x402 USDC Payments

MCP server that exposes Carbon & Cashmere's 177 crypto market intelligence endpoints to your AI agent — with automatic x402 USDC micropayments from your wallet.

What you get

When this MCP is installed, your AI agent can call tools like:

• get_v1_news — analyzed crypto news with sentiment + market impact
• get_v1_stablecoins — total market cap, per-coin supply, chain distribution
• get_v1_onchain_btc — Bitcoin on-chain metrics (hash-rate, mempool, miner flows)
• get_v1_bittensor_leaderboard — top-100 Bittensor miners by emission
• get_v1_mantis_realized_accuracy — MANTIS subnet (UID 253) realized accuracy track-record
• get_v1_intel_snapshot — aggregated market intel snapshot
• … plus subscriptions ($10/day, $50/week, $200/month), sponsor tiers ($500–$50000), and 170+ research endpoints priced $0.003 – $0.50 per call

177 paid endpoints total. Default exposes top 100 (curated by revenue + popularity). Set X402_TOOL_LIMIT=177 to expose all incl. per-coin variants.

Each tool invocation triggers a single USDC micropayment from your configured wallet. No subscription. No API key. Pay only for what you use.

Install

Claude Desktop

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

{
  "mcpServers": {
    "carbon-cashmere": {
      "command": "npx",
      "args": ["-y", "@carbon-cashmere/x402-mcp"],
      "env": {
        "EVM_PRIVATE_KEY": "0x_your_evm_private_key_here",
        "SVM_PRIVATE_KEY": "your_base58_solana_private_key_here_optional"
      }
    }
  }
}

Restart Claude Desktop. The top 100 Carbon & Cashmere tools (curated by revenue + popularity) appear in your tool palette.

Cursor / Cline / Continue

Same JSON config, paste into your MCP settings panel.

Wallet setup

Security: use a dedicated spending wallet, not your main wallet. Your EVM_PRIVATE_KEY sits in plaintext in your Claude Desktop config. Any process on the host with read access to that file gets the key. Treat it as a hot wallet you would not mind losing: fund it with the small amount you plan to spend ($10–$50 in USDC), never with your main holdings.

• EVM_PRIVATE_KEY (required for Base / Polygon / Arbitrum / World / Avalanche payments)
  • 0x-prefixed hex string (e.g. 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80)
  • Fund with USDC on Base mainnet (cheapest, fastest, ~$0.0001 gas per call)
  • One-time cost: ~$10 USDC covers 200+ tool calls
  • Hard cap = wallet balance. A runaway agent loop can spend the funded amount but no more; signing is bounded by USDC available on the chain.
• SVM_PRIVATE_KEY (optional, for Solana payments)
  • base58-encoded Solana keypair bytes
  • Only needed if a tool routes Solana-only

Network whitelist. The MCP server only signs payments for networks that appear in the API's 402-response accepts[] array (currently Base, Polygon, Arbitrum, World Chain, Solana, Algorand, Stellar). Requests routed to any other chain are rejected at the signing layer — your key cannot be tricked into signing for an arbitrary network.

Keys never leave your machine. They live only in your Claude Desktop config and the MCP process running locally. Payment signing happens on your device; only the signed EIP-712 / SIWS payload travels to Carbon & Cashmere.

Discovery-only mode (no keys set)

If you start the MCP server without EVM_PRIVATE_KEY and without SVM_PRIVATE_KEY, it runs in discovery-only mode: tool registration works, free endpoints work, paid tool invocations surface a 402 error to the agent. Useful for: evaluating the tool catalog, MCP marketplace introspection (Glama, npm scrapers), or running in a CI/test environment without funding a wallet.

How it works

Claude Desktop          @carbon-cashmere/x402-mcp        api.carbon-cashmere.de
     |                          (your machine)                 (our server)
     |  call get_v1_news        |                                 |
     |------------------------->|  GET /v1/news                   |
     |                          |-------------------------------->|
     |                          |  402 + PAYMENT-REQUIRED         |
     |                          |<--------------------------------|
     |                          |  sign with EVM_PRIVATE_KEY      |
     |                          |  GET /v1/news + PAYMENT-SIG     |
     |                          |-------------------------------->|
     |                          |  200 OK + data                  |
     |                          |<--------------------------------|
     |  formatted data          |                                 |
     |<-------------------------|                                 |

Built on official Coinbase x402 v2 protocol. Settlement happens on Base USDC (or your chosen chain). Typical latency: ~2-3 seconds per call (one-shot payment + data).

Tool count

By default, the top 100 tools are exposed — curated server-side by a scoring that combines actual settlement volume, USD revenue, exploration weighting, and description quality (the catalog re-ranks without requiring an npm re-publish).

X402_TOOL_LIMIT	What you get
50	Minimal footprint — top free + bundles + a few high-revenue endpoints
100 (default)	High-revenue tier — adds $10–$50 subscriptions, sponsor tiers, weekly/daily reports, on-chain bundles
177	Everything — adds per-coin specialized routes (vol-regime/{coin}, hurst/{coin}, trend/{coin}, …). Recommended only when your agent knows which coin to query.

Override:

"env": {
  "X402_TOOL_LIMIT": "177"
}

Why a default cap: too many MCP tools can degrade an agent's tool-selection accuracy. 100 is the empirical sweet spot for Claude 3.5 Sonnet and newer.

Configuration env

Variable	Required	Default	Description
EVM_PRIVATE_KEY	one of	—	0x-prefixed EVM key for Base/Polygon/Arbitrum
SVM_PRIVATE_KEY	one of	—	base58 Solana keypair
X402_API_BASE	no	https://api.carbon-cashmere.de	API base URL
X402_TOOL_LIMIT	no	100	Max tools exposed (1-177)
X402_LOG_LEVEL	no	info	debug | info | warn | error

Security

• Keys never leave your machine. The MCP server runs locally as a child process of Claude Desktop.
• Pay-per-call cap. Each call costs $0.003–$0.50. Even a runaway loop is bounded by wallet balance.
• No off-chain credentials. No API keys. No OAuth. Pure on-chain settlement.
• Open source. Audit the code yourself: github.com/CarbonCashmere/x402-mcp

License

MIT — see LICENSE.

Links

• Carbon & Cashmere API: https://api.carbon-cashmere.de
• x402 protocol: https://github.com/coinbase/x402
• MCP protocol: https://modelcontextprotocol.io