Description

The Claude Agent SDK fails to initialize on Windows with the error:

OSError: [WinError 193] %1 is not a valid Win32 application

This occurs when the SDK attempts to spawn the Claude Code CLI subprocess.

Environment

• OS: Windows 11
• Python: 3.12
• claude-agent-sdk: 0.1.3
• Claude Code CLI: Installed via npm globally

Root Cause

On Windows, npm creates two files when installing the @anthropic-ai/claude-code package:

1. claude - A POSIX shell script (for Unix/Git Bash)
2. claude.cmd - A Windows batch file

The SDK's _find_cli() method in subprocess_cli.py:62 uses shutil.which("claude"), which returns the path to the bash script. When Python tries to execute this bash script as a subprocess on Windows, it fails because Windows cannot execute shell scripts as Win32 applications.

Steps to Reproduce

import anyio
from claude_agent_sdk import query

async def main():
    async for message in query(prompt="What is 2 + 2?"):
        print(message)

anyio.run(main)

Result:

claude_agent_sdk._errors.CLIConnectionError: Failed to start Claude Code: [WinError 193] %1 is not a valid Win32 application

Expected Behavior

The SDK should successfully detect and use the claude.cmd batch file on Windows, allowing the subprocess to start correctly.

Proposed Solution

Modify the _find_cli() method to:

1. Use claude.cmd on Windows (sys.platform == "win32")
2. Use claude on other platforms
3. Update fallback location paths to include the .cmd extension on Windows
4. Add common Windows npm installation path (AppData/Roaming/npm)

Additional Context

• The issue is Windows-specific and does not affect Linux/macOS
• Workaround: Users can manually specify the CLI path via ClaudeAgentOptions(cli_path='C:\path\to\claude.cmd')
• Similar issues have been reported in other Python projects that spawn npm-installed CLI tools on Windows

Co-Authored-By: Claude [email protected]