McpServer.layerStdio: two stdio servers in one layer graph share one stdio protocol, so the second never reads its stdin

Start by running the supplied reproduction, then read layerProtocolStdio in packages/effect/src/unstable/rpc/RpcServer.ts and layerStdio in packages/effect/src/unstable/ai/McpServer.ts. Trace how layer memoization behaves within Layer.mergeAll and how it interacts with McpServer.layer. Done means documenting the intended model or agreeing on and validating a change that supports the required composition.

This may well be intended: a process has one stdin, so one stdio server per process is a natural model. We're reporting it because we hit it in tests, where two servers each get their own Stdio and the result surprised us.

What we observed

We merged two McpServer.layerStdio servers into one layer graph with Layer.mergeAll, each with its own Stdio. Only the first server answers. The second never reads its own stdin. If the same two servers are built separately, each with its own Layer.build, both answer.

Environment: [email protected] (tag [email protected], 14a3f14), on Node 26.10.0 and macOS. From reading the source, layerProtocolStdio is still a module-level constant on main at 3788b63 (packages/effect/src/rpc/RpcServer.ts:1436-1440).

Reproduction

import { Effect, Layer, Sink, Stdio, Stream } from "effect"
import { McpProtocol, McpServer } from "effect/unstable/ai"

const initialize = (id: number) =>
  JSON.stringify({
    jsonrpc: "2.0",
    id,
    method: "initialize",
    params: { protocolVersion: "2025-11-25", capabilities: {}, clientInfo: { name: "repro", version: "0" } }
  }) + "\n"

const makeStdio = (name: string, id: number, out: Array<string>) =>
  Stdio.layerTest({
    stdin: Stream.make(new TextEncoder().encode(initialize(id))).pipe(Stream.concat(Stream.never)),
    stdout: () =>
      Sink.forEach((chunk: string | Uint8Array) =>
        Effect.sync(() =>
          out.push(`${name} stdout: ${typeof chunk === "string" ? chunk : new TextDecoder().decode(chunk)}`.trim())
        )
      )
  })

const server = (name: string) =>
  McpServer.layerStdio({ name, version: "0.0.0", protocols: [McpProtocol.v2025_11_25] })

const report = (label: string, outA: Array<string>, outB: Array<string>) =>
  Effect.sync(() => {
    console.log(`--- ${label}`)
    console.log(outA.length ? outA.join("\n").slice(0, 150) : "A stdout: (nothing)")
    console.log(outB.length ? outB.join("\n").slice(0, 150) : "B stdout: (nothing)")
  })

const outA: Array<string> = []
const outB: Array<string> = []

const program = process.argv[2] === "separate"
  ? Effect.gen(function*() {
    yield* Layer.build(server("A").pipe(Layer.provide(makeStdio("A", 1, outA))))
    yield* Layer.build(server("B").pipe(Layer.provide(makeStdio("B", 2, outB))))
    yield* Effect.sleep("300 millis")
    yield* report("each server in its own Layer.build", outA, outB)
  })
  : Effect.gen(function*() {
    yield* Layer.build(Layer.mergeAll(
      server("A").pipe(Layer.provide(makeStdio("A", 1, outA))),
      server("B").pipe(Layer.provide(makeStdio("B", 2, outB)))
    ))
    yield* Effect.sleep("300 millis")
    yield* report("both servers in one Layer.mergeAll", outA, outB)
  })

Effect.runFork(Effect.scoped(program))

We ran node repro.ts separate and node repro.ts merged, and captured the output live:

--- each server in its own Layer.build
A stdout: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-11-25","capabilities":{"logging":{},"completions":{}},"serverInfo":{"name":"A","ve
B stdout: {"jsonrpc":"2.0","id":2,"result":{"protocolVersion":"2025-11-25","capabilities":{"logging":{},"completions":{}},"serverInfo":{"name":"B","ve
--- both servers in one Layer.mergeAll
A stdout: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-11-25","capabilities":{"logging":{},"completions":{}},"serverInfo":{"name":"A","ve
B stdout: (nothing)

Our reading

No spec is involved here. This is a question about layer design. MCP's stdio transport has the client launch "the MCP server as a subprocess", so one server per process is the normal production shape. We hit this only in tests and tooling that host several servers in one process.

Where it happens

• RpcServer.ts:1416-1420: layerProtocolStdio is a module-level Layer.effect(Protocol, makeProtocolStdio).
• McpServer.ts:1439-1447: every layerStdio(...) call provides that same constant. Layers are memoized by reference within one build, so as we read it, both servers resolve Protocol to a single instance built over whichever Stdio was reached first. That fits what we see: B's initialize (id 2) is answered on neither stdout.

How we work around it

We don't have a code fix. In @effected/mcp we document the limitation: give each server a fresh layer memo map, through its own ManagedRuntime, Effect.provide(layer, { local: true }), or its own process.

Is this intended?

If one stdio server per memo map is the intended model, a sentence in the layerStdio / layerProtocolStdio docs would have saved us some time. If it isn't intended, one possible direction is for layerStdio to build its protocol layer per call rather than sharing the module constant. We haven't worked through how that would interact with the shared McpServer.layer.