CLI silently uses API key instead of Max plan when ANTHROPIC_API_KEY is in environment

Bug description

When ANTHROPIC_API_KEY is present in the shell environment, Claude Code CLI silently switches from the authenticated Max plan (OAuth) to direct API billing — with no warning or confirmation.

This caused unexpected API charges on an Opus key that was intended only for backend scripts, not for the CLI.

Steps to reproduce

1. Authenticate Claude Code via claude login (Max plan / OAuth)
2. In the same shell, export an API key: export ANTHROPIC_API_KEY=sk-ant-...
3. Open a new Claude Code session
4. The session silently uses the API key (and its billing) instead of the Max plan

Expected behavior

Claude Code should either:

• Warn the user when it detects both an OAuth session AND an ANTHROPIC_API_KEY in the environment, letting them choose which to use
• Prioritize the Max plan login over the environment variable, since the user explicitly authenticated via claude login
• At minimum, show a visible indicator in the UI that the session is using API billing rather than the Max plan

Actual behavior

• No warning is shown
• The CLI silently picks up ANTHROPIC_API_KEY and uses it for all requests
• The user only discovers the issue after checking API usage/billing

Workaround

Add unset ANTHROPIC_API_KEY to ~/.bashrc to prevent the CLI from picking it up. Keep project-specific keys in separate env vars (e.g. ANTHROPIC_API_KEY_PRODUCTION).

Environment

• Claude Code CLI (latest)
• Linux (Ubuntu)
• Max plan subscriber