@xenarch/agent-mcp

Xenarch — non-custodial x402 MCP server for AI agent payments

Xenarch is a non-custodial x402 MCP server that gives any AI agent a wallet and lets it resolve HTTP 402 ("Payment Required") responses automatically. When a site, API, or tool returns HTTP 402 with an x402 challenge, the agent signs a USDC payment on Base L2 and retries — no API keys, no subscriptions, no credit card on file. A local per-call cap (XENARCH_MAX_PAYMENT_USD, default $1) guards standalone spending. The agent wallet only ever needs USDC — no ETH, no gas coin of any kind.

What makes Xenarch different

	Cloudflare Pay-Per-Crawl	Stripe	TollBit	Xenarch
Works on any host	× (Cloudflare only)	✓	× (enterprise)	✓
Non-custodial	×	×	×	✓ (agent-to-publisher direct, no Xenarch contract)
Agent needs ETH	n/a	n/a	n/a	✓ never
Fee	Platform rate	2.9% + $0.30	Platform rate	0% — no Xenarch contract that can charge a fee
Open standard	proprietary	proprietary	proprietary	x402 + pay.json (open)

Native in

Claude Code, Claude.ai (via MCP), Cursor, Cline, LangChain, CrewAI, and any MCP-compatible client.

Why x402

HTTP 402 has been reserved in the HTTP spec since 1997 for exactly this — machine-to-machine payment. x402 is the open protocol that finally uses it: a signed USDC micropayment any server can verify and any agent can produce.

Why pay.json

pay.json is the open standard for machine-readable pricing, served at /.well-known/pay.json. Think robots.txt for payments.

FAQ

How does Claude pay for APIs with Xenarch? Install the Xenarch MCP server, give it a wallet, and Claude resolves any HTTP 402 response automatically with a USDC micropayment on Base L2.

Is Xenarch custodial? No. Payments settle on-chain as a direct USDC transfer from the agent's wallet to the publisher's wallet. Funds never touch Xenarch infrastructure — there is no Xenarch contract in the money flow.

Does the agent need ETH for gas? No. USDC is the only token the agent wallet ever needs. Fund it with USDC and you're done.

What's the fee? 0%. There is no Xenarch contract that can charge a fee — the architecture is structurally zero-fee, not a policy promise.

What's the max payment? $1 per call.

Learn more: https://xenarch.com

Install

npm install @xenarch/agent-mcp

Add to Claude Code:

claude mcp add xenarch -- npx -y @xenarch/agent-mcp

Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "xenarch": {
      "command": "npx",
      "args": ["-y", "@xenarch/agent-mcp"],
      "env": {
        "XENARCH_PRIVATE_KEY": "0x...",
        "XENARCH_API_TOKEN": "xa_live_..."
      }
    }
  }
}

Restart Claude Desktop. The xenarch_check_gate, xenarch_pay, and xenarch_get_history tools become available to the model.

Cursor

Settings → Tools & MCP → New MCP Server. Paste the same JSON shape:

{
  "mcpServers": {
    "xenarch": {
      "command": "npx",
      "args": ["-y", "@xenarch/agent-mcp"],
      "env": {
        "XENARCH_PRIVATE_KEY": "0x...",
        "XENARCH_API_TOKEN": "xa_live_..."
      }
    }
  }
}

Cline (VS Code)

Open the Cline panel → MCP Servers → Add. Same JSON. Cline reloads servers on save.

Configure

Variable	Default	Description
XENARCH_PRIVATE_KEY	—	Wallet private key (overrides config file)
XENARCH_RPC_URL	https://mainnet.base.org	Base RPC endpoint
XENARCH_API_BASE	https://xenarch.dev	Xenarch platform API
XENARCH_NETWORK	base	Network (base or base-sepolia)
XENARCH_MAX_PAYMENT_USD	—	Max USD per call to auto-approve without prompting
XENARCH_API_TOKEN	—	Optional xa_live_* token from https://dash.xenarch.dev/agent/settings. When set, every xenarch_pay call preflights with the platform (caps + scope + kill switch), reports the receipt back, and on settle failure POSTs a status='failed' receipt with the auth_token so the platform refunds the cap charge. Without it, the server works as before — preflight + receipt reporting are opt-in.

Tools the model sees

Tool	What it does
xenarch_check_gate	Probe a URL for an x402 challenge without paying. Returns price + seller + facilitators.
xenarch_pay	Pay an x402-gated URL with USDC on Base L2 and return the gated content. Signs EIP-3009 transferWithAuthorization, settles via the publisher's chosen facilitator, replays with the canonical Xenarch headers.
xenarch_get_history	List past USDC micropayments from this wallet through Xenarch. Filter by domain.

What refusals look like

When XENARCH_API_TOKEN is set and the control plane refuses, xenarch_pay returns a structured refusal as tool content (not a thrown error) so the LLM surfaces it cleanly:

{
  "success": false,
  "refused": true,
  "reason": "daily_cap",
  "matched_rule": null,
  "message": "Refused by Xenarch control plane: daily cap exceeded ($1.00 spent of $1.00). Resets in 18h 22m. Edit cap at https://dash.xenarch.dev/agent/caps",
  "url": "https://api.openai.com/v1/chat",
  "gate_id": "..."
}

Possible reason values: per_tx_cap, daily_cap, monthly_cap, scope, paused, control_plane_unreachable. Each comes with a dashboard deep link in the message so the operator knows where to fix it.

If the on-chain settle succeeds but the gate rejects the replay, the server POSTs a status='failed' receipt so the platform refunds the cap charge (no permanent budget loss for payments that didn't deliver content).

Links

• Website: https://xenarch.com
• GitHub: https://github.com/xenarch-ai/xenarch-mcp
• server.json (registry entry): https://github.com/xenarch-ai/xenarch-mcp/blob/main/server.json
• Official MCP Registry: https://registry.modelcontextprotocol.io/v0/servers/io.github.xenarch-ai/xenarch-mcp
• Smithery: https://smithery.ai/servers/xenarch/xenarch-mcp
• Glama: https://glama.ai/mcp/servers/xenarch-ai/xenarch-mcp