# Wymcp

MCP (Model Context Protocol) server library for Elixir. A Plug-based
implementation of the MCP JSON-RPC 2.0 protocol with support for tools and
optional Bearer token authentication.

> ### API Changes {: .warning}
> This project is a work in progress and the API will change until we reach version 1.0.0.

> ### Duplicated `Origin` is refused in every configuration {: .warning}
> Since 0.4.0 the origin check answers HTTP 400 to a request carrying two or
> more `Origin` header lines, whether or not the mount sets `:origin`. Earlier
> releases ran that check only under a configured allowlist, so a mount
> without one answered such a request normally. RFC 6454 already forbids a
> user agent from sending more than one `Origin` header field — several
> origins travel space-separated inside a single line, which the
> duplicate-header check never inspects — so the new 400 answers a broken
> proxy, not a browser. That 400 fires only on an adapter that preserves
> repeated header lines: see *Supported HTTP adapter* below.

<div data-toc />

## Supported MCP protocol versions

Wymcp is a **dual-era** server: it serves the modern era (`2026-07-28`) and
the legacy era (`2025-11-25`) on the same endpoint. `docs/glossary.md`
defines the two eras; which revisions each accepts, why `2024-11-05` is
refused, and what a client asking for something else is answered are all
documented at `Wymcp.ProtocolVersion`; how a request is sorted into a lane is
at `Wymcp.Plugs.Era`.

## Supported HTTP adapter

Wymcp is developed and tested against [Bandit](https://hexdocs.pm/bandit);
`Plug.Cowboy` is untested, and the difference is observable. Cowboy folds
repeated header lines into one comma-joined value before Plug sees the
request, so a duplicated header never arrives as a duplicate. Under Cowboy a
mount with no `:origin` allowlist accepts a duplicated `Origin`, and a mount
with an allowlist refuses it as a disallowed origin naming the folded value —
neither is the 400 documented here. `Wymcp.Plugs.OriginCheck` documents what
the origin check answers, `Wymcp.Plugs.SingletonHeaders` the other
cardinality checks.

Support for another adapter is welcome as a contribution.

## Getting started

### 1. Add dependency

In `mix.exs`:

```elixir
defp deps do
  [
    {:wymcp, "~> 0.4.0"}
  ]
end
```

### 2. Create a tool

```elixir
defmodule MyApp.Tools.Calculator do
  use Wymcp.Tool

  @impl true
  def name, do: "calculator"

  @impl true
  def description, do: "Basic arithmetic"

  @impl true
  def actions do
    %{
      add: %{
        description: "Add two numbers",
        properties: %{
          "a" => %{"type" => "number"},
          "b" => %{"type" => "number"}
        },
        required: ["a", "b"],
        defaults: %{}
      }
    }
  end

  @impl Wymcp.Tool
  def run_action(:add, %{"a" => a, "b" => b}, _context) do
    {:ok, %{result: a + b}}
  end
end
```

Two framework behaviours a tool author meets next, both documented in full at
their modules: every server exposes a `help` tool that answers at three levels
(`Wymcp.Help`), and a tool can suggest follow-up actions by returning hints
(`Wymcp.Hint`).

### 3. Add config

In `config.exs`:

```elixir
config :wymcp,
  name: "My MCP Server",
  version: Mix.Project.config()[:version] || "0.1.0"
```

### 4. Add route

In `router.ex`:

```elixir
forward "/mcp", Wymcp.Router,
  tools: [MyApp.Tools.Calculator]
```

### 5. (Optional) Add authentication

Implement the `Wymcp.Auth` behaviour and pass it to the router:

```elixir
defmodule MyApp.McpAuth do
  @behaviour Wymcp.Auth

  @impl Wymcp.Auth
  def authenticate(conn) do
    with ["Bearer " <> token] <- Plug.Conn.get_req_header(conn, "authorization"),
         {:ok, user} <- MyApp.Accounts.fetch_user_by_api_token(token) do
      {:ok, Plug.Conn.assign(conn, :current_user, user)}
    else
      _ -> {:error, "Invalid or missing Bearer token"}
    end
  end
end
```

```elixir
forward "/mcp", Wymcp.Router,
  tools: [MyApp.Tools.Calculator],
  auth: MyApp.McpAuth
```

Authentication runs per request on every MCP route — POST, the GET stream,
and DELETE. `Wymcp.Auth` documents the contract and the 401 challenge; `Wymcp.Router`'s `:www_authenticate` option
adds the RFC 9728 discovery hints a spec-following client looks for.

### 6. (Optional) Restrict browser origins

```elixir
forward "/mcp", Wymcp.Router,
  tools: [MyApp.Tools.Calculator],
  origin: ["http://localhost:4000"]
```

`origin:` is an allowlist of `Origin` header values — DNS-rebinding protection
for browser-based clients. `Wymcp.Plugs.OriginCheck` documents which requests
pass and which are refused; a request carrying two or more `Origin` headers is
refused with or without the option.

## Documentation

Wymcp's documentation is published at
[hexdocs.pm/wymcp](https://hexdocs.pm/wymcp) — or build it locally with
`mix docs`:

- [`Wymcp`](https://hexdocs.pm/wymcp/Wymcp.html) — the map: every module,
  what it owns, and why it exists, with the request-flow diagram.
- [Glossary](https://hexdocs.pm/wymcp/glossary.html) — canonical domain
  terms and where each one is defined.
- [MCP 2026-07-28 overview](https://hexdocs.pm/wymcp/mcp-spec-2026-07-28-overview.html)
  — the modern era's conformance map.
- [MCP 2025-11-25 overview](https://hexdocs.pm/wymcp/mcp-spec-2025-11-25-overview.html)
  — the legacy era's conformance map.

The last two are maintainer yardsticks for planning wymcp's next revision, and
they are filed under **Development** in the sidebar; the glossary sits beside
this README, for every reader.