[Bug] Sonnet 5 incorrectly limited to 200K context instead of native 1M window

Bug Description
Title: Sonnet 5 is incorrectly limited to 200K context on a Claude subscription

Claude Code is treating Claude Sonnet 5 as a 200K-context model instead of using its native 1M context window.

Environment:

• Claude Code: v2.1.222
• Commit: fbf49312c284
• Installation: npm-global
• Platform: darwin-arm64
• Authentication: Claude subscription, not API
• Opus 5 with 1M context works correctly on the same account

Steps to reproduce:

1. Start a completely new Claude Code session:

claude --model claude-sonnet-5

1. Run /context.

Actual result:
/context reports:

• Model: Sonnet 5
• Model ID: claude-sonnet-5
• Context window: 42.2k/200k tokens

Expected result:
Sonnet 5 should use its native 1M context window, approximately 967K usable tokens in Claude Code, without requiring a [1m] suffix or usage credits.

Additional findings:

• The issue reproduces in a fresh session launched from /tmp.
• CLAUDE_CODE_DISABLE_1M_CONTEXT is not set.
• DISABLE_AUTO_COMPACT, DISABLE_COMPACT, CLAUDE_CODE_MAX_CONTEXT_TOKENS, CLAUDE_CODE_AUTO_COMPACT_WINDOW, ANTHROPIC_BASE_URL, ANTHROPIC_MODEL, and ANTHROPIC_DEFAULT_SONNET_MODEL are all unset.
• claude doctor reports no installation issues.
• /model sonnet correctly resolves to Sonnet 5.
• /model sonnet[1m] incorrectly returns: “Sonnet with 1M context is not available for your account.”
• This is not only a display issue. The session actually stops with Context limit reached when approaching the reported 200K limit.
• Opus 5 with 1M context works normally on the same subscription and installation.

It appears that Sonnet 5 is being evaluated using the legacy Sonnet extended-context entitlement path instead of being recognized as a native 1M-context model.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.222
• Feedback ID: 42913dd8-4da2-4128-bca0-1b0d2a1770aa

Errors

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