[Bug] claude-opus-4-6 model not recognized despite API availability

Bug Description
Here's the updated bug report with this evidence included:

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Bug: /model claude-opus-4-6 returns "Model not found" despite the model being fully accessible via the API.

Steps to reproduce:

1. Open Claude Code v2.1.62
2. Type /model claude-opus-4-6
3. Error: Model 'Opus 4.6' not found

Expected: Model switches to claude-opus-4-6

Actual: Model not found error

Evidence — API works perfectly:

curl https://api.anthropic.com/v1/messages with model "claude-opus-4-6"
→ Response: {"model":"claude-opus-4-6","type":"message",...} ✅

• Direct API call with the same key returns a valid response from claude-opus-4-6
• Same API key accesses claude-opus-4-6 successfully via Cursor
• GET /v1/models lists claude-opus-4-6 as available for this key
• Other date-suffixed Opus models (e.g. claude-opus-4-20250514) work fine via /model

Root cause hypothesis: Claude Code's internal model validation/allowlist does not include claude-opus-4-6 (no date suffix), even though the API accepts it. The UI tooltip even prompts /model to try Opus 4.6, making this a direct contradiction in the product.

Environment:

• Claude Code v2.1.62
• macOS (Apple Silicon)
• API billing: standard tier

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Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.62
• Feedback ID: ea8c7f4d-fe01-4a88-8a38-7b05cd916d60

Additional Information

[Additional evidence — full API response confirming model works:

curl https://api.anthropic.com/v1/messages (with model "claude-opus-4-6")

Response:
{
  "model": "claude-opus-4-6",
  "id": "msg_01NyZ5c47XtTCHLH1MUBr3sX",
  "type": "message",
  "role": "assistant",
  "content": [{"type": "text", "text": "Pong! 🏓"}],
  "stop_reason": "max_tokens",
  "usage": {"input_tokens": 8, "output_tokens": 16, "service_tier": "standard"}
}

Also, /v1/models lists these as available for this key:
- claude-opus-4-6  ← not recognized by /model
- claude-opus-4-5-20251101
- claude-opus-4-1-20250805
- claude-opus-4-20250514]