[BUG] `/export` concatenates multiple arguments into a single filename when no extension present

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When the first argument has no file extension, all arguments (including paths intended as destinations) are concatenated into a single filename string with spaces preserved, then .txt is appended. This creates invalid file paths that fail with ENOENT.

What Should Happen?

Arguments should be parsed consistently. Either support a <name> <destination> syntax and parse correctly, or reject multiple arguments with a clear error message.

Error Messages/Logs

> /export test ~/Desktop
Failed to export conversation: ENOENT: no such file or directory, open '/Users/rps/Desktop/old-github/claude-code/test ~/Desktop.txt'                                                                             
                                                                                                                                                                                                                     
> /export test /Users/rps/Desktop                                                                                                                                                                                 
Failed to export conversation: ENOENT: no such file or directory, open '/Users/rps/Desktop/old-github/claude-code/test /Users/rps/Desktop.txt'                                                                   
                                                                                                                                                                                                                     
> /export test /Users/rps/Desktop/test.txt
Failed to export conversation: ENOENT: no such file or directory, open '/Users/rps/Desktop/old-github/claude-code/test /Users/rps/Desktop/test.txt'

Steps to Reproduce

1. Open Claude Code in any project directory
2. Run /export test ~/Desktop
3. Observe the error shows the entire string test ~/Desktop.txt used as one filename
4. Run /export test /Users/<you>/Desktop
5. Observe the same concatenation behavior

Claude Model

Opus

Is this a regression?

I don't know.

Last Working Version

_No response_

Claude Code Version

2.1.92 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

The /export command appears to support relative paths with extensions (e.g., /export ../../test.txt correctly resolves the path), but this is undocumented. /help export provides no information about supported arguments, path syntax, or expected behavior.

The presence or absence of a file extension in the first argument fundamentally changes how the command parses input:

• With extension: only first arg used as filename, relative paths resolve, destination arg silently ignored
• Without extension: all args concatenated as filename, relative paths mangled

This suggests the path resolution logic exists but is gated behind extension detection, and the no-extension code path has a different (broken) implementation.

Tested on macOS with both Opus 4.5 and Opus 4.6 — identical behavior.