[BUG] [SandboxDebug] [Sandbox Linux] message breaks JSON parsing in VS Code extension

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?

Description:
When opening the Claude Code panel in VS Code/VSCodium, the extension fails with:
SyntaxError: Unexpected token 'S', "[SandboxDeb"... is not valid JSON

Root cause:
The sandbox outputs debug messages to stdout instead of stderr:
[SandboxDebug] [Sandbox Linux] Seccomp filtering not available (missing binaries for x64). Sandbox will run
without Unix socket blocking (allowAllUnixSockets mode).

This message appears in the JSON output stream, causing JSON parsing to fail.

Environment:

• Claude Code CLI: 2.0.5
• VS Code Extension: 2.0.44
• OS: Ubuntu Linux (kernel 6.8.0-87-generic)
• Editor: VSCodium installed via Snap (also affects VS Code)

Steps to reproduce:

1. Install Claude Code extension in VS Code/VSCodium on Linux
2. Open the Claude Code panel
3. Error appears after a few seconds

Expected behaviour:
Sandbox debug messages should be sent to stderr, not stdout, so they don't interfere with JSON communication.

What Should Happen?

Sandbox debug messages should be sent to stderr, not stdout, so they don't interfere with JSON communication.

Error Messages/Logs

Logs:
  From Output panel (Claude Code):
  2025-11-18 15:39:16.301 [info] From claude: [SandboxDebug] [Sandbox Linux] Seccomp filtering not available
  (missing binaries for x64)...
  2025-11-18 15:39:16.849 [error] Error from Claude (on channel i7u9dfhscs): SyntaxError: Unexpected token 'S',
  "[SandboxDeb"... is not valid JSON

Steps to Reproduce

1. Install Claude Code extension in VS Code/VSCodium on Linux
2. Open the Claude Code panel
3. Error appears after a few seconds

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.5

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_