caudena/fastest_mcp

**MCP servers and clients for Elixir and Phoenix.**
The Model Context Protocol (MCP) lets AI
applications such as Claude, ChatGPT, Cursor, and AI-enabled editors use your
software: the model calls your *tools*, reads your *resources*, and offers your
*prompts* to the user. FastestMCP turns your Elixir or Phoenix application into
an MCP server, with ordinary functions as handlers and OTP supervision
underneath, and includes a client for calling other MCP servers.
Getting Started ·
Core Concepts ·
Phoenix ·
All guides
Installation
Add fastest_mcp to your dependencies in mix.exs:
FastestMCP requires Elixir 1.19 or later.
Quick start with Phoenix
Define a server module. Each tool is a function plus a description and a JSON
Schema that tell the model how to call it. The handlers call MyApp.Notes,
one of your application's contexts; Getting Started
builds it as a small notes store:
Start it with your application, before the endpoint:
Route /mcp to it, in a scope of its own and without the :browser or :api
pipeline; the server module is the plug of its endpoint:
Your app now serves MCP at http://localhost:4000/mcp. Connect an MCP client
to it, for example Claude Code:
or call it from your own code and tests:
Getting Started walks through the whole setup,
including testing it, and Phoenix covers hosts,
authentication, and deployment. Without Phoenix, give the server its own
listener with use FastestMCP.ServerModule, http: [port: 4100], or run it over
stdio; see Transports.
What you can build
| You want to | Use | Guide |
| --- | --- | --- |
| Let a model call functions in your app | Tools | Tools |
| Let an AI application read your data by URI | Resources and resource templates | Resources |
| Offer ready-made prompts to users | Prompts | Prompts |
| Serve MCP from your Phoenix app | A forward in your router | Phoenix |
| Run a local server that a desktop app launches | The stdio transport | Transports |
| Require tokens or OAuth scopes | Authentication and authorization | Authentication and Authorization |
| Log to the client, report progress, keep state | The handler context | Context |
| Share connections and services with handlers | Lifespans and dependencies | Lifespans and Dependencies |
| Ask the user to confirm, or ask the model to write | Elicitation and sampling | Sampling and Elicitation |
| Run long jobs that clients can follow and cancel | Background tasks | Background Tasks |
| Add logging, rate limits, or caching | Middleware | Middleware |
| Combine servers, proxy a remote server, or turn an OpenAPI spec into tools | Providers and mounting | Providers and Mounting |
| Change the tools on offer while the server runs | Dynamic components and visibility | Dynamic Components |
| Show an interactive UI from a tool (MCP Apps) | Protocol extensions | Extensions |
| Call other MCP servers from Elixir | The client | Client |