Agent Check: Verify A2A Agent Cards, MCP Servers & x402 Endpoints

A trust check for agent-to-agent work. Before your AI agent delegates a task to another agent, plugs in a remote MCP server or pays an x402 endpoint, run one cheap check and get a clear JSON verdict.

Three checks in one Actor:

1. A2A Agent Card check: fetches /.well-known/agent-card.json (or the legacy agent.json), recognises A2A v1.0 / v0.3 / custom cards, checks required fields, skills, interfaces (HTTPS, reachable), security schemes and Signed Agent Card. Returns a 0-100 score, a grade A-F, issues and warnings.
2. MCP server probe: a real MCP Streamable HTTP handshake (initialize + tools/list). Protocol version, server name and version, tools and their schemas, latency, and OAuth protected-resource metadata when the server requires auth. No tool is ever called.
3. x402 endpoint probe: one unpaid request, then decodes the 402 Payment Required offer (x402 v1 body or v2 header): network, asset (known USDC or not), price in USD, payTo, testnet flag, resource mismatch and red flags. Nothing is paid.

Private / internal addresses are refused. Live checks only.

Use cases

• An orchestrator (CrewAI, LangGraph, AutoGen...) picks a sub-agent: check its A2A card before sending work.
• A coding agent adds a remote MCP server: confirm it is up, speaks MCP and lists the expected tools.
• A buyer agent is about to pay an x402 endpoint: confirm the price, the network, that the asset is real USDC and not a look-alike token.
• An agent marketplace or registry runs a nightly health check on its listings.

Input

Field	What it does
agentUrls	Agent base URLs, domains or direct card URLs to check.
mcpServers	Remote MCP endpoints (Streamable HTTP) to probe.
x402Endpoints	Paid endpoint URLs to inspect before paying (GET).
queries	Advanced: [{"endpoint": "x402_probe", "params": {"url": "...", "method": "POST"}}]
maxQueries	Safety cap on checks per run (default 1000).

Example:

{
  "agentUrls": ["https://sabli.app"],
  "mcpServers": ["https://mcp.deepwiki.com/mcp"],
  "x402Endpoints": ["https://www.x402financialdata.com/producer-prices"]
}

Output

One dataset item per check. Shortened example from a real check:

{
  "endpoint": "a2a_card_check",
  "input": {"url": "https://sabli.app"},
  "ok": true,
  "result": {
    "card_url": "https://sabli.app/.well-known/agent-card.json",
    "found": true,
    "card_flavour": "v0.3",
    "name": "Sabli",
    "skills_count": 21,
    "interfaces": [{"url": "https://sabli.app/api/a2a", "binding": "JSONRPC", "version": "0.3.0"}],
    "auth_schemes": ["sabli_key", "sabli_oauth"],
    "score": 95,
    "grade": "A",
    "verdict": "conformant",
    "warnings": ["card_not_signed (A2A 1.0 Signed Agent Card absent)"]
  },
  "charged_event": "a2a-card-check",
  "price_usd": 0.01
}

x402 probe (shortened): {"is_x402": true, "x402_version": 2, "service_name": "US Producer Price Index (PPI)", "offers": [{"network": "eip155:8453", "asset_is_known_usdc": true, "price_usd": 0.01, "testnet": false}], "verdict": "payable_looks_safe"}

Pricing

Pay per successful check, $0.01 per A2A card, MCP probe or x402 probe. Errors (card not found, host not found, invalid or private URL) are never charged.

Limits (honest)

• A snapshot check: it does not prove the quality of a paid answer or the honesty of the operator.
• No MCP tool call and no A2A message is sent (no side effects); authentication is not attempted.
• A2A card signatures are detected but not cryptographically verified yet.

Also available pay-per-call over x402 (USDC on Base) and MCP: https://gogeta2-u35175.vm.elestio.app/agents/catalog. Contact: contact@novarithm.com

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Want this whole store under your brand? → https://gogeta2-u35175.vm.elestio.app/agents/clones/en/cases/api-store-for-ai-agents/