> ## Documentation Index
> Fetch the complete documentation index at: https://docs.key0.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

> All HTTP endpoints served by Key0 at a glance.

Key0 exposes a set of HTTP endpoints for agent discovery, payment-gated access, and protocol interoperability. All endpoints are served by a single Express router created via `key0Router()`.

## Endpoints

### Discovery

| Method | Path                      | Auth | Description                                                                                                                    |
| ------ | ------------------------- | ---- | ------------------------------------------------------------------------------------------------------------------------------ |
| `GET`  | `/.well-known/agent.json` | None | Returns the A2A Agent Card describing your agent's capabilities, skills, and x402 extension. Only present when A2A is enabled. |
| `GET`  | `/discover`               | None | Returns all available plans and pricing (the plan catalog).                                                                    |
| `GET`  | `/llms.txt`               | None | Generated machine-readable buyer onboarding guide. Standalone only.                                                            |
| `GET`  | `/skills.md`              | None | Generated markdown workflow guide for buyers and agents. Standalone only.                                                      |

### Payment

| Method | Path           | Auth | Description                                                                                                                                                                               |
| ------ | -------------- | ---- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `POST` | `/x402/access` | None | The x402 HTTP payment endpoint. Handles challenge issuance (`planId` without `PAYMENT-SIGNATURE`) and settlement (`planId` with `PAYMENT-SIGNATURE`). Returns 400 if `planId` is missing. |

### MCP (when `mcp: true`)

| Method | Path                    | Auth | Description                                                                                               |
| ------ | ----------------------- | ---- | --------------------------------------------------------------------------------------------------------- |
| `GET`  | `/.well-known/mcp.json` | None | MCP discovery document. Returns the server name, description, version, and Streamable HTTP transport URL. |
| `POST` | `/mcp`                  | None | MCP Streamable HTTP transport. Exposes `discover_plans` and `request_access` tools.                       |

### Health (standalone server only)

| Method | Path      | Auth | Description                                                                                                                       |
| ------ | --------- | ---- | --------------------------------------------------------------------------------------------------------------------------------- |
| `GET`  | `/health` | None | Returns `{ status: "ok" }`. Only available when running the standalone server, not when mounting the router into an existing app. |

## Authentication Model

Key0 uses the x402 payment protocol instead of traditional API keys: discover a plan, request a challenge (402), sign an EIP-3009 authorization, settle on-chain, receive a Bearer token.

For the full three-phase flow explanation and sequence diagram, see [How It Works](/introduction/how-it-works).

## Base URL

All paths are relative to the host where you mount the Key0 router. For example, if your server runs at `https://api.example.com`, the agent card is at `https://api.example.com/.well-known/agent.json`.

## Common Headers

| Header              | Direction | Description                                                                                                                                                            |
| ------------------- | --------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `payment-required`  | Response  | Base64-encoded `X402PaymentRequiredResponse` JSON. Present on all 402 responses.                                                                                       |
| `www-authenticate`  | Response  | HTTP-standard authentication challenge header. Includes `realm`, `accept`, and optionally `challenge` parameters.                                                      |
| `payment-signature` | Request   | Base64url-encoded `X402PaymentPayload` JSON. Sent by the client to complete payment.                                                                                   |
| `payment-response`  | Response  | Base64-encoded `X402SettleResponse` JSON. Present on successful settlement (200 responses).                                                                            |
| `x-a2a-extensions`  | Request   | When present on `POST /x402/access` (Express only), routes the request to the A2A JSON-RPC handler instead of the standard x402 HTTP flow. Used by A2A-native clients. |

## Error Format

All errors follow a consistent JSON structure:

```json theme={null}
{
  "type": "Error",
  "code": "TIER_NOT_FOUND",
  "message": "Plan \"premium\" not found in plan catalog",
  "httpStatus": 400
}
```

See [Data Models](/api-reference/data-models) for the full list of error codes.