[FEATURE] Include mode (ie. plan) as part of the API payload

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When proxyxing requests via ANTHROPIC_BASE_URL, it'd be nice to be able to determine the "mode" that they are in.

This would help in a couple ways:

• Visibility in to how users use Claude Code
• Control which models are accessible based on the mode from the proxy side

Proposed Solution

Add a new key to the API payload which includes the 'mode' (ie. plan).

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

API and model interactions

Use Case Example

Restricting access to certain models based on the mode the user is in.

ie. Restricting Opus to just "plan" mode

Additional Context

_No response_