Add Elixir SDK to documentation

src/components/cards.tsx

@@ -121,6 +121,17 @@ export function RustSdkCard() {
   );
 }
 
+export function ElixirSdkCard() {
+  return (
+    <Card
+      description="MPP SDK for Elixir — Plug middleware for Phoenix and Plug apps"
+      icon="simple-icons:elixir"
+      title="Elixir"
+      to="/sdk/elixir"
+    />
+  );
+}
+
 export function MppxCreateReferenceCard({ to }: { to: string }) {
   return (
     <Card

src/pages/sdk/elixir/core.mdx

@@ -0,0 +1,89 @@
+---
+imageDescription: "Challenge, Credential, and Receipt types for working with the MPP protocol in Elixir"
+---
+
+# Core types [Challenge, Credential, and Receipt modules]
+
+These modules map directly to HTTP headers — `WWW-Authenticate`, `Authorization`, and `Payment-Receipt` — and can be used independently of the `MPP.Plug` middleware.
+
+## Challenge
+
+Create an HMAC-bound challenge and format it as a `WWW-Authenticate` header:
+
+```elixir
+alias MPP.{Challenge, Headers}
+
+challenge = Challenge.create(
+  secret_key: "your-hmac-secret",
+  realm: "api.example.com",
+  method: "stripe",
+  intent: "charge",
+  request_b64: Base.url_encode64(Jason.encode!(%{amount: "1000", currency: "usd"}), padding: false)
+)
+
+header = Headers.format_www_authenticate(challenge)
+# → "Payment id=\"abc...\", realm=\"api.example.com\", ..."
+```
+
+Verify a challenge ID on the retry request:
+
+```elixir
+{:ok, true} = Challenge.verify(challenge, "your-hmac-secret")
+```
+
+## Credential
+
+Parse an `Authorization: Payment` header into a credential struct:
+
+```elixir
+alias MPP.{Credential, Headers}
+
+{:ok, credential} = Headers.parse_authorization(auth_header)
+
+# Access the echoed challenge fields
+credential.challenge_id
+credential.method
+credential.intent
+
+# Access the payment payload
+credential.payload
+```
+
+## Receipt
+
+Build a receipt and format it as a `Payment-Receipt` header:
+
+```elixir
+alias MPP.{Receipt, Headers}
+
+receipt = %Receipt{
+  status: "success",
+  challenge_id: challenge.id,
+  method: "stripe",
+  reference: "pi_abc123"
+}
+
+header = Headers.format_receipt(receipt)
+# → base64url-encoded JSON
+```
+
+## Headers
+
+The `MPP.Headers` module handles all header parsing and formatting:
+
+| Function | Description |
+|----------|-------------|
+| `format_www_authenticate/1` | Serialize a Challenge to a `WWW-Authenticate` header value |
+| `format_receipt/1` | Serialize a Receipt to a `Payment-Receipt` header value |
+| `parse_authorization/1` | Parse an `Authorization` header into a Credential |
+
+## Type reference
+
+| Module | Description |
+|--------|-------------|
+| `MPP.Challenge` | Server challenge with HMAC-SHA256 ID binding |
+| `MPP.Credential` | Client credential parsed from `Authorization` header |
+| `MPP.Receipt` | Server receipt for `Payment-Receipt` header |
+| `MPP.Headers` | Header parsing and formatting utilities |
+| `MPP.Intents.Charge` | Charge intent request schema (amount, currency, recipient) |
+| `MPP.Errors` | RFC 9457 Problem Detail error types |

