Countersign

A neutral, cross-vendor control plane for AI agents that spend money. Countersign holds the policy, the freeze, and the audit ledger across multiple agent-wallet backends at once — the one thing no single wallet vendor can do, because each only governs its own rail. That aggregation is the moat.

Live version of this loop: countersign.network/demo.html · 60s video

One falsifiable test defines it: can Countersign freeze agents across many backends at once, in under a second, with a unified tamper-evident ledger of every attempt? Proven LIVE across four rails (Coinbase, Turnkey, Openfort, and a Lithic Visa card) in ~432ms on testnet.

This repository is the open-core front door — the Apache-2.0 packages you build against: the integration contract, the typed client, the MCP tools, and the x402 guard. The control-plane "brain" (the policy compiler, the hash-chained ledger, the vendor adapters, and the hosted Core) is separate and proprietary; you reach it over the network via the SDK/MCP, hosted at app.countersign.network.

Quickstart

Drop the kill switch + spend guard into any MCP client (Claude, Cursor, …) — one line:

// claude / cursor mcp config
{ "mcpServers": { "countersign": {
  "command": "npx", "args": ["-y", "@countersign/mcp"],
  "env": { "COUNTERSIGN_URL": "https://app.countersign.network", "COUNTERSIGN_API_KEY": "csk_…" }
}}}

Or wire it into your own agent with the SDK:

import { CountersignClient } from "@countersign/sdk";
const cs = new CountersignClient({ baseUrl, apiKey });

await cs.evaluate({ agentId, amount, asset, venue }); // may this spend happen? (allow / deny / needs_approval)
await cs.freeze();                                     // the kill switch — every backend, < 1s

Get a free testnet key at https://app.countersign.network/start?ref=gh-readme.

Agents paying agents? See examples/guarded-payee — the A2A/AP2 pattern where a payee advertises it is governed and the payer verifies that (and guards its own payment) before any mandate is signed.

Packages (this repo — all Apache-2.0)

Package	Role
@countersign/core	the EnforcementProvider interface, branded ids, the unified policy schema, the fail-closed freeze controller — the integration contract every backend implements
@countersign/api-contract	OpenAPI + typed REST/ws schema — the single source of truth for the Client↔Core wire interface
@countersign/sdk	typed client over the Core API + live ledger subscribe
@countersign/mcp	Countersign as MCP tools — kill switch + spend guard inside any MCP client
@countersign/x402	govern x402 (HTTP-402 machine payments) — guard a payment before it pays
@countersign/verify	verify a ledger entry offline — hash chain, RFC 6962 Merkle inclusion, Ed25519 signatures
@countersign/ap2	govern AP2 (Agent Payments Protocol) — guard an agent-payment mandate before it executes

The proprietary brain (policy compiler to each backend's native controls, ledger, Coinbase / Turnkey / Openfort / Lithic adapters, the hosted Core) lives in a separate private repository.

Prime directives (invariants)

1. Don't build cryptography — integrate vendor MPC/TEE; session keys, never master keys.
2. Build the layer above the wallets; cross-vendor aggregation is the product.
3. Fail-closed: no decision / no backend response ⇒ the transaction does not execute.
4. Backend-agnostic core; no vendor logic leaks past the EnforcementProvider interface.
5. Append-only, hash-chained ledger is the source of truth.
6. Testnet only — mainnet follows a third-party security audit.

Links

• Home: https://countersign.network · Hosted Core: https://app.countersign.network
• npm: @countersign/sdk · @countersign/mcp · @countersign/x402 · @countersign/ap2
• Architecture: docs/architecture.md · Security: SECURITY.md

Apache-2.0. Countersign holds policy, freeze, and a tamper-evident ledger — it never takes custody of funds.