René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`session.disconnect()` does not kill stdio MCP server processes spawned for that session","articleBody":"### Describe the bug\n\nWhen a `CopilotSession` is created with `mcpServers` containing one or more `stdio` entries, the Copilot CLI spawns a child process for each server. When `session.disconnect()` is called, those child processes are **not killed**. They only die when `CopilotClient.stop()` is eventually called.\n\nIn workloads that create many short-lived sessions sharing a single `CopilotClient` (e.g. eval/testing pipelines that run N prompts sequentially), this causes one orphaned MCP server process per session, leading to monotonically increasing memory consumption for the lifetime of the client.\n\n### Process hierarchy\n```\nnode (host process)\n  └── Copilot CLI subprocess          ← spawned by CopilotClient\n        └── node mcp-server.js        ← spawned by CLI when session sends first prompt\n        └── node mcp-server.js        ← spawned for next session, previous one still alive\n        └── ...                       ← accumulates until client.stop()\n```\n\n### Affected version\n\nGitHub Copilot CLI 1.0.49.\n\n### Steps to reproduce the behavior\n\nThe following two files are all that is needed. \n\n### `mcp-server.js` — a trivial stdio MCP server\n\n```js\nimport { McpServer } from \"@modelcontextprotocol/sdk/server/mcp.js\";\nimport { StdioServerTransport } from \"@modelcontextprotocol/sdk/server/stdio.js\";\nimport { z } from \"zod\";\n\nconst server = new McpServer({ name: \"greeting-server\", version: \"0.1.0\" });\n\nserver.registerTool(\n  \"demo_get_greeting\",\n  {\n    description: \"Returns a greeting for the given name.\",\n    inputSchema: { name: z.string() },\n  },\n  async ({ name }) =\u003e ({ content: [{ type: \"text\", text: `Hello, ${name}!` }] }),\n);\n\nconst transport = new StdioServerTransport();\nserver.connect(transport);\n```\n\n### `repro-sdk-only.mjs` — the repro script\n\n```js\nimport { CopilotClient, approveAll } from \"@github/copilot-sdk\";\nimport { execFileSync } from \"node:child_process\";\nimport { fileURLToPath } from \"node:url\";\nimport { dirname, join } from \"node:path\";\n\nconst __dir = dirname(fileURLToPath(import.meta.url));\nconst MCP_SERVER = join(__dir, \"mcp-server.js\");\n\nfunction snapshotProcesses(label) {\n  let lines = \"\";\n  try {\n    lines = execFileSync(\n      \"powershell\",\n      [\n        \"-NoProfile\",\n        \"-Command\",\n        \"Get-CimInstance Win32_Process -EA SilentlyContinue\" +\n          \" | Where-Object { $_.Name -eq 'node.exe' -and $_.CommandLine -match 'mcp-server' }\" +\n          \" | ForEach-Object {\" +\n          \"   $m = [math]::Round((Get-Process -Id $_.ProcessId -EA 0).WorkingSet64/1MB,1);\" +\n          '   Write-Output \"  PID=$($_.ProcessId) Mem=${m}MB Cmd=$($_.CommandLine)\"' +\n          \" }\",\n      ],\n      { encoding: \"utf8\", timeout: 5_000 },\n    ).trim();\n  } catch { /* no processes or powershell unavailable */ }\n\n  const count = lines ? lines.split(\"\\n\").length : 0;\n  const status = count \u003e 0 ? `${count} process(es) still alive ← BUG` : \"0 process(es) alive ✓\";\n  console.log(`\\n[${label}] ${status}`);\n  if (lines) console.log(lines);\n}\n\nconst client = new CopilotClient();\n\n// Step 1 — create session\nconsole.log(\"[1] Creating session with stdio MCP server...