src/pages/sdk/elixir/index.mdx

@@ -0,0 +1,107 @@
+---
+title: "Elixir SDK for MPP"
+imageDescription: "The mpp Elixir library for adding payment-gated endpoints to Phoenix and Plug applications"
+---
+
+import * as SdkBadge from '../../../components/SdkBadge'
+
+# Elixir SDK [Plug middleware for Phoenix and Plug apps]
+
+## Overview
+
+The `mpp` Elixir library provides Plug middleware that adds pay-per-call billing to any Phoenix or Plug application. Mount the plug on a route, set a price, and the middleware handles the full 402 challenge/credential/receipt flow.
+
+<div className="flex gap-2">
+  <SdkBadge.GitHub repo="ZenHive/mpp" />
+  <SdkBadge.Maintainer name="ZenHive" href="https://github.com/ZenHive" />
+</div>
+
+## Install
+
+Add `mpp` to your dependencies in `mix.exs`:
+
+```elixir [mix.exs]
+defp deps do
+  [
+    {:mpp, "~> 0.1"}
+  ]
+end
+```
+
+Then fetch dependencies:
+
+```bash [install.sh]
+$ mix deps.get
+```
+
+## Requirements
+
+- Elixir 1.17+
+- A Plug-compatible application (Phoenix, Bandit, Cowboy)
+
+## Quick start
+
+### Server
+
+Mount `MPP.Plug` in a Phoenix router to gate endpoints behind payment:
+
+```elixir [router.ex]
+defmodule MyAppWeb.Router do
+  use MyAppWeb, :router
+
+  pipeline :paid do
+    plug MPP.Plug,
+      secret_key: "your-hmac-secret",
+      realm: "api.example.com",
+      method: MPP.Methods.Stripe,
+      method_config: %{stripe_secret_key: System.get_env("STRIPE_SECRET_KEY")},
+      amount: "1000",
+      currency: "usd"
+  end
+
+  scope "/api", MyAppWeb do
+    pipe_through [:api, :paid]
+    get "/data", DataController, :show
+  end
+end
+```
+
+The middleware handles the full flow automatically:
+
+1. Request without `Authorization: Payment` header returns `402` with a `WWW-Authenticate` challenge
+2. Client pays off-band (e.g. via Stripe), retries with `Authorization: Payment <credential>`
+3. Valid credential passes through with a `Payment-Receipt` header attached
+4. Invalid credential returns `402` with a fresh challenge and an RFC 9457 error body
+
+### Per-route pricing
+
+Each route can have its own price — just mount the plug with different options:
+
+```elixir [router.ex]
+scope "/api", MyAppWeb do
+  pipe_through :api
+
+  scope "/basic" do
+    plug MPP.Plug, amount: "100", currency: "usd",
+      secret_key: secret, realm: realm, method: MPP.Methods.Stripe
+    get "/data", DataController, :show
+  end
+
+  scope "/premium" do
+    plug MPP.Plug, amount: "5000", currency: "usd",
+      secret_key: secret, realm: realm, method: MPP.Methods.Stripe
+    get "/report", ReportController, :show
+  end
+end
+```
+
+## Design
+
+- **Stateless**: Challenge verification uses HMAC-SHA256 binding — no database or challenge store needed
+- **Explicit configuration**: No `Application.get_env` or ENV fallbacks — pass credentials directly via plug opts
+- **Pluggable methods**: Implement the `MPP.Method` behaviour to add new payment methods beyond Stripe
+
+## Next steps
+
+- [Core types](/sdk/elixir/core): Challenge, Credential, and Receipt modules
+- [Server](/sdk/elixir/server): Plug middleware configuration and usage

