[BUG] Skill description budget uses base context, ignores [1m] extension

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?

## Summary
Skill description budget is capped at 2% of context window per the 2.1.x changelog, but on
claude-opus-4-7[1m] the budget appears to be computed against the base 200K context (~4,000
chars), not the 1M extension (~20,000 chars). This causes heavy skill truncation for paid 1M-context
users.

## Repro

• Model: claude-opus-4-7[1m] (1M context add-on)
• Claude Code: 2.1.133
• ~60 SKILL.md files, ~11,200 chars total description text
• Result: "32 skill descriptions dropped" on session start

11,200 chars fits comfortably in 20,000 (2% of 1M) but blows past 4,000 (2% of 200K). The math is
consistent with the budget being calculated off the base model context, ignoring the [1m] flag.

## Workaround
Archive skills or shorten descriptions to fit the 4K budget — defeats the point of paying for 1M
context.

What Should Happen?

## Expected
1M-context users get the 1M-scaled budget (~20K chars), so paid extended-context plans don't get
throttled on skill auto-routing.

Error Messages/Logs

See previous

Steps to Reproduce

Constant

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Claude Opus prior to 04/30/26

Claude Code Version

2.1.133

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_