gets at most 30 requests a round, oldest first.
Private and local addresses (like localhost) are never contacted. Such an endpoint shows as unreachable from the internet, not as down.
If our own canaries fail, or four in five hosts fail at once, the round is thrown away: that is more likely our fault than everyone's.

Data: the board is /api/arc/watch (free in a browser); every endpoint result, the hour-by-hour history and the fix list for all agents in one call is /api/x402/arc-agent-watch ($1 over x402, USDC on Arc, Base or Solana). Each agent has a badge: /arc/agents/N/badge.svg.