src/pages/sdk/elixir/server.mdx

@@ -0,0 +1,146 @@
+---
+imageDescription: "Protect Phoenix and Plug endpoints with payment requirements using MPP middleware"
+---
+
+# Server [Plug middleware for payment-gated endpoints]
+
+Mount `MPP.Plug` in any Phoenix or Plug router to protect endpoints with payment. The plug handles the full 402 challenge/credential/receipt flow.
+
+## Quick start
+
+```elixir
+plug MPP.Plug,
+  secret_key: "your-hmac-secret",
+  realm: "api.example.com",
+  method: MPP.Methods.Stripe,
+  method_config: %{stripe_secret_key: "sk_..."},
+  amount: "1000",
+  currency: "usd"
+```
+
+## Options
+
+### secret_key (required)
+
+- **Type:** `String.t()`
+
+HMAC-SHA256 key for stateless challenge ID binding. The server recomputes the HMAC on verification — no challenge store needed.
+
+### realm (required)
+
+- **Type:** `String.t()`
+
+Server protection space, included in `WWW-Authenticate` headers. Typically your API domain.
+
+### method (required)
+
+- **Type:** module implementing `MPP.Method`
+
+Payment method module for verifying credentials. Built-in: `MPP.Methods.Stripe`.
+
+### amount (required)
+
+- **Type:** `String.t()`
+
+Price in base units (e.g. `"1000"` for $10.00 USD with Stripe).
+
+### currency (required)
+
+- **Type:** `String.t()`
+
+Currency code, normalized to lowercase (e.g. `"usd"`).
+
+### method_config (optional)
+
+- **Type:** `map()`
+
+Server-only configuration passed to `MPP.Method.verify/2` via `charge.method_details`. Use this for secrets like API keys that the payment method needs but clients must not see. Never serialized into challenges.
+
+```elixir
+method_config: %{stripe_secret_key: System.get_env("STRIPE_SECRET_KEY")}
+```
+
+### recipient (optional)
+
+- **Type:** `String.t()`
+
+Payment recipient identifier included in the charge intent.
+
+### description (optional)
+
+- **Type:** `String.t()`
+
+Human-readable description attached to the challenge.
+
+### expires_in (optional)
+
+- **Type:** `integer()`
+
+Challenge TTL in seconds. Defaults to 300 (5 minutes).
+
+### opaque (optional)
+
+- **Type:** `String.t()`
+
+Base64url-encoded server correlation data included in the challenge.
+
+## Phoenix router example
+
+```elixir
+defmodule MyAppWeb.Router do
+  use MyAppWeb, :router
+
+  pipeline :paid do
+    plug MPP.Plug,
+      secret_key: Application.compile_env!(:my_app, :mpp_secret_key),
+      realm: "api.example.com",
+      method: MPP.Methods.Stripe,
+      method_config: %{
+        stripe_secret_key: Application.compile_env!(:my_app, :stripe_secret_key)
+      },
+      amount: "1000",
+      currency: "usd"
+  end
+
+  scope "/api", MyAppWeb do
+    pipe_through [:api, :paid]
+    get "/data", DataController, :show
+    get "/report", ReportController, :show
+  end
+end
+```
+
+## Accessing the receipt
+
+After successful payment verification, the receipt is available in `conn.assigns`:
+
+```elixir
+def show(conn, _params) do
+  receipt = conn.assigns[:mpp_receipt]
+  json(conn, %{data: "paid content", reference: receipt.reference})
+end
+```
+
+## Custom payment methods
+
+Implement the `MPP.Method` behaviour to add new payment methods:
+
+```elixir
+defmodule MyApp.Payments.CustomMethod do
+  @behaviour MPP.Method
+
+  @impl true
+  def verify(credential, charge) do
+    # Verify the payment and return a receipt or error
+    {:ok, %MPP.Receipt{status: "success", reference: "ref_123", ...}}
+  end
+end
+```
+
+Then use it in your plug configuration:
+
+```elixir
+plug MPP.Plug,
+  method: MyApp.Payments.CustomMethod,
+  # ...
+```

src/pages/sdk/index.mdx

@@ -1,16 +1,17 @@
 ---
 title: "SDKs and client libraries"
-description: "Official MPP SDK implementations in TypeScript, Python, and Rust. Libraries for building MPP clients and servers with full protocol support."
-imageDescription: "Official MPP SDK implementations in TypeScript, Python, and Rust for clients, servers, and agents"
+description: "MPP SDK implementations in TypeScript, Python, Rust, and Elixir. Libraries for building MPP clients and servers with full protocol support."
+imageDescription: "MPP SDK implementations in TypeScript, Python, Rust, and Elixir for clients, servers, and agents"
 ---
 
 import { Cards } from 'vocs'
-import { TypeScriptSdkCard, PythonSdkCard, RustSdkCard, WalletCliCard } from '../../components/cards'
+import { TypeScriptSdkCard, PythonSdkCard, RustSdkCard, ElixirSdkCard, WalletCliCard } from '../../components/cards'
 
 # SDKs [Official implementations in multiple languages]
 
 <Cards>
   <TypeScriptSdkCard />
   <PythonSdkCard />
   <RustSdkCard />
+  <ElixirSdkCard />
 </Cards>