> For the complete documentation index, see [llms.txt](https://quackai.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://quackai.gitbook.io/docs/developer-suite/q402-sign-to-pay-protocol.md).

# Q402 (Sign-to-Pay) Protocol

The Q402 protocol is Quack AI’s gasless payment infrastructure for EVM chains, built on EIP-7702 + EIP-712. It replaces the traditional “approve → transfer → pay” pattern with a single signature flow that is:

* Gasless for users (Q402 relayer sponsors all gas fees)
* Works with any ERC-20 (no token upgrades, no ERC-3009 dependency)
* Policy-aware (spend limits, expiry, whitelists, jurisdictional rules)
* Fully verifiable (on-chain receipts via X-PAYMENT-RESPONSE headers)
* Roadmapped for x402 ecosystem interoperability (future releases)

This system forms the execution backbone of Quack AI’s AI Economy — powering governance automation, agent-to-agent transactions, and RWA settlements across 7 EVM mainnets.

For developers, the entire implementation — including core contracts, SDKs, middleware, and facilitator services — is available here:

* 🔗 GitHub: [github.com/bitgett/Q402-Institutional](https://github.com/bitgett/Q402-Institutional)
* 🔗 Live Platform: <https://q402.quackai.ai>
* 📖 SDK: v1.6.0 · Manifest: v1.6.0
* 📧 Contact: <business@quackai.ai>

### Repository Structure

The Q402 repository is organized into modular packages for flexible integration:

| Directory / Package          | Purpose                                                                          | Tech Stack             |
| ---------------------------- | -------------------------------------------------------------------------------- | ---------------------- |
| /packages/core               | Core SDK and client utilities for creating and signing Q402 payment headers      | TypeScript             |
| /packages/facilitator        | Standalone gas relay and settlement service for executing delegated transactions | Node.js / Docker       |
| /packages/middleware-express | Express.js middleware for instant 402 payment validation                         | TypeScript             |
| /packages/middleware-hono    | Lightweight middleware for Hono-based APIs                                       | TypeScript             |
| /examples                    | Demo apps showing how to integrate Q402 with APIs or dApps across 7 EVM mainnets | JavaScript / Express   |
| /docs                        | Architecture, standards, and deployment guides                                   | Markdown               |
| /specs                       | Core EIP-7702 implementation logic                                               | Solidity + TS bindings |

Each package is published under the @q402 namespace and can be installed independently via pnpm or npm.

### Core Developer Flow

Developers can use the Q402 stack through three main integration paths:

| Flow                | Use Case                                                    | Entry Point                                       |
| ------------------- | ----------------------------------------------------------- | ------------------------------------------------- |
| Client SDK          | Apps or wallets generating sign-to-pay transactions         | @q402/core                                        |
| Server Middleware   | APIs requiring payment validation or premium access         | @q402/middleware-express or @q402/middleware-hono |
| Facilitator Service | Independent relay handling verification and gas sponsorship | @q402/facilitator                                 |

All flows support EIP-712 + EIP-7702 standards, use short-lived authorization tuples, and output a verifiable X-PAYMENT header that can be traced on-chain.

### Example Use Cases

| Scenario                                     | Implementation Path                               |
| -------------------------------------------- | ------------------------------------------------- |
| Gasless API access (e.g., premium endpoints) | Express middleware using @q402/middleware-express |
| DAO proposal execution with policy checks    | Governance API + Facilitator                      |
| RWA payouts and treasury automation          | Governance API + Q402 SDK                         |
| Subscription renewals for data agents        | Facilitator + EIP-7702 integration                |
| Merchant or marketplace settlement           | Batch transactions via Facilitator Service        |

Each transaction includes a receipt hash linking signature → transaction → policy execution — providing a transparent audit trail for compliance and governance monitoring.