There is no HTTP transport, no listening port and no call out to any service — signing happens locally with the standard libraries for each chain, which is the right shape for a signer. The consequence worth reasoning about is where the key goes: it arrives as a parameter on the tool call rather than from the environment, so it passes through whatever your client logs, stores or includes in that call. Pair it with the broadcast package when the signed transaction actually needs to reach a chain, and remember that the two x402 paths hand back a signature or a re-serialized transaction rather than a broadcastable hex.
A local transaction signer exposed over MCP. Six tools cover EVM chains, UTXO chains, Tron, XRP, Kaspa and Solana, each taking the transaction and the key as call parameters and returning something ready to broadcast. It speaks stdio only, listens on no port, and makes no HTTP call to Crypto APIs or anywhere else.
- evm_sign covers Ethereum, Ethereum Classic, BSC, Polygon, Avalanche C-Chain, Arbitrum, Base, Optimism and Tron, either from structured transaction fields or from a pre-built unsigned hex
- evm_sign also signs EIP-712 typed data — the x402 gasless path, such as the EIP-3009 TransferWithAuthorization from a buyer's /authorize — returning a signature rather than a transaction
- utxo_sign covers Bitcoin, Bitcoin Cash, Litecoin, Dogecoin, Dash and Zcash, from a prepared inputs-and-outputs object or a raw unsigned hex with input descriptors
- tron_sign signs with secp256k1 and no TronWeb dependency, and xrp_sign handles XRP from structured fields or unsigned hex
- kaspa_sign uses schnorr (BIP340) through kaspa-wasm, mainnet only
- svm_sign partial-signs a Solana x402 payment, adding only the buyer's source-authority signature and leaving the facilitator's feePayer slot to be signed at settlement
- Most tools return signedTransactionHex ready to broadcast; the x402 paths return a signature or a re-serialized transaction instead
Npx @cryptoapis-io/mcp-signer over stdio. No API key and no account. The private key travels as a tool parameter — privateKey, privateKeys or secret — and is deliberately never read from an environment variable. Signing uses ethers for EVM, bitcoinjs-lib with ecpair and tiny-secp256k1 for UTXO, elliptic for Tron and xrpl for XRP. MIT.
One command — npm install @cryptoapis-io/mcp-signer