Opus/Sonnet 1M context variants broken on Max 20x — misleading 'Rate limit reached' error

Bug Description

The 1M context model variants (opus[1m] / claude-opus-4-6[1m]) selected via /model fail with "API Error: Rate limit reached" despite ample quota remaining. This has been broken for over a week. The error is misleading — it's not a rate limit issue.

The non-1M variants (Default, explicit claude-opus-4-6) work fine with the same account and quota.

Reproduction

Tested systematically by editing settings.json and launching fresh sessions:

| "model" value in settings.json | Context Window | Result |
|---|---|---|
| (key absent — Default) | 200k | Works |
| "claude-opus-4-6" | 200k | Works |
| "opus[1m]" (via /model menu option 2) | 1M | Rate limit error |
| "claude-opus-4-6[1m]" (manual or rewritten by Claude Code) | 1M | Rate limit error |

Steps

1. Run Claude Code v2.1.63
2. /model → select "Opus (1M context)" (option 2)
3. Settings written as "model": "opus[1m]"
4. Send any message → API Error: Rate limit reached
5. /model → select "Default" → works immediately

Additional observations

• The /model menu itself hints at the issue: selecting the stale claude-opus-4-6[1m] shows: "Newer version available · select Opus for Opus 4.6" — but selecting "Opus (1M context)" from the menu gives the same broken ID
• Session JSONL confirms: failed requests show "model":"<synthetic>" with "error":"rate_limit" (never reaches a real model), while Default requests show "model":"claude-opus-4-6" with "service_tier":"standard"

Usage at Time of Error

Current session:   5% used
Current week:     19% used (all models)
Sonnet only:       4% used

Nowhere near any limit.

Impact

• Users who previously selected a 1M context model are permanently broken with no obvious fix
• The "Rate limit reached" error is misleading — users wait for a reset that never helps
• The only workaround is using "Default" (200k context) or manually setting "model": "claude-opus-4-6" and not using /model

Environment

• Claude Code: v2.1.63
• Plan: Claude Max 20x
• Platform: Windows 11 Pro (win32)
• Terminal: Windows Terminal with PowerShell 7
• Shell (Claude Code): bash
• Update channel: latest