[FEATURE] Enable 1M context window for Sonnet 4.6 in Claude Code

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 4.6 supports 1M context at the API level, but Claude Code only offers 1M context for Opus 4.6. Sonnet is significantly cheaper ($3/$15 vs $5/$25 per million tokens) and faster, making it the preferred model for many daily coding tasks. Being limited to 200K context on Sonnet means more frequent compaction events and lost conversation history during longer sessions, forcing users to either switch to the more expensive Opus model or accept degraded context retention.

Proposed Solution

Allow sonnet[1m] as a valid model option in Claude Code, the same way opus[1m] works today. Users should be able to run /model sonnet[1m] to enable 1M context for Sonnet 4.6 sessions.

Alternative Solutions

Currently the only workaround is switching to Opus 4.6 with 1M context (/model opus[1m]), which works but costs ~67% more per token. For tasks that don't require Opus-level reasoning, this is an unnecessary cost increase just to get a larger context window.

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

1. Working on a large codebase where conversation context frequently exceeds 200K tokens
2. Run /model sonnet[1m] — get error: "Sonnet 4.6 with 1M context is not available for your account"
3. Forced to switch to /model opus[1m] just for the context window, despite Sonnet being sufficient for the task
4. With this feature, I could stay on the cheaper, faster Sonnet model while still benefiting from extended context

Additional Context

The 1M context GA blog post confirms 1M is available for both Opus 4.6 and Sonnet 4.6 at the API level with standard pricing. This request is specifically about enabling it in Claude Code, where it's currently Opus-only.