BeamMCP

A Model Context Protocol server core for the BEAM. Protocol handling, two transports — stdio and a stateless Streamable HTTP Plug — and JSON Schema validation, with the tool catalog and the dispatch function injected by the host.

The package holds no tools, no domain, and no policy. It decides what a well-formed request is and refuses one that is not; what a tool does is the host's business.

def deps do
  [{:beam_mcp, "~> 0.3.0"}]
end

~> 0.3.0, not the more usual ~> 0.3. While this package is 0.x it documents wire breaks at the minor position, and it has used that position twice: 0.2.0 removed two fields from results for legacy-declared requests, and 0.3.0 adds the HTTP transport and the ttlMs/cacheScope fields 2026-07-28 requires on tools/list. ~> 0.3 admits 0.4.0, so it would carry you across the next such break on a routine mix deps.update; ~> 0.3.0 does not. The tighter form is deliberate and is not an over-pin to be tidied away.

Two contracts

Injection without a specification is a claim with nothing behind it, so both are declared.

BeamMCP.ToolCatalog — the host names the tools.

defmodule MyApp.Catalog do
  @behaviour BeamMCP.ToolCatalog

  @impl true
  def all do
    [
      %BeamMCP.ToolSpec{
        name: :get_weather,
        command_class: :observe,
        mode: :read_only,
        description: "Read the current weather for a place.",
        input_schema: %{
          "type" => "object",
          "properties" => %{"place" => %{"type" => "string"}},
          "required" => ["place"],
          "additionalProperties" => false
        }
      }
    ]
  end
end

The dispatch callback — the host does the work.

@type dispatch :: (atom(), map(), keyword() -> {:ok, term()} | {:error, term()})

Running it

BeamMCP.Transport.Stdio.run(
  tool_catalog: MyApp.Catalog,
  dispatch: &MyApp.Dispatch.call/3,
  server_name: "my-app"
)

:tool_catalog is required. :dispatch is required for tools/call. :server_name defaults to beam_mcp, and a host that wants its own name in initialize says so.

One schema, one source

A tool's schema lives on its ToolSpec. tools/list advertises that schema and tools/call enforces that schema, so the contract a client is shown and the contract it is held to cannot drift apart. Argument keys are derived from the schema's properties and reach dispatch as atoms — a tool declaring "place" is dispatched %{place: "Oslo"}, not %{"place" => "Oslo"}. Values are passed through unchanged, because turning a string into a domain term is the host's job and a generic layer that guesses has acquired someone else's domain.

A ToolSpec that omits input_schema is a tool with no arguments: it advertises an open empty object, so tools/call refuses nothing and dispatch is handed %{} whatever the client sent.

Validation is a deliberately small subset of JSON Schema — type, properties, required, additionalProperties, and bounds. It refuses rather than guesses, and it is not a general validator.

Transports

stdio — BeamMCP.Transport.Stdio.run/1, newline-delimited JSON-RPC over a pipe.

HTTP — BeamMCP.Transport.HTTP, a Plug serving the 2026-07-28 stateless model at one endpoint: no sessions, no Mcp-Session-Id, no SSE resumability. plug and bandit are optional dependencies; a stdio-only host does not pull them in.

If you add plug to a host that already has this package compiled, run mix deps.compile beam_mcp --force. The module is guarded by Code.ensure_loaded?(Plug), which is evaluated once at compile time and is not a tracked compile-time dependency, so adding the dependency afterwards does not rebuild this package: mix compile reports success and BeamMCP.Transport.HTTP does not exist. Changing this package's version rebuilds it and needs no such step.

Bandit.child_spec(
  plug: {BeamMCP.Transport.HTTP,
         tool_catalog: MyApp.Catalog,
         dispatch: &MyApp.Dispatch.call/3,
         authorize: &MyApp.Auth.check/1,
         allowed_origins: ["https://app.example.com"]},
  port: 4000,
  ip: {127, 0, 0, 1}
)

Or mounted inside an existing router, where forward matches on a path prefix and the Plug serves everything under it:

defmodule MyApp.Router do
  use Plug.Router
  plug :match
  plug :dispatch

  forward "/mcp",
    to: BeamMCP.Transport.HTTP,
    init_opts: [
      tool_catalog: MyApp.Catalog,
      dispatch: &MyApp.Dispatch.call/3,
      authorize: &MyApp.Auth.check/1,
      allowed_origins: ["https://app.example.com"]
    ]

  match _, do: send_resp(conn, 404, "")
end

authorize and allowed_origins are required and have no defaults. Omit either and the Plug raises when it is initialised — at start, not on the first request.

