[BUG] claude --print silently prefers ANTHROPIC_API_KEY over subscription auth

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 ANTHROPIC_API_KEY is set in the shell environment (e.g. via .zshrc), claude --print uses API billing instead of subscription auth
• No warning or prompt that billing mode switched
• Discovered after unexpected API spend -- multiple claude --print sessions billed ~$10 to the API console instead of using Max subscription
• The key was originally placed in .zshrc on Claude Code's own recommendation during a setup conversation

What Should Happen?

The CLI should either warn when it detects both auth methods, or at minimum the recommendation should have come with a caveat
about --print mode billing behavior.

Error Messages/Logs

No error messages — the issue is silent. No warning is shown when claude --print
  switches from subscription auth to API key billing.

Steps to Reproduce

1. Have an active Claude subscription (Max/Pro)
2. Export ANTHROPIC_API_KEY in ~/.zshrc (or any shell profile)
3. Run: claude --print "Hello world"
4. Check API console at console.anthropic.com — the call is billed to the API key, not the subscription
5. No warning or indication that billing mode changed

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.79 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

The key was originally added to ~/.zshrc on Claude Code's own recommendation during a setup conversation. This means every shell
process inherits ANTHROPIC_API_KEY, and claude --print silently bills to the API instead of the subscription.

Discovered after ~$10 in unexpected API charges over a week. The API console showed Sonnet 4.6 usage on days where no SDK/API code
was run — only claude --print invocations.

Suggested fix: warn the user when both subscription auth and ANTHROPIC_API_KEY are available, or let the user choose which takes
priority.

Thank you guys! Best, Moritz