[BUG] Sonnet 4.6 consumes 4.5× more context window % than Opus 4.8 on session start with identical project

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?

On session start with a single "start session" message, Sonnet 4.6
shows 18% context
window used. Opus 4.8 shows 4% after the same message plus a second
reply, in the
same project with identical CLAUDE.md, MEMORY.md, and MCP
configuration.

The project system prompt is ~56k tokens (CLAUDE.md + MEMORY.md +
skills list +
deferred tools). Sonnet exhausts context ~4.5× faster than Opus from a
cold start,
making it impractical for projects with moderately sized instructions.

What Should Happen?

Sonnet 4.6 and Opus 4.8 should show comparable context window % for the
same system
prompt size, or the difference should be documented and explained (e.g.
different
context window sizes).

Error Messages/Logs

Opus 4.8 session stats (from /status):
    claude-opus-4-8: 5.0k input, 1.1k output, 48.3k cache read, 56.3k
  cache write

  Sonnet 4.6 context % after first response: 18%
  Opus 4.8 context % after first response + second message: 4%

Steps to Reproduce

1. Create a project with a CLAUDE.md (~180 lines) + auto-memory

MEMORY.md (~60 lines)

1. Start a session with Sonnet 4.6, type "start session", observe

context % in statusline

1. /clear, switch to Opus 4.8, type "start session", observe context %
2. Sonnet shows ~18%, Opus shows ~4% for identical content

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.179

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Claude Code v2.1.179
macOS, Terminal.app
Login: Claude Max account
Both sessions: xhigh effort, same project directory, same .mcp.json
(playwright only)
Google Drive + M365 MCP connectors active on claude.ai account during
Sonnet session
(cloud connectors — not in .mcp.json)