> ## Documentation Index
> Fetch the complete documentation index at: https://docs.gx402.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Backend overview

> High-level backend role for x402 and how Gx402 middleware ties platforms together.

# Backend overview

As a developer you should think of the backend as the single source of truth for payments. With x402 the provider handles actual transaction execution (on-chain or relayed), but your backend is where you:

* build and store `paymentSession` objects (paymentId, item, amount, user),
* produce the signable `payload` that clients must sign,
* verify signatures the client returns,
* call x402 endpoints to finalize a payment,
* process webhooks, and
* grant game items / update order state.

Gx402's approach: we provide **middleware blueprints** and opinionated server patterns (Express examples in the repo) so every platform (Unity, Unreal, WebGL, Telegram, Farcaster, Mobile) talks the same language to your backend. That reduces cross-platform bugs and ensures consistent auditing.

## Responsibilities (server-side)

* **Session lifecycle**: create → pending → finalized/failed.
* **Signature verification**: check that the signature matches the expected wallet address.
* **Idempotency & replay safety**: prevent double fulfillment.
* **x402 integration**: call x402 create/confirm endpoints when appropriate.
* **Webhooks**: verify and reconcile async events from x402.
* **Auditing**: persist `paymentId`, `orderId`, `txHash`, timestamps, and user ids.

## Quick model for you

Platform (Unity/Web/Unreal) → Middleware  → Your Backend (paymentSession, verify) → x402 API

> Tip: keep the server-side logic intentionally simple: build a deterministic payload, persist session state, verify signatures, call x402, then respond. Complexity belongs in the server.