Support primary/fallback billing (subscription + API key overflow)

Feature Request

Problem: Claude Code currently uses a single active billing method based on a fixed precedence order. There's no way to configure it to use a subscription (Pro/Max) as the primary billing method and automatically fall back to API key billing when the subscription quota is exhausted.

Desired behavior: Allow users to configure a billing chain, e.g.:

1. Use my Claude Pro/Max subscription first
2. When subscription quota is exceeded, automatically switch to API key / AI billing for overages

Current workaround: Users must manually switch between billing methods (remove/add ANTHROPIC_API_KEY, re-login, etc.) when their subscription quota runs out, which interrupts workflow.

Use case: Users with a subscription who want uninterrupted usage — leveraging their included subscription quota first, then paying for overages via API billing rather than being blocked when the subscription limit is hit.

🤖 Generated with Claude Code