[BUG] Opus 4.6 [1m] effective context window capped at 400K instead of 1M on Max plan

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

: Max plan, v2.1.92, Opus 4.6 [1m] selected, effective window 400K instead of 1M, started a few days ago ▛███▜▌ Claude Code v2.1.92
▝▜█████▛▘ Opus 4.6 (1M context) with high effort · Claude Max
▘▘ ▝▝ ~/git/FlatFootScribe

❯ /context
⎿ Context Usage
⛁ ⛁ ⛁ ⛁ ⛁ ⛀ ⛀ ⛶ ⛶ ⛶ Opus 4.6 (1M context)
claude-opus-4-6[1m]
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ 21k/400k tokens (5%)

⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ Estimated usage by category
⛁ System prompt: 7.4k tokens (1.8%)
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛁ System tools: 8.6k tokens (2.1%)
⛁ Memory files: 4.6k tokens (1.1%)
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛁ Skills: 476 tokens (0.1%)
⛁ Messages: 8 tokens (0.0%)
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ Free space: 358k (89.5%)
⛝ Autocompact buffer: 21k tokens (5.3%)
⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶ ⛶
On a Max plan, Opus 4.6 with 1M context is correctly identified in both /status and /context, but the effective context window is 400K tokens instead of 1M. This started within the last few days — 1M was working before.
Environment:

Claude Code version: 2.1.92
Model: claude-opus-4-6[1m] (Default Opus 4.6 with 1M context)
Plan: Claude Max
OS: Linux
IDE: Connected to Antigravity extension
MCP servers: 2 connected

/context output:
Opus 4.6 (1M context)
claude-opus-4-6[1m]
21k/400k tokens (5%)

Estimated usage by category
System prompt: 7.4k tokens (1.8%)
System tools: 8.6k tokens (2.1%)
Memory files: 4.6k tokens (1.1%)
Skills: 476 tokens (0.1%)
Messages: 8 tokens (0.0%)
Free space: 358k (89.5%)
Autocompact buffer: 21k tokens (5.3%)
Expected behavior: Effective window should be 1,000K tokens, not 400K.
Actual behavior: Model is correctly identified as claude-opus-4-6[1m] and labeled "1M context" everywhere, but the effective window is capped at 400K tokens. Autocompact and free space calculations all use 400K as the ceiling.
/status output:
Version: 2.1.92
Session ID: 16e71f2a-5627-49be-9659-a1a87c40ce6c
Login method: Claude Max Account
Model: Default Opus 4.6 with 1M context

What Should Happen?

it should be the full 1 million context

Error Messages/Logs

Steps to Reproduce

sign in on Claude code, or cli or any interface and i get the 400k cap. it worked until a few days ago

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Version: 2.1.92

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_