That is deliberate. This package cannot decide who may call your tools: it has no view of your identity model, and deciding for you would be claiming something it cannot keep. But serving tools/call to anyone who can reach the port is a confused-deputy surface, and a README sentence telling you to authenticate is documentation rather than a control. A required argument with no default is a contract, because you cannot start without answering it. To accept every caller, say so: authorize: fn _conn -> :ok end.

authorize/1 must not read the request body. It runs before this Plug reads it, and Plug.Conn.read_body/2 can be called once: a host that consumes the body in authorize/1 leaves the transport nothing to parse, and the request fails as a parse error rather than as whatever the host meant. Authorize on the Plug.Conn — headers, peer, assigns set by an earlier plug — and if a decision genuinely needs the payload, make it in dispatch/3, which is handed the decoded arguments.

Said plainly, because it is a real limitation and not a preference: body-signature authentication is not possible in authorize/1. The callback runs before the body is read and returns :ok | {:error, reason}, with no way to hand back the conn it read from. A host that reads the body there does not get an error — a small request appears to work because the body is already in the adapter's buffer, and a larger one hangs until the server's read timeout and then returns 408 with the connection dead. Measured: 119 bytes 200, 16 KiB and 200 KiB both 408 after 15.0 s. Today the workarounds are a plug in front of this one that reads the body and re-supplies it, or deciding in dispatch/3. Whether authorize/1 should instead run after the body is read, or be able to hand the conn back, is an open design question on the required-option contract and not something this release settles.

Resources this Plug bounds, and the ones it does not

@max_body_bytes caps a single body at 1 MiB. Three things that is not:

• It is not an aggregate bound. Each in-flight request at the cap costs about 1.05 MiB, measured linear with no plateau to 8,000 concurrent (+8.16 GiB RSS). The concurrent-request ceiling is your HTTP server's: for Bandit/ThousandIsland it is num_acceptors * num_connections, defaulting to 100 × 16,384 = 1,638,400. Setting it is the host's capacity decision, and a number this package picked for you would be one it cannot keep.

• It is not a ceiling on bytes read. What the server reads before refusing is the cap itself: a declared 32 MiB body is refused after read_body/2 returns a partial of exactly 1,048,576 bytes, constant across six socket-buffer settings and four runs. How much the client got onto the wire by then is a different quantity and not a property of this package — the same 24 measurements put it between 1.125 MiB and 7.438 MiB, varying run to run at one fixed buffer size — so there is no number to design against there, only the server-side constant above.

• It is not a time bound. A slow client is held by read_body/2's :read_timeout, which this package does not set and therefore inherits from the server — 15,000 ms under Bandit. That is a whole-body deadline rather than a per-read reset, so a drip client is answered 408 at 15 s rather than held indefinitely. That makes slow connections a transient rather than a hold: they cost memory for at most the timeout, and a legitimate request was still served in under 0.01 s with 12,000 of them in flight.

• It does not bound headers. @max_body_bytes is a body limit; the number and size of request headers are your HTTP server's settings, inherited the same way the read timeout is.

allowed_origins is separate because the specification makes validating Origin a MUST, to prevent DNS rebinding; which origins are legitimate is yours to say. :any is available and must be chosen deliberately. The specification also says a locally-running server SHOULD bind to localhost rather than all interfaces — that is your Bandit option, above, and this package cannot enforce it for you.

What the header requirement does and does not close. The transport requires an MCP-Protocol-Version header on every POST and requires it to match the body, so a request that establishes no protocol era is malformed and refused — that part of the stdio caveat below does not apply here.

It does not close the lifecycle. tools/call over HTTP runs without initialize having been seen, because 2026-07-28 has no initialize and every request stands alone. That is the revision's design rather than a gap, but an earlier draft of this section said "a request with no era established is malformed and refused" in a way that read as covering both, and a reviewer was right that it claimed more than the code supports. Who may call is authorize/1's question, and it is yours.

