Max x20 plan blocked from using Opus 4.6 with 1M context - "Extra usage is required"

Bug description
Starting today (2026-04-05, ~08:00 UTC), I can no longer use Opus 4.6 with 1M context on my Max x20 subscription. Every session immediately shows:

API Error: Extra usage is required for 1M context · run /extra-usage to enable, or /model to switch to standard context

This worked fine before today. I have to downgrade to Opus with 200K context to use Claude Code at all.

Expected behavior
Max x20 plan should include Opus 4.6 with 1M context without requiring extra usage opt-in.

Debug log findings (~/.claude/debug/<session-id>.txt):

• Startup quota check (/v1/messages source=quota_check) returns extra_usage_disabled
• Claude Code then blocks Opus 4.6 (1M context) based on this flag - the block is client-side, no actual API message call is ever made
• The same extra_usage_disabled response appears even on sessions that work fine with 200K context, suggesting the flag is not specific to 1M but is being incorrectly used to gate it
• Request IDs from failed sessions:
• 4fc14606-73ac-433d-b984-dac54c2e03b7 (session 05689d8b)
• d6d10f7c-bd6d-40e6-aad3-1eb6e5a15d98 (session c1488378)

Environment

• Plan: Max x20
• Model: Opus 4.6 (1M context)
• Platform: macOS (Darwin 25.4.0)
• Claude Code version: v2.1.92

Reproduction

1. Open Claude Code with --debug
2. Select Opus 4.6 (1M context) via /model
3. Error appears immediately before any message can be sent
4. Downgrading to Opus 4.6 (200K) works fine