\");\nconst session = await client.createSession({\n  mcpServers: {\n    \"greeting-server\": {\n      type: \"stdio\",\n      command: \"node\",\n      args: [MCP_SERVER],\n      tools: [\"*\"],\n    },\n  },\n  onPermissionRequest: approveAll,\n  streaming: false,\n  workingDirectory: process.cwd(),\n});\nconsole.log(`    Session created: ${session.sessionId}`);\nsnapshotProcesses(\"after createSession (before prompt)\");\n\n// Step 2 — send a prompt (this is when the CLI spawns the MCP process)\nconsole.log(\"\\n[2] Sending prompt...\");\nawait session.sendAndWait(\n  { prompt: \"Use the demo_get_greeting tool to greet Alice.\", mode: \"immediate\" },\n  60_000,\n);\nconsole.log(\"    sendAndWait returned.\");\nsnapshotProcesses(\"after sendAndWait — MCP process spawned\");\n\n// Step 3 — disconnect: MCP process should die here, but doesn't\nconsole.log(\"\\n[3] Calling session.disconnect()...\");\nawait session.disconnect();\nconsole.log(\"    session.disconnect() returned.\");\nsnapshotProcesses(\"after session.disconnect() — should be 0, is NOT ← BUG\");\n\n// Step 4 — stop the client: only now does the process die\nconsole.log(\"\\n[4] Calling client.stop()...\");\nawait client.stop();\nconsole.log(\"    client.stop() returned.\");\nsnapshotProcesses(\"after client.stop() — finally clean\");\n```\n\n### Run it\n\n```bash\nnode repro-sdk-only.mjs\n```\n\n### Observed results\n\n```\nStarting minimal repro for session.disconnect() MCP leak...\n\nMCP server: C:\\Evals\\evaluate\\tests\\evals\\mcp-process-leak\\mcp-server.js\n\n[1] Creating session with stdio MCP server...\n[CLI subprocess] (node:67688) ExperimentalWarning: SQLite is an experimental feature and might change at any time\n[CLI subprocess] (Use `node --trace-warnings ...` to show where the warning was created)\n    Session created: c9939bf6-b45f-41c0-aad9-607aa6ca489f\n\n[after createSession (before any prompt)] 0 process(es) alive ✓\n\n[2] Sending prompt (triggers MCP server spawn)...\n    sendAndWait returned.\n\n[after sendAndWait — MCP process now alive] 1 process(es) still alive ← BUG\nPID=51096 Mem=78.4MB Cmd=node C:\\Evals\\evaluate\\tests\\evals\\mcp-process-leak\\mcp-server.js\n\n[3] Calling session.disconnect()...\n    session.disconnect() returned.\n\n[after session.disconnect()] 1 process(es) still alive ← BUG\nPID=51096 Mem=67MB Cmd=node C:\\Evals\\evaluate\\tests\\evals\\mcp-process-leak\\mcp-server.js\n\n[4] Calling client.stop()...\n    client.stop() returned.\n\n[after client.stop() — processes finally gone] 0 process(es) alive ✓\n```\n\nKey observation: **the same PID survives `session.disconnect()` and only disappears after `client.stop()`**. The process is not a timing artifact — it persists indefinitely until the client is stopped.\n\n\u003e **Note:** `sendAndWait` fully resolves (the Promise settles and `\"sendAndWait returned.\"` is printed) **before** `session.disconnect()` is ever called. The leak is not caused by a pending or unresolved `sendAndWait` — the process survives even after all session work is entirely complete.\n\n\n### Expected behavior\n\n`session.disconnect()` should kill any stdio MCP server processes that were spawned for that session. Callers should not need to know whether a session used MCP servers in order to ensure clean process teardown.\n\n### Additional context\n\n- **`@github/copilot-sdk`**: latest\n- **OS**: Windows 11\n- **Node.js**: v22.22.2","author":{"url":"https://github.com/AlitzelMendez","@type":"Person","name":"AlitzelMendez"},"datePublished":"2026-05-21T00:21:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/3440/copilot-cli/issues/3440"}