What of 2026-07-28 this transport implements

Stated as a list rather than left to be inferred, because a transport that advertises a feature and does not enforce it is worse than one that never advertised it.

Implemented. One POST endpoint; MCP-Protocol-Version required and matched against the body's _meta; Mcp-Method, Mcp-Name and Mcp-Param-{Name} required where the revision requires them and validated against the corresponding body values; =?base64?…?= header values decoded before comparison; Origin validated against a host-supplied allow list; a body size bound; 405 on non-POST; 404 for an unimplemented method and 200 with a JSON-RPC error for an unknown tool. Every header is checked in all of its values, not the first — a duplicated header is the smuggling primitive the specification's validation MUST exists to prevent.

Mcp-Param-{Name} is enforced because tool_definition/1 passes a schema's x-mcp-header annotation through to tools/list verbatim. Advertising that a header is authoritative and then ignoring it gives a client that believes you a silent divergence between the value it routed on and the value that ran.

Not implemented, by design of the revision. Sessions, Mcp-Session-Id, SSE streaming, and SSE resumability — all removed from this revision's transport; and the initialize / notifications/initialized handshake, which 2026-07-28 deleted along with ping.

Those three are refused here rather than merely absent: 404 with -32601. The distinction is not pedantry. The package's core is dual-era and its initialize clause deliberately outranks _meta, because over stdio an initializeis the era discriminator — so before this was refused at the transport, an HTTP caller declaring 2026-07-28 could send initialize and receive 200 with protocolVersion: "2025-11-25", a different revision's version number, while this section said it was not implemented. The refusal lives in the transport and not in the core because HTTP is the carrier that stamps every request modern; stdio's dual-era rule is untouched.

Not implemented, and yours. Binding to localhost (a Bandit option), TLS, request timeouts and connection limits (your HTTP server's settings, not this Plug's), and authentication — authorize/1 is where you put it, and it is required precisely so the decision is yours.

What it speaks

Newline-delimited JSON-RPC over stdio, dual-era: it serves both the current revision and one legacy revision.

server/discover, tools/list, tools/call, shutdown, exit at both eras; initialize and notifications/initialized at legacy only.

A revision, not a carrier, decides the semantics._meta decides only that a request is served statelessly. Which revision the _metanames then decides the method table and the result envelope, so a ping declaring 2025-11-25 through _meta is answered and its result carries no resultType. This matters because -32022 tells a client to pick from supported — which lists 2025-11-25 — and retry the request, so a _meta naming the legacy revision is a message this server asks clients to send.

Two exceptions, and they are exceptions to the row above.server/discover and initialize are matched before the revision switch, so neither is affected by what a _meta declares and neither result is decorated — a server/discover result carries no resultType even under 2026-07-28, where the specification requires one on every result. server/discover is matched first on purpose: on stdio it is the era probe, sent by a client that does not yet know what it is talking to. The missing resultType on it is a known gap, not a design choice.

The session is tracked, not enforced. Nothing in this package refuses a request because initialize has not been seen: every method it implements is served bare, tools/call included — and tools/call executes through the host's dispatch. On stdio that is defensible, because whoever can write to the transport already has the host's privileges. On any transport where that is not true, refusing unestablished callers is the host's job, and this package does not do it for you.

A request naming a revision the server does not support gets UnsupportedProtocolVersionError (-32022) listing what it does support. 2024-11-05 is not supported — it predates the two chosen revisions.

JSON-RPC batching is refused. It was added in 2025-03-26 and removed in 2025-06-18, so it is required by exactly one revision of five and by neither of ours.

Status

Pre-1.0. The API may change. Known gaps are listed above and in CONVENTIONS.md, which is not shipped in the package and so is linked rather than named. It also records how this package is developed — the gate takes no baseline, a probe's population is derived the way the checked mechanism derives it, and CI is unproven until a run exists.

Consumers today: Ultraviolet, and Trinity as a candidate under its own evaluation.

Licence

Apache-2.0. See LICENSE, and NOTICE for attribution.