[BUG] debug mode is sensitive to the env var DEBUG which is too generic and very likely to be declared for the codebase claude-code is used for

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

The env var `DEBUG` is a popular name for application debugging and tends to be set in dev environments. IMO, dev tools relying on the same in released software is bad design.
I had reported something similar for the [gh-cli](https://github.com/cli/cli/issues/5289) tool and there is a lot more reasoning there.

### What Should Happen?

Like the gh-cli folk, please do reconsider deprecating this in favour for a more specifically scoped env var name (eg: CC_DEBUG), or a specific value (eg: DEBUG=cc-debug) or remove it completely since the cli already has a `--debug` option.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Change into a repo for an app that uses `DEBUG=true` env var to enable debugging the app.
2. export DEBUG=True and start the app
3. launch `claude`
4. claude will startup with debug enabled

### Claude Model

Not sure / Multiple models

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.276

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

WSL (Windows Subsystem for Linux)

### Additional Information

https://github.com/user-attachments/assets/52dca25b-4c27-4d76-8626-db21e09f4089

This has been reported multiple times:

1. https://github.com/anthropics/claude-code/issues/346
2. https://github.com/anthropics/claude-code/issues/11015
3. https://github.com/anthropics/claude-code/issues/24
4. https://github.com/anthropics/claude-code/issues/70576

But every time the report has been closed without an actual resolution.

Start by reviewing the CLI's existing --debug behavior and the prior issues linked in the report, especially #346, #11015, #24, and #70576. Done means a generic DEBUG setting from the surrounding application no longer enables Claude Code debugging, while the supported debug option remains clear.

Written by the indexing model from the issue text.