NotImplementedError - user parameter not supported in subprocess creation

[Ellie1206]

Environment

• OS: Windows 10/11
• Python: 3.13
• claude-agent-sdk: (bundled version in notebook-intelligence 4.3.1)
• anyio: 4.10.0

Description

When the Claude Agent SDK attempts to create a subprocess on Windows, it fails with
NotImplementedError because the user parameter is passed to anyio.open_process(), but this
parameter is not supported on Windows.

Error Message

NotImplementedError (no message)

Full traceback shows the error originates from:
File "C:...\asyncio\base_events.py", line 539, in _make_subprocess_transport
raise NotImplementedError

Root Cause

The user parameter for running subprocesses as a different user is a Unix/Linux-specific feature.
On Windows, attempting to use this parameter causes NotImplementedError to be raised in asyncio's
subprocess transport creation.

Location: claude_agent_sdk/_internal/transport/subprocess_cli.py, line ~410

Current code

  self._process = await anyio.open_process(
      cmd,
      stdin=PIPE,
      stdout=PIPE,
      stderr=stderr_dest,
      cwd=self._cwd,
      env=process_env,
      user=self._options.user,  # <-- Not supported on Windows
  )

Proposed Fix

  Conditionally include the user parameter only on non-Windows platforms:

  # Build process options - user parameter is not supported on Windows
  process_options = {
      "stdin": PIPE,
      "stdout": PIPE,
      "stderr": stderr_dest,
      "cwd": self._cwd,
      "env": process_env,
  }

  # Only add user parameter on non-Windows systems
  if platform.system() != "Windows" and self._options.user is not None:
      process_options["user"] = self._options.user

  self._process = await anyio.open_process(cmd, **process_options)

Note: platform is already imported at the top of the file (line ~6).

Steps to Reproduce

1. Use Claude Agent SDK on Windows (e.g., through Intelligent Notebook)
2. Attempt to create a subprocess via the SubprocessCLITransport
3. Observe NotImplementedError in asyncio's _make_subprocess_transport

Expected Behaviour

Subprocess should be created successfully on Windows (without user parameter).

Actual Behaviour

NotImplementedError is raised when attempting subprocess creation, preventing the SDK from functioning on Windows.

Additional Context

• This affects all Windows users of the Claude Agent SDK
• Related to Issue Hide subprocess terminal for Windows system #480 which shows the problematic code
• This is a platform-specific compatibility issue documented in Python's asyncio documentation
• Fix has been tested and confirmed working on Windows 10/11 with Python 3.13
• The user parameter is only meaningful on Unix-like systems where processes can be started as different users