MockStdioResponsePlugin: @stdin.body.id placeholder fails to resolve when messages arrive back-to-back after an id-less message

Description

When using MockStdioResponsePlugin with blockUnmockedRequests: true, the @stdin.body.id placeholder stops resolving correctly for messages that immediately follow an id-less JSON-RPC notification (e.g. notifications/initialized). The placeholder is output as the literal text @stdin.body.id instead of the numeric id from the current message.

Steps to reproduce

Dev Proxy version: 3.1.0
OS: macOS (Apple Silicon)

Config (.devproxy/devproxyrc.json)

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.1.0/rc.schema.json",
  "plugins": [
    {
      "name": "MockStdioResponsePlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "mockStdioResponsePlugin"
    }
  ],
  "mockStdioResponsePlugin": {
    "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.1.0/mockstdioresponseplugin.schema.json",
    "mocksFile": "stdio-mocks.json",
    "blockUnmockedRequests": true
  }
}

Mocks (.devproxy/stdio-mocks.json)

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.1.0/mockstdioresponseplugin.mocksfile.schema.json",
  "mocks": [
    {
      "request": { "bodyFragment": "protocolVersion" },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"protocolVersion\":\"2024-11-05\"}}\n"
      }
    },
    {
      "request": { "bodyFragment": "accept_eula" },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"content\":[{\"type\":\"text\",\"text\":\"ok\"}]}}\n"
      }
    }
  ]
}

Repro script (repro.mjs)

import { spawn } from "node:child_process";

const proc = spawn("devproxy", ["stdio", "--config-file", ".devproxy/devproxyrc.json", "--", "cat"], {
  stdio: ["pipe", "pipe", "pipe"],
});

let buf = "";
proc.stdout.on("data", (c) => {
  buf += c;
  let i;
  while ((i = buf.indexOf("\n")) >= 0) {
    const line = buf.slice(0, i).trim();
    buf = buf.slice(i + 1);
    if (!line) continue;
    console.log("← stdout:", line);
  }
});

// 1. Send initialize (has id) — works fine
proc.stdin.write(JSON.stringify({ jsonrpc: "2.0", id: 1, method: "initialize", params: { protocolVersion: "2024-11-05" } }) + "\n");

// 2. Wait for response, then send notification + accept_eula back-to-back
setTimeout(() => {
  // Fire-and-forget notification (no id field)
  proc.stdin.write(JSON.stringify({ jsonrpc: "2.0", method: "notifications/initialized" }) + "\n");
  // Immediately followed by a request with id
  proc.stdin.write(JSON.stringify({ jsonrpc: "2.0", id: 2, method: "tools/call", params: { name: "accept_eula", arguments: {} } }) + "\n");
}, 500);

setTimeout(() => {
  proc.kill();
  process.exit(0);
}, 3000);

Run

node repro.mjs

Expected behavior

← stdout: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05"}}
← stdout: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"ok"}]}}

Actual behavior

← stdout: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05"}}
← stdout: {"jsonrpc":"2.0","id":@stdin.body.id,"result":{"content":[{"type":"text","text":"ok"}]}}

The first message resolves @stdin.body.id to 1 correctly. The second message (after the id-less notification) outputs the literal placeholder text.

Workaround

Adding a delay (e.g. 100ms) between the notification and the subsequent request makes it work — suggesting the issue is related to how messages are buffered/parsed when they arrive in the same I/O read.

Context

This blocks using MockStdioResponsePlugin to mock MCP servers that follow the standard JSON-RPC handshake (initialize → notifications/initialized → tools/call), where the notification and first tool call are sent synchronously in the same callback.