[FEATURE] Configuration to expose Sonnet (1M Context) for non first party APIs like Vertex / Bedrock

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

Sonnet (1M Context) is hidden in the model picker based on first party API access + some user setting flag

When using Claude Code with non first party APIs, it's still possible to use sonnet[1m] if properly set up. It'd be nice if there was a way to also expose that in the model picker

Proposed Solution

Some configuration env var or setting that when set, makes Sonnet (1M Context) show up in the model picker. On the user / org to ensure it's actually available / beta headers are being properly accepted on the backend API

Alternative Solutions

It's possible to manually select using /model sonnet[1m] or --model sonnet[1m], but native visibility in the model picker to switch onto at any point is a nice convenience and doesn't require extra user knowledge

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

I'm working with Opus and hit a context overflow

1. I want to switch to Sonnet 1m to compact the session / or continue the work on 1m context
2. I can /model sonnet[1m] if I know about it, but if I don't, when I go to /model I'm lost

Additional Context

_No response_