ArborMCP

ArborMCP provides Elixir clients and servers for the Model Context Protocol, with stdio, Streamable HTTP and BEAM-local transports. Its Hex package is arbor_mcp; its Elixir module namespace is Arbor.MCP.*.

Version 2 release candidate preparation. This checkout is the MCP part of the library split, prepared as arbor_mcp 2.0.0-rc.1. The released 1.x package remains ex_mcp; arbor_mcp 2.0 has not been released. The supervised Runtime and accepted API cleanup are implemented; final package and release qualification remain in progress. See the release assessment for the qualified source checkpoints and open gates. Historical 1.x test counts and performance results are not v2 qualification evidence.

ExMCP 1.x remains maintained on codex/maintenance-1.x, with backported fixes and compatible minor releases. Migration to v2 is optional; see the maintenance policy.

ACP clients, agents and the optional vendor adapter bundle are developed in ArborACP. MCP and ACP depend on the small shared ArborRPC package and can be installed independently. ArborRPC has its own repository, with the arbor_rpc Mix project at the repository root.

Upgrading from ex_mcp 1.x? Start with the v1-to-v2 migration guide. It covers package selection, namespace and configuration changes, supervision, HTTP mounting, removed APIs and the RC testing checklist. The RC notes describe the candidate's qualification status and known limits.

Development checkout

For downstream RC testing after publication, use an exact dependency pin:

defp deps do
  [{:arbor_mcp, "== 2.0.0-rc.1"}]
end

This pulls in shared ArborRPC. Add ArborACP or the optional adapter bundle only when your application uses them. RC publication is still pending; the local checkout setup below works without treating the candidate as a published package.

The MCP migration branch is codex/v2-migration; the default master branch still contains ExMCP 1.x. ArborRPC is a separate repository on main. Until the coordinated RC packages are published, select both source checkouts explicitly:

git clone --branch codex/v2-migration https://github.com/trust-arbor/arbor_mcp.git
git clone --branch main https://github.com/trust-arbor/arbor_rpc.git
cd arbor_mcp
export ARBOR_RPC_PATH="$(pwd)/../arbor_rpc"
mix deps.get
mix compile
mix test

ARBOR_RPC_PATH is a local development override. Without it, the package declares a normal Hex dependency with an explicit development/RC prerelease floor; stable releases retain the ~> 2.0 major-compatible range. See the package release guide. For isolated split QA, ARBOR_V2_DEPS can point to an existing directory of dependency sources; ARBOR_V2_BUILD and ARBOR_V2_LOCK select separate build and lock paths. Release checks must also run without those overrides using the packaged artifacts.

The supported minimum is Elixir 1.17 with Erlang/OTP 27. Protocol output uses OTP's JSON encoder. The CI matrix checks multiple Elixir/OTP versions; passing CI and the release gates are required before publishing 2.0.

On macOS/Darwin and Linux, installing the transitive arbor_rpc source package requires a C17 compiler (cc, or the executable selected by CC), including for HTTP-only and BEAM-only applications. Source archives contain reviewed C source, not prebuilt helpers. An installed release includes the built helper and needs no runtime compiler. Windows native subprocess operations are unsupported; framing is separate. Only the qualified platform/architecture matrix is supported.

Client example

{:ok, client} =
  Arbor.MCP.Client.start_link(
    transport: :stdio,
    command: ["mcp-server"],
    protocol_mode: :prefer_modern
  )

{:ok, tools} = Arbor.MCP.Client.list_tools(client, format: :map)
{:ok, result} = Arbor.MCP.Client.call_tool(client, "echo", %{"message" => "hello"}, format: :map)
Arbor.MCP.Client.stop(client)

The latest stable MCP revision is 2026-07-28. :prefer_modern allows evidence-based fallback to the legacy protocol era; :modern_only and :legacy_only select an era explicitly. Library version 2 and the MCP wire revision are separate version identifiers.

Servers and transports

Handlers use Arbor.MCP.Server.Handler and the declarative Arbor.MCP.Server.DSL to define tools, resources and prompts. HTTP, stdio and BEAM use a supervised Runtime per server, bounded handler scheduling and explicit session/subscription storage contracts. Follow the roadmap for the accepted contract.

HTTP applications mount Arbor.MCP.HttpPlug with an explicit Runtime inside an existing Plug/Phoenix server. Standalone HTTP listeners are optional. Install the selected Cowboy or Bandit dependency and choose http_adapter: :cowboy or :bandit; Cowboy remains the default. See the HTTP listener guide for startup, shutdown and mounted-host ownership, and the transport guide for transport contracts.

Guides and examples

• Migrate from v1 to v2
• Version 2 release candidate
• Quickstart
• User guide
• Phoenix mounting
• Server DSL
• Runtime operations
• Configuration
• Security
• Examples
• Development
• Troubleshooting
• Changelog

Published 1.x API documentation is available at hexdocs.pm/ex_mcp. It describes the previous package and namespace. The v2 migration guide is available in this checkout and is included in the package and ExDoc documentation.

For v2 API documentation from the checkout, run mix docs --warnings-as-errors after fetching development dependencies. Published ArborMCP HexDocs will be linked once RC publication is complete. Repository-only release assessments remain separate from the consumer guides shipped in the source package.

Reporting issues

Report suspected vulnerabilities through the private vulnerability reporting form. Use GitHub issues for other bugs and feature requests.

Licensed under the MIT license.