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

# Key Concepts

> Core x402 v2 concepts and how Armory applies them

This page explains the core x402 v2 model and how Armory maps it into practical server and client libraries.

## What Armory Is

Armory is a TypeScript toolkit for implementing x402 v2 payments over HTTP.

* Server middleware packages enforce payment requirements and verify payloads.
* Client packages create and send payment payloads from wallets.
* Shared protocol logic and types live in `@armory-sh/base`.

## x402 v2 in One Flow

1. Client requests a protected resource.
2. Server returns `402` with `PAYMENT-REQUIRED` and an `accepts[]` list.
3. Client selects one requirement, signs payment authorization, and retries with `PAYMENT-SIGNATURE`.
4. Server verifies and settles, then returns data with `PAYMENT-RESPONSE`.

## Three Actors

* Resource server: protects routes and requires payment.
* Client: picks a requirement and signs authorization with a wallet.
* Facilitator: verifies and settles payments.

## Core Objects

* `PaymentRequirements`: server-offered requirement options in `accepts[]`.
* `PaymentPayload`: client-signed payment payload tied to one accepted requirement.
* `SettlementResponse`: settlement result returned from verification/settlement.
* `VerifyResponse`: verification result from facilitator.

## HTTP Header Contract

* `PAYMENT-REQUIRED`: challenge with `accepts[]` and resource metadata.
* `PAYMENT-SIGNATURE`: client payment payload.
* `PAYMENT-RESPONSE`: settlement response.

## Versioning Rule

Armory is v2-only. Requests and responses should use x402 v2 fields and wire format.

## Scheme and Signing

For EVM exact payments, Armory uses EIP-3009 (`transferWithAuthorization`) and EIP-712 signatures.

* Payer signs authorization off-chain.
* Settlement executes token transfer on-chain.
* Nonce and validity windows prevent replay.

## Identifiers You Will See

* Network: CAIP-2 format, for example `eip155:8453`.
* Token/asset: token contract or CAIP asset form.
* Chain/token inputs can often be provided as names, IDs, or CAIP forms.

## How to Apply This in Armory

* Accept payments: start with a middleware package under `/accept-payments/*`.
* Make payments: start with a client package under `/make-payments/*`.
* Extend behavior: use extension docs and client hooks for selection/logging.

## References

* [x402 Spec v2](https://github.com/coinbase/x402/blob/main/specs/x402-specification-v2.md)
* [HTTP Transport v2](https://github.com/coinbase/x402/tree/main/specs/transports-v2/http.md)
* [A2A Transport v2](https://github.com/coinbase/x402/tree/main/specs/transports-v2/a2a.md)