SSIG MCP

SSIG is a local MCP-to-browser signing bridge for EVM, Solana, and Sui. An agent can prepare and simulate a transaction, but it cannot access a private key or approve the wallet prompt. You inspect the request in a retro terminal UI and sign with an extension already installed in your browser.

What it does

• Exposes separate MCP tools for EVM, Solana, and Sui transaction requests.
• Requires an ASCII-only agent explanation for every request.
• Automatically attempts a dry run before adding the request to the queue.
• Serves an authenticated approval UI on loopback by default, with explicit RFC1918 LAN mode.
• Discovers EIP-6963/EIP-1193 EVM wallets, Solana Wallet Standard wallets, and Sui Wallet Standard wallets.
• Supports sign and sign-and-submit modes.
• Persists request state, never keys, in ~/.ssig/requests.json with owner-only permissions.

The UI keeps three pieces of evidence separate:

1. The agent's ASCII explanation (explicitly marked untrusted).
2. The dry-run output.
3. Decoded and exact serialized transaction data.

The browser wallet's own confirmation screen is the final authority.

Install and build

Requirements: Node.js 20 or newer and a compatible browser wallet extension.

npm install
npm run build
npm test

Run it manually with:

npm start

The process prints the terminal URL to stderr. MCP itself uses stdin/stdout, so an MCP client will normally start the process for you.

MCP configuration

Add a local stdio server to your MCP client, using the absolute path to the built entrypoint:

{
  "mcpServers": {
    "ssig": {
      "command": "node",
      "args": ["/absolute/path/to/ssig/dist/server/server/index.js"],
      "env": {
        "SSIG_PORT": "3721"
      }
    }
  }
}

The available tools are:

• request_evm_transaction
• request_solana_transaction
• request_sui_transaction
• get_transaction_request
• list_transaction_requests
• cancel_transaction_request

The three request tools return an approvalUrl. Open that URL in the browser profile containing your wallet extension.

Required ASCII explanation

asciiExplanation rejects non-ASCII characters. The agent should name amounts, assets, recipients, expected state changes, and material risk. For example:

+------------------- SWAP --------------------+
| INPUT  : 100.00 USDC                       |
| ROUTE  : USDC -> WETH via 0xRouter...      |
| MIN OUT: 0.031 WETH                        |
| SIGNER : 0x1234...abcd                     |
| RESULT : USDC decreases; WETH increases    |
| RISK   : slippage, malicious router, MEV   |
+---------------------------------------------+

This explanation is context, not proof. Compare it with the decoded payload, dry run, and extension preview.

Transaction formats

EVM

Pass chainId, an optional expectedSigner, and an EIP-1193 transaction object. Quantities must use canonical JSON-RPC hex (0x0, 0x1, and so on). SSIG checks the active chain and never calls wallet_addEthereumChain; unknown networks must be added by you through a trusted wallet flow.

sign-and-submit uses eth_sendTransaction. sign uses eth_signTransaction, which many consumer wallets intentionally do not support.

Solana

Pass a Wallet Standard chain identifier and a base64-encoded wire transaction:

• solana:mainnet
• solana:devnet
• solana:testnet
• solana:localnet

The transaction can be unsigned or partially signed. The extension receives the exact decoded bytes through solana:signTransaction or solana:signAndSendTransaction.

Sui

Pass a Sui Wallet Standard chain identifier and base64-encoded BCS TransactionData bytes:

• sui:mainnet
• sui:testnet
• sui:devnet
• sui:localnet

SSIG uses Mysten's dApp Kit compatibility layer, including modern and legacy Sui Wallet Standard features.

Dry-run configuration

Simulation destinations are configured by the server owner, never by an MCP tool argument. This prevents an agent from turning the bridge into an arbitrary network client.

EVM

Tenderly is preferred when all three settings are present:

SSIG_TENDERLY_ACCOUNT=my-account
SSIG_TENDERLY_PROJECT=my-project
SSIG_TENDERLY_ACCESS_KEY=secret

If Tenderly is unavailable or fails, SSIG can fall back to eth_call plus eth_estimateGas through a chain-ID map:

SSIG_EVM_RPC_URLS='{"1":"https://ethereum.example/rpc","8453":"https://base.example/rpc"}'

Tenderly needs expectedSigner because the sender is required for an accurate simulation.

Solana

SSIG calls the native simulateTransaction RPC with signature verification disabled and recent-blockhash replacement enabled:

SSIG_SOLANA_RPC_URLS='{"solana:mainnet":"https://solana.example/rpc","solana:devnet":"https://api.devnet.solana.com"}'

Sui

SSIG calls the current Sui Core API simulateTransaction over gRPC:

SSIG_SUI_GRPC_URLS='{"sui:mainnet":"https://fullnode.mainnet.sui.io:443","sui:testnet":"https://fullnode.testnet.sui.io:443"}'

Simulation policy

By default, an unavailable simulator is shown prominently but does not prevent signing. A simulation that executes and reports failure is blocked by default.

# Refuse requests when simulation is unavailable or errors.
SSIG_REQUIRE_SIMULATION=true

# Allow the user to inspect and sign even after a simulated execution failure.
SSIG_BLOCK_FAILED_SIMULATION=false

Simulation is point-in-time and provider-dependent. It cannot guarantee later execution, protect against every state change, or prove the agent's explanation is accurate. Sending a transaction to a simulation provider also reveals its contents to that provider.

Other configuration

Trusted-LAN access

Loopback is the secure default. To open the approval terminal from another machine on the same trusted private network, opt in explicitly and advertise this machine's RFC1918 address:

SSIG_HOST=0.0.0.0 \
SSIG_ALLOW_LAN=true \
SSIG_PUBLIC_HOST=192.168.1.50 \
npm start

LAN mode continues to require the random UI bearer token and accepts only loopback or the exact SSIG_PUBLIC_HOST value in the HTTP Host header. It uses plain HTTP, so use only a trusted LAN; on shared or hostile networks, keep loopback mode and use an SSH port-forward instead.

Variable	Default	Purpose
SSIG_HOST	127.0.0.1	Bind address. LAN mode accepts 0.0.0.0 or a private IPv4 address.
SSIG_ALLOW_LAN	false	Required explicit opt-in for non-loopback binding.
SSIG_PUBLIC_HOST	—	Exact RFC1918 address advertised and allowed in LAN mode.
SSIG_PORT	3721	UI/API port. Use 0 to choose a free port.
SSIG_DATA_DIR	~/.ssig	Persistent request-state directory.
SSIG_REQUEST_LIMIT	1000	Maximum retained requests; pending requests are never pruned.
SSIG_WEB_DIR	bundled dist/web	Override the static browser build location.

Security boundaries

• SSIG binds only to loopback by default. LAN mode is explicit, RFC1918-only, and allowlists one exact Host value to reduce DNS-rebinding risk.
• The approval API uses a random 256-bit bearer token. The browser stores it in session storage and immediately removes it from the URL.
• API responses are no-store, the UI cannot be framed, and a restrictive Content Security Policy is applied.
• The server validates the connected wallet against expectedSigner again when recording completion.
• RPC URLs come only from administrator environment variables.
• The MCP never receives seed phrases, private keys, or extension secrets.
• An agent can still propose a malicious transaction or misleading ASCII explanation. Always read the extension preview and use small-value test transactions first.

Development

npm run typecheck
npm test
npm run build

The browser interface is visually inspired by AnderShell 3000. See NOTICE for attribution.