Co-signer flow — adds OUR Ledger signature to a multi-sig PSBT produced by an external initiator (Sparrow / Specter / Caravan / a peer running this server). Looks up the registered wallet by name, decodes the PSBT, validates every input carries a bip32_derivation entry for our master fingerprint ...
AI agents call sign_btc_multisig_psbt to retrieve information from VaultPilot MCP without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
| Parameter | Type | Required | Description |
|---|---|---|---|
psbtBase64 | string | Yes | Base64-encoded PSBT v0 from the initiator. Every input must carry a `bip32_derivation` entry for our master fingerprint, or we refuse to forward to the device. |
walletName | string | Yes | Name of a previously-registered multi-sig wallet (matches the `name` passed to `register_btc_multisig_wallet`). Refused if no wallet is registered under this na |
Parameters from the server's own tool schema.
Even though sign_btc_multisig_psbt only reads data, uncontrolled read access leaks sensitive information and racks up API costs — an agent caught in a retry loop can make thousands of calls a minute without anyone noticing.
Attacks that exploit this kind of access
Co-signer flow — adds OUR Ledger signature to a multi-sig PSBT produced by an external initiator (Sparrow / Specter / Caravan / a peer running this server). Looks up the registered wallet by name, decodes the PSBT, validates every input carries a bip32_derivation entry for our master fingerprint (defense against being tricked into signing for a foreign tx), forwards to the Ledger device for the on-device output walkthrough (the user MUST verify every output address + amount on-device matches the chat-side verification block before approving), splices our partial signature(s) into the PSBT, returns the partial PSBT for the user to share back to the coordinator. We do NOT finalize or broadcast — that's the initiator's job once they have all M signatures. Phase 2 scope: P2WSH wallets registered via register_btc_multisig_wallet. It is categorised as a Read tool in the VaultPilot MCP MCP Server, which means it retrieves data without modifying state.
sign_btc_multisig_psbt accepts 2 parameters: psbtBase64, walletName. Required: psbtBase64, walletName. The full parameter table on this page comes from the server's own tool schema.
Register the VaultPilot MCP server in PolicyLayer and add a rule for sign_btc_multisig_psbt: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches VaultPilot MCP. Nothing to install.
sign_btc_multisig_psbt is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the sign_btc_multisig_psbt rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for sign_btc_multisig_psbt. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
sign_btc_multisig_psbt is provided by the VaultPilot MCP server (vaultpilot-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.