Bug report form

• I have done a basic search of the issue tracker to find any existing issues that are similar.
• I have checked that my version is at least the latest stable release available via my installation method.

Describe the bug

When using nushell as an MCP server, piping non-nushell (external) commands hangs forever. For example:

echo "hi" | cat

This command never completes when run through the MCP interface.

I have fixed this before in a different context, and IIRC the issue is related to MCP using stdio for JSON-RPC communication. The external commands stdio gets mixed up with the MCP transport layer.

Possible solutions

I previously fixed this by using HTTP streaming for the RPC transport instead of stdio. However, there is probably a way to make stdio work—perhaps by using child process stdio redirection (pseudo-terminals / PTYs?) so the external commands stdio doesnt conflict with the MCP JSON-RPC stdio.

Stdio transport is really nice for integration with tools like Claude Code, so it would be great to find a solution that preserves stdio as a transport option.

How to reproduce

1. Start nushell as an MCP server (nu --mcp)
2. Use an MCP client (e.g., Claude Code) to run: echo "hi" | cat
3. The command hangs indefinitely

Expected behavior

The command should complete and return hi.

Configuration

Running from main branch (commit 08d27ca7e).